## 10 (b) Punnett Squares p86-87

A Punnett square is a chart that shows all the possible combinations of alleles that can result from a genetic cross. Geneticists use Punnett squares to show all the possible outcomes of a genetic cross, and to determine the probability of a particular outcome.

In this case, the Punnett square shows a cross between two hybrid pea plants with round seeds (Rr). The allele for round seeds (R) is dominant over the allele for wrinkled seeds (r). Each parent can pass either of its alleles, R or r, to its offspring. The boxes in the Punnett square represent the possible combinations of alleles that the offspring can inherit.

The completed Punnett square in Figure 7 shows four possible combinations of alleles. The probability that an offspring will be RR is 1 in 4, or 25 percent. The probability that an offspring will be rr is also 1 in 4, or 25 percent. Notice, however, that the Rr allele combination appears in two boxes in the Punnett square. This is because there are two possible ways in which this combination can occur. So the probability that an offspring will be Rr is 2 in 4, or 50 percent.

When Mendel crossed hybrid plants with round seeds, he discovered that about three fourths of the plants (75 percent) had round seeds. The remaining one fourth of the plants (25 percent) produced wrinkled seeds. Plants with the RR allele combination would produce round seeds. So too would those plants with the Rr allele combination. Remember that the dominant allele masks the recessive allele. Only those plants with the rr allele combination would have wrinkled seeds.

In this case, the Punnett square shows a cross between two hybrid pea plants with round seeds (Rr). The allele for round seeds (R) is dominant over the allele for wrinkled seeds (r). Each parent can pass either of its alleles, R or r, to its offspring. The boxes in the Punnett square represent the possible combinations of alleles that the offspring can inherit.

The completed Punnett square in Figure 7 shows four possible combinations of alleles. The probability that an offspring will be RR is 1 in 4, or 25 percent. The probability that an offspring will be rr is also 1 in 4, or 25 percent. Notice, however, that the Rr allele combination appears in two boxes in the Punnett square. This is because there are two possible ways in which this combination can occur. So the probability that an offspring will be Rr is 2 in 4, or 50 percent.

When Mendel crossed hybrid plants with round seeds, he discovered that about three fourths of the plants (75 percent) had round seeds. The remaining one fourth of the plants (25 percent) produced wrinkled seeds. Plants with the RR allele combination would produce round seeds. So too would those plants with the Rr allele combination. Remember that the dominant allele masks the recessive allele. Only those plants with the rr allele combination would have wrinkled seeds.