Inheritance Pattern of Lethal Alleles
- Some genes are lethal when both alleles are present. Lethality can occur before or after birth
- Huntington's disease in humans is caused by a lethal allele, death occurs later in life
- Other examples: Mouse coat color (yellow), Creeper legs in chickens, Manx Cats (no tails)
An example is the "creeper" allele in chickens, which causes the legs to be short and stunted.
- Creeper is a dominant gene, heteroyzous chickens display the creeper phenotype
- If two creeper chickens are crossed, one would expect to have (from mendelian genetics) 3/4 of the offspring to be creeper and 1/4 to be normal
- Instead the ratio obtained is 2/3 creeper and 1/3 normal.
- This occurs because homozygous creeper chickens die.
Manx Cats
Cats possess a gene for producing a tail. The Manx phenotype in cats is produced by an allele that is lethal in its homozygous state. The allele interferes with normal spinal development, in heterozygous cats this results in lack of a tail. Cats that are heterozygous for the trait hav a short tail.
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