4.1: 4.1: Mendalian genetic;Mendalian genetic; monohybrid & dihybridmonohybrid & dihybridInheritanceInheritance : the genetic transmission of : the genetic transmission of

characteristics from parentcharacteristics from parent

of offspring.of offspring.

GenesGenes : Unit of information about : Unit of information about

specific traits & they are passed specific traits & they are passed

from parents to offspring/ Small from parents to offspring/ Small

section of DNA that codes for a section of DNA that codes for a

particular protein. Each gene particular protein. Each gene has a has a

specific location (locus) on a specific location (locus) on a

chromosome.chromosome.

AlleleAllele

Alternative forms of a genetic locus.Alternative forms of a genetic locus.

A single allele for each locus is inherited A single allele for each locus is inherited separately from each parent (e.g. at a separately from each parent (e.g. at a locus for eye colour the allele might result locus for eye colour the allele might result

in blue or brown eyes.in blue or brown eyes.

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LocusLocus : The position on a : The position on a ((plural : lociplural : loci)) chromosome of a gene or chromosome of a gene or

other chromosome other chromosome marker.marker.

SegregationSegregation- the separation of paired - the separation of paired alleles or homologous chromosomes, alleles or homologous chromosomes, especially during meiosis, so that the especially during meiosis, so that the members of each pair appear in different members of each pair appear in different gametes.gametes.

Back crossBack cross : A crossing of a heterozygous: A crossing of a heterozygous organism & one of its organism & one of its homozygous parents.homozygous parents.

DominantDominant : a gene is said to be dominant: a gene is said to be dominantalleleallele if it expresses its phenotype if it expresses its phenotype

even in the presence of a even in the presence of a recessive gene.recessive gene.

GameteGamete : mature male or female : mature male or female reproduction cell (sperm or reproduction cell (sperm or ovum) with a haploid set of ovum) with a haploid set of chromosome (23 for human). chromosome (23 for human).

GenotypeGenotype : the genetic constitution of : the genetic constitution of an organism. an organism.

PhenotypePhenotype : The physical appearance / : The physical appearance /

observable characteristics observable characteristics

of an organism.of an organism.

HeterozygoteHeterozygote : An organism or cell having : An organism or cell having

2 different alleles at 2 different alleles at

corresponding loci on corresponding loci on

homologous homologous

chromosomes. (Aa)chromosomes. (Aa)

HomozygoteHomozygote : An organism whose : An organism whose

genotype is characterized genotype is characterized

by two identical alleles of a by two identical alleles of a

gene. (AA or aa).gene. (AA or aa).

RecessiveRecessive : A gene that is expressed : A gene that is expressed

alleleallele only when it is present in 2 only when it is present in 2

copies or if the other copy iscopies or if the other copy is

missing.missing.

Self-crossSelf-cross : cross involving plants of the: cross involving plants of the

same generationsame generation

Test-crossTest-cross : The mating of an : The mating of an organism organism

to a double recessive in to a double recessive in order to determine order to determine

whether whether it is homozygous or it is homozygous or heterozygous for a heterozygous for a character under character under consideration.consideration.

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True breeding or pure breedingTrue breeding or pure breeding::

Organisms that are homozygous for Organisms that are homozygous for any given genotype & therefore pass it on any given genotype & therefore pass it on to all their progeny in a cross withy a to all their progeny in a cross withy a similar homozygote.similar homozygote.

Reciprocal crossReciprocal cross

Using male & female gametes for 2 Using male & female gametes for 2 different traits & alternating the source of different traits & alternating the source of the gametes.the gametes.

MENDEL’S EXPERIMENTSMENDEL’S EXPERIMENTSMendel was an Austrian monk who studied Mendel was an Austrian monk who studied

the inheritance of characteristic in the inheritance of characteristic in garden garden peas ( peas ( Pisum sativumPisum sativum)) which he grew in which he grew in the vegetable garden in his monastery.the vegetable garden in his monastery.

He choose peas because:He choose peas because: They were They were easy to groweasy to grow.. They had a They had a short life cycleshort life cycle.. Their Their pollination could be controlledpollination could be controlled.. They’ve They’ve easily observable characteristicseasily observable characteristics..

He studied He studied sevenseven characteristics of pea characteristics of pea plants, each of which has 2 contrasting plants, each of which has 2 contrasting alternatives.alternatives.1.1. Seed shapeSeed shape : round/wrinkled : round/wrinkled2. 2. Seed colourSeed colour : yellow/green : yellow/green3. 3. Pod shapePod shape : inflated/constricted : inflated/constricted4. 4. Pod colourPod colour : yellow/green : yellow/green5. 5. Flower colourFlower colour : purple/white : purple/white6. 6. Flower positionFlower position : axial/terminal : axial/terminal7. 7. Stem lengthStem length : tall/short (dwarf) : tall/short (dwarf)

Mendel’s First Law/Law of Mendel’s First Law/Law of SegregationSegregation

A pair of allelic genes that are present in an A pair of allelic genes that are present in an organism for a given character, separate or organism for a given character, separate or segregate segregate from each other during meiosis, so from each other during meiosis, so they end up in a different gametethey end up in a different gamete.. This This segregation is just a chance and there is no segregation is just a chance and there is no choice. choice.

For example when the tall plant which is For example when the tall plant which is heterozygous produces gametes, only one (T) of heterozygous produces gametes, only one (T) of the gene pair goes into one gamete and (t) to the the gene pair goes into one gamete and (t) to the other. In this process whether T gene goes to this other. In this process whether T gene goes to this gamete or that gamete is purely a chance process. gamete or that gamete is purely a chance process.

However if thousand gametes are produced, 500 However if thousand gametes are produced, 500 of them receive dominant. "T" genes and the rest of them receive dominant. "T" genes and the rest (500) receive "t" genes. Thus the segregation is (500) receive "t" genes. Thus the segregation is random, but in equal ratios. random, but in equal ratios.

Monohybrid CrossMonohybrid Cross

In monohybrid inheritance, a pair of In monohybrid inheritance, a pair of contrasting characters was studied.contrasting characters was studied.

Mendel used pure breed plant for parental Mendel used pure breed plant for parental generation (P).generation (P).

For e.g. pea plant have one gene for For e.g. pea plant have one gene for flower colour. The gene for flower colour flower colour. The gene for flower colour has two alleles : purple colur (P) & white has two alleles : purple colur (P) & white colour (p). Therefore, there are 3 possible colour (p). Therefore, there are 3 possible genotypes:genotypes:

PP – homozygous for PPP – homozygous for P

Pp - heterozygous for the 2 allelesPp - heterozygous for the 2 alleles

pp – homozygous for ppp – homozygous for p

He crossed all purple flowers with all white He crossed all purple flowers with all white flowers.flowers.

This is a parental pure-breeding x pure This is a parental pure-breeding x pure breeding.breeding.

P – purple flower p- white flower

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Mendel’s Conclusions on Mendel’s Conclusions on Monohybrid CrossMonohybrid Cross

The FThe F22 plants were a mixture of purple & plants were a mixture of purple &

white flowers in approximate ratio of 3 : 1.white flowers in approximate ratio of 3 : 1.The most striking features of these results The most striking features of these results

were that:were that:

1. 1. there were no flower of intermediate there were no flower of intermediate

colour.colour.

2. there were no white flower in F2. there were no white flower in F11 generation generation

although they reappeared in the Falthough they reappeared in the F22..

From the first observation, Mendel From the first observation, Mendel concluded that characteristics are not concluded that characteristics are not blended together like different colours of blended together like different colours of paint but they are determined by definite, paint but they are determined by definite, discrete particles which he called ‘factors’.discrete particles which he called ‘factors’.

From the second observation, he From the second observation, he concluded that the factor for white colour concluded that the factor for white colour must be carried in the F1 plants, but is must be carried in the F1 plants, but is ‘hidden’ by the factor for purple colour.‘hidden’ by the factor for purple colour.

The white colour factor is expressed only The white colour factor is expressed only in the absence of purple colour factor.in the absence of purple colour factor.

Therefore, each character is controlled by Therefore, each character is controlled by a pair of factors, one factor coming from a pair of factors, one factor coming from each parent.each parent.

As all F1 flowers are purple, the factor for As all F1 flowers are purple, the factor for purple colour must be purple colour must be dominantdominant to the to the factor of white colour, which is the factor of white colour, which is the recessiverecessive factor. factor.

In modern genetic terms, the inheritance In modern genetic terms, the inheritance factor is actually the factor is actually the genegene..

Each character is controlled by a pair of Each character is controlled by a pair of genes.genes.

The different form of a gene are called The different form of a gene are called allelesalleles..

The hereditary factors (genes) do not mix The hereditary factors (genes) do not mix & remain as separate particles during their & remain as separate particles during their transmission from the parents to the transmission from the parents to the offspring.offspring.

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Mendel's observations from these Mendel's observations from these experiments can be summarized in the experiments can be summarized in the principles below :principles below :

Mendel’s first law: Mendel’s first law: law of segregationlaw of segregation -  - the two alleles for a heritable character the two alleles for a heritable character

segregate (separate) segregate (separate) during gamete formation during gamete formation and and end up in different gametes. end up in different gametes. →E.g. Homologous chromosomes line up during →E.g. Homologous chromosomes line up during metaphase I of meiosis and then separate metaphase I of meiosis and then separate forming forming haploid cellshaploid cells; ;

each homologous chromosome had one allele each homologous chromosome had one allele for the trait and they are now in different gamete for the trait and they are now in different gamete cells.cells.

Pairs of chromosomes separate during Pairs of chromosomes separate during meiosis I, gametes are haploid.meiosis I, gametes are haploid.

They carry only one copy of each They carry only one copy of each chromosome.chromosome.

An Aa individual therefore produces 2 An Aa individual therefore produces 2 kinds of gametes, i.e. A & a.kinds of gametes, i.e. A & a.

Aa

aA

aA aAgametes

The inheritance of traits can be explained by The inheritance of traits can be explained by meiosis. meiosis.

Test cross in monohybridTest cross in monohybrid

A A test crosstest cross is carried out is carried out by mating an by mating an organism to a double recessiveorganism to a double recessive in in order to determine whether the order to determine whether the organism organism is is homozygous or homozygous or heterozygousheterozygous for a character under for a character under consideration.consideration.

E.g. A pea plant with purple flower may E.g. A pea plant with purple flower may be either homozygous dominant (PP) or be either homozygous dominant (PP) or heterozygous (Pp). heterozygous (Pp).

If all the progeny of the testcross have If all the progeny of the testcross have purple flowers, then the purple flower purple flowers, then the purple flower parent was probably homozygous parent was probably homozygous dominant since a PP x pp cross dominant since a PP x pp cross produces Pp progeny. produces Pp progeny.

If the progeny of the testcross contains If the progeny of the testcross contains both purple and white phenotypes, then both purple and white phenotypes, then the purple flower parent was the purple flower parent was heterozygous since a Pp x pp cross heterozygous since a Pp x pp cross produces Pp and pp progeny in a 1:1 produces Pp and pp progeny in a 1:1 ratio. ratio.

Mendel’s Second Law/Law of Mendel’s Second Law/Law of Independent AssortmentIndependent Assortment

Mendel also cross peas that different in 2 contrasting Mendel also cross peas that different in 2 contrasting traits.traits.

A dihybrid inheritance is the inheritance of two A dihybrid inheritance is the inheritance of two characteristics that are controlled by a different genes at characteristics that are controlled by a different genes at different loci.different loci.

The resulting (F2) generation did not have 3:1 dominant : The resulting (F2) generation did not have 3:1 dominant : recessive phenotype ratios. recessive phenotype ratios.

Instead of 4 possible genotypes from a monohybrid Instead of 4 possible genotypes from a monohybrid cross, dihybrid crosses have as many as 16 possible cross, dihybrid crosses have as many as 16 possible genotypes.genotypes.

Dihybrid CrossDihybrid Cross

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CCrosses With Two Traitsrosses With Two Traits

A A dihybriddihybrid cross is a mating between parents that differ cross is a mating between parents that differ in two traits. in two traits.

Mendel performed dihybrid crosses by mating two Mendel performed dihybrid crosses by mating two individuals which differed in seed color (yellow and individuals which differed in seed color (yellow and green) and seed shape (round and wrinkled). green) and seed shape (round and wrinkled).

Round seeds (R) are dominant over wrinkled (r) seeds.Round seeds (R) are dominant over wrinkled (r) seeds. Yellow seed color (Y) is dominant over green (y).Yellow seed color (Y) is dominant over green (y). Plants homozygous for round yellow seeds (RRYY) were Plants homozygous for round yellow seeds (RRYY) were

crossed with plants homozygous for wrinkled green crossed with plants homozygous for wrinkled green seeds (rryy). seeds (rryy).

All the F1 progeny were heterozygous for both traits All the F1 progeny were heterozygous for both traits (RrYy) and had round yellow seeds (the dominant (RrYy) and had round yellow seeds (the dominant phenotypes).phenotypes).

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Mendel allowed self-pollination of F1 plants Mendel allowed self-pollination of F1 plants (RrYy x RrYy) to produce an F2 generation. (RrYy x RrYy) to produce an F2 generation.

When he categorized peas from the F2 When he categorized peas from the F2 generation he found a ratio of 315:108:101:32 generation he found a ratio of 315:108:101:32 which approximates a 9:3:3:1 phenotypic ratio. which approximates a 9:3:3:1 phenotypic ratio.

The experimental results supported the The experimental results supported the hypothesis that hypothesis that each allele pair segregates each allele pair segregates independently during gamete formationindependently during gamete formation. .

Mendel tried all seven of his traits in various Mendel tried all seven of his traits in various combinations in dihybrid crosses and found the combinations in dihybrid crosses and found the same 9:3:3:1 in each case. same 9:3:3:1 in each case.

This behavior of genes during gamete formation This behavior of genes during gamete formation is called is called independent assortmentindependent assortment and the and the principle is referred to as principle is referred to as Mendel's Law of Mendel's Law of Independent AssortmentIndependent Assortment..

From the result of this experiment Mendel From the result of this experiment Mendel construct his second Law : construct his second Law : The Law of The Law of Independent AssortmentIndependent Assortment..

During the gamete formation, segregation of the During the gamete formation, segregation of the alleles of one allelic pair is independent of the alleles of one allelic pair is independent of the segregation of the alleles of another allelic pairsegregation of the alleles of another allelic pair

Variation (in the combination of alleles you Variation (in the combination of alleles you inherit) is due to:inherit) is due to:1. crossing over (meiosis)1. crossing over (meiosis)2. independent assortment 2. independent assortment

(meiosis)(meiosis)3. random fertilization3. random fertilization

A test cross-conducted for the dihybrid inheritance exhibited

a ratio of 1 : 1: 1: 1 of each phenotype.

Mandel confirmed the results of his second law by Mandel confirmed the results of his second law by performing a performing a testcrosstestcross::

F1 dihybrid x recessive parentF1 dihybrid x recessive parentYyRr x yyrrYyRr x yyrr

The The inheritance of inheritance of two traits on two traits on different different chromosomes chromosomes can be can be explained by explained by meiosis. meiosis.

4.2:4.2:Crosses that deviate from the Crosses that deviate from the Mendelian InheritanceMendelian Inheritance

In codominance, the heterozygous & the In codominance, the heterozygous & the homozygous individuals have different homozygous individuals have different phenotypes.phenotypes.

The 2 alleles in the heterozygous The 2 alleles in the heterozygous individuals are equally dominant & both individuals are equally dominant & both are expressed simultaneouslyare expressed simultaneously

4.2.1:Codominant Allele

E.g.: 1. The ABO blood system in humans.E.g.: 1. The ABO blood system in humans. - the alleles - the alleles IIAA & & IIBB are codominant. are codominant.

2. The MN blood group system in 2. The MN blood group system in humans.humans. - this grouping is due to antigen - this grouping is due to antigen found on the surface of the RBC.found on the surface of the RBC.

- In this case there are 2 antigen M & - In this case there are 2 antigen M & NN

- The production is determined by a - The production is determined by a gene with 2 alleles:gene with 2 alleles:

LLMM – provide ability to produce M antigen – provide ability to produce M antigen

LLNN – provide ability to produce N antigen – provide ability to produce N antigen Individual LIndividual LMMLLMM – only have M antigen on their RBC – only have M antigen on their RBC Individual LIndividual LNNLLNN – only have N antigen on their RBC – only have N antigen on their RBC Individual LIndividual LMMLLNN – have both antigen M & N on their RBC – have both antigen M & N on their RBC

PP LLMMLLMM x L x LNNLLNN

GG

F1F1 L LMMLLNN

F1 x F1F1 x F1

P LP LMMLLNN x L x LMMLLNN

GG

F2F2

LM LN

LNLMLNLM

Gametes LM LN

LM LMLM LMLN

LN LMLN LNLN

The genotypic ratio:

1 LMLM : 2 LMLN : 1 LNLN

The phenotypic ratio:

1 producing M antigen2 producing both antigen1 producing N antigen

This is different from Mendel’s Law because there are 3 phenotypes in the ratio 1:2:1 in the F2 generation instead of 2 phenotypes (3 : 1).

Another example of Another example of codominance, red cows codominance, red cows crossed with white will crossed with white will generate roan cows. generate roan cows.

Roan refers to cows that Roan refers to cows that have red coats with have red coats with white blotches. white blotches.

When the F1 roan When the F1 roan individuals self-fertilize, individuals self-fertilize, the F2 progeny have a the F2 progeny have a phenotypic ratio of 1 phenotypic ratio of 1 red:2 roan:1 white. red:2 roan:1 white.

One allele is not fully dominant over its One allele is not fully dominant over its partnerpartner, so in the heterozygous condition, , so in the heterozygous condition, the total product is the total product is intermediateintermediate between between that of the dominant & recessive alleles.that of the dominant & recessive alleles.

E.g. E.g. colour of snapdragon flowercolour of snapdragon flower ( (AntirrhinumAntirrhinum)) Heterozygotes for colour alleles have Heterozygotes for colour alleles have pinkpink colours in colours in

contrast to contrast to redred (dominant homozygotes) & (dominant homozygotes) & whitewhite (recessive homozygotes).(recessive homozygotes).

The phenotype ratio for the monohybrid cross then The phenotype ratio for the monohybrid cross then becomes 1 : 2 : 1 instead of 3 : 1.becomes 1 : 2 : 1 instead of 3 : 1.

Cross between a red flowered plant & white flowered Cross between a red flowered plant & white flowered plant.plant.

4.2.2: Incomplete Dominant Allele

Genotypic ratio:1 CRCR : 2 CRCW : 1CWCW

Phenotypic ratio:1 red : 2 pink : 1 white

EpistasisEpistasis The The masking of the phenotypic effect of masking of the phenotypic effect of

alleles at one gene alleles at one gene by alleles of another by alleles of another gene. gene.

A gene is said to be A gene is said to be epistaticepistatic when its when its presence suppresses the effect of a gene at presence suppresses the effect of a gene at another locus. another locus.

Epistatic genes are sometimes called Epistatic genes are sometimes called inhibiting genes because of their effect on inhibiting genes because of their effect on other genes which are described as other genes which are described as hypostatichypostatic..

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For example Mouse ColourFor example Mouse ColourIn mice and other rodents, the gene for pigment In mice and other rodents, the gene for pigment deposition (C) is epistatic to the gene for pigment deposition (C) is epistatic to the gene for pigment (melanin) production. (melanin) production. In other words, whether the pigment can be In other words, whether the pigment can be deposited in the fur determines whether the coat deposited in the fur determines whether the coat color can be expressed. color can be expressed.

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One gene for pigment (melanin) production : One gene for pigment (melanin) production :

B = Black fur, b= brown fur, B = Black fur, b= brown fur, Another gene is the gene for pigment deposition (C) Another gene is the gene for pigment deposition (C) Homozygous recessive for pigment deposition (cc) Homozygous recessive for pigment deposition (cc)

will result in an albino mouse regardless of the will result in an albino mouse regardless of the genotype at the black (B)/brown (b) locus (BB, Bb or genotype at the black (B)/brown (b) locus (BB, Bb or bb)bb)

CC = normal pigment production = CC = normal pigment production = C dominant over c NO pigment produced (recessive).C dominant over c NO pigment produced (recessive). E.g.:A Black mouse BBCC is crossed with an Albino E.g.:A Black mouse BBCC is crossed with an Albino

mouse bbcc. mouse bbcc. All F1 offspring will be black mice BbCc All F1 offspring will be black mice BbCc 

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Then if we cross mice from the F1 generation (BbCc X BbCc), the gametes each mouse could produce would be (BC, Bc, bC, and bc).

4949

BC Bc bC bc

BC BBCC - black BBCc - black BbCC - black BbCc - black

Bc BBcC - black BBcc - albino BbCc - black Bbcc - albino

bC BbCC - black BbCc - black bbCC - brown bbCc - brown

bc BbCc - black Bbcc - albino bbCc - brown bbcc - albino

If epistasis occurs between two nonallelic genes, the phenotypic ratio resulting from a dihybrid cross will deviate from the 9:3:3:1 Mendelian ratio. Even though both genes affect the same character (coat color), they are inherited separately and will assort independently during gamete formation. A cross between black mice that are heterozygous for the two genes results in a 9:3:4 phenotypic ratio: The F2 would be = 9 black : 3 brown : 4 albino

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Multiple alleles can be of more than 2 Multiple alleles can be of more than 2 forms of the allele.forms of the allele.

E.g. human ABO blood group where it has E.g. human ABO blood group where it has 3 different alleles for blood type:3 different alleles for blood type:

IIAA (Type A) (Type A)

IIBB (Type B) (Type B)

iiºº (Type O) (Type O)

We can only have 2 of three alleles in an We can only have 2 of three alleles in an individual.individual.

4.2.3: Multiple Alleles

They combine to They combine to form genotypes form genotypes that result from that result from codominantcodominant..

The different The different genotypes of genotypes of blood will produce blood will produce the following the following phenotypes:phenotypes:

Type O Type AType O Type A

PP iiOO i iOO x I x IAAIIAA

GG

F1F1 I IAAiiOO blood type Ablood type A

F1 x F1F1 x F1 I IAAiiOO x I x IAA i iOO

G G

F2F2

iO IA

iOIAiOIA

Gametes IA Io

IA IAIA IA i O

iO IAiO i o iO

The phenotypic ratio:3 blood type A : 1 blood type O

The genotypic ratio is:

1 IAIA : 2 IA i

o : 1 i o i o

Again, this shows a 1:2:1 ratio forgenotype

Lethal alleles are alleles which bring about Lethal alleles are alleles which bring about death.death.

There are 2 types of lethal alleles:There are 2 types of lethal alleles:• 1) dominant lethal genes (1) dominant lethal genes (cause death even in cause death even in • heterozygous condition)heterozygous condition)

• 2) recessive lethal genes2) recessive lethal genes

4.2.4: Lethal Alleles

The gene The gene yellowyellow (in mice) show unusual patterns of (in mice) show unusual patterns of inheritance.inheritance.

A)  Yellow mice crossed to wild type mice give aA)  Yellow mice crossed to wild type mice give a 1 yellow to 1 wild type (1:1) ratio in the progeny. 1 yellow to 1 wild type (1:1) ratio in the progeny.

Yellow fur is due to a single dominant allele of a Yellow fur is due to a single dominant allele of a gene. gene.

Yy (yellow) X yy (wild type) Yy (yellow) X yy (wild type) 1 Yy (yellow) : 1 yy (wild type).1 Yy (yellow) : 1 yy (wild type).

Recessive lethal genesRecessive lethal genes

B) Yellow mice crossed yellow mice B) Yellow mice crossed yellow mice produce 2 yellow : 1 wild type (2:1) ratio and no produce 2 yellow : 1 wild type (2:1) ratio and no true breeding line of this allele of yellow mice true breeding line of this allele of yellow mice

exists. exists.    

Yy (yellow) X Yy (yellow) Yy (yellow) X Yy (yellow) 2 Yy (yellow) : 1 yy (wild type).2 Yy (yellow) : 1 yy (wild type).Note that the YY class is missing from the Note that the YY class is missing from the 1:2:11:2:1 ratio. ratio.

Yy (yellow) X Yy (yellow) Yy (yellow) X Yy (yellow) 1 YY lethal:  2 Yy(yellow) : 1 yy (wild type).1 YY lethal:  2 Yy(yellow) : 1 yy (wild type).

All the yellow rodents have a genotype YyAll the yellow rodents have a genotype Yy Although the Y allele is dominant for fur colourAlthough the Y allele is dominant for fur colour

it is it is recessive for lethal characteristic.recessive for lethal characteristic.

In polygenic inheritance, the character of In polygenic inheritance, the character of an organism is controlled by more than an organism is controlled by more than one gene.one gene.

The final phenotype characteristic of an The final phenotype characteristic of an organism is the organism is the additiveadditive or or cumulative cumulative effectseffects of these genes with each gene of these genes with each gene contributing a small amount to the contributing a small amount to the phenotype.phenotype.

A polygenic trait is a A polygenic trait is a quantitativequantitative trait that trait that exhibits continuous exhibits continuous variationvariation & is easily & is easily affected by affected by environmental factorsenvironmental factors..

4.2.5:Polygenes/polygenic Inheritance

Variations to a polygenic trait can be Variations to a polygenic trait can be clearly observed in a population.clearly observed in a population.

They are slight different between group of They are slight different between group of individuals in a population for a polygenic individuals in a population for a polygenic trait.trait.

E.g. in human are height, weight, eye E.g. in human are height, weight, eye colour, skin colour & intelligence.colour, skin colour & intelligence.

The human skin colour is controlled by 2 The human skin colour is controlled by 2 pairs of alleles; P, p & H, h.pairs of alleles; P, p & H, h.

P & H are dominant alleles that contribute P & H are dominant alleles that contribute to ‘ dark skin’.to ‘ dark skin’.

p & h are recessive genes.p & h are recessive genes.Degree of Degree of darknessdarkness

PhenotypePhenotype GenotypeGenotype

--

XX

XXXX

XXXXXX

XXXXXXXX

FairFair

Slightly darkSlightly dark

Quite darkQuite dark

DarkDark

Very darkVery dark

pphhpphh

ppHh, PphhppHh, Pphh

PPhh, ppHH, PpHhPPhh, ppHH, PpHh

PPHh, PpHHPPHh, PpHH

PPHHPPHH

PPhh, ppHH, PpHh PPhh, ppHH, PpHh

Human skincolour

Human eye colour

The condition in which different genes are The condition in which different genes are located on the same chromosome.located on the same chromosome.

Linked genes tend to be inherited together.Linked genes tend to be inherited together.The dihybrid inheritance for linked genes The dihybrid inheritance for linked genes

is similar to the mechanism for is similar to the mechanism for monohybrid inheritance since the 2 linked monohybrid inheritance since the 2 linked genes act as a single unit when passed genes act as a single unit when passed from parents to their offspring.from parents to their offspring.

4.2.6: Linked gene

The FThe F22 phenotype ratio is 3 :1 & the phenotype ratio is 3 :1 & the phenotype combination is the same as the phenotype combination is the same as the parents.parents.

The dihybrid phenotype ratio is not The dihybrid phenotype ratio is not obtained.obtained.

A test cross result in a phenotype ratio is A test cross result in a phenotype ratio is not 1:1:1:1.not 1:1:1:1.

E.g. A dihybrid cross for a pair of E.g. A dihybrid cross for a pair of characteristics determined by 2 linked characteristics determined by 2 linked genes is the genes is the inheritance of abdomen inheritance of abdomen shape & wing shape in shape & wing shape in Drosophila Drosophila melanogastermelanogaster..

L dominant allele for wide abdomenL dominant allele for wide abdomen

l recessive allele for narrow abdomenl recessive allele for narrow abdomen

P dominant allele for long wingP dominant allele for long wing

p recessive allele for vestigial wingp recessive allele for vestigial wing

Wide abdomen x narrow abdomenWide abdomen x narrow abdomen

PP long wing long wing vestigial wingvestigial wing

LP/LPLP/LP x lp/lpx lp/lp

G G

F1 LP/lp LP/lp LP/lp LP/lp F1 LP/lp LP/lp LP/lp LP/lp

F1 x F1 Lp/lp x LP/lpF1 x F1 Lp/lp x LP/lp

F2F2 LP/LP LP/lp LP/lp lp/lpLP/LP LP/lp LP/lp lp/lp

Abdomen shape

Wing shape

LP lp

LP

lpLP

lp LP lp

Phenotypic ratio:3 wide abdoment : 1 narrow long wing vestigial

Genotypic ratio:1 LP/LP : 2 LP/lp : 1 lp/lp

•The slanting line (/) is used to indicate linkage.

Effect of crossing overEffect of crossing over

Linked genes are Linked genes are sometimes not sometimes not inherited together.inherited together.

The exchange of The exchange of chromosomal chromosomal contents between contents between non-sister chromatids non-sister chromatids of paired homologous of paired homologous chromosomes takes chromosomes takes place at a place place at a place known as chiasma known as chiasma during prophase during prophase II meiosis.meiosis.

Resulting in cases where F2 having 2 Resulting in cases where F2 having 2 smaller groups with characteristic that smaller groups with characteristic that have been rearranged (have been rearranged (recombinantrecombinant) ) apart from the 2 main group with apart from the 2 main group with combination of characteristic similar to that combination of characteristic similar to that of the P generation of the P generation ((non-recombinant/parental non-recombinant/parental combinationcombination).).

The The test cross result is not 1 : 1 : 1 : 1test cross result is not 1 : 1 : 1 : 1 for F2 genes.for F2 genes.

E.g. E.g. Drosophila melanogasterDrosophila melanogaster have genes: have genes:

CC dominant allele for grey dominant allele for grey

cc recessive allele for black recessive allele for black

WW dominant allele for normal wing dominant allele for normal wing

ww recessive allele for vestigial wing recessive allele for vestigial wing

( GB/NW) (BB/VW)( GB/NW) (BB/VW)

P: P: CW/CW X cw/cwCW/CW X cw/cw

gametes:gametes:

F1:F1: CW/cw ( CW/cw (GB/NWGB/NW))

F1 undergoF1 undergo CW/cw x cw/cw ( CW/cw x cw/cw (test cross parenttest cross parent))

test cross:test cross:

Gametes:Gametes:

F2F2 : : CW/cw cw/cw Cw/cw cW/cwCW/cw cw/cw Cw/cw cW/cw

GB/NM BB/VW GB/VW BB/NWGB/NM BB/VW GB/VW BB/NW

Non-recombinant / Recombinant phenotype Non-recombinant / Recombinant phenotype

parental phenotypeparental phenotype

Body colour

Wing shape

CW cw cwcWCw

cwCW

Sex determination in HumanSex determination in Human Human sex is determined by a pair of Human sex is determined by a pair of sex sex

chromosomeschromosomes called called XX and and YY.. Because these chromosomes do not look Because these chromosomes do not look

alike they are called alike they are called heterosomesheterosomes.. All other chromosomes are called All other chromosomes are called

autosomesautosomes.. Every human cell contains 23 pairs of Every human cell contains 23 pairs of

chromosomes.chromosomes.

Females have 2 large X chromosomes Females have 2 large X chromosomes (XX), male have one chromosome X & one (XX), male have one chromosome X & one Y chromosome (XY).Y chromosome (XY).

During meiosis, the sex chromosomes pair During meiosis, the sex chromosomes pair up & segregate into the daughter cells.up & segregate into the daughter cells.

Males are Males are heterogametic sexheterogametic sex because they because they produce produce different spermdifferent sperm: approximately : approximately 50% contain an X chromosome & 50% 50% contain an X chromosome & 50% have a Y.have a Y.

Females produce Females produce homogametic sexhomogametic sex because all of their eggs contain an X because all of their eggs contain an X chromosome.chromosome.

4.2.7: Sex Linked Genes4.2.7: Sex Linked Genes

Genes located on the sex chromosomes Genes located on the sex chromosomes are called sex-linked genes & the pattern are called sex-linked genes & the pattern of inheritance of such characteristics of inheritance of such characteristics follows the principle of sex determination.follows the principle of sex determination.

Not all genes on sex chromosomes are Not all genes on sex chromosomes are concerned with sex determination.concerned with sex determination.

In human, the Y chromosome carries very In human, the Y chromosome carries very few genes & most of these are not few genes & most of these are not involved in sex determination.involved in sex determination.

The X chromosome carries many genes The X chromosome carries many genes which control non-sexual, sex-linked which control non-sexual, sex-linked characteristics such as:characteristics such as:

- - haemophiliahaemophilia – It is a disease where blood – It is a disease where blood

does not clot normally.does not clot normally.

- - colour blindness in humancolour blindness in human

- - DrosophilaDrosophila eye colour eye colour

XXNN - normal- normal

XXnn - haemophilia - haemophilia

A carrier mother and a haemophiliac father.A carrier mother and a haemophiliac father.

P: P: XXNNXXnn x X x XnnYY

G: G:

F1: F1: XXNNXXnn X XnnXXnn X XNNY XY XnnYY

The phenotypic ratio of their children will be:The phenotypic ratio of their children will be:

1 haemophiliac female1 haemophiliac female

1 carrier female1 carrier female

1 normal male1 normal male

1 haemophiliac male1 haemophiliac male

XN Xn Xn Y

Haemophilia

Colour blindness in humanColour blindness in humanColor blindness is an abnormal condition Color blindness is an abnormal condition

characterized by the inability to clearly characterized by the inability to clearly distinguish different colors of the distinguish different colors of the spectrum. The difficulties can be mild to spectrum. The difficulties can be mild to severe. severe.

It is a misleading term because people It is a misleading term because people with color blindness are not blind. Rather, with color blindness are not blind. Rather, they tend to see colors in a limited range they tend to see colors in a limited range of hues; a rare few may not see colors at of hues; a rare few may not see colors at all. all.

  Colour-blindness is a recessive gene and this is Colour-blindness is a recessive gene and this is why colour-blindness is more dominant in men why colour-blindness is more dominant in men than women. than women.

Men only have one X chromosome and if it Men only have one X chromosome and if it carries the colour-blindness gene then they are carries the colour-blindness gene then they are colour- blind. colour- blind.

If a woman has one chromosome that carries If a woman has one chromosome that carries the gene and one that does not, then she will not the gene and one that does not, then she will not be colour-blind but is said to be a carrier. be colour-blind but is said to be a carrier.

One in twenty men are colour-blind, whilst only One in twenty men are colour-blind, whilst only one in two hundred women have the condition. one in two hundred women have the condition.

For a woman to be colour-blind, in the inherited For a woman to be colour-blind, in the inherited form, her mother must be either a carrier of the form, her mother must be either a carrier of the gene or colour- blind and her father must have gene or colour- blind and her father must have been colour-blind too. been colour-blind too.

Because the colour-Because the colour-blind gene is blind gene is recessive, a colour-recessive, a colour-blind male and a blind male and a normal female will normal female will have no colour-blind have no colour-blind offspring.offspring.

(see Image ) (see Image ) However, the colour-However, the colour-blind gene may skip blind gene may skip a generation a generation because the because the females are carriers. females are carriers.

N on the X chromosome denotes the dominant normal Gene.  n on the X chromosome denotes the recessive colour-blind gene.

If a woman is colour-blind If a woman is colour-blind then she will be able to then she will be able to produce colour-produce colour-blind males, and female blind males, and female carriers.carriers.

(see Image )   If the father is (see Image )   If the father is also colour-blind all offspring also colour-blind all offspring will be colour-blind. This also will be colour-blind. This also means that colour-means that colour-blind females can only be blind females can only be produced where both parents produced where both parents are colour-blind or where the are colour-blind or where the father is colour-blind and the father is colour-blind and the mother is a carrier of the mother is a carrier of the gene, in the inherited form of gene, in the inherited form of Red / Green colour-Red / Green colour-blindness.blindness.

From this Image we From this Image we can see that a normal can see that a normal male and a carrier male and a carrier female will have 1 in 2 female will have 1 in 2 chance of producing chance of producing offspring that do not offspring that do not carry the gene, but they carry the gene, but they have a 1 in 4 chance of have a 1 in 4 chance of producing a male that is producing a male that is affected by the affected by the condition, and the same condition, and the same chance of producing a chance of producing a carrier female. carrier female.

DrosophilaDrosophila eye colour eye colourThe alleles for eye color is The alleles for eye color is

on the on the XX chromosome of chromosome of Drosophila, but not on the Drosophila, but not on the YY. .

Red eye color (w+) is Red eye color (w+) is dominant to white eye dominant to white eye color (w).color (w).

P:P: XXw+w+XXw+w+ x X x XwwYY

G:G:

F1: F1: XXw+w+XXww X Xw+w+XXww X Xw+w+Y XY XwwYY

2 2 Red eyed female : 1 Red eyed male: 1 white eyed maleRed eyed female : 1 Red eyed male: 1 white eyed male

XwXw+Xw+ Y

4.3: GENETIC MAPPING4.3: GENETIC MAPPING

= is a technique used to study the relative position = is a technique used to study the relative position & sequence of genes on a chromosome & can & sequence of genes on a chromosome & can only be carried out if the genes are only be carried out if the genes are linked & linked & crossing over takes placecrossing over takes place..

According to Morgan, due to his study on the X According to Morgan, due to his study on the X chromosome of chromosome of DrosophilaDrosophila melanogastermelanogaster genes are arranges in a linear sequence along genes are arranges in a linear sequence along the chromosome and each gene takes up a the chromosome and each gene takes up a position called position called locuslocus..

The allele for each gene takes up a relatively The allele for each gene takes up a relatively similar locus on the homologous chromosome.similar locus on the homologous chromosome.

Linked genes which are situated Linked genes which are situated very very close to each otherclose to each other usually cannot be usually cannot be separated by crossing over during meiosis separated by crossing over during meiosis

only gametes with a parental only gametes with a parental combination are produced. combination are produced.

When the genes are situated far from each When the genes are situated far from each other, the chance for crossing over to other, the chance for crossing over to occur between them is greatly increased occur between them is greatly increased more gametes of the recombinant type more gametes of the recombinant type being produced.being produced.

• The further the 2 genes are situated, the greater is the chance for the chromatid to break in and rejoin between the genes during crossing over. The crossing over frequency is directly proportional to the distance between the 2 genes.

• (a) Two pairs of sister chromatids align during meiosis.  A1 and B1 are located on the same chromosome.  A2 and B2 are located on a different chromosome. (b) DNA crossover leads to recombination if the chiasma is located between the two loci. 

The formula to determine the % of The formula to determine the % of crossing over or cross over value crossing over or cross over value

(COV)(COV)COV = total no of recombinant x 100%COV = total no of recombinant x 100%

total no of offspringtotal no of offspringE.g. E.g. Test cross progeny dataTest cross progeny data

GenotypeGenotype No of individualNo of individual

AB/abAB/ab

Ab/abAb/ab

aB/abaB/ab

AB/abAB/ab

480480

480480

2020

2020

COV = total no of recombinant x 100%COV = total no of recombinant x 100%

total no of offspringtotal no of offspring

= 40 x 100% = 4%= 40 x 100% = 4%

10001000

One map unit = 1 morganOne map unit = 1 morgan

Map distance = 4 unitsMap distance = 4 units

Chromosome mapChromosome map

A B4 units

E.g. 2:E.g. 2:

An experiment has been carried out & the An experiment has been carried out & the following COV between genes X, Y & Z following COV between genes X, Y & Z has been obtained.has been obtained.

Crossing over betweenCrossing over between cross over valuecross over value X & Y 20%X & Y 20%

Y & Z 15%Y & Z 15%

X & Z 5%X & Z 5%

Draw a chromosome map indicating the relative position genes X, Y & Z.Draw a chromosome map indicating the relative position genes X, Y & Z.

X Z Y

5 15

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4.4: PEDIGREE ANALYSIS4.4: PEDIGREE ANALYSIS

= Family pedigree is a table, chart or = Family pedigree is a table, chart or diagram representing the ancestral history diagram representing the ancestral history of a group of related individuals.of a group of related individuals.

It describes the inter-relationships of It describes the inter-relationships of parents & children across the generation.parents & children across the generation.

GenerationsGenerations are identified with Roman are identified with Roman numerals in a vertical sequence on the left numerals in a vertical sequence on the left hand side.hand side.

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Horizontal lines of squares & circlesHorizontal lines of squares & circles show individuals in each generation, show individuals in each generation, squaressquares for males & for males & circlescircles for females. for females.

A A solid horizontal linesolid horizontal line connecting a connecting a male & female represents a male & female represents a marriagemarriage..

Descending vertical linesDescending vertical lines illustrate illustrate progenyprogeny with the siblings arranged in with the siblings arranged in order of age from left to right.order of age from left to right.

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Numbers identify members of a generation Numbers identify members of a generation along with their spouses.along with their spouses.

Shaded squares & circles represents Shaded squares & circles represents individual expressing a particular trait.individual expressing a particular trait.

Symbols used in a pedigree chart:Symbols used in a pedigree chart:

normal malenormal male

normal femalenormal female

marriagemarriage

affected maleaffected male

affected femaleaffected female

9898

9999

IMPORTANT Date!!!!

100

29/12/13 (Sunday)- due date for genetic inheritance essay of 25 marks (questions in Extra exercise)

2/1/14 (Thursday)- Quiz 4 (Genetic), due date mind map and Extra exercise

6/1/14 (Monday)- due date genetic calculations

101

Pedigree Analysis

IntroductionA pedigree is a diagram of family relationships that uses symbols to represent people and lines to represent genetic relationships. These diagrams make it easier to visualize relationships within families, particularly large extended families. Pedigrees are often used to determine the mode of inheritance (dominant, recessive, etc.) of genetic diseases. A sample pedigree is below.

In the pedigree above, the grandparents had two children, a son and a daughter. The son had the trait in question. One of his four children also had the trait.

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I

II

III

1 2

1

1

2 43

Circle = female

Square = male

Roman numerals = generations

Arabic numerals = individuals within a generation

Coloured = individuals with trait being tracked

In a pedigree, squares represent males and circles represent females. Horizontal lines connecting a male and female represent mating. Vertical lines extending downward from a couple represent their children. Subsequent generations are therefore written underneath the parental generations and the oldest individuals are found at the top of the pedigree.If the purpose of a pedigree is to analyze the pattern of inheritance of a particular trait, it is customary to shade in the symbol of all individuals that possess this trait.In the pedigree above, the grandparents had two children, a son and a daughter. The son had the trait in question. One of his four children also had the trait.

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Clues (non sex-linked)o       Recessive:       §         individual expressing trait has 2 normal parents§         two affected parents can not have an unaffected childo       Dominant:§         every affected person has at least one affected parent§         each generation will have affected individuals

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• Eg: inheritance of hemophilia in the British Royal family

- shows inheritance of single sex-linked recessive gene

PEDIGREE ANALYSISPEDIGREE ANALYSIS

The pedigree for red hair inheritance in three generations illustrates the use of a pedigree. In generation I, two individuals (I-2 & I-3) have red hair. No individuals in generation II show the trait, but it reappears in generation III in individuals III-2 and III-3.The reappearance of the trait in generation III among children whose parents' hair is not red indicates that red hair is a recessive characteristic.  If it was caused by a dominant allele, at least one of the parents would have been carrying it, and therefore would have had red hair.  This is not the case.  With a recessive allele, both parents could be carrying it (as heterozygotes) without showing the trait.  .

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It is not always possible to tell if a recessive gene is sex-linked or autosomal. However, this pedigree does give us enough information to determine that the red hair recessive gene is autosomal.  Individual III-3 is an affected female.  If the allele was X-linked, this female would have received it from both her parents.  This is not possible because her father (individual II-3) would have had red hair (males only have one allele of X-linked genes because they have a Y chromosome instead of a second X chromosome).  The gene cannot be Y-linked because females have the trait.

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2.  Trait “A “PedigreeUse the Pedigree for Trait A to determine the genetic basis of this trait.1.  Does a dominant or recessive allele produce the trait?  Explain.  Recessive because it can skip generations – unaffected individuals can have affected children2.  Is it autosomal or sex-linked?  Explain.   Autosomal  In this case, the male II-4 is unaffected, but passes the trait on, so it cannot be x-linked.3.  What are the genotypes of all the individuals in the pedigree?  (Write them on the pedigree.) 

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4.  What is the genotype of individual IV 2?Explain.  IV-2 may be a carrier 50% chance, although since her siblings are unaffected it is possible that mom was not a carrier, in which case this individual would be normal5.  What is the genotype of individual IV-6?  Explain.   Must be a carrier as one of their children is affected.

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