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.

SB
Answered by Saahil B. Biology tutor

2026 Views

See similar Biology IB tutors

Related Biology IB answers

All answers ▸

What are the arguments for the endosymbiotic theory?


Translation occurs in living cells. Explain how translation is carried out, from the initiation stage onwards.


Please explain patterns of sex linked genetic inhertitance and mention one or more examples of sex linked diseases


Explain the structure of the DNA double helix, including its subunits and the way in which they are bonded together


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 2013–2025

Terms & Conditions|Privacy Policy
Cookie Preferences