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Biology Chapter 11 Genetics Ch 11-1 Genetics • The scientific study of heredity Fertilization • The joining of male and female reproductive cells Gregor Mendel • Austrian monk • “Father of Genetics” • Worked with garden peas • Studied only one trait at a time • Peas he started with were true-breeding o If allowed to self-pollinate, they would produce offspring identical to themselves • Wanted to produce seeds by mating two different plants • Cut off the male parts from one plant and used it to pollinate another plant o Called cross-pollination – produced seeds from two different plants Trait • A specific characteristic that varies from one individual to another • Called the original plants the P (parental generation) • Called the offspring F1 or first filial generation Hybrid • Offspring of crosses between parents with different traits • In Mendel’s time people thought if two organisms with 2 different traits mated then the offspring would show an intermediate between the 2 traits • In each cross he noticed the plants displayed traits of only 1 plant i.e. tall, yellow pea Mendel drew two conclusions • 1st – biological inheritance is determined by factors that are passed from one generation to the next o genes – chemical factors that determine traits o different forms of a gene are called alleles • 2nd The Principle of Dominance o States that some alleles are dominant and others are recessive o An organism with a dominant allele for a particular form of a trait will always exhibit that form of the trait • o An organism with a recessive allele for a trait will exhibit that trait only when the dominant allele for that trait is not present Segregation o Mendel allowed all seven kinds of F1 hybrid plants to produce an F2 generation by self-pollination The recessive allele reappeared in the F2 generation in about ¼ of the plants Mendel suggested that the alleles for tallness and shortness in the F1 plants segregated from each other during the formation of gametes – sex cells o When each F1 plant flowers and produces gametes, the two alleles segregate from each other so that each gamete carries only a single copy of each gene Chapter 11-2 Probabilities and Punnett Squares Probabilities • The likelihood that a particular event will occur • The principles of probability can be used to predict the outcomes of genetic crosses • Predict the average outcome of a large number of events • Cannot predict the precise outcome of an event o Therefore, the larger the number of offspring the closer the numbers will be to the expected values o Single trait heterozygous cross – 3:1 ratio • Punnett Squares o Used to predict and compare the genetic variations that will result from a cross • Organisms that have two identical alleles for a particular trait are considered homozygous – TT homozygous dominant, tt homozygous recessive • Organisms that have two different alleles for a particular trait are heterozygous – Tt • Homozygous individuals are true breeding for a particular trait • Heterozygous individuals are hybrid for a particular trait • Genotype – the genetic make-up of an organism • Phenotype – the outward expression of a genotype Independent Assortment • Mendel wanted to know if seed color had anything to do with a pea being wrinkled or smooth • He performed a two factor cross • 1st he crossed true-breeding plants that produced only round yellow peas with plants that produced only green wrinkled peas • The plants produced only round, yellow peas • • • • • • • • He wanted to know if the 2 dominant alleles would stay together or if they would “segregate independently” F2 plants produced 556 peas 315 round, yellow 32 wrinkled, green 209 had a combination of the phenotypes The results were close to a 9:3:3:1 ratio The Principle of Independent Assortment states that – genes for different traits can segregate independently during the formation of gametes This is where we get genetic variation Beyond Dominant and Recessive Alleles • Not all genes show the simple pattern of dominant and recessive alleles • The majority of genes have more than 2 alleles • Many important traits are controlled by more than one gene • 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 allele • Mirabilis – 4 o’clock RR+WW = RW (pink) • • • • The heterogeneous phenotype is somewhere between the two homozygous phenotypes Codominance • Both alleles contribute to the phenotype • In chickens white and black feathers are codominant • They produce feathers with white and black sections independent of one another, ex. Barred Rock Multiple Alleles • Genes with more than 2 alleles • Does not mean an individual can have more than 2 alleles but that more than 2 alleles for a trait exist in a population • Rabbits page 273 Polygenetic Traits • Traits that are produced by the intersection of several genes • Traits that are controlled by 2 or more genes • Skin color in humans due partly to 4 different genes that control this trait Applying Mendel’s Principles • • • • Beginning of 1900’s American geneticist Thomas Morgan Hunt looked for a model organism to advance the study of genetics Wanted an animal that was small and easy to keep in a lab Decided to use Drosophila melanogaster, the common fruit fly A single pair could produce 100 offspring Genetics and the Environment • Characteristics of an organism are not solely determined by genes • Environmental factors can play a large part Ch 11-4 Meiosis Chromosome Number • Homologous o Each chromosome from 1 parent has a corresponding chromosome from the other parent • Diploid o A cell that contains both sets of homologous chromosomes o Represented by 2n • Haploid o A cell that contains only 1 set of chromosomes o A gamete o Represented by N Phases of Meiosis • A 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 • Involves two distinct divisions – Meiosis I and Meiosis II • By the end of meiosis II a diploid cell has been split into 4 haploid cells Gamete Formation • In male animals haploid gametes produced by meiosis are called sperm, in plants pollen grains contain sperm • Female gametes are called eggs in animals and egg cells in plants • In female animals, cell division at the end of meiosis I and II are uneven, so a single cell becomes an egg and receives most of the cytoplasm • The three other cells are called polar bodies and usually do not participate in reproduction Meiosis I • Interphase I o DNA is replicated, duplicate chromosomes are formed • Prophase I • • • • • o Chromosomes pair with their homologous chromosomes and forms a tetrad 4 chromatids in a tetrad At this stage crossing-over occurs New combinations of alleles are formed Metaphase o Spindle fibers attach to the new chromosomes Anaphase, Telophase, and Cytokinesis are like mitosis Cells that are formed from meiosis I have different alleles from each other and from the original diploid cell Cells that form from Meiosis I have different chemistry and alleles from each other and from the original diploid cell Meiosis II o Cells do not replicate chromosomes o In the end 4 haploid cells are produced- 2 chromosomes each