Duchenne muscular dystrophy is a sex-linked recessive disorder characterized by a progressive loss of muscle tissue. Neither Rudy nor Carla has Duchenne muscular dystrophy, but their first son does...
Duchenne muscular dystrophy is a sex-linked recessive disorder characterized by a progressive loss of muscle tissue. Neither Rudy nor Carla has Duchenne muscular dystrophy, but their first son does have it. If the couple has a second child, what's the probability that he or she will have the disease?
Because it is a sex-linked trait, Carla is a carrier of the X-linked condition. That means she has one X with the normal gene and one X with the Duchenne muscular dystrophy gene. We can represent the normal gene with the symbol XM and the gene for muscular dytrophy as Xm. Her genotype is XM Xm. However, in the heterozygous condition, she is an unaffected female. However, she can pass along either the normal or the muscular dystrophy gene to her children.
Rudy is normal and his genotype has one X with the normal gene--XM and the Y chromosome. Because Rudy is a male, he is hemizygous for any X-linked traits. Whatever genes he gets will be expressed--in this case, he has one copy of the normal gene and therefore, he is normal. The Y chromosome does not have a corresponding gene on it and doesn't play a role in the inheritance of this condition.
In a punnett square between a carrier female XM Xm and a normal male XM Y, the following offspring can be produced: 25% carrier females, 25% normal females, 25% affected males and 25% normal males.
However, if we consider just the male offspring, there is always a 50% chance of producing a male with Duchenne muscular dystrophy or a normal male. That applies to the second or any other male children that may be produced in the future for this couple.
I have included a link with a Punnett square illustrating this situation.