Exceptions to Mendelian Genetics

1. Exceptions to Mendelian Genetics

2. Incomplete Dominance phenotype of heterozygote falls between that of two homozygotes RR: red flower Rr: pink flower rr: white flower

3. Codominance

4. Epistasis Interaction of two or more genes to control a single phenotype. Epistatic gene- gene that determines whether a trait will be expressed, can mask other genes Hypostatic gene- gene whose expression is masked by an epistatic gene Types of Epistasis Duplicate genes Complementary Gene Action Dominant epistasis Dominant suppression Recessive epstasis

5. Duplicate genes Need either A or B wheat kernel color 15:1 ratio

6. Complementary Gene Action Need C and P Flower color in sweet pea 9:7 ratio

7. Dominant Epistasis Color is recessive to no color Fruit color in squash W= white, w= colored, G= yellow, g= green 12:3:1

8. Dominant Suppression Production of the chemical malvidin in the plant primula both synthesis of malvidin is dominant, K, and so is the suppression of it,D. 13:3 ratio

9. Recessive Epistasis coat color in mice need C to express color A 9:3:4 agouti, black, albino

10. Lethal Genes cause death  dominant- usually not studied recessive- change ratio to out of 3 because 1 die ratio becomes 1:2 example yellow mice

11. Penetrance Complete penetrance In a population, 15 individuals carry the dominant allele for achondroplasiaAll 15 are achondroplastic dwarvesPenetrance = 100%Complete penetrance Incomplete penetrance A population has 20 individuals with the dominant allele for neurofibromatosis16 individuals show symptoms of the diseasePenetrance = 80% Expressivity :The degree to which a penetrant genotype is phenotypically expressedExample: neurofibromatosis, differing amount and size of tumors 

12. More Exceptions Pleiotropy- a single gene has multiple effects on phenotype Sickle Cell anemia- physical weakness, slow development, hypertrophy of bone marrow, damage to organs, heart failure Phenocopy- A phenotype caused by the external environment which mimics a phenotype that is genetically caused Examples- rickets from vitamin D deficiency, Improper development of the limbs, to give a flipper-like appearance, Rare dominant allele or taking thalidomide during a particular period of pregnancy

13. Traits Related to Gender Sex-limited traits Genes that affect a particular character that appears in one sex but not the other. These genes are usually not on the sex chromosomes (autosomal). Milk yield in cattle, Size and shape of horns in rams, Secondary sex characters. Ovary, sperm development. Sex-influenced traits Also usually autosomal Appear in both sexes, but in different frequencies or with different genotype-phenotype relationship. Pattern baldness is dominant in males and recessive in females. Need testosterone to be dominant.

14. Sex Determination Sex determination- SRY gene, sex determining region of the Y gene, necessary for testis formation Lacking this SRY makes you female. XX- normal female XY- normal male XXY- Kleinfelter’s male, breast development, difficulty reading and writing, undersized testes, most sterile XO- Turner’s syndrome, female, short stature, lack of ovarian development, cardiovascular problems, infertile, XXX- female, quiet, passive, fertile, but go through menopause earlier than most XYY- male, increased height, more physically active, delayed mental maturation

15. Linkage Some genes are so close together that they do not cross-over as much Two-Point Test Crosses Should yield 2 recombinants with similar frequencies Should yield parental genotype with similar frequencies (# of recombinants/# of progeny) = recombination frequency Recombination frequency of 50% shows the genes are unlinked

17. Three-Point Test Cross Double crossovers have the fewest progeny The only allele that changed is in the middle Gives us overall organization and distances

18. Sex-linked genes Y linked are found on the Y chromosome and only males can inherit them. Very rare. X linked are found on the X chromosome and both can inherit them. Dominant-:females more likely to have dwarfism Recessive: males more likely to have hemophilia, red-green colorblindness, duchene muscular dystrophy male hemizygous