1. a couple of mice with brown fur and black eyes had litter with the following distribution of traits: 9 with brown fur and black eyes, 3 with brown fur and red eyes, 3 with grey fur and black eyes, and one with grey fur and red eyes. propose an explanation.

The genotype of the couple of mice is heterozygous.

Explanation:

First, we going to classify each trait as dominant or recessive allele:

Brown fur: It’s a dominant allele because 12 of 16 mice have brown fur, therefore 75% of the litter have this trait.

Black eyes: It’s a dominant allele because 12 of 16 mice have black eyes, therefore 75% of the litter have this attribute.

Grey fur: It’s a recessive allele because only 4 of 16 mice have this trait, so 25% of the litter have grey fur.

Red eyes: It’s a recessive allele because 4 of 16 mice have red eyes, so 25% of the litter show this trait.

Second, we going to name each phenotype with a letter and use a capital letter for dominant allele or a small letter for recessive allele.  

For the color of the fur, let’s use letter “F”:  

F: Brown fur

f: Grey fur

For the color of the eyes, let’s use letter “E”:

E: Black eyes

e: Red eyes

The genotype of each phenotype is conformed by two letters that indicates, the recessive allele is showed in the phenotype only when both letters in the genotype are small letter.

For example, for fur, there are three combinations:

FF: Brown fur (two dominant alleles)

Ff: Brown fur (A dominant allele and a recessive allele)

ff: Grey fur (Two recessive alleles)  

For eyes, there are also three combinations:

EE: Black eyes (two dominant alleles) (

Ee: Black eyes (A dominant allele and a recessive allele)  

ee: Red eyes (Two recessive alleles)  

When both allele of the genotype are dominant or recessive, it is heterozygous, if there are a dominant allele and a recessive one, it is a homozygous.

Since there are 7 mice that show recessive phenotype, that means the genotype of the couple of mice are heterozygous. Although, both mice show a dominant phenotype, the genotype have a dominant allele and a recessive allele.

The genotype for the couple of mice is:

FfEe x FfEe

So, when we apply the Mendel's third law, we got the results showed in the chart.

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