lecture 18 genetics. outline recombination – crossing over basic genetic concepts genetic terms...

Lecture 18GENETICS

Outline Recombination – crossing over

Basic Genetic concepts

Genetic terms (Genotype, Phenotype, F1…)

Genetic Tools (Punnett Squares, Probabilities, Pedigrees)

Review

Review Alleles – different versions of the same gene

Maternal Allele – the version of the gene from your mother

Paternal Allele – the version of the gene from your father

Independent Assortment Homologous pairs of chromosomes orient randomly at metaphase I of meiosis

Each pair of chromosomes sorts maternal and paternal homologs into daughter cells independently of every other pair

Independent Assortment The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number

For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes

Figure 13.10-1

Possibility 1 Possibility 2

Two equally probablearrangements ofchromosomes at

metaphase I

Figure 13.10-2



metaphase I

Metaphase II

Figure 13.10-3



metaphase I

Metaphase II

Daughtercells

Combination 1 Combination 2 Combination 3 Combination 4

Followed by Random Fertilization

Crossing Over Crossing over produces recombinant chromosomes, which combine DNA inherited from each parent

Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene

Crossing Over In crossing over, homologous portions of two nonsister chromatids trade places

Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome

Figure 13.11-1Prophase Iof meiosis

Nonsister chromatidsheld togetherduring synapsis

Pair of homologs



Pair of homologs

Chiasma

Centromere

TEM



Pair of homologs

Chiasma

Centromere

TEM

Anaphase I



Pair of homologs

Chiasma

Centromere

TEM

Anaphase I

Anaphase II



Pair of homologs

Chiasma

Centromere

TEM

Anaphase I

Anaphase II

Daughtercells

Recombinant chromosomes

Summary of genetic variation Three mechanisms contribute to genetic variation

◦ Independent assortment of chromosomes◦ Crossing over◦ Random fertilization

Figure 13.7-3

Pair of homologouschromosomes indiploid parent cell

Duplicated pairof homologouschromosomes

Chromosomesduplicate

Sisterchromatids Diploid cell with

duplicatedchromosomes

Homologouschromosomes separate

Haploid cells withduplicated chromosomes

Sister chromatidsseparate

Haploid cells with unduplicated chromosomes

Interphase

Meiosis I

Meiosis II

2

1

Figure 14.2

Parentalgeneration(P) Stamens

Carpel

First filialgenerationoffspring(F1)

TECHNIQUE

RESULTS

3

2

1

4

5

Figure 14.3-1

P Generation

EXPERIMENT

(true-breedingparents) Purple

flowersWhite

flowers

Figure 14.3-2

P Generation

EXPERIMENT

(true-breedingparents)

F1 Generation(hybrids)

Purpleflowers

Whiteflowers

All plants had purple flowersSelf- or cross-pollination

Figure 14.3-3

P Generation

EXPERIMENT

(true-breedingparents)

F1 Generation(hybrids)

F2 Generation

Purpleflowers

Whiteflowers

All plants had purple flowersSelf- or cross-pollination

705 purple-flowered

plants

224 whiteflowered

plants

Table 14.1

Terms Trait/Phenotype/Genotype

Generations: Parental, F1, F2

Self pollination vs Cross pollination

True breeding

Hybrid

Mendel’s Model Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring

Four related concepts make up this model

We now know the molecular explanation for this model

1st Concept To Explain 3:1 Pattern in F2 generation

First: alternative versions of genes account for variations in inherited characters

One Gene: Purple flower – White Flower

These alternative versions of a gene are alleles

Each gene resides at a specific locus on a specific chromosome

Figure 14.4

Allele for purple flowers

Locus for flower-color gene

Allele for white flowers

Pair ofhomologouschromosomes

2nd Concept To Explain 3:1 Pattern in F2 generation

Second: for each character (phenotype), an organism inherits two alleles, one from each parent

The two alleles at a particular locus may be identical, as in the true-breeding plants of Mendel’s P generation

Alternatively, the two alleles at a locus may differ, as in the F1 hybrids

3rd Concept To Explain 3:1 Pattern in F2 generation

Third: if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance

In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant

4th Concept To Explain 3:1 Pattern in F2 generation

Fourth: The law of independent segregation: the two alleles for a heritable characteristic (phenotype) separate (segregate) during gamete formation and end up in different gametes

An egg or a sperm get only one of the two alleles

Allele segregation is because homologous chromosomes segregate during meiosis

Figure 14.7

Dominant phenotype,unknown genotype:

PP or Pp?

Recessive phenotype,known genotype:

pp

PredictionsIf purple-floweredparent is PP

If purple-floweredparent is Pp

or

Sperm Sperm

Eggs Eggs

or

All offspring purple 1/2 offspring purple and 1/2 offspring white

Pp Pp

Pp Pp

Pp Pp

pp pp

p p p p

P

P

P

p

TECHNIQUE

RESULTS

Figure 14.9

Segregation ofalleles into eggs

Segregation ofalleles into sperm

Sperm

Eggs

1/2

1/2

1/21/2

1/41/4

1/41/4

Rr Rr

R

R

RR

R

R

r

r

r

r r

r

Figure 14.8

P Generation

F1 Generation

Predictions

Gametes

EXPERIMENT

RESULTS

YYRR yyrr

yrYR

YyRr

Hypothesis ofdependent assortment

Hypothesis ofindependent assortment

Predictedoffspring ofF2 generation

Sperm

Spermor

EggsEggs

Phenotypic ratio 3:1

Phenotypic ratio 9:3:3:1

Phenotypic ratio approximately 9:3:3:1315 108 101 32

1/21/2

1/2

1/2

1/41/4

1/41/4

1/4

1/4

1/4

1/4

9/163/16

3/161/16

YR

YR

YR

YRyr

yr

yr

yr

1/43/4

Yr

Yr

yR

yR

YYRR YyRr

YyRr yyrr

YYRR YYRr YyRR YyRr

YYRr YYrr YyRr Yyrr

YyRR YyRr yyRR yyRr

YyRr Yyrr yyRr yyrr

Figure 14.UN02

Chance of at least two recessive traits

ppyyRr

ppYyrr

Ppyyrr

PPyyrr

ppyyrr

1/4 (probability of pp) 1/2 (yy) 1/2 (Rr)

1/4 1/2 1/2

1/2 1/2 1/2

1/4 1/2 1/2

1/4 1/2 1/2

1/16

1/16

2/16

1/16

1/16

6/16 or 3/8

The ability to curl your tongue up on the sides (T, tongue rolling) is dominant to not being able to roll your tongue. A woman who can roll her tongue marries a man who cannot. Their first child has his father's phenotype. What are the genotypes of the mother, father, and child?

What is the probability that a second child won't be a tongue roller?

http://www.ksu.edu/biology/pob/genetics/defin.htm



Often inheritance patterns are more complicated

Many heritable characters are not determined by only one gene with two alleles

Basic principles of segregation and independent assortment apply even to more complex patterns of inheritance

Examples of single gene not following Mendelian patterns

Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations:

◦ When alleles are not completely dominant or recessive◦ When a gene has more than two alleles◦ When a gene produces multiple phenotypes

Degrees of Dominance Complete dominance: phenotypes of the heterozygote and dominant homozygote are identical

Incomplete dominance, the phenotype of F1 hybrids is in between the phenotypes of the two parental varieties

Codominance, two dominant alleles affect the phenotype in separate, distinguishable ways

Figure 14.10-1

P Generation

Red White

Gametes

CWCWCRCR

CR CW

Figure 14.10-2

P Generation

F1 Generation

1/21/2

Red White

Gametes

Pink

Gametes

CWCWCRCR

CR CW

CRCW

CR CW

Figure 14.10-3

P Generation

F1 Generation

F2 Generation

1/21/2

1/21/2

1/2

1/2

Red White

Gametes

Pink

Gametes

Sperm

Eggs

CWCWCRCR

CR CW

CRCW

CR CW

CWCR

CR

CW

CRCR CRCW

CRCW CWCW

Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain

◦ At the organismal level, the allele is recessive◦ At the biochemical level, the phenotype (i.e., the enzyme activity level) is

incompletely dominant◦ At the molecular level, the alleles are codominant

Multiple Alleles Most genes exist in populations in more than two allelic forms

The ABO blood group in humans are determined by three alleles

Single Gene codes for an enzyme that attaches a specific carbohydrate to the surface of the RBC ◦ IA allele – The enzyme adds the A carbohydrate◦ IB allele – The enzyme adds the B carbohydrate◦ i allele – Adds neither

Figure 14.11

Carbohydrate

Allele

(a) The three alleles for the ABO blood groups and their carbohydrates

(b) Blood group genotypes and phenotypes

Genotype

Red blood cellappearance

Phenotype(blood group)

A

A

B

B AB

none

O

IA IB i

iiIAIBIAIA or IAi IBIB or IBi

Pleotrophy Most genes have multiple phenotypic effects, a property called pleiotropy

Pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease

Some traits may be determined by two or more genes

Epistasis In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus

Labrador retrievers and many other mammals, coat color depends on two genes

One gene determines the pigment color (with alleles B for black and b for brown)

The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair

Figure 14.12

Sperm

Eggs

9 : 3 : 4

1/41/4

1/41/4

1/4

1/4

1/4

1/4

BbEe BbEe

BE

BE

bE

bE

Be

Be

be

be

BBEE BbEE BBEe BbEe

BbEE bbEE BbEe bbEe

BBEe BbEe BBee Bbee

BbEe bbEe Bbee bbee

Polygenic Inheritance Quantitative characters are those that vary in the population along a continuum

Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype

Skin color in humans is an example of polygenic inheritance

Nature vs. Nurture

lecture 18 genetics. outline recombination – crossing over basic genetic concepts genetic terms...

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