Who was Gregor Mendel?
Born Johann Mendel, Gregor Mendel (1822-1884) was a 19th century monk who is often regarded as the "Father of Genetics". Mendel was born in a small village in the Austrian Empire, to Anton and Rosine Mendel. Their family was in poor financial condition, and he became a friar to get an education without having to pay for it. He studied heredity in his monastery's garden, using pea plants as his test subjects. Although it was not realized at the time, Mendel's work gave us basic understanding about how genes and heredity worked.
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Mendel's Experiments
Mendel experimented with common pea plants (pisum sativum), observing traits like flower color and height. Luckily for Mendel, pea plants happen to be the perfect test subject—they follow the simple inheritance pattern that we now know as Mendelian genetics. Mendel found that the reason offspring look like parents is that the parents pass down bits of information called genes. These genes are in each and every cell of the organism, in their DNA.
A genotype is the genetic makeup of an organism, usually used to refer to the genes corresponding to a certain trait like eye color. Alleles are the different possible types of genes. If there is a dominant gene/allele present in a genotype, that dominant gene will show and be the phenotype, even if the other gene/allele is recessive. However, if there are no dominant genes present (two recessive) than the recessive allele will show in the phenotype. A genotype with two identical alleles is said to be homozygous, a genotype with mixed alleles is said to be heterozygous.
To express dominant and recessive genes in writing, we use single letters. Let's say that in a flower, the allele for the flower petals being Pink is dominant over the allele for the flower petals being White. We would use the first letter of Pink (P) for both alleles, because pink is dominant. However, the dominant allele would be uppercase and the recessive allele would be expressed as lowercase. A gene for pink would be P, whereas a gene for white would be p.
To express genotypes, we use the letters mentioned above. For a homozygous Pink flower (Remember, homozygous means both genes are identical) we would write PP. For a heterozygous (one of each) flower, we would write Pp. For a homozygous white or recessive flower, we would use pp.
A genotype is the genetic makeup of an organism, usually used to refer to the genes corresponding to a certain trait like eye color. Alleles are the different possible types of genes. If there is a dominant gene/allele present in a genotype, that dominant gene will show and be the phenotype, even if the other gene/allele is recessive. However, if there are no dominant genes present (two recessive) than the recessive allele will show in the phenotype. A genotype with two identical alleles is said to be homozygous, a genotype with mixed alleles is said to be heterozygous.
To express dominant and recessive genes in writing, we use single letters. Let's say that in a flower, the allele for the flower petals being Pink is dominant over the allele for the flower petals being White. We would use the first letter of Pink (P) for both alleles, because pink is dominant. However, the dominant allele would be uppercase and the recessive allele would be expressed as lowercase. A gene for pink would be P, whereas a gene for white would be p.
To express genotypes, we use the letters mentioned above. For a homozygous Pink flower (Remember, homozygous means both genes are identical) we would write PP. For a heterozygous (one of each) flower, we would write Pp. For a homozygous white or recessive flower, we would use pp.
Punnett Squares
Mendel found out how simple genetics functioned, and was able to predict the traits of offspring before the actually developed based on the parents. Today, we can do this much simpler using what's known as a punnett square. A diagram from Reginald C. Punnett that shows the possible outcome of breeding, and what traits might result. It is a square divided in 4, with one parent’s genes on the top and one on the left, with the outcomes displayed at the intersection of each gene. Each parent can contribute one of their two genes, and all the possible combinations of these genes equal the possible genotypes found in their children.
To use a Punnett Square is very simple—after writing the genotypes of the parents in their respective spaces, we just write down the combination of genes where the two genes intersect. |
EXAMPLE: Heterozygous Punnett Square
For this example, we have a cross between two parent flowers, that are both Heterozygous. This means they both have one of each gene (They are both Rr). If an offspring is homozygous dominant (red) or recessive (white), then they will be red or white respectively. If they are homozygous like their parents, they will be Red because the dominant R gene shows.
In this case, there is a 25% chance of the genotype being RR (Homozygous Dominant/Red), a 25% chance of rr (Homozygous Recessive/White), and a 50% of Rr (Heterzygous). The phenotype is a 75% chance of red and a 25% chance of white. |
EXAMPLE: Homozygous Punnett Square
This Punnett Square is a cross between two flowers—one that is Homozygous Dominant (red, RR), and one that is Homozygous Recessive (White, rr). If an offspring has both dominant or recessive genes, they will be either homozygous Red or White respectively. If they have 1 of each gene, they will be heterozygous—but since Red is dominant, the phenotype will be Red.
In this case, each parent can only provide a R or r gene respectively. The outcomes have 100% genotypes of Heterozygous, or Rr. The phenotypes are 100% Red, since R is dominant. |