(a) Define the term hybridization (b) State five aims of crop improvement. (c) Describe a monohybrid cross as explained by Mendel's Law of segregation. Illu...

(a) Hybridization

Hybridization is the crossing of two genetically different plants, usually different varieties of the same species, in order to obtain hybrid offspring which combine desirable inherited characters of both parents.

(b) Aims of crop improvement

1. To produce crop varieties with higher yield.
2. To improve the quality of produce, such as size, taste, nutritive value and keeping quality.
3. To produce varieties resistant to pests and diseases.
4. To produce varieties adapted to adverse or varying climatic and soil conditions, for example drought or poor soils.
5. To produce early-maturing, uniform varieties with good harvesting and processing qualities.

(c) Monohybrid cross and Mendel's Law of Segregation

A monohybrid cross is a genetic cross involving only one pair of contrasting characters. Mendel's Law of Segregation states that the two alleles controlling a character separate during gamete formation, so that each gamete receives only one allele. The alleles unite randomly again at fertilization.

For example, let T represent the allele for tallness and t the allele for shortness. Tallness is dominant over shortness.

In the parental generation, a pure tall plant, TT, is crossed with a pure short plant, tt. Every first filial generation plant receives T from the tall parent and t from the short parent; hence all the F₁ plants are Tt and tall.

When two F₁ plants are selfed, Tt × Tt, the alleles segregate during gamete formation. Each parent produces two types of gamete, T and t. Random fertilization gives:

• F₂ genotypic ratio: 1 TT : 2 Tt : 1 tt.
• F₂ phenotypic ratio: 3 tall : 1 short.

The reappearance of short plants (tt) in the F₂ generation shows that the alleles did not blend in the F₁ generation; they segregated into separate gametes.