Mendelian genetics and punnett squares

Genetics is the science that studies inheritance, or the way parents transmit certain traits to their descendants. And Mendelian genetics, refers to Gregor Mendel—an Austrian monk—who studied inheritance by experimenting on pea plants.

He cross-pollinated the flowers of different plants together, took the seeds the developed from the pairing, planted those seeds, and took careful notes on the types of peas that resulted in the subsequent generations.

As a monk he was just trying to find his inner peas (peace)! Now in addition to having lots and lots of peas in his garden, he helped to formulate two important laws; the law of segregation and the law of independent assortment.

So to start out - Mendel took plants with violet flowers and plants with white flowers and crossbreed them. This original group of flowers are called the P generation, as in “parent,” and then when he obtained some peas, he planted them and got more plants and the flowers in this offspring generation was called F1, or filial one.

It turned out that the F1 generation consisted of all violet flowers, so he called the violet trait “dominant,” while the white trait which appeared to be lost in the F1 generation, was called “recessive.” Next, Mendel let the violet flowers in the F1 generation cross-pollinate amongst themselves, and when they formed peas - he planted them again.

He got more plants and the flowers from that second generation of plants he called filial two or F2. It turned out that some of the plants in this F2 generation had white flowers whereas other plants had purple flowers!

In fact, the ratio was about 3 violet flowering plants for every 1 white flowering plant. Based on this experiment, Mendel drew a few conclusions.

First, since the F1 violet flowers had some offspring plants that produced violet flowers and other offspring plants that produced white flowers, it meant that the F1 plants must have contained both of these elements.

The inheritable elements of pea plants are its the gametes, so that meant that the gametes of the F1 plant, contained either the dominant violet trait or the recessive white trait.

The F2 plants are created with one gamete from each parent. And Mendel worked out that the white flowering plants resulted when they receive both white flower elements, and that plants that had at least one violet flower element from either parent would produce violet flowers.

Mendel didn’t know this at the time, but the “element” he was referring to were segments of DNA called genes that encoded flower color.

These genes were located on specific parts of chromosomes, called loci. Different versions of a gene are called alleles, and in the case of the flowers there were two alleles - a white and violet allele for flower color.

A helpful way to visualize Mendel’s experiment is to use a Punnett square. Imagine a box with four squares within in, were we put the genetic information of one parent—or genotype—on the horizontal row and the other parent on the vertical column.

The dominant allele, represented with a capital letter, codes for a violet flower; and the recessive allele, represented with a lowercase letter, codes for a white flower.

The letter we choose doesn’t matter, so let’s use capital P for the violet flower allele, and a lowercase p for the white flower allele.

In the P generation Mendel used pure-breeding plants, so their genotype was two of the same alleles. In other words, both of the parent plants in this generation were homozygous for flower color trait.

“Homo” meaning same, and zygous referring to the male and female alleles. The violet pea plant had two of the same dominant alleles “PP”, and therefore had all violet flowers, whereas the white pea plant had two of the same recessive alleles “pp”, and therefore had all white flowers.

Now the observable trait that results from the genotype is called the phenotype - in this case the phenotype is the flower color.

So when the violet and white flowering plants were cross-bred, each offspring got a dominant allele from the violent flower parent and one recessive allele from the white flower parent.

Since the two alleles are different, these plants rare all heterozygous - meaning that they have “hetero” or different alleles for the flower color trait.

The phenotype of these heterozygous plants was that they all had violet flowers, because the dominant P allele masks the recessive p allele.

Now, when we breed any two of these heterozygous plants in the F1 generation, we can make a new Punnett square, with the Pp genotype of one parent on the horizontal row and the same Pp genotype of the other parent in the vertical column.

Now when we use the Punnett square we get 1 offspring with the PP genotype, 2 with the Pp genotype, and 1 with the pp genotype.

The three plants with at least one P allele will all have a violet flower phenotype, and the one plant with the homozygous pp genotype will have a white flower phenotype.

This was the ratio of plants that Mendel observed in the F2 generation and it helped to establish the law of segregation which states that alleles segregate, and that offspring acquire one allele from each parent.

Now, it turns out that in addition to flower color, Mendel also observed the seeds of his pea plant - specifically their color and texture.

He noted whether the seeds were yellow, which we’ll call the dominant big Y allele, or green, the recessive little y allele, and whether the seeds were round, which we’ll call the dominant big R allele, or wrinkly, the recessive little r allele.

As before, Mendel started with pure breeding plants. One of them was homozygous dominant for both traits, which means it was YY genotype for the color trait, and RR for the seed texture trait.

So this plant’s phenotype was that it had yellow, round seeds. The other plant was homozygous recessive for both traits, which means it had yy genotype for color trait and rr genotype for texture trait.

So it’s phenotype was that it had green, wrinkled seeds. So Mendel cross-pollinated these two plants, and the result was that all of the plants in the F1 generation got YR from one parent and yr from the other parent and therefore were YyRr.

So far, so good, but then Mendel bred two of these F1 plants with one another, and things got interesting. Let’s put this in a Punnett square.

For these two traits, there are 4 different combinations for each parent; YR, Yr, Ry, and ry. When we crossbreed the plants, we can expect that the F2 generation will have seeds that have four different types of phenotypes: 9 are yellow and round—these have at least one dominant Y and one dominant R, 3 are yellow and wrinkled—those that have at least one dominant Y and two r, 3 are green and round—those that has two y’s and at least one dominant R, and 1 that’s green and wrinkled—the one that has two y and two r.

And that’s what Mendel got, a 9:3:3:1 ratio! This helped establish the law of independent assortment - that the genes for seed color and seed texture were assorting independently of each other, that they weren’t somehow influencing one another.

In other words, having one trait did not make having another trait any more or less likely. This law is generally true, except in certain situations, like when two genes are located really near one another on a chromosome.

When that happens, it’s called genetic linkage, and the two genes start to move together more often than not, and therefore don’t assort independently.

All right, as a quick recap, the law of segregation states that inherited alleles are separated when producing gametes, and the law of independent assortment states that alleles get distributed to offspring randomly, and without regard to what other allele the offspring may have received.