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McMaster University
Lovaye Kajiura

WEEK 7: MONDAY, OCTOBER 11TH (No lecture- Thanksgiving Holiday), WEDNESDAY OCTOBER 15TH TO FRIDAY, OCTOBER 18TH, 2013 CHAPTER 10: PATTERNS OF INHERITANCE PHYSICAL BASIS OF INHERITANCE Genes are segments of DNA at specific locations called loci (singular locus) on chromosomes Figure 10-1 The relationship among genes, alleles, and chromosomes 1. Mutations are the source of alleles 2. An organism’s two alleles may be the same or different PRINCIPLES OF INHERITANCE DISCOVERED Fig. 10-4 Cross of pea plants that are true-breeding for white or purple flowers MENDEL’S EXPERIMENTS WITH A SINGLE TRAIT GREGOR MENDEL: - controlled breeding, selected true-breeding parents - prevented self-fertilization of pea plants by removing male reproductive organs from the flowers - permitted cross-fertilization by using pollen from a different plant - crossed true-breeding lines of garden pea plants - Mendel observed pairs of contrasting forms (Fig. 10-10) - Mendel followed inheritance for 3 sexual generations: P = parental generation F1 = first filial generation F2 = second filial generation - unlike previous scientists, Mendel carefully counted the offspring and kept detailed records of his experiments - when reciprocal crosses were conducted, it did not matter whether the genetic determinants for seed shape are situated in the female or male parent.results were identical, all progeny had round seeds Fig. 10-5 Self-fertilization of first filial generation plants with purple flowers Fig. 10-6 The distribution of alleles in gametes Fig. 10-8 Determining the outcome of a single trait cross Fig. 10-9 Punnett square of a test cross MENDEL'S PRINCIPLE OF SEGREGATION Two alleles (paired) for each locus segregate from one another during gamete formation. Two alleles are present for each locus in adults. The paired condition is restored during fertilization by the random fusion of gametes. Each gene occupies a specific locus in the DNA on a chromosome. A diploid nucleus has two sets of homologous chromosomes and therefore two copies of each locus, or gene- one copy of each homolog. For a diploid nucleus, we say that it has alleles at each locus. The use of the term "allele" allows us to include the situations where the two alleles differ. The locus for seed shape occupies a specific position on the homolog. ALLELES - are variant forms of a gene - alleles of the same gene differ in their nucleotide sequence DOMINANT ALLELE - is fully expressed in a heterozygote ex. genetic determinant for round seeds RECESSIVE ALLELE - is masked in a heterozygote ex. genetic determinant for wrinkled seeds PHENOTYPE - organism's appearance GENOTYPE - organism's genetic makeup (or the alleles that are present in the individual) A HOMOZYGOUS individual - has two copies of the identical allele for a specific gene - for example SS or ss. A HETEROZYGOUS individual - has two different alleles for a specific gene - for example Ss COMPLETE DOMINANCE - an allele is fully expressed in the phenotype of the heterozygote which masks the phenotypic expression of the recessive allele - homozygous dominant and heterozygous dominant both visually appear the same MONOHYBRID CROSS - a mating (or cross) between two parents, that are heterozygous for a specific gene - a Punnett Square may be used to predict the result of the mating Offspring genotypes: ¼ PP : ½ Pp : ¼ pp 1:4 PP : 1:2 Pp : 1:4 pp Offspring phenotypes: ¾ purple plants ¼ white plants 3:4 1:4 Fig. 10-11 Predicting genotypes and phenotypes for a cross between parents that are heterozygous for two traits MENDEL’S EXPERIMENTS WITH TWO TRAITS DIHYBRID CROSS - mating (or cross) between two parents that are heterozygous for both of the two genes which are being examined -start with true breeding parents: P1 SSYY X ssyy Gametes SY sy F1 SsYy "double heterozygote" For the F2 generation, use the "FOIL rule". What is the "FOIL rule"? • “firsts, outers, inners, lasts” to figure out the gametes to be used in the Punnett square: SY Sy sY sy Fig. 10-1
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