GS-1 Biological Sciences Question 31

[vijkrishna3659]

Sickle-cell anemia is a recessive trait.
According to the explanation, you are saying that a ss has no sickle-cell anemia, when in reality, ss means the person has anemia. Furthermore, SS or Ss would mean the person has regular hemoglobin. Ss is a carrier for the trait. Thus, when I did by crosses using this notation, I got the answer D. I don't understand why B. is correct and D. is wrong.

[admin]

Notation is up to you but that does not impact the answer. The Explanation used a different notation but that's OK, let's use yours (which is the classical way of doing it):

SS homozygous normal

Ss heterozygous, has the trait

ss homozygous thus has the disease

The cross, using your notation, in the second generation would then by SS x Ss. When you do the Punnett square you will get:

2 SS: 2 Ss

In other words, 1 normal : 1 sickle cell trait which is answer choice B.

[melissalyn4392]

If the heterozygous male in generation one (the square on the left) is replaced with a homozygous male for the normal condition; neither of the parents would have the trait. Therefore the male in the 2nd generation would also by homozygous. In the Punnett square explanation the parents are shown as one being Ss and one being ss. Shouldn't they be SS and ss?

[calena7178147]

The explanation in the book is correct. It may be confusing because the sickle cell disease is represented with SS (not ss); or you may be confused by which generation they are talking about. Here is the new pedigree with genotypes.



Therefore, crossing the second generation parents would be ss X Ss, as stated above. The Punnet square above is correct, and results in a 1:1 ratio of normal homozygous (ss) to heterozygous carriers (Ss).