Exploring Mendelian Genetics Chapter 11.3 Exploring Mendelian Genetics

Independent Assortment Mendel wanted to know if one pair of alleles assorted independently from another. Meaning does the gene, for example, for seed color determine whether a seed is round or wrinkled? To figure this out Mendel decided to look at a cross and follow 2 different genes.

The Two-Factor Cross: F1 Mendel decided to cross a true-breeding plant with round yellow peas with a true breeding green wrinkled pea plant. All of the offspring were round and yellow

The Two-Factor Cross: F2 Mendel knew that the F1 plants had a genotype of RrYy (they were all heterozygous for those traits). He crossed F1 plants to see if the dominant traits would stay together and the recessive traits stay together. He found that 9/16 had both dominant phenotype, 1/16 had both recessive phenotypes, what he also found were the remaining had mixed traits. 3/16 were round and green and 3/16 were wrinkled and yellow. That showed Mendel that the traits assorted independently – they were not connected to one another.

Principle of Independent Assortment – Genes for different traits can segregate independently during the formation of gametes. Independent assortment gives more genetic variations found in living organisms.

A Summary of Mendel’s Principles Mendel figured 4 principles: Genes are the individual units that determine inheritance of biological characteristics. Genes are passed from parents to offspring. When there are 2 or more forms (alleles) of a gene for a single trait, there will be a dominant form and a recessive form. Most adult organisms will have 2 copies of each gene. These genes will segregate (separate) when gametes are formed. The alleles for different traits usually segregate independently from one another.

Beyond Dominant and Recessive Alleles Many organisms have a much more complex pattern of inheritance. Some alleles are neither dominant or recessive, and many traits are controlled by multiple alleles or multiple genes.

Incomplete Dominance If we cross a red flower with a white flower sometimes we get pink flowers. This is incomplete dominance – neither allele is completely dominant over the other one. The heterozygous phenotype is a mix of the 2 homozygous phenotypes.

Codominance Codominance – both alleles are expressed. For example we cross a white chicken and black rooster we could get a black and white checkered chick. Another example would be blood types – you could be A, AB, BB, or O. AB would be codominant.

Multiple Alleles Multiple alleles are when there are more than 2 possible alleles. Again blood type could be an example, the alleles are A, B, and O.

Polygenic Traits Polygenic Traits are traits that are controlled by more than 1 gene. Example: There are 3 genes that control the fruit fly eye color. Skin color in humans is also an example.

Applying Mendel’s Principles Thomas Hunt Morgan decided to try Mendel’s ideas on an animal – he decided to use a fruit fly. Fruit flies were easy to test.

Genetics and the Environment Genes and the environment work together to produce certain characteristics. For example: genes may affect a sunflowers height and the color of it’s flowers but those factors can also be affected by climate, soil conditions, and the availability of water.