Question

1. Duchenne muscular dystrophy (DMD) is caused by a relatively rare X-linked recessive allele. It causes progressive muscular wasting, and usually leads to death before age 20. What is the probability that the first son of a woman (who does not have the disease but whose father is affected) will be affected?

2. Hemophilia in humans is inherited as an X linked recessive trait. A woman whose father is hemophiliac marries a man with normal clotting ability. What is the probability that her first child will have hemophilia? Assume that the woman's mother is homozygous dominant.

3.In the ABO blood system in human beings, alleles A and B are codominant and both are dominant to the O allele.  In a paternity dispute, type AB women claimed that one of four men was the father of her type A child.  What blood types could the father of the child be on the basis of the evidence given?

Answer 1

Ans - x linked recessive traits affect males more than females . Because males have XY chromosomes and the X would carry the defective gene. In females XX are the chromosomes, so the other X chromosome suppresses the effects of defective X chromosome .

Here since the father of the woman had the disease his genotype would be X*Y. Now the woman would inherit one normal X chromosome from mother and one affected X* chromosome from father so her genotype is X*X. She is a carrier of the disease .  

There is a 50% chance that the son of this woman would inherit the defective X* chromosome and 50% chance that he would inherit X chromosome . So his genotype could be X*Y (50%) ,or XY (50%). Totally a 50 percentage chance to be affected .

2. Here again the same situation , the father is hemophilic and the disease is X linked recessive . So the fathers fathers would be X​​​​​​h​​​​​Y. Now his daughter would inherit a normal X chromosome from mother and the detective X​​​​​​h​​​​​Xh chromosome from father . So her genotype is X​​​​​​​​​​h​​​​​X. Now she is a carrier of the disease , because in females one normal X chromosome supresses the affected X chromosome unlike in males who inherit only one set of X chromosomes . But in this case its ientioned her mother eas homozygous dominant meaning both her X chromosome were affected . So her daughter would surely inherit any on eif the affected X chr from mother and the affected X chromosome from father making her also homoxygous dominant.

She marries a man with normal chromosomes say XY

So the cross is. XY ×. X​​​​​​h​​​​​X​h

	x​​​​​​h	X​​​​h
X	X​​​​​​h​X	Xh X
Y	X​​​​​​h Y	X​​​​​​h Y

The probabilty

That her first child will have hemophilia is 50% and the child would be hemophilic only if its a male because the females would inherit the normal X chromosome from father to supresses the affects of affected X chr from mother.

3. Since mother is AB ,father could be A type or O type or forB type or B type , any four of the rypes would produce an offspring to be A positive .

The chances are 50% in case of fathers blood type A or type O . And 25% in type AB and type B.

if this helps please give it a positive rating thank you.