A good starting point is the discoveries of Gregor Mendel. He grew up working in his father’s orchards and became famous (after his death) for his work with pea plants. Mendel found that traits are passed on intact, not simply mixed together. Some traits are dominant and some are recessive.
A more modern approach to genetics is the use of twins, siblings & strangers. In theory, identical twins have the same prenatal environment and the same genetics. Fraternal twins will have the same prenatal environments and different genetics. Siblings have different environments and different but related genetics. And strangers have everything different.
The goal is to find out more about dominant, recessive & carrier traits. Each trait is a combination of two genes, one from Mom and one from Dad. If both contribute a dominant gene, the offspring will be a dominant-dominant trait holder. If both contribute a recessive, the offspring will be recessive-recessive.
If either contributes a recessive gene, the offspring will be a dominant-recessive (Mom-Dad) or recessive-dominant (Mom-Dad) holder. This combination will make them carriers of both gene types.
Genetics also considers which chromosome was involved. Every cell in the body (except the sperm and ovum) has 46 chromosomes. Two combine to determine sex and produce traits that are sex-linked, unique to each sex. Some traits are sex-limited (not on X or Y but impact each sex differently). Sex-limited is a subtype of the primary category of autosomal genes.
All of our genetic code is contained in these sex-linked, sex-limited & autosomal genes.
Here is a whole lesson on genetics.