Lethal allele

A variant of a gene that causes death when present in a particular genotype, often before reproduction. Its effect may depend on whether one or two copies are inherited.

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Dominance relationship: The relationship between alleles that determines how their effects appear in a heterozygote. A lethal allele can be dominant, recessive, or lethal only in a particular genotype.
Yellow coat color in mice: A coat-color trait in mice associated with an allele that is lethal when homozygous. The viable yellow heterozygotes produce fewer surviving offspring than a simple dominant model predicts.
Allele: One of the alternative forms of a genetic sequence found at a particular locus. A lethal allele is a gene variant whose effect depends on the genotype carrying it.
Segregation distortion: A departure from the expected equal transmission of alleles from a heterozygous parent. Death of offspring with one genotype can make surviving progeny appear to violate Mendelian ratios.
Homozygosity: The state of carrying two identical alleles at a genetic locus. Many recessive lethal alleles cause death only when both inherited copies are lethal.
Manx cat: A cat breed characterized by a naturally short or absent tail, associated with a dominant allele. The tail allele is lethal in homozygous embryos, while heterozygotes can survive.
Genotype: The genetic makeup of an organism at one or more loci. Lethality attaches to particular allele combinations, not necessarily to a single allele in isolation.
Hardy–Weinberg principle: A population-genetic model relating allele and genotype frequencies under specified assumptions. Selection against lethal genotypes can shift populations away from its expected genotype proportions.
Heterozygosity: The state of carrying different alleles at a genetic locus. A heterozygote may survive while transmitting a recessive lethal allele.
Achondroplasia: A genetic condition that causes disproportionate short stature and is usually due to a dominant FGFR3 variant. Two copies of the common achondroplasia-causing variant are typically lethal before or shortly after birth.