This document discusses lethal alleles, which are alleles that cause death in an organism. It defines lethal alleles and provides a brief history of their discovery through early studies of coat color inheritance in mice. The document outlines four types of lethal alleles: early onset alleles that cause death early in life, late onset alleles that cause death late in life, conditional alleles that only cause death under certain environmental conditions, and semi-lethal alleles that only kill some individuals, not all. It provides the example of the Y gene in mice, which causes a yellow coat color but is lethal when present in the homozygous dominant state (YY), though not in the heterozygous or recessive states.

1. Lethal allele
Harshraj Subhash Shinde
KKW, CABT, NASHIK

2. Outline
• Defination and Introduction
• History
• Types of lethal allele
• Example of lethal allele- Coat colour in mice

3. Introduction
• “Allele which cause an organism to die is known as lethal allele”
• Certain genes are absolutely essential for survival. Mutation in these genes
creates lethal allele
• Lethal alleles are dominant or recessive
• Fully dominant lethal allele kills organism in both homozygous and
heterozygous condition
• Certain lethal alleles kills organisms in homozygous condition only.

4. History
• Lethal genes were first discovered by Lucien Cuénot while studying the
inheritance of coat colour in mice.
• He expected a phenotype ratio from a cross of 3 yellow:1 white, but the
observed ratio was 2:1.
• Allele was lethal in homozygous dominant condition

5. Types of lethal alleles
• Lethal alleles falls into four categories.
1. Early onset- lethal alleles which result in death of an organism at
early stage of life for example during embryogenesis
2. Late onset- lethal allele which kills organism at their final stage of
life are known as late onset allele
3. Conditional- lethal allele which kill organism under certain
environmental conditions only.
e.g., temperature sensitive alleles kills organism at high
temperature. But they don’t kill any organism at low temperature.

6. Cont..
4. Semi lethal – Lethal allele which kill only some individuals of
the population but not all are know as semi lethal.

7. Example
• Y gene in mouse. Y gene is responsible for yellow coat color
in mouse. In recessive condition it produces white colour.
• Allele Y is lethal only when it present in homozygous
dominant condition (YY).
• But this allele is not lethal in heterozygous dominant (Yy) and
homozygous recessive (yy) condition

8. Example