Genetics
It has long been known that kids resemble their parents, and that this phenomenon remains true throughout species. Two tall people will likely have a taller kid. Two really fluffy dogs will probably have fluffy puppies. A bush that grows blueberries will produce another bush that has blueberries. Humans have known about the ability to selectively breed for desired traits thanks to this principle since the dawn of agriculture thousands of years ago.
You are probably familiar with the role of genes and DNA in this, but that wasn't always the case. For a while, it was simply believed that offspring were a blend of their parents' traits. Then, everything changed when a friar with a green thumb started fiddling around with some peas.
Gregor Mendel
Mendel was an Augustinian friar from the 1800s who is famously lauded as the "Father of Genetics". At the monastery in which he lived, Mendel studied pea plants and uncovered many of the rules behind how heredity works and why "kids resemble their parents" as was mentioned above.
Thanks to his meticulous counting of hundreds of thousands of peas over several years, Mendel established these basic rules of genetics that are now referred to as the laws of Mendelian Inheritance.
Some Basic Terminology
There are some important terms that are used often in Genetics and essential to understand first.
The functional unit of heredity is a gene. Genes are segments of DNA that code for a particular trait. In other words, they have the recipe for a certain protein. Genes are found at a certain spot on a chromosome known as the genetic locus (loci if plural). The locus is the specific position that the gene is found - kind of like its address on the chromosome.
Genes exist in different versions, meaning they can code for different versions of the same protein. These differences result in different traits, such as one version coding for type A blood and one coding for type B blood. These different versions are known as alleles. If an individual has two identical alleles for the same gene, they are considered homozygous for that trait. If they have two different alleles for the same trait, they are heterozygous. For some genes, an individual might only have one copy (such as things on the X chromosome for males), in which case they are known as hemizygous.
The combination of alleles an individual has - in other words, their genetic makeup - is known as their genotype. The physical expression of these genes is known as the phenotype.
Mendel famously took strains of "true breeding" pea plants (what we now know as homozygous) and crossed them. This initial cross was known as the P generation, and their kids were known as the F1 generation (the first filial generation, coming from the Latin word for offspring). All of the F1 displayed the same phenotype, but when they were crossed to produce the F2, the other trait "reappeared" at a smaller ratio.
The Law of Segregation
Most individual organisms are diploid, meaning they have two alleles for each trait. These alleles segregate (or separate) during meiosis, ensuring that only one allele is in each gamete (sex cell). This allows for offspring to inherit one allele from each parent, reestablishing the pair of alleles.
While Mendel didn't know about DNA, he deduced that parents must have two forms of each "factor" (what he called genes) and that only one would be passed on to each kid.
The Law of Dominance
The trait that appeared unchanged in the F1 heterozygous offspring were named dominant, while the trait that seemed to "skip a generation" was named by Mendel as being recessive. Dominant alleles will mask a recessive allele and show their phenotype even if there is only one copy. Recessive alleles will result in a phenotype only if it is the only version of the gene present. By convention, dominant alleles are written with a capital letter (A) and recessive alleles are written with a lowercase letter (a).
In basic Mendelian Inheritance, you can expect the following phenotypes with the corresponding genotypes:
Dominant phenotype: AA, Aa
Recessive phenotype: aa
The Law of Independent Assortment
Organisms are not as simple as only having one trait. When Mendel looked at multiple genes at the same time, he came to the conclusion that alleles at different loci are inherited independently of each other. In other words, different traits are not necessarily inherited together. Essentially, you can mix and match traits.
