Ch 11- Introduction to Genetics

1. Ch 11- Introduction to Genetics • Genetics- scientific study of heredity • Gregor Mendel- father of genetics, laid the foundation of the science of genetics • Used garden peas for his experiments • He cross-breed true-breeding plants with different characteristics and study the results

2. Genes and Dominance • Mendel studied seven different pea plant traits • Trait- specific characteristic, such as seed color or plant height • P (parental) generation-original pair of plants • F₁ (first filial) generation- offspring of original pair of plants

3. What did Mendel notice with the offspring? • All F₁ offspring had the characteristic of only one of the parents • What did Mendel conclude about inheritance? • Traits are inherited through the passing of factors from parents to offspring • Genes- chemical factors that determine traits • The traits studied were controlled by one gene that occurred in two contrasting forms • Alleles- different forms of a gene • What is the principle of dominance? • Some alleles are dominant and others are recessive • Allele for tall plants-dominant, allele for short-recessive

4. Segregation • He allowed F₁ plants to self-pollinate – produced F₂ generation • Results of F₂- traits controlled by recessive allele represented one fourth of F₂ plants • Segregation- separation of alleles • Gametes- sex cells • What happens during segregation? • The two alleles segregate from each other so that each gamete carries only a single copy of each gene • Each F₁ plant produces two types of gametes, those with allele for tallness and those with allele for shortness • Capital T represents a dominant allele, lowercase t represents recessive allele. Result= F₂ generation with new combination of allele

5. Sec 2- Probability and Punnett Squares • Probability- the likelihood that a particular event will occur • What is chance of that a coin lands heads up or heads down? • How is coin flipping relevant to genetics? • The way in which alleles segregate is completely random, like a coin flip • The principles of probability can be used to predict the outcomes of genetic crosses

6. Punnett Squares • Punnett squares- diagram that shows gene combinations as a result of genetic crosses • Punnett squares can be used to predict and compare the genetic variations that will result from a cross • Homozygous- organisms that have two identical alleles for a particular trait- TT or tt • Heterozygous- organisms that have two different alleles for the same trait- Tt • Phenotype- physical characteristics • All tall plants have same phenotype • Genotype- genetic makeup • Genotype of one third of tall plants is TT while two thirds is Tt

8. Sec 3- Exploring Mendelian Genetics • Does the gene that determines whether a seed is round or wrinkled in shaped have anything to do with the gene for seed color? • Mendel’s crossed plants that produced only round yellow peas (genotype RRYY) with plants that produced wrinkled green peas (genotype rryy) • All offspring produced round yellow peas • Yellow and round are dominant over green and wrinkled • Mendel allowed the F₁ generation to self-pollinate to produce an F₂ generation

9. What did Mendel find out? • F₂ plants produced 556 seeds. 315 seeds were round and yellow, 32 were wrinkled and green. 209 seeds had combinations of phenotypes and alleles that weren’t found in parents • What does this mean? • The alleles for seed shape segregated independently of those for seed color • Genes for seed shape and seed color in pea plants do not influence each other’s inheritance • Results-9:3:3:1 ratio -punnett squares predict this • What is the principle of independent assortment? • Genes for different traits can segregate independently during the formation of gametes

11. Dominant and Recessive Alleles • Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes • Incomplete dominance- one allele is not completely dominant over another • Red plants (RR) crossed with white plants (WW)- pink (RW) • Codominance- both alleles contribute to the phenotype • Black feathered chicken (BB) crossed with white (WW)- all speckled offspring (BBWW) • Multiple alleles- many genes have more than two alleles • Human genes for blood type • Polygenic traits- traits controlled by two or more genes • Variation in skin color • Applying Mendel’s Principles- Thomas Hunt Morgan uses fruit flies to test Mendel’s principles • Produce large # of offspring

12. Sec 4- Meiosis • Where are genes located? • On chromosomes in cell nucleus • Fruit fly, Drosophila, has 8 chromosomes • 4 from male parent, 4 from female parent. These sets of chromosomes are homologous- each of the 4 chromosomes that came from the male parent has a corresponding chromosome from the female parents • Diploid- cell that contains both sets of homologous chromosomes • Represented by 2N • For Drosophila, the diploid number is 8, 2N=8 • Diploid cells contain 2 complete sets of chromosomes and 2 complete sets of genes • Haploid- gametes of sexually reproducing organisms, contain only single set of chromosomes, only a single set of genes • For Drosophila, haploid number is 4, N=4

13. Phases of Meiosis • Meiosis- process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell • 4 haploid cells are genetically different from one another and from the original cell • Involves two distinct divisions- meiosis I and meiosis II • Produces gametes

14. Meiosis I • Interphase, prophase, metaphase, anaphase, telophase • Interphase I- each chromosome is replicated • Prophase I- each chromosome pairs with its corresponding homologous chromosome to form a tetrad • 4 chromatids in tetrad • Crossing over- chromosomes exchange portions of chromatids • Produces new combinations of alleles • Telophase I and Cytokinesis- Nuclear membranes form, cells separate into two cells

15. Meiosis II • After meiosis I, the two cells have sets of chromosomes and alleles that are different from each other and from diploid cells • Prophase II- meiosis I results in two haploid daughter cells, each with half the number of chromosomes as original cell • Metaphase II- chromosomes line up in a similar way to the metaphase stage of mitosis • Anaphase II- sister chromatids separate and move toward opposite ends of the cell • Telophase II and Cytokinesis- meiosis II results in four haploid daughter cells • Four daughter cells contain just 2 chromosomes each • Meiosis

16. Gamete Formation • In males, meiosis results in four equal sized gametes called sperm • In females, only one large egg cell results from meiosis • Other three cells called polar bodies are not involved in reproduction • How is mitosis different than meiosis? • Mitosis results in the production of two genetically identical diploid cells, whereas meiosis produces four genetically different haploid cells • Comparison of Meiosis and Mitosis

17. Sec 5- Linkage and Gene Maps • Thomas Hunt Morgan- researched fruit flies in 1910 and discovered that chromosomes assort independently, not individual genes • Each chromosome is actually a group of linked genes • How did Mendel manage to miss gene linkage? • Six of the seven genes he studied are on different chromosomes, the two genes found on same chromosome are so far apart they assort independently

18. Gene Maps • If two genes are found on the same chromosome, does this mean that they are linked together forever? • Crossing over during meiosis separates genes on same chromosomes • What does this lead to? • Genetic diversity • Alfred Sturtevant- created gene map showing the relative locations of each known gene on one of the Drosophila chromosomes • He showed genes close to each other on chromosome are usually inherited together • His method has been used to construct genetic maps- including maps of human genome