Question

Your friend Joe has a younger brother Sam and two older sisters. Sam has a strange disorder that has caused his hair to turn green. The doctor says that this is caused by a single gene that is inherited and that boys are more likely to have green hair than girls. However, Joe, his sisters, and both of his parents have a normal hair color. Joe does not know much biology and is confused.

Explain to Joe how his brother's condition could have arisen even though Joe has normal hair. Then discuss the likelihood that a) Joe, and b) Sam will have children with green hair. Assume that they both married women with normal hair color. You will need to include information on meiosis and transmission of genes to assure that Joe understands. HINT: As part of your answer, indicate the percent of girls vs. boys that you predict would have green hair if fathered by Joe vs. Sam.

Answer 1

This condition is the sex linked recessive disorder or condition. This is X linked disorder. In this situation the abnormal condition is caused due to the presence of both recessive alleles of X chromosome.

Suppose this is caused by Xh allele while the X allele is normal. When an offspring will have both Xh allele or with no X allele this disorder can be caused.

In reproduction during gamete formation there is meiosis division in which homologous chromosomes gets segregated in two gametes. XY is sex pair of chromosomes which also get segregated during the gamete formation. One of the gamete receives X while the other receives Y chromosome from father. From mother who is XX, both of the gametes receive X chromosome. After fertilization, the XY genotype leads to a male and the XX genotype leads to a female offspring.

In a male offspring the XY is normal condition and the XhY is abnormal, because there is no other X allele which can compensate it. In girls XX and XhX is normal condition due to the presence either one or two X allele. XhXh will cause the hair to be green because of absence of normal X allele.

So in this case Sam has genotype of XhY type while all other siblings have normal X allele. This condition arose due to the inheritance of one Xh allele from the mother. It means mother has the genotype of XhX.

Now in next generations, when both the brothers will marry to women with normal hair colors, then there will be all the offsprings with normal hair color in case of Joe. While in case of Sam also there will be no boy with green hair because a father can give his X chromosome to a girl baby only. However, all the girls will be carrier of this trait because of the X chromosome inherited from the father.