lecture06 patterns inheritance sv.ppt

8/17/2019 Lecture06 Patterns Inheritance Sv.ppt

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Introduction to Genetics

Lecture 6

Outline

• How are traits passed from parents to offspring?

• Testing hypotheses

• Mendel’s experiments

• Mendel’s principles

• More complex patterns of inheritance

Questions Mendel Was Trying to

Answer?

What are the basic patterns in the transmission of traits

from parents to offspring?

H1: blending inheritance

H2: inheritance of acquired characteristics


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Concept of “blending of inherited traits”

X

Progeny inter-

mediate in size

Once trait is

“blended,”

can’t get the

original types

back

offspring: blended combination of parents

Mechanism to explain

evolution:

Jean-Baptiste de Lamarck

(1744-1829)

The inheritance of

acquired characters

Gregor Mendel

• Heredity: the transmission of traits from parents tooffspring

(1822-1884)

•

Question: why offspringresemble their parents and howtransmission of traits occurs

• studied heredity in a scientificway

• “particulate” hypothesis ofinheritance


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TerminologySELF-POLLINATION:

pollen from stamen fertilizes egg in carpel of same flower

CROSS POLLINATION:

fertilization of one plant by pollen from another plant

Terminology

character: heritable feature that varies among individuals

trait: a variant for a character

gene: hereditary determinant for a trait

allele: different version of a gene

genotype: the alleles found in an individual

phenotype: physical appearance that expresses the genotype

homozygous: an organism with a pair of identical alleles for

a character

heterozygous: an organism with 2 different alleles for a

character

Mendel’s Experiments

• true-breeding: individuals that produce offspring

like themselves over many generations of self-

pollination

• hybridization: created by mating two true-

breeding varieties

P – parental generation (true-breeding parents)

F1 – first generation

F2 – second generation


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Peas Can Be Self-Pollinated or Cross-Pollinated

Self-pollination

Female organ

(receives pollen)

Eggs

Male organs(produce pollengrains, whichproduce malegametes)

Cross-pollination

CROSS-POLLINATION

3. Transfer pollen to thefemale organs of the

individual whose maleorgans have beenremoved.

2. Collect pollen from adifferent individual.

1. Remove male organsfrom one individual.

SELF-

POLLINATION

true-breeding white X true-breeding red hybrid

Mendel Studied Seven Characters That Were Variable in

Pea PlantsTrait Phenotypes

Seed shape

Seed color

Pod shape

Pod color

Flower color

Flower andpod position

Stem length

Tall Dwarf

Terminal (at tip)Axial (on stem)

Purple White

YellowGreen

Inflated Constricted

Green Yellow

Round Wrinkled

Character /Trait

Cross Pollination


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Traits in First and Second Generations

Variations in Inherited Characters

Mendel’s Law of Segregation


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Phenotype vs Genotype

Does the type of gamete affect the inheritance of seed shape?

Reciprocal Cross

What Mendel Observed

• For all seven pairs of contrasting traits:

1. The F1 generation showed only one of the two parental traits

• dominant trait (trait expressed)

• recessive trait (not expressed)

•

In genetics, the terms dominant and recessive identify onlywhich phenotype is observed in individuals carrying two

different genetic traits

2. The F2 generation showed an ~ 3:1 ratio of the

dominant:recessive parental traits


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Mendel Used the Scientific Method

Mendel repeated his experiments for all the traits of pea plants

Mendel repeated his experiments for all the traits of pea plants

Mendel’s Principle of Segregation

Gametes

p P

P

p

Gametes

pp Pp

Pp PP

Explaining the 3:1 ratioof phenotypes in the F2 generation

– 2 alleles of each gene

must segregate intodifferent gamete cellsduring gamete (eggand sperm) formation,then come backtogether when an eggis fertilized by a spermto form a zygote


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A Cross Between 2 Homozygotes and 2 Heterozygotes

Probabilityis 100%

50%

25%

Probabilityis 25%

Mendel’s Hypothesis

• Particulate Inheritance -

heritable factors (genes)

retain their individuality

from one generation to the

next

contradicts blending

inheritance and

contradicts inheritance of

acquired characteristicsGenes on homologous chromosomes

We now know that alleles are distributed to gametes during me iosis

Mendel’s Model

1. peas have two versions of each gene – different

versions of genes are called alleles (phenotype, genotype)

2. alleles do not blend together

3. each gamete contains one allele of each gene

4. males and females contribute equally to the genotype

of their offspring (homozygous, heterozygous)

5. Some alleles are dominant to other alleles


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Testcross

• used to determine an

individual’s actual geneticcomposition

• A purple-flowered plant can behomozygous (PP) orheterozygous (Pp)

in a TEST CROSS, the knownindividual is a ___________

Dihybrid Cross: A Cross between Pure

Lines for Two Traits

YyRr x YyRr

Chromosome Theory of Inheritance

• genes are located on chromosomes

• links meiosis with patterns of inheritance

1903: Walter Sutton and Theodor Boveri carefully observed meiosis;

noted that chromosome number was reduced by half and

hypothesized that chromosomes are composed of Mendel’s

hereditary determinants, which we now call genes

- meiosis explains Mendel's principle of segregation and independent

assortment


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Separation of alleles during anaphase of meiosis 1 is responsible for

Mendel’s Principle of Segregation

Meiosis IAlleles segregate

Recessive allelefor seed shape

G a m e t e s

Chromosomes replicate

Rr parent

Dominant allelefor seed shape

Meiosis II

Principle of segregation: Each gamete carries onlyone allele for seed shape, because the alleles havesegregated during meiosis.

Meiosis and the Principle of Independent Assortment

Meiosis I

Replicated chromosomesprior to meiosis

G a m e t e s

Alleles for seed shape

Meiosis II

Principle of independent assortment: The genes for seed shape and seed colorassort independently, because they are located on different chromosomes.

Meiosis II

Meiosis I

Alleles for seed color

1/4 RY 1/4 ry 1/4 Ry 1/4 rY

Chromosomes can line up intwo ways during meiosis I

R

R y y r

Y Y

R R

R R

R R

r r

r r

r r

r

Y Y

Y Y

Y Y

y y

y y

y y y

y

y y

y y

R R

R R

R R

r r

r r

r r

Y Y

Y Y

Y Y

Law of Probability

Probability of r being present

in both gametes (homozygous)

at fertilization is

! (egg has r) x ! (sperm has r)

! x ! = " for rr in F2! x ! = " for RR in F2


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Law of Probability

Heterozygous example

F1 gametes can combine

to produce Rr offspring in

2 mutually exclusive ways:

dominant allele can come

from either egg or sperm,

but NOT both

-for F2 heterozygote

" + " = ! for Rr

Probability in a Dihybrid Cross

Fig. 14.8

YyRr x YyRr

Y = yellow

R = round

y = green

r = wrinkled

Probability: Dihybrid Cross

YyRr x YyRr

1)

Seed color

Punnett square: Yy x Yy

YY homozygous yellow = "

Yy heterozygous yellow = !

yy = homozygous green"

Y y

Y

y

Yy

Yy

YY

yy


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YyRr x YyRr

1) Seed shape

Punnett square: Rr x Rr

RR = homozygous round"

Rr = heterozygous round !

rr = homozygous wrinkled"

R r

R

r

Rr

Rr

RR

rr


Question:

What is the probability of obtaining a pea,

yellow in color and round in shape,

homozygous for both traits? (refer to previous two

slides)

YYRR = " (YY) x " (RR) = 1/16

Check results with Fig. 14.8


Question:

What is the probability of obtaining a pea,

yellow in color (heterozygous) and round in

shape, (homozygous)?

YyRR = ! (Yy) x " (RR) = 1/8

Check results with Fig. 14.8


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More Complex Patterns of

Inheritance

– Incomplete dominance

– Codominance

– Pleiotropic effects

– Epistasis

– Polygenic inheritance

– Environmental effects

Incomplete Dominance

the heterozygotes have an intermediate phenotype

since alleles of a gene are not always clearly

dominant or recessive

– Example:

• color in snapdragons and four o’clock flowers

Incomplete Dominance in Snapdragons

F1: appears to support blending hypothesis


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Incomplete Dominance(snapdragons)

Explanation?

R allele: has functional enzyme for

producing ____ pigment

r allele: has a non-functional

enzyme

Codominance

– produces a

heterozygote

phenotype that is a

combination of that of

the two homozygotes

– Ex.

Roan color in horses

Unlike incompletedominance, both alleles

are expressed

Frequency of Dominant Alleles

Dominant alleles are not always the

more common alleles in a

population

• Polydactyly

–


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Multiple Alleles

Pleiotropic Effects

• Single geneinfluences many traits

(can have multiplephenotypic effects)

– Cystic fibrosis

– Marfan syndrome

Epistasis• one gene affects the action of another gene

in many cases, the alleles at one gene mask or reduce the effectsof alleles at a different gene

E – determines if color pigment will be deposited in fur,

B gene determines how dark pigment is; b = brown, B = black


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Epistasisone gene affects the action of another gene

Epistasis• Interaction between two genes where one modifies the

phenotypic expression of the other

– F1 generation: all purple

– F2 generation: ratio of 9 purple:7 white

– Mendelian genetics predicts a 9:3:3:1 ratio

–

Why is the ratio modified?

• There are two genesthat contribute to

kernel color

– B! production of

pigment

– A! deposition of

pigment

•

gene expression – to produce pigment a

plant must possess at

least one functional

copy of each gene


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Genes at Different Loci Can Interact to Influence a TraitCrosses between chickens with different comb phenotypes giveodd results

Parentalgeneration

Rose comb Pea comb

All Walnut combs

F1

Walnut combs Rose combs Pea combs Single combs

F2

New combphenotype

Another newcomb phenotype

!

9 : 3 : 3 : 1

Different Genes Can Interact to Influence a Trait

Parentalgeneration

A genetic model to explain the results

F2

F1

Rose combRRpp

Pea combrrPP

All Walnut combsRrPp

rrppSingle comb

R_ppRose comb

R_P_Walnut comb

rrP_Pea comb

Two genes (R and P )interact to producecomb phenotype

Blanks in a genotypemean that either allele canbe present

!

Polygenic Inheritance

Characters in a population vary in gradations along a continuum;

referred to as quantitative characters


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Human Height

•

polygenic

–

produced by actions of many

genes

– Quantitative characters

usually indicate polygenic

inheritance; an additive

effect of 2 or more genes

– each gene adds a small

amount to the value of the

phenotype

Extremes are muchrarer than the

intermediate values

Skin pigmentation

is under polygenic control

continuous variation

According to this model, 3 genes

affect color of skin

A,B,C: dark skin alleles

a, b, c: light skin alleles

Alleles have cumulative effect:

AaBbCc or AABbcc will have the

same genetic contribution to skin

darkness

Environmental influences/exposure to sun?

The Environment and Phenotype

• Norm of Reaction


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Environmental Effects

•

expression of some

genes is influenced byenvironmental factors:temperature

Color resemblessnowy background

in winter

Color resemblestundra background

in summer•

Some alleles are heat-

sensitive

– Arctic foxes

Summary

Traits determined by polygenes may givepatterns of inheritance resembling concept of

“blending of traits”

Environmental variables: temperature, nutrition,

light affect gene action

Multifactorial characters: many factors, genetic

and environmental, collectively influence

phenotype

Patterns of Inheritance

Applying Mendel

s rules to humans

• Mutations cause genetic disorders

•

The inheritance of disorders is studied by looking at

pedigrees (family trees)


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Pedigree analysis:

dominant allele

Pedigree analysis:

recessive allele

Pedigree of a Family with an Autosomal Recessive

Disease

Carrier male Carrier female

Affectedmale

Affectedfemale

I

II

III

IV

E a c h r o w r

e p r e s e n t s a g e n e r a t i o n

Carriers(heterozygotes)are indicatedwith half-filledsymbols

Albinism: Recessive Trait


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Albinism: Recessive Trait

•

allele is rare in the

population

• both parents must

be heterozygous

carriers to have an

affected child

Sickle-Cell Anemia: Recessive Trait

Normal red blood cell Sickled red blood cell

Smooth shape allows foreasy passage through

capillaries

Irregular shapecauses blockage of

capillaries

Sickle-Cell Anemia: Recessive Trait

•

Autosomal inheritance: autosomal recessive trait in

which the protein hemoglobin is defective


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Tay-Sachs Disease: Recessive Trait

Enough enzymeto prevent CNS

deterioration – affected

individuals

cannot break

down specific

lipids

– lipids

accumulate in

brain cells

– children die

when young

Absence of enzyme, hexosamidase A: without hexA, fatty substances build up on

nerve cells in body and brain progressive degeneration of central nervous

system

Affected femaleI

II

III

IV

Unaffected male

If a child shows the trait,then one of the parentsshows the trait as well

Huntington’s Disease: autosomal dominant trait that causes

progressive deterioration of brain cells: fatal

gene sequenced in 1993: Huntington allele is near the tip ofchromosome 4

Family with an Autosomal Dominant Disease

Recognizing autosomal dominant disorders:

Achondroplasia: A Dominant Trait

People without achondroplasia: homozygous for recessive allele

Recessive allele: much more prevalent than dominant allele

heterozygotes have

the dwarf phenotype


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Phenylketonuria (PKU)

•

autosomal recessive genetic disorder• individuals lack the enzyme, phenylalanine hydroxylase,

that converts phenylalanine to tyrosine

• phenylalanine and phenylpyruvic acid accumulate in the

body and interfere with the development of the nervoussystem, causing seizures and mental retardation

Newborn Screening

Newborn screening:

determine the concentration of Phe and

the ratio of Phe to tyrosine, both of whichwill be elevated in PKU

Changing the environmentalfactor, by consuming a diet low

in phenylalanine, usually results

in normal development

newborn screening

Genetic Testing and Counselling

Tests for identifying carriers for autosomal recessive diseases:

sickle-cell anemia, cystic fibrosis, Tay-Sacks, etc

lecture06 patterns inheritance sv.ppt

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