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Different ratios occur in crosses with single gene pairs or two gene pairs. What types of ratios are likely to occur in crosses dealing with a single gene pair?

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Answer:

Genotype ratio: 1, 1:1, 1:2:1

Phenotype ratio: 1, 3:1

Explanation:

Single gene pair cross is also known as monohybrid cross. This means that only one gene usually with two alleles is observed and it express one trait.

For example, if we name the gene for a certain trait with A, the possible genotypes are AA (dominant homozygous), aa (recessive homozygous) and Aa (heterozygous). Possible crosses are:

P: AA  x  AA

F1 : all of them are AA

The same is with aa x aa (all of the offspring are with aa genotype)

P: AA  x  Aa

F1: AA Aa AA Aa  (genotype ratio 1:1) (phenotype ratio 3:1)

The same genotype ratio is  in aa x Aa (offspring will have aa Aa aa Aa-(genotype ratio 1:1) (phenotype ratio 1:1)

P: Aa x Aa

F1: AA Aa Aa aa (genotype ratio 1:2:1) (phenotype ratio 3:1)

P: AA x aa

F1: Aa Aa Aa Aa (1)

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