Find the gentype and modes of inheritance?

The pedigree below depicts a family from Bloomington that is affected by familial acrophobia. The frequency of affected individuals in the population at large is negligible - this is a very rare trait.





http://www.flickr.com/photos/26031843@N0...





a) Which modes of inheritance can definitely be excluded by the pedigree. For each one excluded, give your reasoning.


• Autosomal recessive:


• Autosomal dominant:


• X-linked recessive:


• X-linked dominant:


• Y-linked:


b) Which mode of inheritance is most likely and why?


c) Based on your answer to part b, give the genotype for each pedigree individual listed below. Define your symbols.


• III-5


• III-6


• IV-9


• IV-11


• V-9


d) Based on your answer to part b, if individual V-1 marries V-4, what is the probability that their first child will be affected? Would your answer be different for a first male child or a first female child? Why or why not?

Find the gentype and modes of inheritance?
Norsi, I’ll assume that square is male, circle is female, and coloured shapes are the ones affected. That’s the usual standard, but if my assumption isn’t correct, my answers won’t be either.





A) Autosomal recessive: No, can’t be excluded. The condition is clearly a recessive trait, and although statistically it is improbable that 15 males and only one female would be affected, it is not impossible.


Autosomal dominant: Yes, can be excluded. The acrophobia was not present in I-1 or I-2, and yet turned up in one of their children (II-2), and dominant traits must be present in at least one parent if they are present in the child.


X-linked recessive: No, can’t be excluded. In X-recessive all sons of an affected mother would be affected themselves, and this is demonstrated by IV-9’s children. Also, the condition did not express itself in a female until IV-9, and those siblings were the first instance where both parents had evidence of the condition in their family.


X-linked dominant: Yes, can be excluded. III-7 did not pass the condition on to all of his female children. He did pass his one and only X-chromosome on to all of them.


Y-linked recessive: No, because the condition is present in both males and females.





B) X-linked recessive, because only one in 16 of the affected individuals on this chart was female. This uneven gender distribution is unlikely in an autosomal disease.





C) The symbols I used are XA for normal X chromosome, Xa for affected X chromosome, and Y for a Y chromosome.


III-5 = XA, Y


III-6 = XA, Xa


IV-9 = Xa, Xa


IV-11 = Xa, Y


V-9 = XA, Xa





D) If V-1 (genotype XA, Xa) marries V-4 (genotype Xa, Y), the probability of an affected first child is 50%. This is the same regardless of gender, because there is a 50% chance that any child would receive V-1’s healthy XA chromosome, which would prevent the recessive condition from expressing itself. If the child received V-1’s Xa chromosome instead, the genotype would be either (Xa, Xa) or (Xa, Y). Either one would express the condition in their phenotype.





Good luck with the rest of your biology homework!