extensions and exceptions to mendel s laws n.
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Extensions and Exceptions to Mendel’s Laws. Genetics Chapter 5. Exceptions to Mendel. Mendel studied simple dominant/recessive traits, but many inherited traits are not as simplistic.

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exceptions to mendel
Exceptions to Mendel
  • Mendel studied simple dominant/recessive traits, but many inherited traits are not as simplistic.
  • If methods of allele transmission do not occur in the proportions that Punnett squares predict, the nature of the genemay be more complex.
polygenic traits
Polygenic Traits
  • Many traits are controlled by more than one gene
  • For example, eye color is not just “B=brown, bb=blue” but is controlled by approximately 10 different genes working together.
lethal alleles
Lethal Alleles
  • Lethal = deadly before birth (miscarriage) OR deadly before reproductive age.
  • Huntington’s = non lethal to population even though it’s lethal to the individual
  • Sometimes inheriting two dominant alleles is lethal
  • HH= dead Hh=hairless hh=hairy
multiple alleles
Multiple Alleles
  • Some genes have more than two alleles (instead of A and a, perhaps A, B, d, f…)
  • Example: PKU (lacks enzyme to break down phenylalanine)
  • There are more than 300 possible alleles for this gene. Depending on which you have, you may be phenotypically normal, severe PKU, moderate PKU, or mild PKU.
incomplete dominance
Incomplete Dominance
  • The alleles are neither dominant nor recessive. Homozygous genotypes each have their own trait, and the heterozygous genotype has a different or intermediate trait.
  • Example: snapdragons
  • Both alleles are expressed in a heterozygote
  • Example: BB chickens have black feathers, WW chickens have white feathers, and BW chickens have both black and white feathers.
  • Example: ABO blood type






  • One gene masks the effects of another gene (do not confuse with dominant and recessive)
  • Usually the two genes affect related structures: Example: a recessive hairless gene (hh) will cover up the effects of the hair color gene in mice since you can’t see hair color if there is no hair!
  • Refers to what % of individuals that have the genotype will have the phenotype.
  • Huntington’s = 100% penetrant because everyone who has the genotype is affected
  • Polydactyly = incompletely penetrant because about 20% of those with the genotype have normal fingers/toes
  • Severity or extent of a phenotype
  • Again with polydactyly – some who have the genotype have only an extra tip of a toe or finger; some will have many full extras. It is variably expressive.
  • One gene = many effects – can be difficult to trace
  • Can be Mendelian but it is a disorder with many symptoms/controls several functions/has more than one effect
  • Ex: Porphyria variegata – the missing enzyme causes porphyrin to accumulate in many body systems causing many effects
  • This is an environmentally-caused trait that appears to be inherited/mimics an inherited trait
  • Ex: Limb birth defect caused by the drug thalidomide, but it mimics the inherited defect phocomelia
genetic heterogeneity
Genetic Heterogeneity
  • When several genes produce the same phenotype (redundancy)
  • Ex: 132 genes can cause hearing loss. If mom is deaf by two recessive alleles on gene 3 and dad is deaf by two recessive alleles on gene 17, the child will not necessarily be deaf.