12. (4 pts) Two male and three female colonists from Earth settle on an uninhabi
ID: 134935 • Letter: 1
Question
12. (4 pts) Two male and three female colonists from Earth settle on an uninhabited Earthlike planet. The colonists and their offspring mate randomly for generations. All five of the original colonists had free ear lobes (E); two of them were heterozygous for the trait (Ee). The allele for free ear lobes is dominant to the allele for attached ear lobes (e). What is the expected percentage of people in the population who would show attached ear lobes (ee)? Please show your work. (hint: find the allele frequencies of E and e first) A. Arter many generations, the population on this planet shows an unusually hich frequency of sex-linked color blindness. What feature of small populations would explain this finding? B.Explanation / Answer
A] There are 5 individuals in the population.
Two individuals are heterozygous-- Ee
Three individuals are homozygous -- EE
Therefore, the allele frequency of E and e alleles in the population is 2:8 or 1:4
Or, e is 25% of the population and E is 75% of the population.
According to Hardy-Weinberg equilibrium :
p= 25% or 25/100
or, p=. .25
q= 75% or 75/100
or, q= .75
p + q = 1
.25 + . .75 = 1
B] The gene for sex-linked colour blindness is present on the X-chromosome and is recessive
In it is the males who are most affected by this disorder. This is so because the gene can express itself in males if they have inherited it from their mother. [ In males, the Y-chromosome comes from father and the X-chromosome comes from mother ].
A female can become a carrier in two ways-- one if the father is affected and carries the defective gene on his X-chromosome, or secondly if the daughter inherits the carrier X- chromosome from her mother.
Colour blindness can be detected in females only if both the X-chromosomes carry the defective gene. Normally this is quite rare but in the given scenario, the population is very small and chances of inbreeding are very high. In such a case, even one carrier female or one affected male can cause the gene to accumulate in the population, resulting in the unusually high frequency of sex-linked colour blindness. Most probably, many females are also affected by this disease because inbreeding might have resulted in the inheritance of two defective X-chromosomes; one from affected father and the other from a carrier mother.
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