Deviations of Mendelian inheritance

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  • Deviations from Mendelian inheritance
    • Lethal alleles
      • conditional lethal alleles
        • may kill an organism only when certain environmental conditions prevail
      • One that has the potential to cause the death of an organism, result of mutation in essential genes, usually recessive
    • Sex-influenced traits
      • traits where on allele is dominant in one sex but recessive in the opposite sex
        • Baldness in humans
    • Sex-limited traits
      • traits that only occur in one of the two sexes, determined by sex hormone levels
        • Breast development (females), beard development (males)
    • Multiple alleles
      • the presence of more than two alleles of a gene in a population
        • PKU, CF
    • Incomplete dominance
      • the heterozygote has a phenotype that is intermediate between the corresponding homozygote phenotypes
    • Co-dominance
      • the presence of both alleles in heterozygous genotypes can be detected, has own phenotype
    • Over- dominance
      • heterozygote is fitter than both of the corresponding homozygotes
        • Sickle cell anaemia
    • Under-dominance
      • heterozygote is less fit than either of the homozygotes
    • Epistasis
      • Recessive
        • homozygous recessive at one locus(w) prevents the other locus(m) from exerting its phenotype
          • phenotype of wwmm = wwMm
          • 9:3:4
      • Dominant
        • dominant allele at one locus(B) prevents the other locus(A) from exhibiting homozygous recessive phenotype
          • phenotype A_B_ = aaB_
          • 12:3:1
      • Dominant and Recessive
        • dominant allele at one locus(A) produces the same phenotype as homozygous recessive at other locus(B)
          • phenotype A_BB=A_Bb=A_bb
          • 13:3
      • Duplicate recessive
        • complete dominance at both loci
          • phenotype A_bb=aaB_=aaB_ , A_B_ is different
          • 9:7
      • Duplicate dominant
        • presence of dominant allele at either loci produces dominant phenotype
          • phenotype A_B_=A_bb=aaB_ , aabb is different
          • 15:1
      • Duplicate gene with cumulative effects
        • dominant allele at either locus, both not both, produces same phenotype
          • phenotype A_bb=aaB_ , aabb distinct
          • 9:6:1
      • when one gene masks and effects the phenotype of another
        • gene that does this is called a modifier or epistasis gene
    • Penetrance
      • whether a genotype is expressed as phenotype
        • proportion of organisms whose phenotype matches genotype for given character is penetrance of genotype
        • always expressed = completely penetrant
        • not always expressed = incompletely penetrant
    • Expressivity
      • genes that are expressed to different degrees in different organisms, variable expressivity
      • Polydactyly
    • Pleiotrophy
      • single-gene disorder with many symptoms, or gene that controls several functions or has more than one effect
        • porphyria
    • Heterogeneity
      • Genetic
        • different genes that produce the same phenotype
          • 132 separate genes control hearing loss
      • Allelic
        • multiple alleles at same gene locus
          • 1600 allele mutations in CFTR that cause CF
    • Phenocopies
      • environmental caused trait that appears to be inherited is phenocopy
        • Siamese cats

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