 Gregor Mendel and the Punnet Square.  Probability Genetics 1

Review of Standards 2b,2c,2d Genetics: the scientific study of heredity. In other words, how genes, the basic unit of heredity, are passed from one organism to its offspring. The most influential person in the study of genetics was Gregor Mendel. He was a high school teacher and a gardener. While working in the garden, he conducted an experiment that would help us understand genetics, and how we inherit traits, or specific characteristics, from our parents.

Review of Standards 2b,2c,2d Plants and animals both inherit genes from their parents. Both plants and animals have male and female sex cells that come together to form a new and unique offspring. First we will talk about plant sex cells and later we will discuss animal sex cells

Review of Standards 2b,2c,2d All you need to know, is that the stigma and ovary make up the female part and the stamen makes up the male part.

Review of Standards 2b,2c,2d Mendel grew pea plants in his garden. He noticed that different pea plants had different characteristics, that is, different pea plants looked different. Some pea plants were tall, some were short By crossed, Mendel took male parts of one tall pea plant and fertilized the female part of a short plant.  Fertilization is when male and female reproductive cells join during sexual reproduction When Mendel crossed short plants with tall plants, What do you think he got?

Review of Standards 2b,2c,2d If you thought that he got MEDIUM sized pea plants, you were wrong! This is why: The characteristic of height is a trait. The chemical factors that determine traits are called genes. The gene for height can be tall (T) or short (t). These two different forms of the gene, (T and t) are called alleles. Traits are passed from both parents, so you always have at least two for every trait. This is very important to understanding genetics.

Review of Standards 2b,2c,2d Now that you know that every trait is determined by at least two genes, let’s talk about Mendel’s experiment. Mendel crossed tall plants (TT) with short plants (tt). These pea plants were called true-breeding, meaning that if they were allowed to pollinate (or fertilize) themselves, they would produce offspring identical to themselves Because these tall and short plants have the same two alleles for the tall trait, they are called homozygous When he crossed the tall plants (TT) with the short plants (tt), he expected to find medium plants, but instead all of them were tall.

Review of Standards 2b,2c,2d The reason that all the offspring were tall is because the tall (T) allele is dominant over the short (t) allele, which is recessive. When a dominant and a recessive are mixed, the dominant is always expressed. Remember, there are always two genes, one from each parent. They way to predict the offspring of a “cross” is to use a Punnett Square.

Review of Standards 2b,2c,2d All of the offspring are (Tt) because the tall (T) is dominant over the short (t) Result: offspring are all tall (Tt)

Review of Standards 2b,2c,2d Next, Mendel crossed the offspring together and because they are all tall (Tt), he expected that their offspring would all be tall, but he was wrong. NOTE: The offspring were mixed, (Tt). When the alleles are different, they are called heterozygous. (“Hetero-” always means different. Like, heterosexuals like members of the opposite sex) When Mendel crossed his new, heterozygous pea plants (Tt) with each other, what happened?

Draw the Punnett Square in your notes and fill in the four boxes to find Mendel’s results

Final Thoughts… What were your results? Why did some of the offspring from two tall (Tt) pea plants produce some short pea plants? How is that possible? Does this mean that every fourth offspring will be short? It is likely that approximately ¼ will be short.