Chapter 4- Multiple Alleles & Gene Interactions

I. Allelic interactions
        A. complete dominance
                1. heterozygote has same phenotype as homozygote
                2. e.g., Pisum sativum traits studied by Mendel 
        B. incomplete (or partial) dominance
                1. heterozygote has phenotype intermediate between 
                 2 homozygotes
                2. e.g., flower color in Mirabilis jalapa (fig. 4.1)
	    	3. e.g. palomino horses
        C. overdominance
                1. heterozygote has phenotype more extreme than either
                 homozygote
                2. e.g., plant weight in Arabidopsis thaliana (p. 87)
        D. codominance
                1. phenotypes of both alleles are present in heterozygote
                2. usually found in genes that produce identifiable protein 
                 products, examples:
                        a. seed proteins in Phlox pilosa
                        b. human blood groups (fig. 4.9)
                        c. coat color in female cats
        E. to determine dominance relationship between 2 alleles:
                1. do monohybrid & dihybrid crosses- examine ratios 
II. Genes can have more than 2 alleles
        A. but each individual can only carry 2 alleles!
        B. allelic series
                1. alleles can be dominant over some alleles & 
                 recessive to other alleles
                        a. i.e., dominance is a relationship betw. alleles!
                        b. e.g., Drosophila eye color alleles at white 
                         gene (p.89)
                        c. e.g., sickle cell anemia (HbA/Hbs- see p. 89-90)
                                i. HbA is completely dominant
                                        a. with respect to (=WRT) visible 
                                         phenotype of cell sickling
                                ii. HbA is incompletely dominant
                                        a. WRT sickling in test tube under
                                         low oxygen concentrations
                                iii. HbA/Hbs is overdominant
                                        a. WRT malarial resistance
                                iv. HbA/Hbs is co-dominant
                                        a. WRT protein products
III. Gene interactions
        A. cause modified dihybrid F2 ratios
                1. epistasis
                        a. mutant allele at one locus masks or blocks
                         expression of mutant allele at second locus
                                i. allele that masks or blocks is epistatic allele
                                        a. recessive
                                                i. e.g., coat color in mice
                                                ii. gives 9:3:3:4  (fig. 4.14)
                                        b. dominant
                                                i. e.g., fruit color in summer
                                                 squash (p. 101)
                                                ii. gives 12:3:1 ratio
                2. pleiotropy
                        a. one gene, several phenotypes
                        b. e.g., seed coat color/flower color in 
                         Mendel's peas
                3. penetrance
                        a. percentage of genetically mutant individuals who 
                         show mutant phenotype
                        b. partial penetrance
                                i. less than 100% individuals with mutant 
                                 genotype show mutant phenotype
                                ii. e.g., Blackpatch in Drosophila
                4. expressivity
                        a. range of severity in mutant phenotype
                                i. variable expressivity
                                        a. not all affected individuals show same 
                                         degree of mutation
                5. lethal alleles
                        a. mutations in genes that perform necessary function
                        b. can modify Mendelian ratios by eliminating individuals 
                         in particular genotypic classes if lethal before birth
                                i. e.g., yellow mutation in mice
                                ii. e.g., Manx allele in cats
			c. modified ratios may not be seen at birth if lethal acts later
				i. e.g. Huntington's Disease
	     	6. sex-influenced genes
			a. heterozygote phenotype is dependent upon sex chromosomes
			b. e.g., pattern baldness in humans