Theory question 104
Mendelian Inheritance(a) Complete the following Punnett squares of a dihybrid cross between two rats. One has black fur (BB) and short tail (tt), the other has brown fur (bb) and long tail (TT).
[A 4 by 4 grid: the corner is marked X, the gametes Bt run across the top and the gametes bT run down the side.]
(b) How many of the offspring will have: (i) black fur and short tail; (ii) brown fur and long tail; (iii) black fur and long tail; (iv) brown fur and short tail.
(c) If there were twenty (20) chromosomes in the leaf cell of a plant, how many chromosomes would be in each of the following cells of the plant? (i) pollen grain (ii) guard cell (iii) ovule (iv) root cell
Show the model answer
(a) Parent BBtt gives only Bt gametes; parent bbTT gives only bT gametes. Every box of the square is BbTt.
(b) All 16 offspring are BbTt: black fur is dominant over brown and long tail is dominant over short, so (i) 0; (ii) 0; (iii) 16 (all); (iv) 0.
(c) Pollen grain and egg cells are made by meiosis, so they are haploid: (i) pollen grain - 10; (ii) guard cell - 20; (iii) ovule (egg cell) - 10; (iv) root cell - 20.
Work out the gametes first; each homozygous parent can make only one kind of gamete.