(And who’s that Punnett guy?)

Gregor Mendel was a monk. An Austrian monk. He lived during the 1800s: July 20, 1822 – January 6, 1884, to be exact. In addition to being a monk he was also a teacher. At his monastery, he became interested in pea plants. Mr. Mendel is known as the “father of genetics” because of the work he did with pea plants.

Mendel came up with what are now called The Laws of Inheritance. He came up with these while experimenting with his pea plants. He made 3 choices with his pea plants: 1. control over breeding 2. only using purebred plants. 3. “either-or” traits

Hey Mendel, what’s a purebred? Well, in peas, the sex organs are located in the flower. Peas have both male and female sex organs. Stamens are male. Pistils are female. In nature, pea plants pollinate/fertilize themselves. If they do this long enough they become purebred, meaning: Every generation looks like the one before.

Mendel chose to study 7 different “either–or “traits in his pea plants. Pea shape: Round or Wrinkled Pea Color: Yellow or Green Flower Color: Purple or White Pod Shape: Smooth or Constricted Pod Color: Green or Yellow Flower Position: Axial or Terminal Plant Height: Tall or Short Most of these traits were coded by genes on separate chromosomes. This will be important later.

Our good friend, Mendel, wanted to mate purebred plants with certain traits with other purebred plants with other traits. Like: a tall plant with purple flowers and a short plant with white flowers. But if the plants pollinated themselves, how could Mendel accomplish his goal? Easy: he cut off the male sex organs (top) so the plants could not self-pollinate and then manually pollinated each pistil from a chosen stamen (bottom).

From these crosses, or mating of 2 organisms, Mendel came up with his Laws of Inheritance: 1.Law of Segregation: Organisms inherit 2 copies of each gene, one from each parent. Organisms donate only one copy of each gene in their gametes – meaning the 2 copies of each gene separate during Meiosis.

Mendel continued his experiments and crosses and came up with another Law of Inheritance: 2. The Law of Independent Assortment: Allele pairs separate independently of each other during meiosis. In other words, different traits appear to be inherited separately. How do you keep track of all these alleles going different ways? Well...

R.C. Punnett, a British Geneticist (20 June 1875 – 3 January 1967) Not long after Mendel published his work, this guy came up with a way to keep track of all those alleles when they split up during Meiosis. His invention even helps us find out where they wind up in offspring – it’s called a PUNNETT SQUARE!

While working with his pea plants, observed:

Then, The top of the square and left side represent the genes of the parents. Each letter represents an allele that could be present in a parental gamete based upon their genotype. The four squares inside the box represent possible genetic make ups or genotypes of offspring depending on which sperm fertilizes which egg. organized the information into this, a Punnett Square:

There are 2 types of crosses we’ll be studying using the Punnett Square. The first is a monohybrid cross. A monohybrid cross is a cross of 2 organisms that examines the inheritance of one specific trait, like Flower color. F and f represent the alleles for flower color in pea plants.

Monohybrid crosses come in 3 varieties or kinds: 1.) Homozygous – Homozygous Example: FF x ff 2.) Heterozygous – Heterozygous Example: Ff x Ff 3.) Heterozygous – Homozygous Example: Ff x ff or Example: Ff x FF There is a special kind of cross we use to help us determine the genetic make up of an organism when we don’t know it. This type of cross is a test cross. To do a test cross, you breed the organism with the unknown genotype with one with a recessive phenotype. A recessive phenotype means the organism MUST be homozygous recessive for that trait. The offspring of the test cross will tell you the genotype of the unknown organism based upon their appearance or phenotype.

This a Punnett Square for a Heterozygous – Heterozygous monohybrid cross: And this is a Punnett Square for a Heterozygous – Homozygous monohybrid cross:

The second type of cross we’ll be studying is called a dihybrid cross. A dihybrid cross is used to examine the inheritance of two different traits like pea shape and pea color. Punnett Squares for dihybrid crosses look like this:

Good show, old chap! I’d rather be gardening.