Genetics Study Notes: Mouse Coat Color, Epistasis, and Genetic Linkage

Recessive Lethal Alleles

Definition and Example in Mice

Recessive lethal alleles are mutations that cause death when present in a homozygous state, but heterozygotes may survive and display a phenotype. In mice, the agouti (A) and mutant yellow (AY) alleles provide a classic example.

• AY allele is dominant for coat color (yellow) but recessive for lethality (AYAY mice die before birth).

• Crossing two AYA mice yields a 2:1 ratio of yellow to agouti offspring, not the expected 3:1 Mendelian ratio, because AYAY genotype is lethal.

Example:

Genotype	Phenotype
AA	Agouti
AYA	Yellow
AYAY	Lethal (dies before birth)

Why do AYAY mice die? The AY allele is caused by a 120,000 nucleotide deletion that:

• Deletes the regulatory region of the agouti gene, causing it to be always expressed.

• Deletes part of the adjacent Merc gene; if both copies are affected, lethality occurs.

Mouse Coat Color Genetics

Genotypes and Phenotypes

Mouse coat color is determined by the interaction of alleles at the agouti locus:

• Agouti (AA or Aa): Hair has bands of black and yellow pigment.

• aa: Hair is solid black (loss-of-function mutation).

• AAY or aAY: Hair is solid yellow (gain-of-function mutation).

Example:

Genotype	Coat Color
AA or Aa	Agouti (banded)
aa	Black
AYA or AYa	Yellow

Sample Crosses and Expected Ratios

Crosses between different genotypes yield specific phenotypic ratios. For example:

• Cross: AYa × Aa

• Possible offspring: AYA, AYa, Aa, aa

• Phenotypes: yellow, yellow, agouti, black

Example Punnett Square:

	A	a
AY	AYA	AYa
a	Aa	aa

Epistasis

Definition and Examples

Epistasis occurs when the expression of one gene masks or modifies the expression of another gene. This phenomenon is important in understanding complex genetic traits.

• Mouse coat color: Interaction between agouti and albino loci.

• ABO blood group: Bombay phenotype (involves the FUT1 gene).

Epistasis in Mouse Coat Color

The presence of any color in mouse coat is dependent on at least one functional copy of another gene, B. Homozygous bb mice are albino, regardless of the agouti locus genotype. Thus, the B locus is epistatic to the agouti locus.

• Agouti (AA or Aa, B-): Banded color

• Black (aa, B-): Solid black

• Yellow (AYA or AYa, B-): Solid yellow

• Albino (bb): No pigment, regardless of agouti genotype

Dihybrid Cross and Modified Ratios

When crossing agouti (AABB) × albino (aabb), all F1 are agouti (AaBb). F2 generation from AaBb × AaBb yields a modified phenotypic ratio due to epistasis:

Character	F2 Phenotypes	Modified Ratio
Agouti	9/16
Albino	3/16
Black	3/16
Albino	1/16
		9:3:4

This 9:3:4 ratio is a hallmark of recessive epistasis.

Genetic Linkage

Definition and Principles

Genetic linkage is an exception to Mendel's law of independent assortment. Genes located close together on the same chromosome tend to be inherited together, rather than assorting independently.

• Linked alleles are more likely to be inherited together.

• If genes are not linked, there is a 50% chance alleles will be inherited together and 50% chance they will be separated by recombination.

Measuring Linkage: CentiMorgans (cM)

The distance between linked genes is measured in centiMorgans (cM). One cM corresponds to a 1% recombination frequency between two genes.

• If two genes are 1 cM apart, there is a 1% chance they will be separated by recombination and a 99% chance they will be inherited together.

Equation:

Recombination frequency (%) = (Number of recombinant offspring / Total offspring) × 100

$\text{Recombination frequency} = \frac{\text{Number of recombinant offspring}}{\text{Total offspring}} \times 100$

Summary Table: Mouse Coat Color Genetics

Genotype	Coat Color	Notes
AA or Aa, B-	Agouti	Banded pigment
aa, B-	Black	Loss-of-function
AYA or AYa, B-	Yellow	Gain-of-function
Any, bb	Albino	B locus epistatic

Additional info:

• Epistasis is a key concept in understanding non-Mendelian inheritance patterns.

• Genetic linkage is fundamental for chromosome mapping and understanding inheritance in eukaryotes.