Going back to high school biology, we know we are all comprised of segments of DNA. This DNA contains all of the genetic material that was handed down from our parents. As humans, we are Diploids. We get half of our chromosomes from our mother and half from our father. In total, humans have 46 Chromosomes (23 pairs). 22 of these pairs are autosomes (look the same in males and females), and the last one is a sex chromosome.
Of each of the sets of chromosomes we get from our parents, 50% of the material handed down from each parent is a copy of their genetic makeup. The other 50% are random combinations of genes (Mendell’s Law of Independent Assortment) that come together during meiosis. Meiosis is reduction division. From each parent, we get 1 set of chromosomes. During meiosis, the parental cells are divided by half so that in each egg and each sperm, there is only one set of chromosomes that will be paired together to form offspring.
Genes are distinct sequences of nucleotides forming parts of these chromosomes, and Alleles are specific forms or variants of genes located at specific positions on specific chromosomes. Different Alleles result in the different traits we exhibit. We get one Allele from each parent. That makes up the genotype. One of the Alleles we receive will be dominant and the other recessive. The dominant Allele is what will show up. This is called a phenotype. An example would be eye color. If our mother passed on a blue eye color recessive gene and our father passed on a brown eye dominant gene, even though we have both allele’s, we will have brown eyes.
Mendel was a scientist, Augustinian friar and abbot of St. Thomas’ Abbey. He is known as the father of genetics. He began all of the research summarized below. His experiments were conducted with peas.
Pea plants have both male and female reproductive parts. Therefore, they can self-pollinate or cross pollinate with another plant.
His first test would have been F1 hybrids. He crossed plants that produce yellow seeds with ones that produced green seeds. His first observation was that the offspring of these plants always had yellow seeds.
His next test is would we know as F2 hybrids, or the following generation. They put one yellow to green seeds at a consistent ratio of 3:1 (in monohybrid crosses, it was 9:3:3:1 in dihybrid crosses).
The 3:1 ratio occurs in later generations as well.
The results of these experiments led Mendel to a few conclusions:
- inheritance of each trait is determined by units that are passed on to decendents unchanged (Genes)
- an individual inherits one such unit from each parent for each trait
- that trait may not show up in an individual but can still be passed on to further generations
Based on these conclusions, he established three principles for what we know as Mendelian inheritance:
- Principle of Segregation
Every organism carries two alleles for each trait, and that during meiosis, these alleles separate so that each gamete contains only 1 allele.
2. Law of Independent Assortment
Alleles for separate traits are passed independently of one another from parents to offspring. The biological selection of an allele for one trait has nothing to do with the selection of an allele for any other trait.
3. Law of Dominance
Recessive alleles will always be masked by dominant alleles
So what have we learned? Let’s look at an example of a monohybrid cross and an example of a dihybrid cross using a Punnett Square.
If a man has Brown Eyes (B) and a woman has Green Eyes (b), what are the odds the offspring will have Green Eyes? I denote Brown with a capital ‘B’ because Brown is dominant, and Green with a lower case ‘b’ because Green is recessive.
As you can see from above, there is a 3:1 ratio. So there is a 25% chance of having green eyes.
Let’s look at a dihybrid cross. This is a model for Mendel’s pea experiment. ‘R’ represents dominate allele for round seed, while ‘r’ represents a recessive allele for wrinkled. ‘A’ will represent the dominant color yellow, and ‘a’ will represent the recessive allele for the color green.
Dominant traits mask recessive traits. So ther are nine combinations for round yellow, three that have round green, three that are wrinkled yellow, and one that is wrinkled green. The ratio is 9:3:3:1.