AP Biology 2.1 Essential Life Process Information

What is the simplest interpretation for these results?

Here are the potential answers…

Okay, here we go.

To keep it simple, let’s first assume that feather color is a single trait under the [Bird spreading its wings]

control of a somatic gene on a single chromosome.

This already eliminates answers C and D, since dihybrid crosses involve two different traits,

and sex-linkage involves the two sex chromosomes, rather than the other somatic chromosomes.

This leaves us with options A and B. First, let’s think about what phenotypes

we’re seeing.

The phenotype is what’s physically displayed, while the genotype is what is encoded genetically. [Girl and guy looking puzzled]

So taking a look at this question, we see three phenotypes: black, white, and gray. [Three birds on a branch]

Now we need to think about what their genotypes are…

We're thinking bell-bottom, but…don't hold us to that. [Three phenotypes wearing jeans]

Since feather color is a trait controlled by a single allele, when a new baby bird is

born, there are three possibilities for its genotype: (1) it inherits two dominant alleles

– this is called “homozygous dominant” (2) it inherits a dominant and a recessive

allele, or “heterozygous”, or (3) it inherits two recessive alleles – called “homozygous

recessive”.

Huh.

Really thought "bell-bottoms" would be a choice for genotype…

Anyway.

The question is, which color is dominant?

Let’s assume black feathers are dominant here. [Black birds and white birds in a field]

If that’s the case, in a monohybrid cross, birds that are heterozygous should be black,

as black feathers would dominate over the gene for white feathers.

But we don’t see that – we see birds with gray feathers. [Black, Gray and White birds in a field]

This means that we are likely to have incomplete dominance – where heterozygotes show a mixture

of the dominant and recessive alleles for a trait.

Which means the answer for this question is (A).

Glad we came to the right answer.

Now we can dihybrid happy. [A gravestone for Science McScienceton]