Mr & Mrs Smith have a child who has Cystic Fibrosis, yet they do not personally suffer from the condition. Explain, with the use of genetic crosses, how this is possible and, if they had another child, how likely the second child will have Cystic Fibrosis

As neither of the parents have the condition, it therefore is assumed to be a Homozygous Recessive condition, so requires both the recessive alleles for the disease to be present in an individual (eg ff). As the parents do no suffer, yet hold the allele to pass on the condition, they must be carriers (Ff). By drawing out the genetic grid, you can see that, by both parents being carriers, they have the possibility to have offspring with the homozygous dominant (FF), heterozygous carriers (Ff) and the homozygous recessive (ff), the latter will cause the disease.
As for how likely it is that a second child will also have the condition, from the genetic cross, you make 1 x FF, 2 x Ff and 1 x ff. Therefore there are 4 possible combinations, and one of those will be the homozygous recessive, ff. Therefore there is a 25% chance of the second offspring having the condition.

Answered by Jack N. Biology tutor

3247 Views

See similar Biology A Level tutors

Related Biology A Level answers

All answers ▸

How is an action potential transmitted across a cholinergic synapse?


Describe the process of transcription


Explain the process of oxidative phosphorylation and the electron transport chain in the production of ATP in respiration.


Not all mutations result in a change to the amino acid sequence of the encoded polypeptide. Give an explanation.


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 2013–2024

Terms & Conditions|Privacy Policy
Cookie Preferences