Genetics Study Notes- Sep 10

Recessive Lethal Alleles

Definition and Examples

Recessive lethal alleles are mutations that cause death when present in a homozygous state, but may allow survival in heterozygotes. These alleles can affect both phenotype and viability, as seen in mouse coat color genetics.

• Wild-type agouti (A) and mutant yellow (AY) alleles in mice.

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

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

Genetic Mechanism

• The AY allele is caused by a 120,000 nucleotide deletion.

• This deletion removes the regulatory region of the agouti gene, causing it to be always expressed.

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

Punnett Square Example

Cross: AYA x AYA

	A	AY
A	AA	AYA
AY	AYA	AYAY (lethal)

Phenotypic ratio: 2 yellow : 1 agouti (AYA : AA), AYAY genotype is lethal.

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).

• AYA or AYa: Hair is solid yellow (gain-of-function mutation).

Example Table: Genotype to Phenotype

Genotype	Phenotype
AA, Aa	Agouti (banded)
aa	Black (solid)
AYA, AYa	Yellow (solid)

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 B loci.

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

Epistasis in Mouse Coat Color

• Presence of any color depends on at least one functional copy of another gene, B.

• bb homozygotes have albino coat, regardless of agouti genotype.

• B locus is epistatic to the Agouti locus (B masks the effect of A).

Genetic Cross Example

P1 cross: agouti (AABB) x albino (aabb) F1: all agouti (AaBb) F2 cross: agouti (AaBb) x agouti (AaBb)

Genotype	Phenotype
A_B_	Agouti
aaB_	Black
__bb	Albino

Modified phenotypic ratio: 9 agouti : 3 albino : 4 black (9:3:4)

Genetic Linkage

Definition and Principles

Genetic linkage refers to the tendency of genes located close together on the same chromosome to be inherited together. This is an exception to Mendel's law of independent assortment.

• Linked alleles are more likely to be inherited together than to segregate independently.

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

Measuring Linkage: CentiMorgans (cM)

• The distance between linked genes is measured in centiMorgans (cM).

• 1 cM = 1% recombination frequency.

• 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:

$\text{Recombination Frequency} = \frac{\text{Number of Recombinant Offspring}}{\text{Total Number of Offspring}} \times 100\%$

Summary Table: Mouse Coat Color Epistasis

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

Modified ratio: 9:3:4

Additional info:

• Product law can be used to calculate expected ratios in dihybrid crosses.

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

• Linkage mapping is fundamental for locating genes on chromosomes and understanding genetic diseases.