The genotypes of three pure lines of corn plants bred by a geneticist are:
• a/a ; B/B ; C/C
• A/A ; b/b ; C/C
• A/A ; B/B ; c/c
The geneticist wants to combine the 3 genes of a, b, and c as a pure line.
a) An effective crossing program, which can be used to gain a pure line of a/a; b/b; c/c would be:
• Cross any of the two plants given above to get progeny. The progeny is selfed.
• We get 1/16th of the progeny with two of the desired genes in a homozygous state.
• This progeny is then crossed with the third plant from the above list getting a triple heterozygous plant.
• This triple heterozygote is then selfed to obtain the desired genotype as 1/64th of the progeny.
b) The phenotypes and the frequencies for each stage can be given in the following way:
• The plant a/a; B/B ; C/C is crossed with A/A; b/b; C/C giving the progeny of A/a; B/b; C/C. Selfing of this plant will give the following progeny:
9:3:3:1
9 – A/- ; B/- ; C/C
3 – a/a; B/- ; C/C
3 – A/- ; b/b; C/C
1 – a/a; b/b; C/C
• Only 1/16th of the progeny has a/a; b/b; C/C with two of the desired genes in a homozygous state.
• This plant a/a; b/b; C/C is then crossed with A/A; B/B; c/c to give a triple heterozygous plant – A/a; B/b; C/c.
• This triple heterozygote is then selfed to obtain the ratio of 27:9:9:9:3:3:3:1. The desired genotype of a/a; b/b; c/c occurs as 1/64th of the progeny.