Theory question 104
Mendelian Inheritance(a) Explain briefly how the process of meiotic division contributes to variation in a population
(b) State Mendel's first and second laws of inheritance
(c) A pure breeding brown coloured rat, BB was crossed with a pure breeding white rat bb. By means of diagram only, show the genotypes of the offspring up to the second filial generation.
Show the model answer
(a) Crossing over between homologous chromosomes at prophase I exchanges genes; independent (random) assortment of chromosomes at metaphase I gives many combinations; gametes therefore differ, and random fertilisation combines them, giving variation
(b) 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 separate independently of each other when gametes form
(c) Parents: BB × bb; gametes B and b; F₁: all Bb (brown)
F₁ × F₁: Bb × Bb; gametes B, b × B, b
F₂: BB, Bb, Bb, bb (Punnett square) – genotypic ratio 1 : 2 : 1; phenotypes 3 brown : 1 white
In (c) a diagram only is asked for: show parents, gametes and offspring in a Punnett square.