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Chapter 2

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University of Toronto Mississauga
Fiona Rawle

Chapter 2 - Mendelian Genetics Principal Points - The genotype is the genetic makeup of an organism, whereas the phenotype is the observable characteristic (produced by interaction b/w genotype environment) - Genes provide the potential for the development of characteristics; this potential can be affected by interactions with other genes and with the environment - PRINCIPLE OF SEGREGATION – two members of gene pair segregate from each other in the formation of gametes - PRINCIPLE OF INDEPENDENT ASSORTMENT – members of different gene pairs are transmitted independently of one another during gamete production - To determine an unknown genotype (usually individual with dominant phenotype), a TEST CROSS is made with a homozygous recessive individual, that tells the genotype of unknown Genotype and Phenotype - HEREDITY TRAITS are transmitted from one generation to another (aka characters) - These traits are controlled by genes - Genes provide potential for developing a particular phenotypic characteristic - Genes are a starting point for determining the structure and function of an organism and the route to the mature phenotypic state is highly complex, involving many interacting biochemical pathways - Actions of other genes and products + Genotype + Environmental influences and random developmental events  Phenotype (expression of physical trait) Mendel’s Experimental Design - Mendel cross-bred pea plants PISUM SATIVUM for its characteristics such as colour, shape etc - For a Genetic Cross: Two diploid individuals differing in phenotype are allowed to produce haploid gametes by meiosis. Fusion of male and female gametes produces zygotes from which the diploid progeny individuals are generated. The phenotypes of the offspring are analyzed to provide clues to heredity of those phenotypes - SELF-FERTILIZATION/SELFING – anthers at the ends of the stamen produce pollen (male gametophyte) which lands on the pistil (female gametophyte) within the same flower, thus fertilizing the plant - CROSS-FERTILIZATION/CROSS – fusion of male gametes (pollen) and female gametes (eggs) of two different plants - Mendel worked with TRUE BREEDING/PURE BREEDING STRAINS – the characteristics stay unchanged from parents to offspring over generations - Mendel studied seven characteristics: flower and seed coat colour, seed colour, seed shape, pod colour, pod shape, stem height, and flower position. Monohybrid Crosses and Mendel’s Principle of Segregation - P GENERATION – parental generation - F 1 First filial generation - MONOHYBRID CROSS – crosses between true-breeding strains of peas that differed in a single trait. o Example: cross smooth seed plants with wrinkled seed plants - RECIPRICAL CROSS – when the parental traits are switched such as male with smooth seed becomes female with smooth seeds, while female with wrinkled seed becomes smooth male - PRINCIPLE OF UNIFORMITY in F – all 1ffspring in the first generation are alike - Mendel found that F gen1ration may all be same phenotype, but they did not breed true i.e. they had different genome - Each factor (gene) exists in alternative forms, called ALLELES - DOMINANT is the allele that is able to express its trait in the phenotype of the F 1 - RECESSIVE is the allele whose trait is not expressed in the phenotype of the F 1 - True-breeding individuals that contain the same specific allele for a particular gene are said to be HOMOZYGOUS for that gene - Where there are two different alleles for a particular gene are said to be HETEROZYGOUS - PUNNETT SQUARE is used to determine all possible progeny of a cross - Mendel concluded that: o Results of reciprocal crosses were always the same o All 1 progeny resembled one of the parental strains, indicating the dominance of one allele over the other o In the F2generation, the parental trait that had disappeared in the F 1 generation reappeared. Furthermore, the trait seen in the F was 1 always found in the F a2 about three times the frequency of other trait Principle of Segregation - Recessive characters, which are masked in the F from a 1ross b/w two true- breeding strains, reappear in a specific proportion in the F  tw2 members of a gene pair (alleles) segregate (separate) from each other during the formation of gametes, half the gametes carry one allele, while the other half carries the other allele - The genes are located on the LOCUS of a CHROMOSOME - At the gene level, members of a pair of alleles segregate during meiosis and that each offspring receives only one allele from each parent - GENE SEGREGATION parallels the separation of homologous pairs of chromosomes at anaphase I
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