Question

Given below is a highly simplified representation of the human sex chromosomes from a karyotype. The gene a and b could be of:
A. Colourblindness and body height
B. Attached ear lobe and Rhesus blood group
C. Haemophilia and red-green colourblindness
D. Phenylketonuria and haemophilia

Answer 1

Hint: TA human being receives two sets of chromosomes from each parent. Each set comprises of 23 chromosomes out of which 22 are autosomes and one is sex chromosome.

Complete Step by Step Answer:
The genes for red-green colorblindness are X linked, that is to say they are located on the X chromosome. Genes located on this gene can be inherited from one X chromosome of the father and either of the two X chromosomes of the mother. With this disease a person will face problems seeing colours like red, green and yellow.

Similarly, genes for haemophilia are sex linked. Haemophilia refers to a disease in which the blood lacks clotting factors 8 and 9. Thus, when a haemophiliac person bleeds, he/she keeps bleeding without clotting which leads to great loss of blood.

Rh factor or the Rhesus factor is a protein antigen found on the surface of red blood cells. A person containing this antigen is categorised as Rh positive and a person lacking it is termed as Rh negative.

Attached earlobes is an autosomal recessive trait. The dominant allele is for free lobes and is written as ‘F’. The recessive allele is written as ‘f’. Therefore, it is imperative for both the parents to have at least one ‘f’ allele for it to be transmitted to the offspring.

In this question, since both the genes are located on the same chromosome, we can say that these genes belong to one chromosome. Owing to this fact, we can also infer that the chromosome is X chromosome as the genes for both these traits are present on X chromosome.
Therefore, the correct answer for this question is C


Note: Haemophilia and red-green colour blindness usually occurs more in boys than girls. This is because these are X linked diseases. Females have one more X chromosome than males. Having just one haemophilic X chromosome will make them a carrier but they won’t show the disease. Males on the other hand have only one chromosome. So, even if they get one haemophilic X chromosome, they will be affected.