Mendelian (Basics) Genetics

This covers the basics of genetics. We’ll be talking about Gregor Mendel, the father of modern genetics. Mendel was a priest that was interested in plants. He is famous for his experiments in pea genetics. Mendel found that some traits like; seed shape, flower color, and plant height had interesting traits.

Let’s look at red and white flower color. If Mendel breed a pure line: breeding a red flower with a red flower and then discarding all the white flowers. He found after a few generations the red flowers only produced red flower offspring and no white flowers. This is a purebred line for red flower, the only genes are for red flowering. Mendel did the same thing for the white flowers, having white flowering plants that only produce white flower offspring. What do you think happened when he crossed a pure white flower with a pure red flower? The answer is he got all red flowers. These red flowers that where crossed from two purebred lines are hybrids. Mendel then crossed the hybrids with other hybrids from the same generation. The plants he got we mostly red, 3/4th red and 1/4th white to exact.

Basic genetics

Diagram showing Mendel’s crosses.
From: wikipedia.org

 

The first step (#1) in the diagram is the purebred flowers. The second (#2) is the hybrids what came from the purebred line. The third (#3) is the hybrid cross, breeding two hybrids together. Mendel was pretty smart and from this he figured out two things that (#2) the hybrids had two pieces of genetic information one from each of the parents.  That one of the pieces of information was dominate over the other, this is seen in the hybrids. Recap:

  • Genetic information is inherited from both parents.
  • When two different traits are present in an individual one is dominate and will be seen.

Before we keep talking about how smart Mendel was we need to cover two vocab words, Gene and Trait. With the pea flower color example there were two traits white color and red color. When humans are trying to understand nature they write it down as (R) for red and (r) for white. The reason they use the letter R is because red is the dominate trait. In the diagram they use (R) and (w) that was for demonstration and scientist use (R) and (r). Ok, so know you know what a  trait is what’s a gene. A gene is the genetic information for a trait, so the gene for flower color in pea plants is (R). Little r for white color and big R for red color, these two traits are on the same gene. You can think of it like clothes you can wear a shirt (the gene for flower color) but what’s on the shirt maybe be different each day (the color of the flower red or white.)

Now back to Mendel. Mendel wasn’t content crossing two traits he wanted to know what happened when he was crossing four or more traits. This is called a dihybrid cross, crossing two hybrids from 4 traits. We’ll be looking at flower color and seed color. For seed color Mendel found that the color yellow was dominate to the Green seed coat color. The gene is denoted (Y) for yellow and (y) for green. There are three possible gene combinations for hybrids with two traits. For seed coat color it would be (YY),  (Yy), and (yy). There should be four because a two by two square gives you four outcomes, but (Yy) is the same as (yY) the order does’t matter. In the diagram below showing the dihybrid cross there are nine possibilities three from one hybrid cross three from the other. A three by three square gives you nine possibilities. Check it out:

 

 

Basic Genetics

A Dihybird Cross of Flower Color and Seed Coat Color. From: wikimedia.org 

 

You’ll notice that (RrYy) is the most common with 1/4th probability overall. That’s because (Rr) is the same as (rR) so it effectively doubles the probability for that combination. You might also notice from this cross that its all probability and all of the chance are dispersed. A (YY) has the same probability in (RR) as it has in (rr).

Here’s another visual this one of Cat genetics. It might clear up any confusion you have on the rations:

 

Dihybrid cross, basic genetics

Cat Genetics
From: wikimedia.org 

 

Mendel saw these rations and came up with another conclusion.

Mendel’s Laws of Genetics:

1. Law of Segregation: We covered this one already, that we inherit two genes one from each parent.

 2. Law of Independent Assortment: This one is what we were just talking about with dihybrid crosses. The red flower color is not linked to yellow seed coats. This means that genes from other chromosomes will not effect genes on another chromosome.

You might be thinking, “What? You just threw in this word chromosome” Yes that’s because these are laws, but in the real world they are more like guidelines. In the real world some genes are linked to each other and do have a higher probability when another gene is inherited. There are other examples of exceptions, but we’ll cover that later. Thanks for reading, any questions?

Leave a Reply