Question

in garden peas, genes controlling height, flower color, and seed size are on the third autosome

gene map:

height-----10 mu------- color-------------20mu------ seed size

a purebred tall, red flowered, large-seeded plant is crossed with a purebred short, white flower, small seeded plant. All of the offspring are medium height, pink, and small seeded.

A. What proportions of the pollen produced by the F1 will carry the tall, red flowered and large seed alleles?

B. if the F1 is allowed to produce an F2 what proportion of the F2 will be tall, red flowered and large-seeded?

Answer 1

The description of F₁ i.e. medium height, pink flower and small seeded indicates that the genes for height and flower color show incomplete dominance because the phenotypes in F₁ is intermediate in extremes i.e. medium height (parents are tall and short), pink flowers (parents have red flowers and white flowers). But the gene for seed size shows dominance of small seeds over large ones.

Answer A)

Let the genotypes for height are TT (tall), tt (short), Tt (medium height);

genotypes for flower color are RR (Red), rr (white), Rr (pink)

genotypes for seed size are SS, Ss (small seeded) and ss (large seeded)

Genotype of first parent : TTRRss, Phenotype : tall, red flowered, large-seeded , gamete formed : TRs

Genotype of second parent : ttrrSS, Phenotype : short, white flower, small seeded, gamete formed : trS

As the parents are pure breeding, the genotype of F₁ would be TtRrSs with give phenotype i.e. medium height, pink and small seeded.

For knowing the genotype of pollen of F₁, we must consider linkage because the genes are less than 50 map units apart. If the genes are 50 or more map units apart are unlinked.

Now pollens formed could be with TRs, trS (parental types), TRS, trs (Recombination due to cross over between gene of color and seed size), TrS, tRs (Recombination due to cross over between gene of height and color), Trs, tRS (Recombination due to double cross overs)

Recombination frequency (%) = map units

Now, Recombination frequency between gene of height and color = 10%

Recombination frequency between gene of color and seed size = 20%

Recombination frequency of double cross overs = Recombination frequency of first gene pair (i.e between height and color) x  Recombination frequency of first gene pair (i.e between color and seed size)

Recombination frequency of double cross overs = 0.1 x 0.2 = 0.02 or 2%

Now frequency of parental type = 100 - (Sum of all recombinant frequencies)

frequency of parental type = 100 - (20+10+2)

frequency of parental type = 100-32 = 68%

Frequency of each parent type = 68/2 = 34 %

Proportions of the pollen produced by the F₁ which will carry the tall, red flowered and large seed alleles = 34 %.

Answer B)

The genotype of F₂ with tall, red flowered and large seeded would be TTRRss i.e of the parent which means both the gamete should be TRs genotype.

Now frequency to have that gamete is 34% and two of same type is (34%)² = 11.56 %

Proportion of the F₂ to be tall, red flowered and large-seeded = 11.56 %