# A misnomer?

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:

:smack:[ol]
[/ol]

Replace every instance of 17 in the post above with 16.

No, those are phenotype ratios.

AABB, AaBB, AABb, and AaBb all have the same phenotype but different genotypes.

AAbb, and Aabb have the same phenotype but different genotypes.

aaBB and aaBb have the same phenotype but different genotypes.

aabb is a phenotype which is only generated by one genotype,

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.