A female tomato plant, which is trisomic for chromosome 6, is crossed with a normal diploid male plant. This male is homozygous for the recessive allele for potato leaf with the genotype p/p.
The F1 progeny gives a trisomic plant. This plant is then backcrossed to the male parent.
a) The cross is P/P/p x p/p
Since we need to assume that the p gene is on chromosome 6, the gametes of the trisomic plant need to be considered. The gametes from the trisomic parent will occur in the following proportions:
• 1/6 p
• 2/6 P
• 1/6 P/P
• 2/6 P/p
As gametes with p genotype alone can give potato leaves, this indicates that out of the 4 genotypes shown above, only the first genotype will show potato leaves. Since the ratio of this genotype is 1/6, this indicates that out of every 6 plants, 5 will have normal leaves while 1 will have potato leaves. Hence, the ratio of normal-leaved plants to potato-leaved plants would be 5:1.
b) If the p gene is not on chromosome 6, a backcross with a parent would yield a 1:1 ratio. This would indicate a normal backcross done of a F1 progeny with a parent. The ratio of normal leaved plants to potato leaved plants would be 1:1.