Fig. 1 shows a pedigree drawn during an investigation into Duchenne muscular dystrophy, DMD. DMD is an X-linked recessive disorder that causes progressive w...
Question 1 Report
Fig. 1 shows a pedigree drawn during an investigation into Duchenne muscular dystrophy, DMD. DMD is an X-linked recessive disorder that causes progressive weakening of skeletal muscles. Filled squares show males with DMD. The laboratory has offered DNA testing to females in this family because some may carry the altered allele without having the disorder.
(a) Describe why DMD is more common in males than in females. [3]
(b) What is the most likely genotype of the mother of the affected male in generation II? Use XD and Xd. [2]
(c) Give the genotype of an affected male. [1]
(d) Complete a genetic diagram for a carrier female and an unaffected male. [3]
(e) Give one advantage of DNA testing for the unaffected females in this family. [1]
Answer Details
(a) DMD is more common in males because males have one \(X\) chromosome and one \(Y\) chromosome. A male with \(X^dY\) has only one copy of the X-linked gene, so \(X^d\) is expressed because there is no normal allele on a second X chromosome. [3]
(b) The mother of the affected male is most likely \(X^D X^d\). She is an unaffected carrier but can pass \(X^d\) to a son. [2]
(c) An affected male has genotype \(X^dY\). [1]
(d) A carrier female is \(X^D X^d\) and an unaffected male is \(X^D Y\).
| \(X^D\) | \(Y\) | |
|---|---|---|
| \(X^D\) | \(X^D X^D\) | \(X^D Y\) |
| \(X^d\) | \(X^D X^d\) | \(X^dY\) |
The gametes are \(X^D\), \(X^d\), \(X^D\), and \(Y\), giving offspring \(X^D X^D\), \(X^D X^d\), \(X^D Y\), and \(X^dY\). [3]
(e) DNA testing can identify unaffected females who are carriers and allow informed reproductive decisions. [1]
Download The App On Google Playstore
Everything you need to excel in your exams
