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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