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Chapter 13

BIO316 Chapter 13 - Genetics (part two)

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University of Toronto Mississauga
Don Mc Kenzie

CHAPTER 13: GENETICS  The basic unit of heredity is the gene (composed of DNA located on chromosomes)  When genes exist in more than one form, they are called alleles  The genetic makeup of an individual is the individual’s genotype o The physical manifestation of genetic makeup is the individual’s phenotype Mendelian Genetics  Gregor Mendel took true-breeding individuals with different traits, mated them, and statistically analyzed the inheritance of the traits in the progeny A. Mendel`s First Law: Law of Segregation o Genes exist in alternate forms (alleles) o An organism has two alleles for each inherited trait (one from each parent) o The two alleles segregate during meiosis  The gametes carry only one allele for any given trait o If two alleles in an individual organism are different, only one is fully expressed (dominant) while the other is silent (recessive)  Organisms that contain two copies of the same allele are homozygous  Organisms that carry two different alleles are heterozygous 1. Monohybrid  A cross between two true-breeding organisms with focus on a single trait  The individuals being crossed are the parental (P) generation  The progeny are the filial (F) generation 2. Punnett Square  One way of predicting the genotypes expected 3. Testcross  Only with a recessive phenotype can genotype be predicted with 100 percent accuracy  If a dominant phenotype is expressed, the genotype can be either can be homozygous dominant or heterozygous  Only homozygous recessive organisms always breed true  Here an organism with a dominant phenotype of unknown genotype is crossed with a homozygous recessive gene  The appearance of the recessive phenotype in the progeny indicates that phenotypically dominant parent is heterozygous B. Mendel’s Second Law: Law of Independent Assortment 1. Dihybrid Cross  Parents differing in two traits (which assort independently) is crossed  Known as independent assortment  The F generation will produce 4 different phenotypes in the ration of 1 9:3:3:1 2. Statistical Calculations  The probability of producing a genotype that requires the occurrence of two independent events is equal to the product of the individual probabilities that these events will occur (i.e. 1/16)  The probability of producing a genotype that can be the result of more than one event is equal to the sum of the individual probabilities that these events will occur 3. Problem Solving (see p. 217) The Chromosomal Theory of Inheritance  Diploids species have chromosomes pairs (homologues)  In diploids, alleles for a given trait are segregated o One allele is located on one chromosome o The other allele is found on its homologue A. Segregation and Independent Assortment o During the first meiotic division, the homologous pairs separate, and following cytokinesis, the number of chromosomes per cell is reduced from 2N to N  This is the step in meiosis during which segregation and independent assortment occur B. Nonindependent Assortment: Genetic Linkage o Not all traits assort independently in a dihybrid cross  These crosses (AaBb x AaBb) produce an F1 genotypic ration of 1:2:1  4 AABB: 8 AaBb: 4 aabb  The segregation pattern for a dihybrid cross is like that of a single trait monohybrid cross o Genes A and B are linked (i.e. located on the same chromosome) and so they are inherited together o Tightly linked genes recombine at a frequency close to 0 percent whereas weakly linked genes recombine at frequencies approaching 50 percent C. Recombination Frequencies: Genetic Mapping o Recombinant chromosomes arise from the physical exchange of DNA between homologous chromosomes paired during meiosis through crossing over (genetic recombination) o The degree of genetic linkage is a measure of how far apart two genes are on the same chromosome  The probability of a crossover b/w two points is generally directly proportional to the distance b/w the points  Pairs of genes that are apart from each other on a chromosomes have a higher probability of being separated during crossing over o Recombination frequencies can be used to construct a genetic map  One map unit is defined as a 1 percent recombinant frequency  Recombination frequencies are roughly additive  If genes are found on a map in the order XYZ, the recombination frequency b/w X and Y and between Y and Z will be roughly equal the recombination frequency between X and Z  See Figure 13.6 Variations on Mendelian Genetics A. Incomplete Dominance o Some progeny phenotypes are apparently blends of the parental phenotypes  Example: snapdragons o When a homozygous dominant red snapdragon is crossed with a homozygous recessive white snap dragon, the progeny are 100 percent pink o When the progeny are self-crossed, they produce red, pink and white progeny in a 1:2:1 ratio, respectively o Heterozygotes are intermediates of the phenotypes B. Codominance o Occurs when multiple alleles exist for a given gene and more than one of them is dominant o Each dominant allele is fully dominant when combined with a recessive allele but when the two dominant alleles are present, the phenotype is the result of the expression of both dominant alleles (e.g. the inheritance of ABO blood groups) A B  Blood type is determined by three different alleles: I , I , and i  I and I are both dominant to i  Those who are homozygous I or heterozygous I i have blood type A B B B  I I and I i are blood type B  Those with ii have blood type O A B  I and I are c
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