Genetics and Probability

Probability refers to the chances of something happening. Probability can be used to predict. In genetics, probability can be determined using

a chart called a Punnett Square.

Remember!

Most genes are in pairs (two alleles) One allele is from the mother (egg cell), one from

the father (sperm cell). They combine in the zygote.

The two alleles can be the same for a trait (homozygous).

The two alleles can be different (heterozygous). Some traits are dominant, while others are

recessive.

Important terms:

Phenotype: The physical trait itself, sometimes visible.

Genotype: The genetic combination of alleles, can be homozygous dominant, homozygous recessive, or heterozygous.

(Do you remember what alleles are?)

But it’s not alwaysquite so simple!

Multiple allele traits: some traits are controlled by more than two alleles, so the probabilities become much more complicated. (ex human blood types)

Some traits are controlled by more than one gene (ex. Human hair color, skin color, eye color).

Incomplete dominance: when two alleles combine, but show up as a third - different - trait. (ex red flowers X white flowers, offspring have pink flowers)

Codominance: when the alleles are neither dominant nor recessive. Both traits can show up. (ex brown cattle X white cattle, giving a mixed cow called a roan)

Mutations: when entirely new traits accidentally appear. These new traits might be favorable or not!

Punnett Squares

Show the possible gene combinations (possible genotypes) of a certain trait when two parents produce offspring.

Show the probability of certain traits showing up in the offspring.

Show how some genes are “hidden” but can can be passed on to offspring.

Time to try some Punnett Squares: