Genomics - Mendelian genetics: Nursing

Genetics is the science that studies inheritance patterns, or the different ways parents transmit certain traits to their descendants. And Mendelian genetics refers to the discoveries of Gregor Mendel, an Austrian monk, who studied inheritance by experimenting on pea plants. He cross-pollinated the flowers of different plants, took the seeds he developed from the pairing, planted those seeds, and took careful notes on the types of peas that resulted in the subsequent generations. Now, in addition to having lots and lots of peas in his garden, he rhelped to lay the foundation for understanding how traits are passed from one generation to another.

So to start out, Mendel took plants with two different traits, purple flowers and white flowers. He called this original group of flowers the “P” generation, as in “parent.” Then, he crossbread the flowers, and called their offspring generation F1, or filial one. It turned out that F1 consisted of all purple flowers, so he called the purple trait dominant, while the white trait which appeared to be lost in the F1 generation, was called recessive.

Next, Mendel let the purple flowers in the F1 generation cross-pollinate amongst themselves, and he called the offspring generation from these plants F2 or filial two. It turned out that some of the plants in this F2 generation had white flowers, while most of the other plants had purple flowers!

Mendel didn’t know this at the time, but the traits he was studying were genes, which are regions of DNA that carry information for specific features like flower color. For human traits, genes carry information for traits like eye color or blood type. Single gene traits have just one single pair of genes encode a trait, while polygenic traits have 2 or more genes influence a trait.

Now, our genes are scattered among 23 pairs of chromosomes, with one pair supplied by one parent and the other pair supplied by the other parent. The 22 of these pairs are somatic, or autosomic pairs, while the 23rd is a sex chromosome pair. In females, the sex chromosomes are two X chromosomes, whereas in males, there is one X and one Y chromosome. Alright, now each chromosome has a lot of different genes, but it can also carry different versions of the same gene, called alleles, which can be either dominant or recessive.

A helpful way to visualize this is to use something called a Punnett square to predict the probability that a certain gene will be inherited. We’ll use the example of the gene that codes for a taste receptor found in our tongues that allows us to taste bitterness. Individuals who recoil when they sense a bitter taste when tasting leafy greens or grapefruit juice are referred to as “tasters,” while those who don’t are called “nontasters.” So, there seems to be a genetic reason why some people dislike broccoli!

Now, to make a Punnett square, imagine a box with four squares within it, where we can input the genetic information of the parents, called a genotype, on the horizontal row. First, we put each of the alleles of one parent on the horizontal row, and the alleles of the other parent on the vertical column. So, each parent supplies one allele of a gene, which can be dominant, represented with a capital letter “T” which codes for being a taster; or recessive, represented with a lowercase letter “t” which codes for being a nontaster.

So, let’s say that one parent has two alleles for being a nontaster, or tt. This parent is homozygous for this genetic trait because both alleles are the same type. Then let’s say that the other parent has one taster allele and one nontaster allele, or Tt. This parent is heterozygous for this genetic trait because there are two different types of alleles. Now, we can fill out the Punnett square by taking one letter from the top and one from the side and filling in each of the squares, like this.