Exploring Mendelian Genetics

1. Exploring Mendelian Genetics 11-3

2. 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

3. 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

5. 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

6. 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

8. 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

9. Summary of Mendel’s Principles • Inheritance of biological characteristics is determined by genes. Genes are passed from parents to their offspring

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

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

12. Alleles for different genes usually segregate independently of one another

13. 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

14. 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)

16. 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)

18. 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

20. Polygenic Traits • Traits controlled by two or more genes • Show a wide range of phenotypes • Ex: human skin color and eye color

21. 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

22. 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

23. 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