Transcript Y or y
Coat Color in Mice 2 different genes determine only 3 different phenotypes, rather than 4 phenotypes typical of a dihybrid cross Homozygous, recessive genotype at C-locus is epistatic to genotype at B-locus Another epistasis example flower color in peas • Flower color is determined by two different genes • The pigment in colored flowers is produced by a two-step process Genotype C_P_ C_pp ccP_ ccpp Flower color Enzyme activities Flowers colored; Functional anthocyanin enzymes from produced both genes Flowers white; p enzyme not no anthocynain functional produced Flowers white; c enzyme not no anthocynain functional produced Flowers white; P and c enzyme no anthocynain not functional produced The result is therefore a ratio of 9 flowered plants: 7 white plants Pleiotropic genes Yellow and gray coat color in mice • In 1904, researchers begin with a truebreeding strain of gray mice crossed with yellow mice • The F1 generation was 50% gray and 50% yellow – Yellow must be dominant to gray – The yellow mice must have been heterozygotes Yellow and gray coat color • Next a cross of two yellow mice was made – One predicts a 3:1 ratio of yellow to gray mice – The result was a 2:1 ratio of yellow to gray mice The ratio of 2:1 suggests a lethal gene • In the heterozygous condition, the Y allele causes a yellowing of the coat • In the homozygous condition, the Y alleles produce enough gene product to cause the mouse to die • The Y allele is said to be pleiotropic; it affects more than one phenotypic characteristic Punnett Square predictions Male Female Y y Y YY Yy y Yy yy Phenylketonuria - another example of pleiotropy • Metabolic defect caused by homozygous recessive alleles for enzyme phenylalanine hydroxylase Phenylketonuria - another example of pleiotropy • Primary effect of mutant gene is to cause toxic substances to build up in the brain, leading to mental impairment • The mutant gene also affects: – the synthesis of melanin pigment, resulting in PKU patients having light brown or blond hair – Posture – Organ function Figure 10.18a Fruit color is highly variable in bell peppers. Figure 10.18b Crosses between pure lines produce novel colors. Parental generation X Yellow Brown F1 generation Red Self-fertilization F2 generation Red 9/16 Yellow 3/16 Brown 3/16 Green 1/16 Figure 10.18c Model to explain 9 : 3 : 3 : 1 pattern observed above: Two genes interact to produce pepper color. Genotype Color Explanation of color R-Y- Red Red pigment + no chlorophyll rrY- Yellow Yellow pigment + no chlorophyll R-yy Brown Red pigment + chlorophyll rryy Green Yellow pigment + chlorophyll Gene 1 Gene 2 R = Red Y = Absence of green (no chlorophyll) r = Yellow y = Presence of green (+ chlorophyll) (-) = R or r (-) = Y or y Skin color in corn snakes Gene interactions in corn snakes • Two loci – One allele causes black pigment to be deposited (dominant allele is B+ and recessive is b) – One allele causes orange pigment to be deposited (dominant allele is O + and recessive is o)