1. Assume that long ear lobes manifess in a population as an autosomal dominant
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Question
1. Assume that long ear lobes manifess in a population as an autosomal dominant trait that exhibits 50% penetrance. A person who is heterozygous for long lobes mates with a person who has typical ear lobes. What is the probability that their first child will have long ear lobes?
a. 50%
b. 30%
c. 15%
d. 25%
e. 35%
2. If 4 distinct loci, A,B,C and D contribute equally and incrementally to the height of a plant and each locus has 2 distinct allele (one completelety dominat over the other), how many different phenotypes exist for this trait?
a. 4
b. 8
c. 9
d. 12
e. 13
Explanation / Answer
1. D
All the possible genotypes for earlobe phenotype can be AA, Aa, aA, aa. Therefore the probability to be heterogygous (Aa, aA) = 2/4 = 0.5.
Now, if the trait exhibits 50% penetrance, then the probability to express it is 0.5.
Thus, the probability of being heterozygous (AND) to express the long earlobe = 0.5 X 0.5 = 0.25
2. A
If they show complete dominance, then the homogygous dominants are not distinguishable from the heterpgygotes. Thus, the traits are govenrd by:
Trait-1 - dominant homogygous/heterogygous at one loci, homogygous recessive at the rest
Trait-2 - dominant homogygous/heterogygous at two loci, homogygous recessive at the rest
Trait-3 - dominant homogygous/heterogygous at three loci, homogygous recessive at the rest one
Trait-4 - dominant homogygous/heterogygous at all the four loci.
Please note that when it is said that there is a comnibation of two, three or four loci, it does not matter which of the four loci combine, as it is said that they contribute equally. For example, an allele combination of AA, BB, cc, Dd will give the same phenotype as AA, bb, CC, Dd.
Please also note that there is one more possible trait, exhibited when all the four loci will ve homogygous recessive. If '5' would be an option, then I would have chosen this option.
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