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Lethal allele
Lethal alleles (also referred to as lethal or lethals) are alleles that cause the death of the organism that carries them. They are usually a result of mutations in genes that are essential for growth or development. Lethal alleles can be recessive, dominant, conditional, perinatal, or postnatal after an extended period of apparently normal development depending on the gene or genes involved.
Lethal alleles may specifically refer to embryonically lethal alleles, in which the fetus will never survive to term. Such alleles are a cause of non-Mendelian patterns of inheritance, such as the observation of traits in a 2:1 ratio.
Lethal alleles were first discovered by Lucien Cuénot in 1905 while studying the inheritance of coat colour in mice. The agouti gene in mice is largely responsible for determining coat colour. The wild-type allele produces a blend of yellow and black pigmentation in each hair of the mouse. This yellow and black blend may be referred to as 'agouti' in colour. One of the mutant alleles of the agouti gene results in mice with a much lighter, yellowish colour. When these yellow mice were crossed with homozygous wild-type mice, a 1:1 ratio of yellow and dark grey offspring were obtained. This indicated that the yellow mutation is dominant, and all the parental yellow mice were heterozygotes for the mutant allele.
By mating two yellow mice, Cuénot expected to observe a usual 1:2:1 Mendelian ratio of homozygous agouti to heterozygous yellow to homozygous yellow. Instead, he always observed a 1:2 ratio of agouti to yellow mice. He was unable to produce any mice that were homozygous for the yellow agouti allele.
It was not until 1910 that W. E. Castle and C. C. Little confirmed Cuénot's work, further demonstrating that one quarter of the offspring were dying during embryonic development. This was the first documented example of a recessive embryonic lethal allele.
Lethal alleles can also refer to any allele that can result in a terminal condition.
A pair of identical alleles that are both present in an organism that ultimately results in death of that organism are referred to as recessive lethal alleles. Though recessive lethals may code for dominant or recessive traits, they are only fatal in the homozygous condition. Heterozygotes will sometimes display a form of diseased phenotype in addition to an apparently dominant phenotype, as yellow mice are particularly susceptible to diabetes and obesity.
An example of a lethal allele in humans are the BRCA mutations; inheriting one defective BRCA allele results in a greatly increased risk of breast cancer and ovarian cancer, while inheriting both defective alleles is embryonically lethal in almost all cases. For live cases, inheriting both mutations lead to a grave prognosis where survival almost never extends beyond childhood. This is because the BRCA mutations also result in a severe subtype of Fanconi anemia (FA-S for BRCA1, FA-D1 for BRCA2), itself an extremely rare medical condition.
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Lethal allele
Lethal alleles (also referred to as lethal or lethals) are alleles that cause the death of the organism that carries them. They are usually a result of mutations in genes that are essential for growth or development. Lethal alleles can be recessive, dominant, conditional, perinatal, or postnatal after an extended period of apparently normal development depending on the gene or genes involved.
Lethal alleles may specifically refer to embryonically lethal alleles, in which the fetus will never survive to term. Such alleles are a cause of non-Mendelian patterns of inheritance, such as the observation of traits in a 2:1 ratio.
Lethal alleles were first discovered by Lucien Cuénot in 1905 while studying the inheritance of coat colour in mice. The agouti gene in mice is largely responsible for determining coat colour. The wild-type allele produces a blend of yellow and black pigmentation in each hair of the mouse. This yellow and black blend may be referred to as 'agouti' in colour. One of the mutant alleles of the agouti gene results in mice with a much lighter, yellowish colour. When these yellow mice were crossed with homozygous wild-type mice, a 1:1 ratio of yellow and dark grey offspring were obtained. This indicated that the yellow mutation is dominant, and all the parental yellow mice were heterozygotes for the mutant allele.
By mating two yellow mice, Cuénot expected to observe a usual 1:2:1 Mendelian ratio of homozygous agouti to heterozygous yellow to homozygous yellow. Instead, he always observed a 1:2 ratio of agouti to yellow mice. He was unable to produce any mice that were homozygous for the yellow agouti allele.
It was not until 1910 that W. E. Castle and C. C. Little confirmed Cuénot's work, further demonstrating that one quarter of the offspring were dying during embryonic development. This was the first documented example of a recessive embryonic lethal allele.
Lethal alleles can also refer to any allele that can result in a terminal condition.
A pair of identical alleles that are both present in an organism that ultimately results in death of that organism are referred to as recessive lethal alleles. Though recessive lethals may code for dominant or recessive traits, they are only fatal in the homozygous condition. Heterozygotes will sometimes display a form of diseased phenotype in addition to an apparently dominant phenotype, as yellow mice are particularly susceptible to diabetes and obesity.
An example of a lethal allele in humans are the BRCA mutations; inheriting one defective BRCA allele results in a greatly increased risk of breast cancer and ovarian cancer, while inheriting both defective alleles is embryonically lethal in almost all cases. For live cases, inheriting both mutations lead to a grave prognosis where survival almost never extends beyond childhood. This is because the BRCA mutations also result in a severe subtype of Fanconi anemia (FA-S for BRCA1, FA-D1 for BRCA2), itself an extremely rare medical condition.