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Chromosomes come in pairs and contain genes, which code for inherited traits. If a gene is always expressed when present, it is called dominant and is represented by a capital letter. A dominant gene will be expressed whether an individual is pure and has two copies of the dominant gene or is hybrid and contains one dominant copy and one copy of another gene. In the case of hybrids, the dominant gene hides the expression of the recessive gene. A recessive gene is expressed only when it is pure, and it is represented by a lowercase letter.
In pea plants, the gene for round pea shape is dominant and the gene for wrinkled pea shape is recessive. A plant that produces wrinkled peas must be pure for the wrinkled gene, meaning it must contain two copies of the wrinkled gene. A plant that produces round peas can either be pure for the round gene or be hybrid and have one gene for round shape and one gene for wrinkled shape.
Punnett Squares are charts used to predict the odds of specific gene combinations in offspring. Below is a Punnett Square of a cross between a pea plant purebred for wrinkled peas and a pea plant that produces round peas.

Which of the following describes the offspring of this cross?
 

Explanation

Since a plant that produces wrinkled peas must contain two copies of the wrinkled gene (r), half of the offspring in the chart contains the genes for the wrinkled peas (rr) and thus will produce wrinkled peas. The offspring in the other half of the chart contains one for round peas (R) and one for wrinkled peas (r). Since the gene for round pea shape is dominant and the gene for wrinkled pea shape is recessive, these offspring will produce round peas.

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