B. A recessive digenic disorder (100% penetrant) occurs ONLY when one has the aa
ID: 89782 • Letter: B
Question
B. A recessive digenic disorder (100% penetrant) occurs ONLY when one has the aabb genotype. Its found that 15 people out of 10,000 in this population have the disorder. What would be the expected frequency of the disorder if the Aa locus and the Bb locus were segregating independently? Is the actual frequency significantly different from this? A very rare disorder is shown to depend on the genotype at three unlinked autosomal loci, A/a, B/b and C/c and that to be affected one must be homozygous for the rare allele at all three loci, that is, aabbcc. The MZ twin disorder concordance is 100%, implying that the penetrance is 100%. Calculate the expected dizygotic twin concordance for presence of the affected trait, that is, if one DZ twin shows the trait, what is the chance that the 2^nd twin will be affected?Explanation / Answer
Answer b:- Given 15 people out of 10,000 are affected by recessive digenic disorder caused by aabb trait
Therefore, genotype frequency for aabb
= 15/10000 = 0.0015.
Now, if the locus for Aa abd Bb segregate independetly, (law of independent assortment-the law states that allele pairs separate independently during the formation of gametes. Therefore, traits are transmitted to offspring independently of one another) the genotype frequency ration obtained is 9:3:3:1. (according to punnett square for dihybrid cross )
Now, as per ratio from punnett square, 1 in 16 has the combination aabb. Therefore, 625 in 10000 will have aabb trait.
Thius, expected genotype frequency after independent segregation of Aa and Bb
= 625/10000 = 0.625
The actual frequency is 0.0015 ,which is significantly different from frequenct 0.625.
Answer 3:- Dizygotic twin concordance for aabbcc trait when the parents are heterozygous for all three alleles ie; parents are AaBbCc is described below.
The trihybrid cross will result into the ratio 27:9:9:9:3:3:3:1 (total -64)for the next generation, where only 1 in 64 will show the trait of aabbcc.
Thus number of concordant = 1, where as number of concordant +number of discordant = 64
Therefore, Concordance rate = Number of concordant/ (number of concordant + Number of discordant)
Concordance rate = 1/64 = 0.016
Concordance % = 0.016 *100 = 1.6%
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