Tuesday, Oct. 10 - Genetics - Probability of Traits

Key Terms

Genotype – The genetic makeup of an organism

Phenotype – Observable traits in an organism.

Punnett Square – A chart used to predict the possible genotypes and phenotypes of offspring.


Inheriting Traits

The genetic makeup of an organism is called the genotype, as opposed to the phenotype, which is the characteristics you can see. Going back to the parents – the blue-eyed mom (phenotype) has a genotype of bb. The brown-eyed dad (phenotype) could have a genotype of BB or Bb.

Some traits, such as – believe it or not – the type of earwax a person has, is determined by just two alleles, one dominant and one recessive. Others, are controlled by multiple alleles or multiple genes, such as hair color and eye color.


Punnett Square

A Punnett Square shows how alleles of two parents could possibly combine in their children.

So let’s take those tall (T) and short (t) pea plants.

Let’s assume one plant has Tt (so it is tall) and the other plant has Tt(so it is tall, too)

For possible combinations of kids, you have: TT, Tt, Tt and tt

So these are all the possible combinations that the parents could produce. You notice that three out of the four carry the dominant gene and would appear (phenotype) tall. So in that case, you could say there is a 75% chance of the children being tall (3 out of 4 or ¾) and 25% chance of the children being short (1 out of 4 or ¼).

It’s important to note here that each child has the same 75% chance of being tall. This table applies to each child equally. It DOES NOT mean the first three kids will be tall and the fourth will be short. Think of it as rolling the dice. Each time you roll is a new chance for doubles.

This is probability.

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