Alleles that cause an organism to die only when present in homozygous condition are called lethal alleles. The gene involved is considered an essential gene.
When Mendel's Laws were rediscovered, geneticists believed that mutations would only alter the appearance of a living organism. However, it was discovered that a mutant allele could cause death. When an essential gene is mutated, it can result in a lethal phenotype. If the mutation is caused by a dominant lethal allele, the heterozygote for the allele will show the lethal phenotype, the homozygote dominant is impossible. If the mutation is caused by a recessive lethal allele, the homozygote for the allele will have the lethal phenotype.
An example of lethal alleles in humans is achondroplasia, a genetic condition which causes dwarfism. Affected individuals are all heterozygotes, as the accumulation of two mutant alleles is lethal and results in stillbirths.
When Mendel's Laws were rediscovered, geneticists believed that mutations would only alter the appearance of a living organism. However, it was discovered that a mutant allele could cause death. When an essential gene is mutated, it can result in a lethal phenotype. If the mutation is caused by a dominant lethal allele, the heterozygote for the allele will show the lethal phenotype, the homozygote dominant is impossible. If the mutation is caused by a recessive lethal allele, the homozygote for the allele will have the lethal phenotype.
An example of lethal alleles in humans is achondroplasia, a genetic condition which causes dwarfism. Affected individuals are all heterozygotes, as the accumulation of two mutant alleles is lethal and results in stillbirths.
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They're not - UNLESS they happen to be homozygous for a lethal mutation. Eg., homozygous checker (C) in pigeons, simply produces checker marked wingshields. On the other hand, homozygous dominant opal produces embryos that either die in the shell or soon after hatch.
It's not the homozygosity that's lethal, it's the fact that IF a mutation is semi-lethal or lethal that when there is no other information in the cell to follow, all the cell's machinery can run is the "lethal programming."
It's not the homozygosity that's lethal, it's the fact that IF a mutation is semi-lethal or lethal that when there is no other information in the cell to follow, all the cell's machinery can run is the "lethal programming."