Mendelian Genetics (6-8 Grade)

Gregor Mendel, was a monk in Austria in the mid-1800s who raised peas in the monastery gardens. While breeding his peas, he made some big discoveries about genetics. The peas had several traits he could see. Some plants were tall and some were short. Some had wrinkled pods and some had smooth pods. Some pods were green and some where yellow. The flowers were white or purple. Mendel looked at each trait and learned how they were passed down to the offspring plants. Since plants breed using pollen, Mendel controlled which plants pollinated other plants. This was how he discovered many important genetic rules.

How an individual looks and what their genetic code is sometimes do not match up. This is the difference between genotype and phenotype. The genotype is the actual genetic make up of an individual. The phenotype is what that individual looks like.

Traits that show up more often are called dominant traits. Traits that show up less often are called recessive traits. If an individual with dominant traits breeds with an individual with recessive traits, this can result in hybrid offspring. Hybrids can look like they have dominant traits (phenotype), but actually be hybrid (genotype). Hybrid plants are different from dominant plants even if they looked the same.

Each gene has two chances at a trait – two copies, two alleles. So a hybrid plant can be carrying the allele for a recessive trait even if you can’t see it. So, a hybrid plant might be tall like its dominant parent, but it still could have an allele for shortness that you don’t see. This is the difference between genotype and phenotype. The genotype is the actual genetic make up of an individual. The phenotype is what that individual looks like.

This can be illustrated with a simple chart called a Punnett Square using the example of tall pea plants verses short pea plants. When two hybrid plants breed, one in four of the offspring are short. This is a 3:1 ratio.