Solution to Punnett Square 5

 

 

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Take the problem one step at a time!

Answer the question that is being asked.

Answer all the questions.

 

This disease is inherited as sex-linked dominant disease.  An affected male marries a homozygous recessive female.  Draw a Punnett square of the possible offspring.  What is the likelihood that any child they have will be affected.  Would the parents of the father (grandparents of these offspring) have been affected?

 

 

This disease is inherited as sex-linked dominant disease. 

1. Recognize how the disease in inherited.

a. Sex-linked means that the gene is carried on the X chromosome. 

b. Dominant means that any person having a dominant gene will be affected.

 

 

An homozygous dominant male marries a homozygous recessive female.  Draw a Punnett square of the possible offspring.

2. Determine the genotypes of the parents.

Affected male is XA Y (he is affected, since his X is mutant)

Homozygous recessive female is X  X (homozygous means two of the same allele, recessive here)

 

 

Draw a Punnett square of the possible offspring.

3. Set up the Punnett square with the genotypes of the above two parents.

 

                                    |      XA                Y          

                     X    |      XA X            X Y

X    |      XA X            X Y

 

 

What is the likelihood that any child they have will be affected?

4. Determine how many possibles would be affected according to how the disease in inherited (See help #1).

 

                                    |      XA                Y          

                     X    |      XA X            X Y

X    |      XA X            X Y

 

 

 

Any offspring having an XA allele would be affected, all daughters.

So 2 of 4 (2/4 = ½),  there is a 50% chance that any child would have the disease, but all affected children would be female.

 

 

Would the parents of the father (grandparents of these offspring) have been affected?

A male receives his X from his mother, so an affected male (XAY) would have had an affected mother (XAX or XAXA) in order for her to be able to give a mutant X to her son.  Sons don’t get the X from father, father gives the Y to make the child male.

His mother would have to be affected.  We can’t tell whether his father was affected or not.