Colour blindness is sex linked and recessive. What is the probability that the son of a carrier female and an unaffected male will be affected by the condition?

The key bit of knowledge required to answer this question is that colour blindness is an X chromosome linked condition. Say the allele for normal vision is (X^N) and the allele for colour blindness is (X^n). A female can be heterozygous for these two alleles and be a carrier. She will remain unaffected by the condition because the allele for normal vision is dominant to the recessive, defective colour blindness allele. She can however, pass her defective X chromosome on to her children. A male who is heterozygous for the allele will always be affected, as he does not have a second X chromosome to cancel out the effect of the defective X^n (colour blindness) allele.
In our question, the female is a carrier and must be (X^N, X^n). The male is unaffected and must be (X^N, Y). Using a simple Punnett grid, the possible outcomes are (X^N, X^n), (X^N, X^N), (X^N, Y), (X^n, Y). The only outcome that produces an affected child is (X^n, Y). The chance that a child will be affected is therefore 1/4. However, since the question asks the probability of having an affected SON, and only two of these outcomes result in a male child, the correct answer to this question is 1/2.

Answered by Saahil B. Biology tutor

571 Views

See similar Biology IB tutors
Illustration of a video tutorial

Need help with Biology?

One to one online tuition can be a great way to brush up on your Biology knowledge.

Have a Free Meeting with one of our hand picked tutors from the UK’s top universities

Find a tutor

Related Biology IB answers

All answers ▸

What is hCG and what does it do?


Describe the three types of fatty acids.


What is the role of receptors in mediating steroids and protein hormones?


Outline the role of insulin and glucagon in the regulation of glucose and outline type 1 and type 2 diabetes.


We're here to help

contact us iconContact usWhatsapp logoMessage us on Whatsapptelephone icon+44 (0) 203 773 6020
Facebook logoInstagram logoLinkedIn logo

© MyTutorWeb Ltd 2013–2022

Terms & Conditions|Privacy Policy