Extensions and Exceptions to Mendel’s Laws

1. Extensions and Exceptions to Mendel’s Laws Genetics Chapter 5

2. 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.

3. 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.

4. 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

5. 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.

6. 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

7. Codominance • 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 B B W W + =

8. Epistasis • 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!

9. Penetrance • 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

10. Expressivity • 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.

11. Pleiotropy • 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

12. Phenocopy • 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

13. 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.