Lethal Alleles

Introduction to Lethal Alleles

  • Lethal alleles: Alleles that can cause the death of an organism, typically during early development.

  • Focus on recessive lethal alleles, which require homozygous genotype (e.g. YY) for lethality.

  • Less emphasis on dominant lethal alleles, which require only one allele to cause death, making live offspring impossible.

Pleiotropic Genes

  • Lethal alleles often included in pleiotropic genes.

  • Pleiotropy: A single gene influences multiple phenotypes.

  • Example: A gene can cause both lethality and other traits like fur color.

  • Phenotypes may have independent dominance patterns.

Monohybrid Cross for Lethal Alleles

  • To test for lethality, perform a monohybrid cross (cross between two heterozygotes).

  • Example: In mice, alleles for fur color:

    • Y = dominant yellow fur color

    • y = recessive white fur color

  • Expected Mendelian ratio from crossing two heterozygous mice (Yy x Yy) would typically be 3:1 (yellow:white).

  • If observed ratio is 2:1 instead, it signifies that the Y allele is also causing lethality when homozygous (YY).

Phenotypic Outcomes

  • Only YY individuals die, hence the 2:1 ratio reflects:

    • 2 yellow heterozygous (Yy) offspring.

    • 1 white homozygous (yy) offspring emerges alive.

  • Gene Y illustrates both a lethal and a phenotypic characteristic (fur color).

Recap Questions

  1. Are lethal alleles only recessive?

    • Answer: No, lethal alleles can also be dominant, but these never appear in offspring as both homozygous and heterozygous genotypes result in death.

  2. What offspring genotypes will survive if you cross two yellow mice?

    • Answer: Only heterozygous (Yy) and homozygous recessive (yy) individuals survive. Homozygous dominant (YY) will not survive.

  3. What will be the ratio of the offspring from two yellow mice?

    • Answer: A ratio of 2:1 (yellow to non-yellow) emerges when considering lethality - two heterozygous yellow and one white.

  4. What would be the ratios if crossing one yellow mouse and one non-yellow mouse?

    • Cross: Yellow (Yy) with Non-yellow (yy).

    • Outcomes: 50% yellow heterozygous offspring (Yy) and 50% non-yellow homozygous recessive offspring (yy). No homozygous YY present, thus no lethality.