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Posted: December 28th, 2022

In chickens, the shape of the comb is determined by the interaction of alleles at two loci, (A, a and B, b)

Assignment 2
Question 1. In chickens, the shape of the comb is determined by the interaction of alleles at two loci, (A, a and B, b). The comb of a chicken is the fleshy protuberance on the top of its head that is used to regulate body temperature and to display courtship behavior. The shape of a chicken’s comb is determined by the interaction of alleles at two loci: the A locus and the B locus.

At the A locus, there are two alleles: A and a. The A allele codes for a larger comb, while the a allele codes for a smaller comb. At the B locus, there are also two alleles: B and b. The B allele codes for a comb with a more upright, V-shaped appearance, while the b allele codes for a comb with a more flat, rounded appearance.

The interaction of these alleles determines the final shape of the comb. For example, if a chicken has the genotype AABB, it will have a large, V-shaped comb. If it has the genotype Aabb, it will have a smaller, rounded comb. There are many other possible combinations of alleles at these two loci, resulting in a wide range of comb shapes.

It is important to note that the shape of a chicken’s comb is just one of many characteristics that are determined by genetics. Other characteristics, such as feather color, body size, and egg production, are also influenced by the interaction of various alleles. There are four possible comb shapes—walnut, rose, pea, and single. The walnut shape results from A_B_ genotype. Rose comb is produced from A_bb. Pea comb is produced from aaB_. Single comb is produced from aabb. For each of the following crosses, determine the phenotypes and proportions of each produced in the offspring. (2 marks each)
A. AABB × aabb
B. AaBb × aabb
C. AaBb × AaBb
D. Aabb × Aabb
E. Aabb × aaBb
F. Aabb × aabb
Question 2. A particular species of pea plant produces either purple or white flowers. A homozygous plant with purple flowers is crossed with a homozygous plant with white flowers. All F1 offspring produced purple flowers. The F1 were self-fertilized, and the following ratios in the F2 were obtained: 9 purple: 7 white.
A. Deduce the genotypes of the parental plants in this cross (2 marks)
B. Outline a possible biochemical pathway to account for the flower colour in this particular plant (4 marks)
Question 3. A plant from a pure-breeding strain which produces sickle shaped fruit is crossed with a plant from another pure-breeding strain which produces round fruit. All of the F1 plants bear sickle shaped fruit. Upon self-fertilization of the F1 plants, the following outcome is observed: 9/16 sickle shaped; 6/16 oblong shaped; 1/16 round fruit. Determine the genotypes of these F2 plants. (6 marks)
Question 4. A particular population of horses exhibit three different colours: yellow, brown, and white. Upon mating a series of these horses, the following results are obtained as shown below:
MATING PROGENY (number + Phenotype)
1 Yellow × yellow 12 yellow: 6 brown: 5 white
2 Brown × brown All brown
3 White × white All white
4 Yellow × brown 7 yellow: 6 brown
5 Yellow × white 10 yellow: 11 white
6 Brown × white All yellow
A. Suggest an explanation for the inheritance of coat colour in these horses (2 marks)
B. Using symbols of your choice, deduce the genotypes of all parents and offspring in matings 1–6 shown above. (6 marks)
Question 5. Answer the following:
A. If a single gene with a series of four alleles (A1, A2, A3 and A4) exists in a given diploid (2n) species, how many alleles will be present in: (2 marks each)
o (i) A chromosome
o (ii) A pair of chromosomes?
o (iii) An individual member of the species?
B. How many genotypic combinations would occur in the entire species? Show them. (1 mark)
Question 6. In a particular diploid species, a specific locus on a chromosome is multi-allelic (b1, b2, b3 and b4), whilst on another chromosome, two alleles of a second gene have been identified at a particular locus (c+ and c). Calculate how many different genotypes with respect to these two sets of alleles can possibly occur in a particular population of this species. (2 marks)

Question 7. A series of mutant strains (a through f) of a particular mold are not able to grow on minimal media unless they are supplemented by one or more of the compounds Q through V as shown below.
Strain Supplements
Q R S T U V
a 0 0 0 + 0 0
b 0 + 0 + + +
c + + + + + +
d 0 0 0 + 0 +
e 0 + 0 + 0 +
f + + 0 + + +
• + = growth 0 = no growth
Draw a plausible metabolic pathway supported by the data provided in the Table above, showing the position of each metabolic intermediate in the pathway, which step is blocked by each mutant and explain your answer. (8 marks)
Question 8. In a particular plant, red pigment is produced from a colourless precursor through an enzymatic reaction. Purple pigment may be produced from the red pigment through hydroxylation via another enzyme. (In a particular cross of two purple plants, the offspring produced were: 80 purple: 27 red: 37 white)
a. What is the term used for this type of gene interaction? Assuming there is a gene for each enzyme, with dominant (functional) and recessive (non-functional) alleles for each, deduce the genotypes of the parental plants that, when crossed, produce the following offspring: 80 purple, 27 red, 37 white. (3 marks)

b. Show the possible genotypes for the offspring and explain why they are a particular colour. (3 marks)

Be sure to indicate which enzymes are represented by the symbols you use.

Question 9. 200 cherry seeds were planted, all of which were derived from crosses of the same parents. The following describes the numbers and phenotypes of the trees produced:

12 – yellow cherries, hairless stems

16 – yellow cherries, very hairy stems

27 – yellow cherries, patchy short haired stems

33 – red cherries, very hairy stems

38 – red cherries, hairless stems

74 – red cherries, patchy short haired stems

From this data, deduce the number of genes responsible for these phenotypes (cherry colour and stem hairs) and the dominance relationship(s) of the alleles of these genes. State the genotypes of the parents and expected phenotypic ratios of the 200 offspring based on your deductions. (6 marks)

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