Moth



Blackwell Publishing

Molecular and Mendelian Genetics - What are the laws of heredity?

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Mendelian ratios

Mendelian ratios express the proportions of different genotypes in the offspring of parents of particular combinations of genotypes.

• What proportions of different genotypes result from a cross between an AA male and anAA female?

After meiosis, all the male's gametes contain the A gene and all the female gametes also contain the A gene. They combine to produce AA offspring. The Mendelian ratio is therefore 100% AA offspring.

• What proportions of different genotypes result from a mating between an AA homozygote and an Aa heterozygote?

Again, all the AA individual's gametes contain a single A gene. When a heterozygote reproduces, half its gametes contain an A gene, and half an a. The pair will produce AA:Aa offspring in a 50:50 ratio.

• What proportions of different genotypes result from a cross between two heterozygotes?

Both male and female produces half a gametes and half A gametes: if we consider the female gametes (eggs, or ovules), half of them are a, and half of them will be fertilized by a sperm, and half by A sperm; the other half are A, and half of them will be fertilized by a sperm and half by A sperm. The resulting ratio of offspring is 1/4 AA : 1/2 Aa : 1/4 aa.

These ratios follow from Mendel's laws of heredity: the law of segregation and the law of independent assortment.

Figure: Mendelian ratios for an AA x AA cross, an AA x Aa cross and for an Aa x Aa cross.

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