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Introduction to Genetics. 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
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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
Gregor Mendel – Father of Genetics • Born in 1822 • Austrian monk • Gardener and teacher • Good observations • Good with numbers
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.
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
Pea Plants • Mendel looked at 8 traits • Examples: seed shape, flower color (purple/white), etc.
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?
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
Mendel Controls Breeding • Had to control self-pollination • Had to prevent pollination from all other sources
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)
Terms you should know! Same phenotype (physical appearance), different genotype (genetic make-up) Phenotype = tall, tall Genotype = AA, Aa
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
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
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?
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)
Punnett Square Solution • Now cross two of the offspring • Heterozygous purple= Pp (F1 #1) • Heterozygous purple= Pp (F1 #2) • Offspring are called F2
Phenotype: Physical appearance Genotype: Genetic composition Purple PP (homozygous) Pp (heterozygous) Purple Pp (heterozygous) Purple pp (homozygous) White Phenotype versus genotype
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)
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 (PpPp) 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
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
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!)