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Download Genes and Heredity - Nova Scotia Department of Education
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Genes and Heredity Test Cross Multiple Alleles Incomplete Dominance Test Cross Sheep- white wool is desirable because black wool is too brittle and difficult to dye, W=dominant allele - white wool w=recessive allele - black wool “How would you know that a ram is homozygous for the white phenotype?” W w W W w Ww ww w Ww Ww w Ww ww w Ww Ww The test cross is always performed between an unknown genotype and a homozygous recessive genotype . (what is it here?) WHAT ARE THE OFFSPRING IN EACH CASE?(%) Multiple Alleles It is possible to have more than two different alleles for one trait - multiple alleles, examples: blood types, eye colour in Drosophila (fruit flies) Phenotypes Wild type, Apricot, Honey, White. Red or wild type is dominant to apricot is dominant to honey is dominant to white. Capital letters and superscripts are used. Crosses That Involve Two Traits When two pairs of contrasting traits are considered in a cross, it is described as a DIHYBRID CROSS. The same principles apply to such a cross as apply to a monohybrid cross. It is more complex because there are more possible combinations. Law of Independent Assortment RR = pure breeding round rr = pure breeding wrinkled YY = pure breeding yellow yy = pure breeding green Phenotype = pure yellow, round parent, the genotype is RRYY Phenotype = pure green, wrinkled parent, the genotype is rryy The offspring produced when these cross (F1) are heterozygous for both the yellow and round genotypes. YYRR Yellow/round cross with yyrr green/wrinkled Gametes formed by meiosis (the alleles for these genes are on separate chromosomes) are YR YR yr yr When these are crossed, the F1 generation looks like this: YR YR yr YyRr YyRr yr YyRr YyRr If a cross is considered between a pure green, round pea plant and a pure yellow wrinkled pea plant, the resulting off spring are yellow, round (all YyRr). See Figure 24.18. Inheritance of the gene for colour is not affected by either the wrinkled or round alleles. This is a demonstration of the LAW OF INDEPENDENT ASSORTMENT. Shape and colour are inherited independently. Progressing to the F2 generation, (let the yellow, round plants self-fertilize), each heterozygous yellow, round plant can produce four different phenotypes, YR, yR, Yr, yr A modified Punnet square, dihybrid cross diagram is used to show the phenotypes of the F2 generation. Gametes YR yR Yr YR YYRR YyRR YYRr YyRr yyRR YyRr yyRr yR YyRR yr Yr YYRr YyRr YYrr Yyrr yr YyRr yyRr Yyrr yyrr
