Tut 13 - Genetic Basis for Variation (Student)

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 flo wered, 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 symbols, co nstruct a genetic diagram to illustrate the outcome of the above crosses. ………………………………………………………………………………………………..[6]

St Andrew’s Junior College Genetics & Inheritance 2022 H1 Biology 2 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 rumbunnies Wild phenotype (facial warts, facial freckles) 45 Facial warts, freckle-less 48 Smooth face, freckle-less face 46 Smooth face, Facial freckles 51 (a) Draw a genetic diagram to show the cross described. ………………………………………………………………………………………………..[5]

St Andrew’s Junior College Genetics & Inheritance 2022 H1 Biology 3 (b) If you are given a Rumbunny with Facial Warts and Facial Freckles, explain how you would derive its actual genotype. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………..[5] [Total: 10]

St Andrew’s Junior College Genetics & Inheritance 2022 H1 Biology 4 QUESTION 3 Fruit flies ( Drosophila melanogaster) have been used extensively in investigation of inheritance. They are particularly suited to study because of their short life cycle and many different mutant forms. In one investigation, red -eyed females with grey bodies were crossed with brown -eyed males with black bodies. All the F1 generation had red eyes and black bodies. When these F1 flies were crossed, the following offspring were produced: Phenotype No. of flies Red-eyed, black-bodied 110 Red-eyed, grey-bodied 39 Brown-eyed, black-bodied 37 Brown-eyed, grey-bodied 13 (a) State the dominant characters for eye color and body color. ………………………………………………………………………………………………..[1] (b) State the genotypes of the parent flies, using suitable symbols. ………………………………………………………………………………………………….. ………………………………………………………………………………………………..[3] (c) Draw a genetic diagram to show how the F2 offsprings were produced. ………………………………………………………………………………………………..[6]

St Andrew’s Junior College Genetics & Inheritance 2022 H1 Biology 5 (d) Determine the results of a test cross conducted on the F1 offsprings. ………………………………………………………………………………………………..[5] [Total: 15]

St Andrew’s Junior College Genetics & Inheritance 2022 H1 Biology 6 Genetics & Inheritance Codominance, incomplete dominance, multiple alleles, sex-linked genes STUDY AID Pedigree trees, based on their traits, can be classified into 4 inheritance patterns: 1 Autosomal recessive traits 2 Autosomal dominant traits 3 X-linked recessive traits 4 X-linked dominant traits (Important ! The following table has been compiled to assist you to quickly identify the mode of inheritance in pedigrees but you will still need to perform a genetic analysis to confirm your prediction) Autosomal dominant trait E.g. woolly hair. Some info (a) A relatively large proportion of the members in the family will be affected with the d isease because all it requires is just one copy of the disease allele for manifestation of the disease. (b) Each affected individual will have at least one affected parent. (c) Equal chance of disease manifestation for both females and males. Autosomal recessive trait E.g. albinism – affected person lacks pigment in the hair, skin and eyes. Some info (a) Lower proportion of the members in the family will be affected with the disease because two copies of the disease allele are required for manifestation of the disease. (b) Equal chance of disease manifestation for both females and males. (c) Offspring of I -1 and I -2 (as well as I -3 and I - 4) are not affected with the disease although I-2 (and I-3) has the disease. (d) Offspring of II -3 and II -4 (ie. III -2) has t he disease, suggesting carrier status of both parents. Both II-3 and II-4 inherited a copy of the disease allele from one of their parents. NO Skipping of generation Generic reason. Try to state exact evidence why it is NOT X-linked instead Recessive: Diseased parent can have offspring without disease (skips generation) Generic reason. Try to state exact evidence why it is NOT X-linked instead. e.g. If it is X-linked, all diseased mothers e.g. I2, must have affected sons, but II1 is normal

St Andrew’s Junior College Genetics & Inheritance 2022 H1 Biology 7 X-linked recessive trait E.g. Haemophilia – blood clotting disease where a person can die from a simpl e bruise which results in internal bleeding. Some info (a) More males affected than females in the pedigree (b) Scenario 1 - If both parents are normal and yet child (son) is affected, female parent must be a carrier of the disease allele. (c) Scenario 2 - If male parent is affected and female parent a carrier, both male and female offspring have 50% chance of having the disease. (d) Scenario 3 – If male parent is normal and female parent has the disease, all female offspring will are carriers of the disease allele whil e all male offspring will have the disease. X-linked dominant trait E.g. A rare form of rickets Some info (a) Each affected individual will have at least one affected parent. (b) Higher probability for female to be affected than males. Recessive: Diseased parent can have offspring without disease (skips generation) Alternative clue: If it is X- linked, all diseased mothers must have affected sons NO Skipping of generation Alternative clue: If it is X- linked, all diseased fathers must have affected daughters

St Andrew’s Junior College Genetics & Inheritance 2022 H1 Biology 8 Revision on Symbol Representation: Scenario Representation Example Co-dominance Allelic symbols (B vs P vs Y) should be superscripted against a base gene alphabet e.g. C CB , CP, CY Incomplete dominance Allelic symbols (B vs b) should be superscripted against a base gene alphabet e.g. C CB, Cb X-linked condition (with one allele dominant to another) Allelic symbols (e.g. N, n) should be superscripted against the letter X (representing X chromosome). Y chromosome has no corresponding allele for the same gene XN, Xn, Y X-linked condition with 2 co- dominant alleles Allelic symbols (e.g. B, G) should be superscripted against the letter X (representing X chromosome). Different allelic symbols are used instead of capital letter vs small letter, because of co-dominance. Y chromosome has no corresponding allele for the same gene XB, XG, Y

St Andrew’s Junior College Genetics & Inheritance 2022 H1 Biology 9 QUESTION 4 [MULTIPLE ALLELES] [Time : 12min] The background colour of the shell of the snail Cepaea nemoralis may be brown, pink or yellow. The colour is controlled by a single gene