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Chapter 11 -- Basic Genetics Inheritance Gregor Mendel 1822-1884 Mendel’s pea garden in Brno Mendel’s paper Verhandlungen des naturforschenden Vereines in Brünn. Brünn: Vereines, 1865. Why did Mendel choose peas? Mendel’s Peas Two Characters Seed Coat Flower color Two traits per character Rough Smooth Two traits per character Purple flowers White flowers Traits are alternative forms of characters white or purple smooth or rough “Brother Mendel! We grow tired of peas!” Controlling the mating No question as to parentage of offspring Compare Mendel’s experimental design • 1. Start with true breeding plants. • 2. Cross alternative forms among these truebreeders. P Generation (true-breeding parents) Purple flowers White flowers F1 Generation (hybrids) F2 Generation All plants had purple flowers Mendel’s experimental design • 3. Allowed progeny (F1) to self-fertilize and examined offspring • 4. Count the number of offspring of each type P Generation (true-breeding parents) Purple flowers White flowers F1 Generation (hybrids) F2 Generation All plants had purple flowers What Mendel foundmonohybrid crosses • Monohybrid crosses: examining one trait at a time P Generation (true-breeding parents) Purple flowers White flowers Monohybrid Cross F1 Generation (hybrids) F2 Generation All plants had purple flowers What Mendel found F1 generation P Generation (true-breeding parents) Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers P Generation (true-breeding parents) Purple flowers White flowers F1 x F1 F1 Generation (hybrids) F2 Generation All plants had purple flowers What Mendel found F2 generation F1 x F1 F2: purple and white 705 purple flowered plants 224 white flowered plants 3:1 ratio F1 Generation (hybrids) All plants had purple flowers F2 Generation 224 white 705 purple 3:1 ratio Mendel’s 7 pea traits Mendel’s model 1. Alternative versions of genes lead to variations in inherited characters. Alternative versions are called alleles 2. Each individual receives 2 alleles. – – homozygotes: 2 copies of identical allele heterozygotes: two different alleles Mendel’s model 3. In heterozygotes only the dominant form of allele will be expressed. – genotype: alleles present – phenotype: physical appearance 4. Alleles segregate from one another into gametes (Law of segregation) Chromosomes, genes and alleles …….. Allele for purple flowers Locus for flower-color gene Allele for white flowers Homologous pair of chromosomes Genes control characters e.g., flower color Genes control characters Traits are alternative forms of characters e.g., white or purple Genes control characters Traits are alternative forms of characters Alleles are alternative forms of a gene Human Chromosomes Genes and Alleles Variation within a population Mendel used symbols to interpret results • Capital letter for dominant allele; lower case letter for recessive allele Example flower color: P = dominant allele p = recessive allele Phenotype: purple Genotype: PP or Pp Phenotype: white Genotype: pp 3 Phenotype Genotype Purple PP (homozygous Purple Pp (heterozygous 1 2 1 Purple Pp (heterozygous White pp (homozygous Ratio 3:1 Ratio 1:2:1 1 The testcross Cross plant with unknown genotype with homozygous recessive The Punnett square Reginald C. Punnett PP x pp p p P Pp Pp P Pp Pp The testcross Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp If PP, then all offspring purple: If Pp, then 2 offspring purple and 1 2 offspring white: p 1 p P p p P Pp Pp Pp P Pp P Pp Pp pp pp Different types of Inheritance Dominant Recessive purple Pp PP white pp Incomplete Dominance Incomplete dominance When alleles are not fully dominant or fully recessive in heterozygotes Multiple Alleles: ABO Blood Group Codominance No single allele is dominant Each allele has its own effect. Codominance Multiple Alleles: ABO Blood Group • Type A: adds galactosamine – IAIA, IAi • Type B: adds galactose – IBIB, IBi • Type AB: adds galactosamine and galactose – IAIB • Type O: no sugar added – ii ABO Blood Groups Pleiotropic Effects One allele have multiple effects Environmental effects THE CHROMOSOMAL BASIS OF INHERITANCE Chromosomes and Mendelian Inheritance Chromosomes, genes and alleles …….. and loci Allele for purple flowers Locus for flower-color gene Allele for white flowers Homologous pair of chromosomes Thomas Hunt Morgan (1866 – 1945) Drosophila Human Chromosomes Sex Linkage XX XY female male Human sex chromosomes Sex Linked traits Color blindness Male pattern baldness Chromosomal problems Chromosomal problems Klinefelter syndrone Recessive genes can be carried without being expressed Queen Victoria and hemophilia Sickle cell anemia
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