Why is the 9:3:3:1 ratio for dihybrid crosses constantly referred to as the “phenotypic ratio”?
Shouldn’t it be the “genotypic ratio” because different genotypes often result in the same phenotype - thanks to simple dominance? :dubious::dubious::dubious:
For example:
Let’s cross AaBb and AaBb. A and B both exhibit simple dominance.
The number of resulting unique phenotypes is 4 - not 17 (as implied by 9+3+3+1).
However, the number of resulting unique genotypes is 17. :eek:
Huh? I just ran across a problem asking for “the number of different phenotypes possible for the progeny of the cross AaBb x AaBb, where A and B exhibit simple dominance.”
The answer was 4. I chose 16 - 16 phenotypes, right? :dubious:
There are 4 unique phenotypes, arising from 9 unique genotypes.
There is more than one way to generate some of those genotypes.
All of these four genotypes give the same phenotype:
AABB - only one way to make this one
AaBB - two ways
AABb - two ways
AaBb - four ways
1 + 2 + 2 +4 = 9
These two genotypes give the same phenotype
aaBB - only one way to make this genotype
aaBb - two ways
1 + 2 = 3
These two genotypes give the same phenotype
AAbb - only one way to make this genotype
Aabb - two ways
1 + 2 = 3
This genotype gives another phenotype
aabb - only one way to make this genotype
1 = 1
Thus in the F2 generation the ratio of the resulting four different phenotypes will be 9 : 3 : 3 : 1.
I’m getting the sickening feeling that my attempted self-pedagogy is causing me to absorb more mistakes than actual knowledge :eek:. Allow me to make sense of what you have written; the fault lies not with you but with me … hopefully this will put another misconception of mine to rest.
No, 4, but with different numbers of individuals in each class
A-B- the A B phenotype (9/16 individuals here)
aaB- the a B phenotype (a recessive phenotype) (3/16)
A-bb the A b phenotype (b recessive)(3/16)
aabb both a and b recessive phenotypes (1/16)
For the A gene you can get the dominant phenotype two genotypic ways: AA or Aa, but same phenotype. The recessive phenotype only appears if aa.