How to solve this Genetics problem: A cattle breeder is upset to find that a cal

Question

How to solve this Genetics problem: A cattle breeder is upset to find that a calf has been aborted spontaneously during early fetal development. He is concerned that the mother and the bull who sired the calf carry a recessive lethal mutation, as both are descendants of the same blood line (i.e. inbred). (SEE p. 227 in your book for a discussion about recessive lethal mutations, and the effect on expected phenotypic frequencies).
The breeder already had noted that some calves from this bloodline had produced aberrant, shaking behavior when young, but had outgrown the condition. In order to test whether the situation is caused by such a recessive lethal mutation, the breeder decides to cross the bull with several other cows whose calves sometimes show the “shakes”.

1) What prediction can the breeder make about the probability that a calf that reaches full-term will show the “shakes”, assuming that calves with the “shakes” are actually heterozygous for the recessive lethal mutation?

Answer: _________________________

2) Assuming that the bull and each of the cows are heterozygous for a recessive lethal mutation, what is the probability that at least one of the three calves from these crosses will show the “shakes”?
Answer:__________________________

Explanation / Answer

Because the bull is alive, we can assume that he is not homozygous recessive. Similarly, since one of his calves died prenatally, we can assume his calf to be homozygous recessive, which would require the sire to be heterozygous for this trait. We can call the sire "Aa". When we cross the sire with other cows whose children sometimes show shakes, it shows that the cows are probably also Aa. Now we can calculate the probability of living calves that "shake." By making a punnet square, we can see that all possible offspring are as follows: AA, Aa, Aa, aa. We neglect the aa offspring since we wish to know the probability that a calf will shake if it lives full-term. Because those that shake are of Aa, we know that 2/3 calves that live full term will shake.

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