question_answer

                A man and a woman, who do not show any apparent signs of a certain inherited disease, have seven children (2 daughters and 5 sons). Three of the sons suffer from the given disease but none of the daughters are affected. Which of the following mode of inheritance do you suggest for this disease?  

A)                 Autosomal dominant                    

B)                 Sex-linked dominant

C)                 Sex-limited recessive                                    

D)                 Sex-linked recessive

Correct Answer: D

Solution :

                A genetic characteristic that is determined by genes located on the sex chromosomes (X or Y) are called sex linked characteristic and the condition where expression of sex linked genes is limited to one sex is called sex limited characteristics. In given problem, disease is the result of sex-linked recessive genes. As neither man nor woman shows signs of disease it means woman would be carrier for disease causing gene. In their children none of the daughter is suffering from disease, while the sons are suffering, it means daughters are also carrier (i.e., X-linked recessive).                 Suppose, genotype of man = X Y                 Genotype of woman \[={{X}^{d}}X\]                 (d-disease causing gene)                                 For each delivery the probability for each combination is 25%. So, among seven children 2 normal daughter, 3 diseased sons and 2 normal sons are possible.