IndietroExtensions of Mendelian Genetics
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Alleles are alternative forms of a gene that can produce different phenotypes.
The wild-type allele is the allele that occurs most frequently in nature and is usually dominant.
Dominant alleles are shown by italic uppercase letters (e.g., D), recessive alleles by italic lowercase letters (e.g., d), and wild-type alleles by italic letters with a superscript + (e.g., e+).
Incomplete dominance occurs when the heterozygote phenotype is intermediate between the two homozygotes, showing partial dominance.
Codominance is when both alleles in a heterozygote are fully expressed, such as in the ABO blood group system.
Heterozygote advantage occurs when heterozygous individuals have greater reproductive success than either homozygote, e.g., sickle-cell trait provides malaria resistance.
Lethal alleles are mutations in essential genes that can cause death, often inherited recessively where homozygous individuals do not survive.
Epistasis is when alleles of one gene mask the phenotypic effects of alleles of another gene.
Complementation analysis determines if two mutations causing similar phenotypes are in the same gene or different genes by crossing mutants.
Pleiotropy is when a single gene affects multiple phenotypic traits, such as Marfan syndrome.
Modifier genes are alleles of one gene that alter the phenotypic expression of alleles of another gene.
Gene redundancy occurs when multiple genes perform the same function, so loss of one gene has no phenotypic effect.
X-linkage refers to genes located on the X chromosome, showing distinct inheritance patterns, especially in males who are hemizygous.
Males express X-linked recessive traits if they inherit one mutant allele; females must be homozygous recessive to express the trait, otherwise they are carriers.
Sex-limited inheritance means a phenotype is expressed only in one sex, despite both sexes carrying the genotype.
Sex-influenced inheritance means the phenotype expression is influenced by the individual's sex but can occur in both sexes.
Penetrance is the percentage of individuals with a genotype who actually express the associated phenotype.
Expressivity is the degree or range of expression of a mutant phenotype among individuals with the same genotype.
Environmental factors can influence phenotypes, such as temperature-sensitive alleles affecting coat color in arctic foxes or Himalayan rabbits.
A conditional lethal allele causes death only under certain environmental conditions, like temperature-sensitive lethals in Drosophila.
The Manx cat has a dominant mutation causing tail shortening in heterozygotes but is lethal in homozygotes, resulting in a 1:2 ratio of viable offspring.