Theory question 107
Mendelian Inheritance(a) Explain briefly the following terms:
(i) Phenotype; (ii) hybrid; (iii) dominant character
(b) Two varieties of maize, one with yellow (Y) grains and the other with purple (y) grains were crossed and all their offsprings had yellow grains. When the offsprings were crossed with the parent variety with purple grains, half of the resulting offsprings had yellow grains, while the other half had purple grains. Using labelled diagrams only;
(i) Deduce the genotype of the grains of the first filial generation,
(ii) Show the results of the cross between the first filial generation and the parent purple variety.
(c) Mention two ways in which the application of genetics is beneficial to agriculture.
Show the model answer
(a)(i) Phenotype: the visible or measurable characteristics of an organism (e.g. tall, yellow), produced by its genotype and the environment.
(ii) Hybrid: an offspring of two parents that differ in one or more characters (genetically unlike), e.g. Yy.
(iii) Dominant character: a character that shows in the offspring even when only one allele for it is present, masking the other (recessive) character.
(b)(i) Parents: YY (yellow) × yy (purple); gametes: Y and y; F1: all Yy – yellow grains. So the F1 genotype is Yy.
(ii) F1 × purple parent: Yy × yy; gametes: Y, y and y, y; offspring: Yy, Yy, yy, yy; phenotypes: 2 yellow : 2 purple, i.e. 1 : 1 (50% yellow, 50% purple). (Diagrams must label parents, gametes and offspring.)
(c) Any two: breeding high-yielding crops and animals; producing disease- and pest-resistant varieties; producing early-maturing varieties; producing drought-tolerant crops; improving quality, e.g. more protein or oil; producing hybrids with hybrid vigour.
Set out each cross in full: parents, gametes, offspring genotypes and phenotypes.