Punnett Squares: Predicting Offspring

0:00Two purple pea plants are crossed. What will their offspring look like?

0:05Flower colour in peas comes from one gene with two versions, called alleles.

0:11Big P, for purple, is dominant: one copy is enough. Small p, for white, is recessive: it needs two copies.

0:21Both parents have genotype big P small p, the alleles they carry, and phenotype purple, the trait we see.

0:30Each gamete, or sex cell, gets one allele. A Punnett square puts one parent's gametes on top, the other's at the side.

0:40Each box takes one allele from each parent: big P big P, big P small p, big P small p, and small p small p.

0:50Counting gives 1 big P big P, 2 big P small p and 1 small p small p: a 1 to 2 to 1 genotype ratio.

1:01Any box with a big P is purple: 3 purple to 1 white. So each offspring has a 3 in 4 chance of purple, or 75 percent.

1:12But four offspring will not always split three to one: each one is a separate chance.

1:18All four could be purple, or two could be white. Large numbers come close to 3 to 1.

1:26Is a purple plant big P big P or big P small p? A test cross with a white plant, small p small p, tells us.

1:35If it is big P big P, every box is big P small p, so all the offspring are purple.

1:43If it is big P small p, half the boxes are small p small p: a 1 in 2 chance of white.

1:51So any white offspring reveals big P small p. Many offspring, all purple, suggest big P big P.

2:00A Punnett square shows chances: two big P small p parents give each offspring a 3 in 4 chance of purple.