Monohybrid crosses and test crosses

Last time

Before we start: last time, in Mitosis, meiosis and stem cells, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Predicting children

Two brown-eyed parents can have a blue-eyed child. How? Genetics lets us predict the chances, using a genetic diagram called a Punnett square. Today: the key words, the monohybrid cross, ratios, and the test cross.

The key words

Genotype is the genetic make-up, the two alleles, such as B b. Phenotype is the observable feature, such as brown eyes. Homozygous means two identical alleles, B B or b b. Heterozygous means two different alleles, B b. A dominant allele shows when present, written with a capital letter. A recessive allele shows only when there are two, written small.

Dominant and recessive

B B and B b both show the dominant phenotype, because one dominant allele is enough. Only b b shows the recessive phenotype. So two parents who are both B b can have a b b child.

The Punnett square

Here is a cross between two heterozygous parents, P p and P p. Write one parent's gametes across the top and the other's down the side, then fill each box. The offspring are P P, P p, P p and p p: a three to one ratio of dominant to recessive.

Reading the result

Three of the four boxes have at least one P, so they show the dominant feature. One box, p p, shows the recessive feature. So the ratio is three to one, and the probability of a recessive child is one in four: zero point two five, or twenty-five percent.

Probabilities are chances

Each child has the same chance, independently of the others. A family of four children will not always be three dominant to one recessive. Large numbers of offspring get close to the expected ratio.

The test cross

An organism with the dominant phenotype could be homozygous or heterozygous. You cannot tell by looking. Cross it with a homozygous recessive, b b. If any offspring show the recessive feature, it must be B b. If all show the dominant, it is probably B B.

Test cross results

Here are both possibilities, crossing with t t. T T times t t gives all T t: all tall. T t times t t gives half T t and half t t: half tall, half short.