I have the following assignment where I am to look at the "tree" (not sure the english word) and assign whether or not they can be autosomal dominant or recessive as well whether they can be X-linked recessive.

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My way to try and complete this assignment is by drawing a punnet's square but I quickly run into some confusion. When I draw my square, I know that is I'm checking for autosomal dominant, then I would draw the mother as A'a, the A' denoting the sick dominant allele. What I'm not sure is this: Can't I also draw her as A'A'? That would also mean she would be autosomal dominant but with 2 sick alleles. What about the father? Do I draw him as Aa, aa or AA? I'm a bit confused here!


1 Answer 1


Let's go step my step here.

Going by your notation I'll call the square as Male and Circle as female. So from this information I can confidently say that you disease is not X linked dominant.


Because Females carry XX, that would mean to be X linked dominant she had to be X'X' for the disease allele to manifest it's phenotype. But if it were X'X, it would lead to a loss in fitness. And since the Male is normal and XY he does not carry the disease which tells us that the disease is definitely not X linked dominant.

Therefore in a cross of X'X' and XY you get individuals X'X and X'Y in the ratio 1:1

Coming to AD Autosomal Dominant may mean A'A' as the disease allele is present in homozygous condition. A heterozygous condition would lead to loss in fitness, not necessarily a disease.

A Father may be AA or AA'

so this cross would give you individuals

  1. AA' for A'A' x AA
  2. A'A A'A' for AA' x A'A'

so in this case you always get the disease in autosomal dominant, what remains is recessive.

AA or AA' Father x A'A' Mother

Products AA' for AA x A'A' or AA' and A'A' for AA' x A'A'

which tells us that the disease is actually Autosomal recessive on the mother.

The father's genotype for the allele is AA or homozygous dominant

and the disease is autosomal recessive for the disease in the mother.

  • $\begingroup$ Why can the father not be Aa or aa? $\endgroup$
    – Paze
    Commented May 28, 2015 at 15:53
  • $\begingroup$ the Aa that you are talking about is the AA' condition and the aa or A'A' cannot exist because in the diagram the father is a normal individual. So he can be either homozygous dominant for the wild type fit allele and heterozygous leading to a loss in fitness. $\endgroup$ Commented May 28, 2015 at 15:56
  • $\begingroup$ I don't think loss of fitness is relevant in this situation. In any case, there is nothing in the question to suggest that the disease is incompletely dominant and would result in a loss of fitness in a heterozygote $\endgroup$
    – C_Z_
    Commented May 28, 2015 at 16:06

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