ShmoopTube

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And here's your Shmoop du jour, brought to you by genotypes.

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Now available in relaxed fit, bootcut, and skinny leg. [Genotypes wearing variations of jeans]

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Okay, here's our question…

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A bird with gray feathers mates with a bird that also has gray feathers.

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They have 4 offspring with white feathers, 9 with gray feathers, and 4 with black feathers. [Birds flying together]

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What is the simplest interpretation for these results?

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Here are the potential answers…

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Okay, here we go.

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

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control of a somatic gene on a single chromosome.

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

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

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

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we’re seeing.

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

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

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Now we need to think about what their genotypes are…

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

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

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

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

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

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recessive”.

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Huh.

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

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Anyway.

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The question is, which color is dominant?

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

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

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

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

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

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of the dominant and recessive alleles for a trait.

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

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Glad we came to the right answer.

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

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