4.3 Theoretical genetics & 10.2 HL Genetics. 4.3 Theoretical genetics: Objectives 1- Define genotype, phenotype, dominant allele, recessive allele, codominant

4.3 Theoretical genetics&

10.2 HL 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

Mendel needed to explain

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-

2-


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

Four Hypotheses 3-

4- Law of segregation:


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

-

-


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

Testcross1-

2-

3-


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 1-

2-

3-


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

A-

B-C-Dominant inheritanceA-

B-


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) dominance1-

2-

3-


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

A-

B-

C-


Many genes have more than two alleles in the population

CodominanceA-

B-

C-


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

Female Male

XR Xr XR Y

XR YXR XR

YXR

XR

Sperm

Eggs

Xr XR Xr YXr

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

A-

B-

C-

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

XRXr

XrXr

XRY

XrY

Example• Predict the ratio of offspring phenotypes for the

following:Female heterozygous for colorblindness colorblind

male?

Example

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 XHXh XhXh XHY XhY

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 TOPICS10.2 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

Mendel needed to explain


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

Law of independent assortment


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 GenesA-

B-

Example studied by Bateson and PunnettA-

B-

C-


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 overA-

B-

C-

Or


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 inheritanceA-

B-


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

Documents

4.3 Theoretical genetics & 10.2 HL Genetics. 4.3 Theoretical genetics: Objectives 1- Define genotype, phenotype, dominant allele, recessive allele, codominant