Question

Please be as clear and succinct as possible. In 1875, Charles Darwin described a family as follows: "Ten men, in the course of four generations, were furnished, in both jaws taken together, with only four small and weak incisor teeth and with eight posterior molars. The men thus affected have very little hair on the body, and become bald early in life... It is remarkable that no instance has occurred of a daughter being affected...Although the daughters are never affected, they transmit the tendency to their sons; and no case has occurred of a son transmitting it to his sons.” In additional families reported to have this disorder, some females seemed to be very mildly affected, with small or malformed teeth and abnormal sweat glands. The lack of hair and sweat glands was patchy, with only some areas of skin affected. What is the most likely cause of these mildly affected females? Explain.

Answer 1

The trait as described is due to some changes(recessive allele) on the X chromosome. This can be said because it is noticed that most of the men are affected as they have only one X chromosome and if the defective chromosome is present the trait is shown. Also the women are not affected as they have 2 copies of x chromosome and as the trait is recessive , then in the heterozygous condition,the disease is not seen in females, as it gets masked by the dominant allele on the other X chromosome. Also, the females are carrier for the disease in the heterozygous condition.

Some females that are heterozygous for the trait show some mild effects. The concept of doasge compensation is prevalent in humans as the females have a double copy of X chromosome, and this statesthat somatic cells of any sex express equal levels off gene products for X-linked products. To achieve this, in the somatic cells there is random silencing of any of the X chromosome as females have two X xhromosomes.As the males have only one X chromosome, there is no silencing there. In the process of this random silencing, if the Normal X chromosome gets silenced then the defective one carrying the recessive allele gets expressed thus showing some mild symptoms of the disease.