1. In humans, right-handedness (R) is dominant to left-handedness (r), and brown
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Question
1. In humans, right-handedness (R) is dominant to left-handedness (r), and brown eyes (B) are usually dominant to blues eyes (b).
a) What are the genotypes of gametes produced by a person heterozygous for both traits?
b) What are the genotypes of gametes for a person heterozygous for brown eyes, but homozygous recessive for left-handedness?
c) Set up a Punnett square for a couple that is heterozygous for both traits. What is the probability that if their first child is brown-eyed and left-handed that the second child is blue-eyed and left-handed?
d) A right-handed, blue-eyed woman marries a right-handed, brown-eyed man. They have two children, one left-handed and brown-eyed and the other right-handed and blue-eyed. By a later marriage with another man, who is also right-handed and brown-eyed, she has seven more children, all of whom are right-handed and brown-eyed. What are the genotypes of the woman and her two husbands?
Explanation / Answer
1)
a) The genotypes of gametes produced by a person heterozygous for right handedness (Rr) and for brown eyes (Bb) are RB, Rb, rB and rb.
b) The individual is heterozygous for brown eyes (Bb) but homozygous recessive for left handedness (rr). So, the genotype of the individual is rrBb. The gametes produced by the individual are rB, rb, rB and rb.
c) The couple is heterozygous for both traits.
RrBb x RrBb
The probability of first child being brown eyes and left handed is 3/16 (18.75%) and the probability of the second child being blue eyed and left handed is 1/16 (6.25%).
d) In the first marriage, the mating between right handed blue eyed woman and right handed brown eyed man produced left handed brown eyed and right handed blue eyed individuals. Left handedness is present in the offspring. Therefore, the right handedness present in the parents should be in heterozygous state. Similarly, blue eyes are seen in the offsprings. So, brown eyed genes must be in heterozygous state in both the parents.
In the second marriage, all of the children are right handed and brown eyed. So, the second husband must be homozygous for both right handedness and brown eyes.
Therefore, the genotype of the woman is RrBb. The genotype of the first husband is RrBb. The genotype of the second husband is RRBB.
RB Rb rB rb RB RRBB RRBb RrBB RrBb Rb RRBb RRbb RrBb Rrbb rB RrBB RrBb rrBB rrBb rb RrBb Rrbb rrBb rrbbRelated Questions
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