Introduction to Genetics

1. Introduction to Genetics

2. Genetics Terms to Know • Trait • Hybridization • Monohybrid cross • True-breeding/purebred • Allele • Dominant, Recessive • Homozygous, Heterozygous • Phenotype, genotype • P generation, F1 generation, F2 generation • Testcross • Dihybrid cross • Law of Segregation • Law of Independent Assortment • Incomplete dominance • Codominance • Multiple alleles • Sex-Linked • Punnett Square

3. Gregor Mendel – Father of Genetics • Born in 1822 • Austrian monk • Gardener and teacher • Good observations • Good with numbers

4. Before Mendel… • The Greeks – Males and females contribute a fluid and at conception, fluids compete to determine sex. • Middle Ages – Spontaneous Generation – Living organisms can arise from non-living substances (i.e. flies arise from meat). • William Harvey – Females had eggs because chickens have eggs.

5. Gregor Mendel’s Plants • Mendel studied characteristics of PEA plants • Fortunate choices - lucky • Peas perfectly suited to genetic research • Lots of offspring, short generation time • Number of stable varieties with distinct traits

6. Pea Plants • Mendel looked at 8 traits • Examples: seed shape, flower color (purple/white), etc.

7. What happens when… • A green pea is crossed (mates) with a yellow pea? • Mendel’s cross results in only yellow peas. WHY? • Mendel crosses those yellow peas with each other? • Mendel’s cross results in both yellow and green peas. WHY?

8. Mendel Lucks Out! • Every trait selected is a single gene inheritance • Every trait is a simple dominance relationship • Traits were easily distinguishable • Picked for ease – got lucky they all worked • All plants were purebreds – means they were true-breeding, always gave the same offspring

9. Mendel Controls Breeding • Had to control self-pollination • Had to prevent pollination from all other sources

10. Terms you should know! • Gene – A sequence of DNA that codes for a trait • Mendel called it a factor, genes and chromosomes had not yet been discovered • Gene Locus – Location of a particular gene on a chromosome • Alleles – Alternate versions of a gene • Homozygous – having identical alleles (AA or aa) • Heterozygous – having two different alleles (Aa)

11. Terms you should know! Same phenotype (physical appearance), different genotype (genetic make-up) Phenotype = tall, tall Genotype = AA, Aa

12. Terms you should know! • Allele A is DOMINANT to allele a. • Aa and AA are tall plants. • Allele a is RECESSIVE to allele A. • aa is the only combination for short plants. AA, Aa, and aa are gentoypes Tall and short are phenotypes

13. Hybridization • Crossing of 2 purebred or true breeding parents that are different from each other to produce a hybrid. • The parents are homozygous. • The hybrid is heterozygous. P =Parents F1 =Offspring, 1st generation

14. Genetics Problems

15. P Generation (true-breeding parents)  Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers F2 Generation Flower Color Cross • Purple is dominant to white. • True-breeding purple-flowered pea plants and white-flowered pea plants were crossed (symbolized by the “”). • If the parents are true-breeding, are they homozygous or heterozygous? • What is the genotype for the purple flower? • What is the genotype for the white flower? • Both purple-flowered plants and white-flowered plants appeared in the F2 generation. In Mendel’s • experiment, 705 plants had purple flowers, and 224 had white flowers. • What is the simplified ratio of purple to white? • What is the genotype of the white flower? • What are the possible genotypes of the purple flowers?

16. Punnett Square Solution • True-breeding purple genotype = PP (parent 1) • True-breeding white genotype = pp (parent 2) • Offspring are called F1 • All offspring are Pp (heterozygous)

17. Punnett Square Solution • Now cross two of the offspring • Heterozygous purple= Pp (F1 #1) • Heterozygous purple= Pp (F1 #2) • Offspring are called F2

18. Phenotype: Physical appearance Genotype: Genetic composition Purple PP (homozygous) Pp (heterozygous) Purple Pp (heterozygous) Purple pp (homozygous) White Phenotype versus genotype

19. Each true-breeding plant of the parental generation has identical alleles, PP or pp. Gametes (circles) each contain only one allele for the flower-color gene. In this case, every gamete produced by one parent has the same allele. P Generation  Appearance:Genetic makeup: Purple flowerPP White flowerspp Gametes: p P Union of the parental gametes produces F1 hybrids having a Pp combination. Because the purple- flower allele is dominant, all these hybrids have purple flowers. When the hybrid plants produce gametes, the two alleles segregate, half the gametes receiving the P allele and the other half the p allele. F1 Generation Appearance:Genetic makeup: Purple flowersPp Gametes: Mendel’s Law of Segregation (first generation, F1 offspring)

20. Each true-breeding plant of the parental generation has identical alleles, PP or pp. Gametes (circles) each contain only one allele for the flower-color gene. In this case, every gamete produced by one parent has the same allele. P Generation  Appearance:Genetic makeup: Purple flowerPP White flowerspp Gametes: p P Union of the parental gametes produces F1 hybrids having a Pp combination. Because the purple- flower allele is dominant, all these hybrids have purple flowers. When the hybrid plants produce gametes, the two alleles segregate, half the gametes receiving the P allele and the other half the p allele. F1 Generation Appearance:Genetic makeup: Purple flowersPp Gametes: 1/2 1/2 p P F1 sperm This box, a Punnett square, shows all possible combinations of alleles in offspring that result from an F1 F1 (PpPp) cross. Each square represents an equally probable product of fertilization. For example, the bottom left box shows the genetic combination resulting from a p egg fertilized by a P sperm. p P F2 Generation P Pp PP F1 eggs p pp Pp Random combination of the gametes results in the 3:1 ratio that Mendel observed in the F2 generation. 3 : 1 Law of Segregation, 2nd generation

21. Phenotype Genotype Purple PP (homozygous) 1 Pp (heterozygous) 3 Purple 2 Pp (heterozygous) Purple pp (homozygous) White 1 1 Phenotypic Ratio 3:1 Genotypic Ratio 1:2:1 Ratios

22. Homework • Book page 199 • Practice Problems One-Factor Crosses • #1 and 2 • Prepare for open-note quiz (these two problems could definitely be on the quiz!)