In this exercise you are asked to play the role of a genetic counselof adviglng

Question

In this exercise you are asked to play the role of a genetic counselof adviglng healthy young couples who are at the beginning stages of their pregnancies. Based on their own phenotypes, as well as those of their parents and grandpar ents, they seek your help in predicting those of their offspring. (Hint: You might find it helpful to diagram each situation, using uppercase letters to indicate domi- nant genes and lowercase letters to indicate recessive genes.) 1. A dark-skinned woman, whose parents and grandparents all had very dark skin, marries a light-skinned man, whose parents and grandparents were all very light-skinned. What is the probable skin tone of their offspring? 2. A man with red hair (recessive) marries a woman with black hair whose mother had red hair. What are the chances that their first child will have red hair? Black hair? 3. A woman and a man both have brown eyes, but their first child has blue eyes. What are the chances that their second child will have blue eyes? 4. An apparently health couple has one normal daughter and then a son who develops a learning disability, an X-linked recessive disease. What are the chances that the boys'sister is a carrier of the learning disability? What chance is there that she will develop the learning disability herself?

Explanation / Answer

1. Let's consider the allele for dark complexion is DD and similarly the allele for light skin as dd. The gametes for by dark complexion is D and the gametes for light skin complexion is d. Probability of offspring getting either which complexion is depends on which is the dominant phenotypic feature here. Suppose the dark complexion is the dominant phenotypic feature respective of light skin probability of the offspring getting dark complexion is quite high as they will have the allele combination of Dd that is heterozygous combination of allele will be present in offspring. They will carry the light skin complexion allele as a recessive trait and the light can appear in their offspring complexion.

2. Man allele will be rr and female allele will be Rr so their offspring will have allelic combination of rR, rr, rR, rr means 50 percent of offspring will have black hair and 50 percent will have red hair combination. Here female is the carrier of red hair so her offspring too will represent that character.

3. Probability of their second child having blue eyes is less than 25 %.

As female has brown eyes and male too has brown eyes either of them is a carrier of blue eyes phenotypic feature or both the couple is a carrier for blue eyes character . Male allele can be represented as Bb and female allele as Bb, here B represent the character brown and b is blue. So offspring will have BB, Bb, Bb, bb. So suppose their first child has allele combination of bb and he got blue eyes so chances of other child having brown eyes is more than 50 percent because either one of allele out of three such as BB, Bb, Bb will be inherited by offspring.

4. Suppose here mother is the carrier for x linked reccesive trait as daughter is normal so mother will have a allele combination of XXc where Xc is the trait which is responsible for causing the disease. So from father he will have normal allele combination of XY . And their daughter will get allele combination of XX that is normal genetic combination from both parents, as you can see it's a 50 percent probability that mother will pass which X allele in her offspring. Here in a similar way son got the allele combination of YXc from his parents as the disorder is x linked son will be affected.

So merely daughter will get this disorder the chances of this is 50-50 percent. All depends on which X trait offspring are getting.

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