Tut 13 - Genetic Basis for Variation

St Andrew’s Junior College Genetics & Inheritance 2022 H1 Biology 1 Full Name: Civics group: 21S Index no.: Date: Core Idea 2: Genetics and Inheritance Dihybrid cross & Test Cross Tutorial 13 QUESTION 1 A homozygous white -flowered, long -stemmed tobacco plant, Nicotiana affinis , was crossed with homozygous pink fl owered, short-stemmed plant. The alleles for white flower and long stem are dominant to those for pink flower and short stem. The above cross gave rise to the F1 generation. The F1 plants were selfed to produce the F2 generation. Using suitable symbol s, construct a genetic diagram to illustrate the outcome of the above crosses. ………………………………………………………………………………………………..[6] Key: Let W represent the allele for white flowers. Let w reresent the allele for pink flowers. Let L represent the allele for long stems. Let l represent the allele for short stems. The allele for white flowers (W) is dominant to allele for pink flowers (w). The allele for long stems (L) is dominant to allele for short stems (l). Parental phenotypes: white flowers, long stems x pink flowers, short stems Parental genotype: WWLL wwll Parental gametes: WL wl F1 genotypes: WwLl F1 phenotypes: All white flowers, long stems Selfing F1: WwLl x WwLl F1 gametes: WL Wl wL wl WL Wl wL wl F1 genotype, determined by Punnett square: F1 x F1 cross Tip: When 2 homozygous parents of contrasting traits are crossed, traits observed in F1 are dominant traits.

St Andrew’s Junior College Genetics & Inheritance 2022 H1 Biology 2 If no punnett sq: 1 WWLL 1 WWll 1wwLL 1wwll 2 WWLl 2 Wwll 2 wwLl 4 WwLl 2 WwLL F2 phenotypes: White, long stem: White, short stem : pink, long stem: pink, short stem F2 phenotypic ratio: 9 : 3 : 3 : 1 [Total: 6] Mark scheme: 1 Key 2 Parental genotypes 3 Parental gametes – (Gametes must be circled) 4 F1 genotype 5 F1 phenotypic ratio Selfing: 6 F1 genotypes (both heterozygous) 7 F1 gametes – (Gametes must be circled) 8 F2 genotypes / Punnett square 9 Genotypes correspond to phenotypes / legend for Punnett square 10 F2 phenotypic ratio 11 Correct presentation of genetic diagram (correct sequence of presentation) Tip: Points in yellow are likely areas awarded marks. Always provide a full presentation of answers.

St Andrew’s Junior College Genetics & Inheritance 2022 H1 Biology 3 QUESTION 2 In rumbunnies, facial warts (W) is dominant to smooth face (w) and freckled face (F) is dominant to freckle-less face (f). Rumbunnies that are heterozygous for both facial warts and freckles were crossed with smooth-faced and freckle-less rumbunnies. The resultant offspring were as follows: Phenotype No. of rumbun