2. CODOMINANCE, INCOMPLETE DOMINANCE AND MULTIPLE ALLELES

1. 2. CODOMINANCE, INCOMPLETE DOMINANCE AND MULTIPLE ALLELES Compiled by Siti Sarah Jumali Level 3 Room 14 Ext 2123

2. Lecture outline • 2nd Topic – extention of Mendelian Genetics • Codominance • Incomplete dominance • Multiple alleles • Lethal alleles • Epistasis • Polygenic inheritance • Linked genes • Crossover value and genes mapping • Sex linked genes

3. TWO ALLELES

4. CODOMINANCE • Co- means together codominant means equal in dominance • alleles are approximately equal effect in individuals; alleles are equally detectable in individuals. • Phenotypes for both alleles are exhibited in the heterozygote • The hybrid shows neither of the parents’ trait, instead, a third, different phenotype • Examples of this is blood types; ABO, iAiB.

5. Pay attention on how to write codominance; Writing it is with superscript

6. example

7. Calico cat

8. Unlike Law of Segregation.. • Using the calico cat as example, The genotype for fur can be represented as CB or CO. • CBis for black color and • CO denotes orange color • Therefore the codominance is written as • CBCBx COCO  100% CBCO

9. Common situation of Codominance • Common phenotype used is roan fur in cattle • Cattle can either be all red RR; all white WW or; Roan RW

10. Roan cattle

11. Other example • Human blood type AB • Two types of protein A and B appear together on the surface of blood cells

12. Question: • What is the probability of a child having type AB blood if one of the parents is heterozygous for A blood and the other is heterozygous for B blood? • What other genotypes are possible for children of these parents?

14. INCOMPLETE DOMINANCE • A form of intermediate inheritance in which one allele for a specific trait is not completely dominant over the other allele. • The heterozygote has an intermediate phenotype between that of either homozygote • This results in a combined phenotype. Pink Snapdragon Rosendahl

15. Example • In cross-pollination experiments between red and white snapdragon plants, the resulting offspring are pink. • The dominant allele that produces the red color is not completely expressed over the recessive allele that produces the white color. 

16. Incomplete dominance eg • Crossing of organisms that has two different phenotypes produces offspring with another different phenotype which is a blend of the parental traits • Examples??????

17. Snakes

18. Test cross for Incomplete dominance

19. Punnet Square for Incomplete dominance

20. Try this • 1. Predict the phenotypic ratios ofoffspring when a homozygous white cow is crossed with a roan bull. • 2. What should the genotypes & phenotypes for parent cattle be if a farmer only wanted a cattle with white fur • 3. A cross between a black cat & a tan cat produces a tabby pattern (black and tan fur together)? • A) What pattern of inheritance is this? • B) What percent of kittens would have tan fur if a tabby cat is crossed with a black cat?

21. THREE ALLELES

22. MULTIPLE ALLELES • More than three alleles • Many genes have multiple alleles • Three or more different alleles • Exclude dominant and recessive effects • All alleles show its own effects in inheritance • Examples: Blood type, hair color

23. Multiple alleles cont’d • Multiple alleles - gene has several allelic forms • Example: blood type is determined by multiple alleles • IA= A antigen on red blood cells • IB= B antigen on red blood cells • i= Neither A nor B antigen on red blood cells • Possible phenotypes and genotypes for blood type: • This is an example of codominancebecause both IAand IBare fully expressed

24. Several genes and the environment can influence a single multifactorial characteristic • Polygenic inheritance occurs when a trait is governed by two or more genes • Multifactorial traits - controlled by polygenes subject to environmental influences

25. Polygenic inheritance: Dark dots stand for dominant alleles; the shading stands for environmental influences

26. Iris color

27. The Human Blood

29. Skin color

30. An interesting example is coat color in rabbits • Four different alleles • C (full coat color) • cch (chinchilla pattern of coat color) • Partial defect in pigmentation • ch(himalayan pattern of coat color) • Pigmentation in only certain parts of the body • c (albino) • Lack of pigmentation • The dominance hierarchy is as follows: • C > cch > ch > c • Figure 4 illustrates the relationship between phenotype and genotype

31. Phenotype Genotype • Agouti (wild type) c+c+, c+cch, c+ch, c+c • Chinchilla (mutant) cchcch • Himalayan(mutant) chch,chc • Light grey cchch, cchc • Albino (mutant) cc FIGURE 4

32. Why is that? • Caused by tyrosinase; producing melanin • Two types of melanin: eumelanin (black pigment) and phaeomelanin (orange/yellow pigment)

34. The himalayan pattern of coat color is an example of a temperature-sensitive conditional allele • The enzyme encoded by this gene is functional only at low temperatures • Therefore, dark fur will only occur in cooler areas of the body • This is also the case in the Siamese pattern of coat color in cats