4.3 Theoretical genetics. 4.3 Theoretical genetics: Objectives 1- Define genotype, phenotype,...

4.3 Theoretical genetics

4.3 Theoretical genetics: Objectives

1- Define genotype, phenotype, dominant allele, recessive allele, codominant alleles, locus, homozygous, heterozygous, carrier and test cross.

2- Determine the genotypes and phenotypes of the offspring of a monohybrid cross using a Punnett grid.

3- State that some genes have more than two alleles (multiple alleles).

4- Describe ABO blood groups as an example of codominance and multiple alleles.

5- Explain how the sex chromosomes control gender by referring to the inheritance of X and Y chromosomes in humans.

6- State that some genes are present on the X chromosome and absent from the shorter Y chromosome in humans.

7- Define sex linkage. 8- Describe the inheritance of color blindness and hemophilia as

examples of sex linkage. 9- State that a human female can be homozygous or

heterozygous with respect to sex-linked genes. 10- Explain that female carriers are heterozygous for X-linked

recessive alleles. 11- Predict the genotypic and phenotypic ratios of offspring of

monohybrid crosses involving any of the above patterns of inheritance.

12- Deduce the genotypes and phenotypes of individuals in pedigree charts.

Heredity SL.pptxHeredity Student version.pptxheredity.ppt

DefinitionsAllele: an alternative form of a gene, occupying a specific locus. Genotype: the genetic constitution of an organismPhenotype: the characteristics or appearance (structural, biochemical ..) of an organismDominant allele: is the allele that has the same effect on the phenotype whether it is present in the homozygous or heterozygous state. recessive allele: an allele that has an effect on the phenotype only when present in the homozygous state. codominant alleles: Pairs of allele that both affect phenotype when present in a heterozygous state. locus: particular position on the homologous chromosomes of a geneHomozygous: having two identical alleles of a gene heterozygous: having two different alleles of a gene.carrier: an individual that has one copy of a recessive allele that causes a genetic diseases in individuals that are homozygous. test cross: testing a suspected heterozygote by crossing it with a known homozygous recessive.

Flower color White

Axial

Purple

Flower position Terminal

YellowSeed color Green

RoundSeed shape Wrinkled

InflatedPod shape Constricted

GreenPod color Yellow

TallStem length Dwarf

Mendelain Genetics

Example of a monohybrid cross • Parental generation: purple flowers white flowers• F1 generation: all plants with purple flowers • F2 generation: of plants with purple flowers

of plants with white flowers Mendel needed to explain

• Why one trait seemed to disappear in the F1 generation• Why that trait reappeared in one quarter of the F2

offspring

Copyright © 2009 Pearson Education, Inc.

P generation(true-breedingparents)

Purple flowers White flowers

F1 generation All plants havepurple flowers

F2 generation

Fertilizationamong F1 plants(F1 ´ F1)

of plantshave purple flowers

3–4 of plants

have white flowers

1–4

Mendel’s law of segregation describes the inheritance of a single character

Four Hypotheses 1. Genes are found in alternative versions called alleles; a

genotype is the listing of alleles an individual carries for a specific gene

2. For each characteristic, an organism inherits two alleles, one from each parent; the alleles can be the same or different

– A homozygous genotype has identical alleles – A heterozygous genotype has two different alleles


Mendel’s law of segregation describes the inheritance of a single character

Four Hypotheses 3. If the alleles differ, the dominant allele determines the

organism’s appearance, and the recessive allele has no noticeable effect

– The phenotype is the appearance or expression of a trait– The same phenotype may be determined by more than one

genotype

4. Law of segregation: Allele pairs separate (segregate) from each other during the production of gametes so that a sperm or egg carries only one allele for each gene


P plants

1–2

1–2

Genotypic ratio1 PP : 2 Pp : 1 pp

Phenotypic ratio3 purple : 1 white

F1 plants(hybrids)

Gametes

Genetic makeup (alleles)

All

All Pp

Sperm

Eggs

PP

p

ppPp

Pp

P

pP

pP

P

p

PP pp

All

Gametes

F2 plants

Homologous chromosomes show the alleles for each character

For a pair of homologous chromosomes, alleles of a gene locate at the same locus

• Homozygous individuals have the same allele on both homologues

• Heterozygous individuals have a different allele on each homologue


Gene loci

Homozygousfor thedominant allele

Dominantallele

Homozygousfor therecessive allele

Heterozygous

Recessive allele

Genotype:

P Ba

P

PP

a

aa

b

Bb

Geneticists use the testcross to determine unknown genotypes

Testcross• Mating between an individual of unknown genotype and

a homozygous recessive individual• Will show whether the unknown genotype includes a

recessive allele• Used by Mendel to confirm true-breeding genotypes


B_

or

Two possibilities for the black dog:

Testcross:

Genotypes

Gametes

Offspring 1 black : 1 chocolateAll black

Bb

bb

BB

Bb bb

B

b

Bb

b

bB

Genetic traits in humans can be tracked through family pedigrees

A pedigree • Shows the inheritance of a trait in a family through

multiple generations• Demonstrates dominant or recessive inheritance• Can also be used to deduce genotypes of family members


Freckles

Widow’s peak

Free earlobe

No freckles

Straight hairline

Attached earlobe

Dominant Traits Recessive Traits

Ff

Female MaleAffected

Unaffected

First generation(grandparents)

Second generation(parents, aunts,and uncles)

Third generation(two sisters)

Ff Ff

Ff

Ff Ff

Ff

ff

ff ff ff

ff

FF

FF

or

or

Many inherited disorders in humans are controlled by a single gene

Inherited human disorders show• Recessive inheritance

– Two recessive alleles are needed to show disease– Heterozygous parents are carriers of the disease-causing allele– Probability of inheritance increases with inbreeding, mating

between close relatives

• Dominant inheritance– One dominant allele is needed to show disease– Dominant lethal alleles are usually eliminated from the

population


Parents NormalDd

Offspring

Sperm

Eggs

ddDeafd

DdNormal(carrier)

DDNormalD

D d

DdNormal(carrier)

NormalDd´

Book page 163

Incomplete (codominance) dominance results in intermediate phenotypes

Incomplete (codominance) dominance• Neither allele is dominant over the other• Expression of both alleles is observed as an intermediate

phenotype in the heterozygous individual• Phenotype ratio = genotype ratio= 1:2:1


P generation

1–2

1–2

1–2

1–2

1–2

1–2

F1 generation

F2 generation

RedRR

Gametes

Gametes

Eggs

Sperm

RR rR

Rr rr

R

r

R r

R r

PinkRr

R r

Whiterr

Many genes have more than two alleles in the population

Multiple alleles• More than two alleles are found in the population(to

control one trait)• A diploid individual can carry any two of these alleles• The ABO blood group has three alleles, leading to four

phenotypes: type A, type B, type AB, and type O blood


Many genes have more than two alleles in the population

Codominance• Neither allele is dominant over the other• Expression of both alleles is observed as a distinct

phenotype in the heterozygous individual• Observed for type AB blood


BloodGroup(Phenotype) Genotypes

O

A

ii

IAIA

orIAi

Red Blood Cells

Carbohydrate A

AntibodiesPresent inBlood

Anti-AAnti-B

Reaction When Blood from Groups Below Is Mixedwith Antibodies from Groups at Left

Anti-B

O A B AB

BIBIB

orIBi

Carbohydrate B

AB IAIB —

Anti-A

BloodGroup(Phenotype) Genotypes

O

A

ii

IAIA

orIAi

Red Blood Cells

Carbohydrate A

BIBIB

orIBi

Carbohydrate B

AB IAIB

AntibodiesPresent inBlood

Anti-AAnti-B

Reaction When Blood from Groups Below Is Mixedwith Antibodies from Groups at Left

Anti-B

O A B AB

—

Anti-A

BloodGroup(Phenotype)

O

A

B

AB

SEX CHROMOSOMES

AND SEX-LINKED GENES


Female Male

XR XR Xr Y

XR YXR Xr

YXr

XR

Sperm

Eggs

R = red-eye alleler = white-eye allele

Then he crossed F1 X F1F2 generation did conform the expected 3:1

ratio but there was a twist; the white eye trait showed up only in males. All the females had red eyes, whereas half of the males had white half of them had red.

Why were there no white eyed females??

Female Male

XR Xr XR Y

XR YXR XR

YXR

XR

Sperm

Eggs

Xr XR Xr YXr

Because all of the females took(inherit) at least one dominant allele from their father, so they had either XRXR or XRXr genotypes(all red phenotype). But for males, since they took their X chromosome only from mother, half of them had recessive half had dominant alleles(half white half red phenotype).

Then he made another experiment to prove his idea of eye color gene is carried on the X chromosome

Female Male

XR Xr Xr Y

XR YXR XR

YXr

XR

Sperm

Eggs

Xr Xr Xr YXr

Sex-linked genes are located on either of the sex chromosomes

• X-linked genes are passed from mother to son and mother to daughter

• X-linked genes are passed from father to daughter

• Y-linked genes are passed from father to son

Sex-linked genes exhibit a unique pattern of inheritance


HUMAN SEX LINKED TRAITS


In humans, as in drosophilas, the Y chromosome carries much less genetic information than the X chromosome.

1) One of the X linked inheritance is color blindness called Daltonism. They cannot distinguish red&green colors XRXR homozygote normal vision female

XRXr heterozygote normal vision female XrXr homozygote color blind female XRY normal vision male XrY color blind male

• Predict the ratio of offspring phenotypes for the following:

Female heterozygous for colorblindness colorblind male?

XBXb XbY ¼ XBXb, ¼ XbXb, ¼ XBY, ¼ XbY = ¼ females with full color vision + ¼ colorblind females + ¼ males with full color vision + ¼ colorblind males

2) Haemophilia is a disease in which blood clotting doesn’t occur.

Clotting is a complex series of reactions. Each step of it requires specific enzymes. One of them is controlled by a gene on the X chromosome called factor VIII(antihaemophilic globin)

XHXH normal femaleXHXh carrier femaleXhXh haemophilic femaleXHY normal maleXhY haemophilic male

QueenVictoria

Albert

Alice Louis

Alexandra CzarNicholas IIof Russia

Alexis

There is no prior history of hemophilia in Queen Victoria’s family, so it is thought that the allele arose by spontaneous mutation in the sex cells of one of her parents

HL TOPICS

10.2 Dihybrid crosses and gene linkage

• Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.

• Distinguish between autosomes and sex chromosomes. • Explain how crossing over between non-sister

chromatids of a homologous pair in prophase I can result in an exchange of alleles.

• Define linkage• Explain an example of a cross between two linked genes. • Identify which of the offspring are recombinants in a

dihybrid cross involving linked genes.

The law of independent assortment is revealed by tracking two characters at once

Example of a dihybrid cross

• Parental generation: round yellow seeds wrinkled green seeds

• F1 generation: all plants with round yellow seeds • F2 generation: of plants with round yellow seeds

of plants with round green seeds of plants with wrinkled yellow seeds of plants with wrinkled green seeds

Mendel needed to explain• Why nonparental combinations were observed • Why a 9:3:3:1 ratio was observed among the F2 offspring


9/163/163/161/16

The law of independent assortment is revealed by tracking two characters at once

Law of independent assortment

• Each pair of alleles segregates independently of the other pairs of alleles during gamete formation

• For genotype RrYy, four gamete types are possible: RY, Ry, rY, and ry


P generation

1–2

Hypothesis: Dependent assortment Hypothesis: Independent assortment

1–2

1–2

1–2

1–4

1–4

1–4

1–4

1–4

1–4

1–4

1–4

9––16

3––16

3––16

1––16

RRYY

Gametes

Eggs

F1

generation

SpermSperm

F2

generation

Eggs

Gametes

rryy

RrYy

ryRY

ryRY

ry

RY

Hypothesized(not actually seen)

Actual results(support hypothesis)

RRYY rryy

RrYy

ryRY

RRYY

rryy

RrYy

ry

RY

RrYy

RrYy

RrYy

rrYYRrYY

RRYyRrYY

RRYy

rrYy

rrYy

Rryy

Rryy

RRyy

rY

Ry

ry

Yellowround

Greenround

Greenwrinkled

Yellowwrinkled

RY rY Ry

Genes on the same chromosome tend to be inherited together

Linked Genes• Are located close together on the same chromosome• Tend to be inherited together

Example studied by Bateson and Punnett• Parental generation: plants with purple flowers, long

pollen crossed to plants with red flowers, round pollen

• The F2 generation did not show a 9:3:3:1 ratio

• Most F2 individuals had purple flowers, long pollen or red flowers, round pollen


Purple longPurple roundRed longRed round

Explanation: linked genes

Parentaldiploid cellPpLl

Experiment

Purple flower

PpLl Long pollenPpLl

Prediction(9:3:3:1)

ObservedoffspringPhenotypes

284212155

215717124

Mostgametes

Meiosis

PL

pl

PL

PL pl

pl

Fertilization

Sperm

Mostoffspring Eggs

3 purple long : 1 red roundNot accounted for: purple round and red long

PL PL

PL

PL

plPL

pl

pl

plpl

Crossing over produces new combinations of alleles

Linked alleles can be separated by crossing over• Recombinant chromosomes are formed• Thomas Hunt Morgan demonstrated this in early experiments• Geneticists measure genetic distance by recombination

frequencyOr• The probability that two genes will be seperated by crossing

over is proportional to the physical distance between them in the chromosome.


Gametes

Tetrad Crossing over

Ba baa b

A BA B A b

ExplanationG L

g l g l

g lGgLl

(female)ggll

(male)

G L g l g L

g l

g l

g l g l

g l

g l

G L

SpermEggs

Offspring

g L

G l

G l

Mutant phenotypes

Shortaristae

Blackbody(g)

Cinnabareyes(c)

Vestigialwings(l)

Browneyes

Long aristae(appendageson head)

Graybody(G)

Redeyes(C)

Normalwings(L)

Redeyes

Wild-type phenotypes

A single character may be influenced by many genes

Polygenic inheritance• Many genes influence one trait• Skin color is affected by at least three genes


P generation

1–8

F1 generation

F2 generation

Fra

ctio

n o

f p

op

ula

tio

n

Skin color

Eggs

Sperm1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

aabbcc(very light)

AABBCC(very dark)

AaBbCc AaBbCc

1––64

15––64

6––64

1––64

15––64

6––64

20––64

1––64

15––64

6––64

20––64

P generation

1–8

F1 generation

F2 generation

Eggs

Sperm1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

aabbcc(very light)

AABBCC(very dark)

AaBbCc AaBbCc

1––64

15––64

6––64

1––64

15––64

6––64

20––64

Fra

ctio

n o

f p

op

ula

tio

n

Skin color

1––64

15––64

6––64

20––64

4.3 Theoretical genetics. 4.3 Theoretical genetics: Objectives 1- Define genotype, phenotype,...

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