Biology 2B

Genetics

Genetic problems

• A monohybrid cross is a cross of individuals looking at a characteristic inherited at one gene locus

• A test cross is crossing an individual back to a homozygous recessive individual in order to determine whether it is a carrier

• A Punnett square is a tool used in genetics• Genotype refers to the alleles present in an

individual• Phenotype refers to the characteristic shown

by the individual

b

B

b

b

Bb

bb

Bb

bb

Autosomal inheritanceBoth males and females have 2 alleles for the characteristicHomozygous individuals have 2 alleles the same and produce

gametes with only 1 type of alleleHeterozygous individuals have 2 different alleles and produce

two types of gametes with each alleleAt fertilisation gametes combine so the new individual has 2 of

each allele – one from each parentWe can show the probabilities of allele combinations from

different crosses by using a Punnett square

Autosomal dominant/recessive

Individuals with two dominant alleles show the dominant phenotype

Individuals with two recessive alleles show the recessive phenotype

Individuals with one of each allele show the dominant phenotype

BB

bb

Bb

Autosomal dominant recessive crossesCrossing a homozygous dominant

individual with a homozygous recessive individual leads to offspring who are all heterozygous and show the dominant trait

Crossing two heterozygous individuals leads to 1 homozygous dominant individual, showing the dominant trait : 2 heterozygous individuals, showing the dominant trait :1 homozygous recessive individual, showing the recessive trait

BB bb

Bb Bb

BB Bb

Bb bb

B b

B

b

Example – dominant recessive problem

A heterozygous black male mouse mates with a homozygous brown female mouse. Black fur is dominant over brown fur. What is the probability of having:

a) a homozygous black offspring? 0% b) a heterozygous black offspring? 50% c) a homozygous brown offspring? 50%

Bb bb

b b

B Bb Bb

b bb bb

Autosomal co-dominance

Individuals with two of the 1st allele show the first trait

Individuals with two of the 2nd allele show the second trait

Individuals with one of each allele show a mixture of both traits

SBSB

SWSW

SBSW

Autosomal co-dominant crossesCrossing an individual homozygous

for one allele with an individual homozygous with the second allele leads to offspring showing a mixture of the two traits

Crossing two heterozygous individuals leads to 1 homozygous individual showing the first trait : 2 heterozygous individuals showing the mixed trait :1 homozygous individual showing the second trait

SBSB SWSW

SBSW SBSW

SB SW

SB SBSB SBSW

SW SBSW SWSW

Example – co-dominance problem

Two heterozygous grey sheep are mated. Black wool is co-dominant to white wool. What is the probability of having:

a) a black offspring? 25% b) a grey offspring? 50% c) a white offspring? 25%

SBSW SBSW

SBSB SBSW

SBSW SWSW

SB SW

SB

SW

Sex linked inheritanceMales and females have different chromosomesMales can only show 2 phenotypes (ie males can not

be carriers)Females can show 3 phenotypes (if codominant) or 2

phenotypes (if dominant recessive, with a carrier)You need to show alleles on the X chromosome (Y

chromosomes don’t carry an allele)

Genetics problems 1Tabby is dominant to black coat colour

in cats. Predict the genotype and phenotype ratios expected from a cross between a hybrid tabby and a black cat.

Tongue rolling is dominant to non-tongue rolling in humans. Predict the genotype and phenotype ratios expected from a cross between a heterozygous tongue roller and a non tongue roller.

Short hair is dominant to long hair in cats. Predict the genotype and phenotype ratios expected from a cross between a hybrid short-haired cat and a long hair cat.

Astigmatism is dominant to normal vision in humans. Predict the genotype and phenotype ratios expected from a cross between a person heterozygous for astigmatism and a normally sighted person.

Genetics problems 1Tabby is dominant to black coat colour in cats.

Predict the genotype and phenotype ratios expected from a cross between a hybrid tabby and a black cat.

Astigmatism is dominant to normal vision in humans. Predict the genotype and phenotype ratios expected from a cross between a person heterozygous for astigmatism and a normally sighted person.

T = tabby t = blackCross is Tt x ttGenotypes ½ Tt, ½ ttPhenotypes ½ tabby, ½ black

Tongue rolling is dominant to non-tongue rolling in humans. Predict the genotype and phenotype ratios expected from a cross between a heterozygous tongue roller and a non tongue roller.

Short hair is dominant to long hair in cats. Predict the genotype and phenotype ratios expected from a cross between a hybrid short-haired cat and a long hair cat.

t t

T Tt

ttt Tt

tt

h h

H Hh hh

h Hh hh

t t

T Tt tt

T Tt tt

a a

A Aa aa

a Aa aa

T = tongue roller t = non tongue rollerCross is Tt x ttGenotypes ½ Tt, ½ ttPhenotypes ½ tongue roller, ½ non tongue roller

H = short h = longCross is Hh x hhGenotypes ½ Hh, ½ hhPhenotypes ½ short, ½ long

A = astigmatism a= normal visionCross is Aa x aaGenotypes ½ Aa, ½ aaPhenotypes ½ astigmatism, ½ normal

Genetics problems 2

Predict the genotypes and phenotypes expected from crossing a white flowered plant and a red flowered plant

Predict the genotypes and phenotypes expected from crossing a pink flowered plant and a white flowered plant

Predict the genotypes and phenotypes expected from crossing a pink flowered plant and a red flowered plant

Predict the genotypes and phenotypes expected from crossing 2 pink flowered plants

Red flower colour is co-dominant with white flower colour in snap dragons, with the heterozygote showing pink flowers

Genetics problems 2

Predict the genotypes and phenotypes expected from crossing a white flowered plant and a red flowered plant

Predict the genotypes and phenotypes expected from crossing a pink flowered plant and a red flowered plant

Red flower colour is co-dominant with white flower colour in snap dragons, with the heterozygote showing pink flowers

Predict the genotypes and phenotypes expected from crossing a pink flowered plant and a white flowered plant Predict the genotypes and phenotypes expected

from crossing 2 pink flowered plants

FRFR = red, FWFW = white, FR FW = pink

FWFW x FRFR all FWFR pink

FWFW x FW FR

FRFR x FW FR

FRFW x FR FW

FRFW

FR FW

FRFR FR

FR FRFR FRFW

FR

FW

FWFW FRFW

FW

FW

FRFW FWFW

FR FW

FRFR FRFW FR

FWFW FW FRFW

½ FRFR = red, ½ FRFW = pink

½ FRFR = red, ½ FRFW = pink 1 FRFR = red : 2 FR FW = pink : 1 FWFW = white

Genetics problems 3

What genotypes and phenotypes are expected if a haemophiliac male marries a homozygous normal female?

What genotypes and phenotypes are expected if a haemophiliac male marries a heterozygous female?

What genotypes and phenotypes are expected if a normal male marries a heterozygous female?

What is the probability of having a son with haemophilia, if a haemophiliac male marries a heterozygous female?

Haemophilia is a disease carried as a sex-linked recessive trait.

Genetics problems 3

What genotypes and phenotypes are expected if a haemophiliac male marries a homozygous normal female?

What genotypes and phenotypes are expected if a normal male marries a heterozygous female?

Haemophilia is a disease carried as a sex-linked recessive trait.

What genotypes and phenotypes are expected if a haemophiliac male marries a heterozygous female?

What is the probability of having a son with haemophilia, if a haemophiliac male marries a heterozygous female?

XHXH = normal female, XHXh = carrier female, XhXh = haemophiliac femaleXHY = normal male, XhY = haemophiliac male

XHXH x XhY XHXh x XHY

XHXh x XhYXHXh x XhY

Y

XH

Xh

XH XH XH

XHXH XHXh

XHY

XH

Xh

XHXh XHXh

XH Xh

XHXhXh XhXh

Xh

Xh XHXh XhXh

XHY XHY Y XhY

Y XHY XhY Y XHY XhY

All females carriers XHXh all males normal XHY

½ females are XHXh = carrier, ½ females are XhXh = haemophiliac ½ males are XHY = normal, ½ males are XhY = haemophiliac

Probability is 1/4