In humans pattern baldness is a sex-influenced trait, whose degree of expression is controlled by the sex of the individual, such that it is autosomal dominant in males while autosomal recessive in females. On the other hand, colour blindness and haemophilia are both
recessive X-linked traits.
B = bald; b = non-bald; C = normal vision; c = colour blind; H = normal blood clotting; h = haemophilia.
a. Give the genotypes for individuals in the P and F1 generations for the matings below. Assume that no crossing over has occurred.
Mating 1
P: bald, non-colour blind, non-haemophiliac female X non-bald, colour blind, haemophiliac male
F1: bald, colour blind, haemophiliac male
Mating 2
P: bald, colour blind, non-haemophiliac female X bald, non-colour blind, haemophiliac male
F1: non-bald, non-colour blind, non-haemophiliac female
b. Suppose that the male and female from the F1s marry, what are the probabilities of the male and female offspring outlined below?
i. haemophiliac but otherwise normal
ii. colour blind and haemophiliac but normal for baldness
iii. colour blind but otherwise normal
iv. bald, colour blind and haemophiliac
To determine the genotypes for individuals in the P and F1 generations, we can assign the corresponding genotype letters to the traits described:
B = bald
b = non-bald
C = normal vision
c = color blind
H = normal blood clotting
h = hemophilia
a. Mating 1:
P: bald, non-color blind, non-hemophiliac female (BbcCCh) X non-bald, color blind, hemophiliac male (bbccHh)
F1: bald, color blind, hemophiliac male (BbCcHh)
Explanation: Since baldness is autosomal dominant in males, the male offspring will be bald because the father is non-bald. Color blindness and hemophilia are both recessive X-linked traits, so the male offspring will inherit both traits from the mother.
Mating 2:
P: bald, color blind, non-hemophiliac female (BbCcHH) X bald, non-color blind, hemophiliac male (BbCcHh)
F1: non-bald, non-color blind, non-hemophiliac female (bbCChh)
Explanation: In this case, baldness is autosomal recessive, so the female offspring will be non-bald because both parents are carriers (Bb). Color blindness and hemophilia are both recessive X-linked traits, so the female offspring will not inherit these traits because the father is non-color blind and non-hemophiliac.
b. Considering the F1 offspring from Mating 2:
i. Haemophiliac but otherwise normal (hemophilia only):
The male offspring will have the genotype BbCcHh and the female offspring will have the genotype bbCChh. Since hemophilia is a recessive X-linked trait, the male offspring has a 50% chance of being haemophiliac (Hh), while the female offspring will not be haemophiliac (HH).
ii. Color blind and haemophiliac but normal for baldness:
The male offspring will have the genotype BbCcHh and the female offspring will have the genotype bbCChh. Since color blindness is a recessive X-linked trait, the male offspring has a 50% chance of being color blind (Cc), while the female offspring will not be color blind (CC). Hemophilia will be present in both male and female offspring.
iii. Color blind but otherwise normal:
The male offspring will have the genotype BbCcHh and the female offspring will have the genotype bbCChh. Since color blindness is a recessive X-linked trait, both male and female offspring have a 50% chance of being color blind (Cc). They will not have hemophilia or baldness.
iv. Bald, color blind, and hemophiliac:
The male offspring will have the genotype BbCcHh and the female offspring will have the genotype bbCChh. Both male and female offspring have a 25% chance of being bald (Bb), a 50% chance of being color blind (Cc), and a 50% chance of having hemophilia (Hh).
Remember, these probabilities assume that no crossing over has occurred.