Through sperm or egg, each parent passes on 1 gene for every trait to the offspring. The offspring receives 1 gene from the female's egg and 1 gene from the male's sperm. The offspring has 2 genes for every trait.

Genes take on different forms, called an allele. The alleles for hair color are brown, blonde, red, black, and so on. Some alleles are dominant and others are recessive.

Reinforce your understanding of genes and alleles using the video linked above.

A Punnett square is a special tool used to predict the possible genes of an offspring based on those of the two parents. In a Punnett square, the possible genes are represented with letters. Dominant genes are uppercase letters, while recessive genes are lowercase letters.

To understand Punnett squares, there are a few more vocabulary words you need to know. A parent has two copies of a gene, but only gives one copy to an offspring. When an individual has two different copies of a gene, it is called heterozygous. Having one dominant gene and one recessive gene is heterozygous. Think of it like being a hybrid. When an individual has two identical copies of a gene, it is called homozygous. Having two dominant genes or two recessive genes is homozygous. Think of it like being purebred.

If a parent has one dominant gene and one recessive gene for a trait (or it heterozygous) then about half the time the dominant gene will be passed on, and half the time the recessive gene will be passed on. If a parent has two dominant genes for a trait (or is homozygous), then every time the dominant gene will be passed on. If a parent has two recessive genes for a trait (or is homozygous), then every time the recessive gene will be passed on.

A Punnett square shows us how parents can pass on their genes. An example of a Punnett square is shown in the image below and is recapped in the video linked above. Make sure you examine the image and read it thoroughly. You can review the video if needed.

Genes determine the physical traits of offspring. The combination of genes each individual has is called the genotype. A genotype can be homozygous or heterozygous. In the sample Punnett square picture above, both parent dogs were homozygous.

The physical appearance of the genes is called the phenotype. We can observe the phenotype in the organism. In the example picture above, you can observe that one dog has black fur, while the other has brown fur.

When interpreting the Punnett square, remember that the recessive trait "hides" when paired with a dominant one. Since there are 4 boxes, each box represents 1/4 or a 25% chance.

If needed, review the video from class today using the link above.

One parent is homozygous for black fur (represented with BB) and the other is homozygous for brown fur (represented with bb). Each parent passes on one gene to the offspring.

The offspring are all heterozygous for black fur (represented with Bb).