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Chapter 10: Mendel, Genes, and Inheritance (textbook summary)

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Biology 1201A
Michael Gardiner

Mendel, Genes, and Inheritance 10.1 The Beginnings of Genetics: Mendel’s Garden Peas: Blending theory of inheritance: Believed before Mendel’s theories introduced that stated that parents’ blood mixed to create a middle in the offspring - He studied characters, heritable characteristics such as flower color, seed shape - Traits were known to be variation in character - Characters are passed to offspring in the form of discrete hereditary factors (genes) - Sometimes characters appear in offspring unchanged, sometimes they skip a generation Mendel Chose True-Breeding Garden Peas for his Experiments - Mendel prevented self-fertilization and pollenated flowers with different ones to create cross-pollination (cross) - He used true-breeding plants that did not pass traits that change from one generation to the next Mendel First Worked with Single-Character Crosses - Used purple flowers to pollinate white ones and vice versa. F1 Results were only purple flowers. Disproved blending theory - However, the F2 offspring of these cross-bred flowers were purple and white (3:1) P Generation: parental plants used in the initial cross F1 Generation: First generation of offspring from the cross F2 Generation: Offspring of F1 generation Mendel’s Single-Character Crosses Led Him to Propose the Principle of Segregation 3 Hypotheses: 1. The adult plants carry a pair of factors that govern the inheritance of each character. For each character, an organism inherits one factor (gene) from each parent - There are 2 alleles of the gene that govern flower color in garden peas (1 for purple, 1 for white) 2. If an individual’s pair of genes consists of different alleles, one allele is dominant over the other, recessive, allele - The dominant one is simply the one that determines the appearance of the organism - Dominant alleles do not directly inhibit recessive alleles 3. The 2 alleles of a gene segregate and enter gametes singly Principle of Segregation: Half the gametes carry one allele, the other half, the other allele - When gametes (haploid) come together to form diploid > zygote nucleus. 1 allele for the character from male gamete and 1 from the female, reuniting the pairs Diploid: Organisms with 2 copies of each gene (The 2 alleles of a gene in diploid may be different or identical) Allele: Different versions of a gene that produce different traits of a character Homozygous: Contains either 2 dominant or 2 recessive alleles (ex. PP and pp) Heterozygous: Contains 2 different alleles of a gene (ex. Pp) Monohybrid: Offspring of parents with different traits (Heterozygous offspring) Monohybrid Cross: A cross between 2 individuals that are each heterozygous for the same pair of alleles (ex. Pp x Pp) Genotype: genetic constitution of an organism Phenotype: Outward appearance Mendel Could Predict Both Classes and Proportions of Offspring from His Hypotheses: Probability: The possibility that an outcome will occur if it is a matter of chance (0(0%)-1(100%)) - The probabilities of all the possible outcomes must = 1 when added together The Product Rule in Probability - Individual probabilities are multiplied (the outcomes are independent of each other) The Sum Rule in Probability - Several different events all give the same outcome - The probability that either event A, B, or C will occur equals the probability of event A + probability of B + probability of C Probability in Mendel’s Crosses - Same rules of probability apply to Mendel’s Crosses - F1 Parent 1 = Pp, F1 Parent 2 = Pp. To create PP offspring: ½ possibility from Parent 1, ½ possibility from parent 2. ½ x ½ = ¼. Same fraction for pp offspring. - To create Pp offspring: p from P1 and P from P2 or P from P1 and p from P2. Sum rule is applied: Each of the ways to get Pp has an individual probability of ¼, so you add them together: ¼ + ¼ = ½ - Possibility of purple outcome in PpxPp: rule of addition. ½ + ¼ = ¾ Mendel Used a Testcross to Check the Validity of His Hypotheses Testcross: A cross between an individual with the dominant phenotype and a homozygous recessive individual. You get a 1:1 ratio - This determines whether an individual with a dominant trait is a heterozygote or a homozygote, because these cannot be distinguished phenotypically - Heterozygous of half display the dominant and half the recessive trait. - Homozygous if all the offspring display the dominant trait Mendel Tested the Independence of Different Genes in Crosses Dihybrid: a zygote produced from a cross that involves two characters. Ex. Round and yellow seeds (RRYY) x wrinkled and green seeds (rryy) Dihybrid Cross: A cross between two individuals that are heterozygous for two pairs of alleles - This gives a ratio of 9:3:3:1 Hypothesis: The alleles of the genes that govern the two characters segregate independently during formation of gametes Mendel’s Pri
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