Exploring Mendelian Genetics

11-3

Independent Assortment

Mendel wondered if the segregation of one pair of alleles affect the segregation of another pair of alleles

To find the answer, Mendel did an experiment to follow two different genes as they passed from one generation to the next

Two-Factor Cross

Mendel crossed true-breeding plants that produced only round yellow peas with a plant that produced wrinkled green peas

All of the F1 offspring were round and yellow– Proving round and yellow are dominant

While the cross didn’t prove if segregation happened, it provided the hybrid plants needed for the F2 generation

To get the F2 generation, Mendel crossed two of the F1 offspring– RrYy x RrYy

556 seeds were produced, with 4 different characteristics

315 seeds were yellow and round– Dominant traits

32 were green and wrinkled

– Recessive traits 209 had a combination of traits

– ~1/2 were green and round Recessive and dominant

– ~1/2 were yellow and wrinkled Dominant and recessive

Phenotype ratio 9:3:3:1

Mendel discovered the principle of independent assortment

– Genes for different traits can segregate independently during the formation of gametes

– Accounts for many genetic variations among organisms

Summary of Mendel’s Principles

Inheritance of biological characteristics is determined by genes. Genes are passed from parents to their offspring

When there are 2 or more alleles, some forms are dominant and others recessive

In sexually reproducing organisms, each adult has 2 copies of each gene – genes are segregated when gametes form

Alleles for different genes usually segregate independently of one another

Beyond Dominant and Recessive Alleles

Despite the importance of Mendel’s work, there are exceptions

– Some alleles are neither dominant or recessive

– Some traits are controlled by multiple alleles or multiple genes

Incomplete Dominance

One allele is not completely dominant over another allele

Heterozygous individual is a blend of the two homozygous phenotypes

Ex: red (RR) x white (WW) = pink (RW)

Codominance

In heterozygous individuals, both traits appear at the same time

They are not blended together like in incomplete dominance

Ex: heterozygous chickens that have erminette coloration (black and white feathers)

Multiple Alleles

Genes that more than two alleles are said to have multiple alleles

Individuals still only have 2 copies of a gene, but there is variation in the alleles they can inherit

Ex: rabbit fur and blood types

Polygenic Traits

Traits controlled by two or more genes

Show a wide range of phenotypes

Ex: human skin color and eye color

Applying Mendel’s Principles

Thomas Hunt Morgan wanted to test Mendel’s principles on organisms other than plants

He needed an animal that produced large numbers of offspring, in a short amount of time

Used Drosophila melanogaster – common fruit fly

Soon, other biologists tested all of Mendel’s principles on a variety of organisms and discovered that the principles applied to them as well

Genetics and the Environment

Characteristics of any organism are not based only on the genes they inherit

Also based on the interaction of their genes with the environment

Genes provide a plan for development, but how that plan unfolds also depends on the environment