Alleles that cause an organism to die only when present in homozygous condition are called lethal alleles.
The following exercise will help you know how to identify a lethal allele in Genetics’ problems.
The cross between two hybrids of F1 of genotypes AaBb gives the following phenotypes:
36 [AB], 12[Ab], 19[aB] and 6[ab].
1- Calculate the proportions of the descendants.
2- Knowing that the genes are independent, how can you explain such a result?
3- Make the factorial analysis verifying the above phenotypic proportions.
1- Proportion Calculation:
- Divide by the smallest number:
[Ab] 12/6= 2
[aB] 18/6= 3
[ab] 6/6= 1
- Find out the total: Total = 6+2+3+1=12
- Write the proportions:
2- Explanation of the Results:
The genes are independent, and this is a selfing between individuals of F1, this means that the resulting theoretical proportions of the phenotypes of F2 should be 9/16[AB]; 3/16[Ab]; 3/16 [aB]; 1/16[ab], the total of the genotypes will be 16.
|Theoretical Results||Real Results|
Comparing the Theoretical to the Real results, we got a change (less) proportions for the first 2 phenotypes, this means that a lethal allele caused the death of 4 out of the theoretical 16 genotypes (16-12= 4) in the first 2 phenotypes: [AB] and [Ab] phenotypes.
3- Factorial Analysis:
Genotypes of F1*F1: AaBb * AaBb
Gametes: ¼ AB, ¼ Ab, ¼ aB, ¼ ab for each parent
Table of cross:
-The genotypes that will enable us to find out the lethal allele are:
We look at the phenotypes that have a change in the real proportions when compared to the theoretical ones:
- 1/16AAbb, 2/16Aabb: all have [Ab] and in the real phenotypes there are 3-2=1 missing (dead) so the missing should be AAbb because if the dead was of Aabb, we would have been missing 2 not 1!
- 1/16AABB, 2/16AaBB, 4/16 AaBb, 2/16 AABb: all have [AB] and in the real phenotypes there are 9-6=3 missing (dead) so with comparison to the above results, the missing genotypes should carry AA (1AABB, 2AABb).
So if we consider that the lethal allele is A in the homozygous state, the given results will be verified.