Theory question 103
Basic Genetics(a) Explain four causes of damage to maize during storage.
(b) Two heterozygous tall plants, Tt, were crossed in an experiment. Using a genetic diagram, (i) show the possible offspring that could result from the cross. (ii) determine the: I. genotypic ratio of the offspring. II. phenotypic ratio of the offspring.
(c) State Mendel's first and second laws of inheritance.
(d) Mention two forage grasses.
Show the model answer
(a) Any four: insects (weevils, moths) bore into and eat grains; rodents eat and contaminate grain; moulds/fungi rot grain when it is damp; high moisture causes heating and sprouting; birds; poor store hygiene/leaking roofs.
(b)(i) Parents Tt × Tt; gametes T, t and T, t; offspring TT, Tt, Tt, tt.
(ii) I. Genotypic ratio 1 TT : 2 Tt : 1 tt. II. Phenotypic ratio 3 tall : 1 short.
(c) First law (segregation): the two factors (alleles) for a character separate during gamete formation so each gamete gets only one. Second law (independent assortment): factors for different characters are passed on independently of each other.
(d) Any two: Panicum maximum (guinea grass); Pennisetum purpureum (elephant grass); Andropogon gayanus (gamba grass); Cynodon dactylon; Brachiaria spp.
Show gametes clearly in the cross.