Chapter 11 The Work of Gregor Mendel Working with Punnett Squares Inheritance Patterns

The Father of Genetics

Summary of Mendel’s Work Mendel worked in the garden of a monastery and studied seven different traits in the pea plants there and what happened when pea plants with certain characteristics are allowed to breed. Mendel kept very detailed and extensive records of the results of his experiments which was vital to helping other scientists understand biological inheritance.

A Summary of Mendel’s Principles The inheritance of characteristics is determined by genes, which are passed from parents to offspring. Some genes are dominant (which means they show in the individual) and some are recessive (which means they rarely show in the individual) Genes are segregated from each other independently of one another. In most sexually reproducing organisms, each adult has two segregated copies of each gene – one from each parent.

What happened when two tall pea plants were crossed? When Mendel crossed two tall plants are crossed they produced all tall offspring.

What happened if two of the offspring from the two tall parent plants are crossed? They are still all tall because they are “true-breeding” (or also called purebred).

What would happen if two short plants were “crossed” or bred with each other? Mendel crossed two short pea plants together and the offspring were all short.

What happened when two of the offspring from the short pea plants were crossed with each other? Mendel crossed two of the short pea plants offspring and found that their offspring were still all short.

What would happen if a short and a tall pea plant were crossed? Mendel crossed a “true-breeding” tall pea plant with a “true-breeding “ short pea plant. All the offspring were tall! These offspring are called “hybrids” because they are a mix of both the tall and short traits.

What happens if two offspring from the tall and short parent combination are crossed? You get a ratio of 3:1 offspring. Three tall pea plants and one short pea plant. Why? The tall trait is dominant over the recessive short trait. So are these hybrids or true-breeding? A Punnett Square can help answer that question.

The Punnett Square Allele from Parent 1 Alternate Allele from Parent 1 Combination of alleles from parent 1 and parent 2 Combination of alternate allele from Parent 1 and allele from Parent 2 Alternate Allele from Parent 2 Combination of alternate allele from Parent 2 and allele from Parent 1 Combination of alternate alleles from parent 1 and parent 2

Overview of Terms Alleles – the different forms of a gene; alleles are represented by an alphabet letter on a Punnett square, like a variable in an algebraic equation Heterozygous – also called “hybrids” is the combination of two different alleles Homozygous – also called “true-breeding” or “purebred” is the combination of two alleles that are the same. Phenotype – the physical display of a trait Genotype – the alphabetical combination of the alleles that an individual has

Cheat sheet to Punnett Square results Aa = heterozygous or hybrid individual, the genotype is “Aa” and the phenotype is whatever description is associated with the “A” allele (unless this is incomplete or codominant traits) AA = homozygous or true-breeding individual, the genotype is “AA” and the phenotype is whatever description is associated with the “A” allele aa = homozygous or true-breeding individual, the genotype is “aa” and the phenotype is whatever description is associated with the “a” allele CAPITAL letters represent dominant alleles lower case letters represent recessive alleles Capital letters are written first in a genotype

Beyond Mendel’s Dominant and Recessive Alleles Four Shades of Gray

Incomplete Dominance Incomplete dominance occurs when two different dominant alleles are present for the same trait. Then a “mixture” of the two dominant allele physical traits is a possible phenotype. “AA” = white flowers phenotype “RR” = red flowers phenotype “AR” = (the dominant red and dominant white will mix) pink flower phenotype

Codominance Codominance occurs when two different dominant alleles are present for the same trait. Then both dominant physical traits appear in that individual. It’s NOT mixed it’s “some of this and some of that” both present “AA” = white feathers phenotype “BB” = black feathers phenotype “AB” = the animal will have both white and black feathers present as a phenotype

Multiple Alleles In multiple alleles, the trait has more than two possible alleles for that trait. Blood type is a perfect example. It has “A”, “B”, and “O” alleles. Genotype Phenotype AA Blood type A AO BB Blood type B BO AB Blood type AB OO Blood type O

Polygenic Traits Polygenic traits are traits that are controlled by combinations of many different genes These kinds of traits will have a wide range of phenotypes Skin color, eye color, hand width, height, hair color are all excellent examples of polygenic traits. An example genotype might look like this: “AaBBccDdFFgg”

Take a breath! You survived a crash course in Punnett Square genetics Now review these notes and prepare yourself to complete worksheets creating and interpreting various Punnett Squares