Theory question 104
Mendelian Inheritance(a) List three transmittable characters in (i) human beings (ii) plants
(b) State three ways in which genetics has contributed to improvement of agriculture
(c) A homozygous tall parent was crossed with a homozygous short parent. The offspring in F1 generation were all tall. An offspring of F1 generation was then crossed with another homozygous short individual (i) Illustrate the crosses with suitable diagrams (ii) What is the phenotypic ratio of offspring in the F2 generation? (iii) What is the genotypic ratio of offspring in the F2 generation?
(d) What is the probability that an individual that has genotype Tt will produce a gamete with T gene? Briefly explain your answer.
Show the model answer
(a)(i) Any three: blood group, skin colour, height, eye colour, tongue rolling, sickle cell trait, albinism
(ii) Any three: flower colour, seed shape, plant height, fruit size/colour, disease resistance
(b) Any three: breeding high-yielding varieties; disease- and pest-resistant crops and animals; early-maturing varieties; drought-tolerant varieties; better quality products (e.g. more milk, meat, eggs); hybrid vigour from cross-breeding
(c)(i) Let T = tall, t = short; parents TT × tt; gametes T and t; F₁ all Tt (tall)
Back-cross: Tt × tt; gametes T, t × t; offspring Tt, Tt, tt, tt
(ii) Phenotypic ratio: 1 tall : 1 short
(iii) Genotypic ratio: 1 Tt : 1 tt
(d) Probability = 1/2 (50%): during meiosis the two alleles separate (segregate) into different gametes, so half the gametes carry T and half carry t
Show parents, gametes and offspring for both crosses, with a key.