BIOL 1106 GENETICS Lecture 3

1. BIOL 1106GENETICSLecture 3 Dr. Lawrence Ramsden Department of Botany

2. The Complications • Multiple alleles • Incomplete dominance • Codominance • Epistasis • Lethal alleles • Expression • Continuous variation • Environmental effects

3. Multiple Alleles • A gene may have many alleles not just 2 • But, a single diploid individual can only carry 2 alleles

4. Dominance • Where there is complete dominance the phenotype of the heterozygote is the same as that of the homozygote dominant. • Incomplete or partial dominance can lead to the expression of intermediate phenotypes. • Andalusian blue, colour produced by CB/CW heterozygote from Black x White chickens.

6. Codominance • In codominance both phenotypes are expressed in the heterozygote, not an intermediate type as in incomplete dominance • In ABO blood group system heterozygous individuals produce both A & B antigens.

8. Gene Interactions • No gene acts by itself in determining phenotype. • Genes can interact to produce new phenotypes. • or, Genes can interact to hide the expression of phenotypes,- Epistasis.

9. Chicken Combs • Phenotype determined by interaction of alleles of 2 genes, R & P • 4 comb types; • Rose R/- p/p • Walnut R/- P/- • Pea r/r P/- • Single r/r p/p • True breeding if the alleles are homozygous

12. F2 9:6:1 ratio in Squash • 2 genes A & B can interact • 3 phenotypes • Spherical A/- b/b or a/a B/- • Long a/a b/b • Disc A/- B/-

14. Epistasis • Genes interact to mask another genes phenotype. • No new phenotypes are produced. • Can be dominant or recessive.

15. Agouti 9:3:4 ratio • Rodent coat colour and pattern. • Agouti, albino or black. • Albino lacks any coat colour. • Black lacks yellow colour in agouti pattern. • But, you can’t have a coat pattern without any colour! so even A/- c/c will be albino, and the recessive c/c masks the A allele

17. Flower colour, 9:7 ratio • White variety crosses sometimes produce purple flowered F1 • Purple F1 x F1 gives 9:7 purple:white • Due to interaction between dominant alleles • Purple C/- P/- • Duplicate recessive white

19. Lethal Genes • Yellow coat colour in mice is an essential gene. • Yellow heterozygotes bred to non-yellow mice give 2:1 yellow non-yellow ratio. • Yellow AY/AY homozygotes are aborted. • Yellow dominant for coat colour. • Recessive with respect to lethality.

21. Environmental Effects • Fur colour in Himalayan rabbits; • above 30°C all white • at 25°C normal pattern with dark extremeties. • Cooled below 25°C, more dark patches.

22. Neurofibromatosis • Penetrance, the phenotype does not always reflect the genotype. • Expressivity, the degree to which the phenotype is expressed for a given genotype.

23. Continuous Variation • Many phenotype characters are the product of the action of a large number of genes. • So the phenotype is the product of independent assortment in the individual genes. • In a population we will see gradual differences, or continuous variation.

24. Linkage • Non-independent assortment of genes • Linked genes, located on same chromosome • Genetic recombination, where progeny show new combinations of alleles not present in the parents

26. Total flies = 2,441 • Total parental phenotypes = 1,541 • Total recombinant phenotypes = 900 • % recombinants = 36.9 • If there was independent assortment we would predict 50% recombinants • Conclusion, the genes are linked

27. Crossing Over

28. Linkage Maps • Use linked genes to determine physical location on chromosome. • The greater the separation between 2 genes the higher the probability that crossover will occur. • 1% recombination = 1 map unit (mu or cM) • Multiple cross-overs can also occur

30. Avoiding multiple cross-overs • Use closely-linked genes (within 10 mu) • Use a three-point test cross including a third allele. • The products of a double crossover will be the least frequent. • Can help establish the order of the genes on the chromosome. • Interference can happen between cross-overs.