Chapter 11 The Basic Principles of Heredity pre mendel 1 2 all hybrid plants of true breed genertically pure parents similar when hybrids mate off spring show mixture of traits Mendells Model of Inheritance 1 Principles of Segregation 2 Principles of Independent Assortment gene pairs related to meiosis alignment during metaphase before reproduction 2 alleles carried by parent myst be separated segregated meiosis members of any gene pair segregate from one another independently of members of other phenotype physical apperance of organism genotype genetic constitution of organism P generation parental generation genetically pure Genes hereditary factors dominant masks presence of recessive unit vs recessive both can be passed on to next generation unchanged unblended and unaffected by the environment alleles alternate form of gene 2 dominant and recessive location of particular gene on chromosome and type of gene Locus ex locus for seed color locus for seed shape Monohybrid Cross inheritance of 2 different alleles of a single locus homozygous true breeding two alleles are identical BB or bb heterozygous 2 different alleles Bb Dihybrid Cross ex BBss Bbss BBSs BbSs 2 locus Test cross individual of unknown genotype crossed with homozygous recessive individual 1 1 phenotypic ratio Punnent Square grid of possible combinations of egg and sperm Chromosomes and Heredity Linked Genes tend to be inherited together tendency for a group of genes on the same chromosome to be inherited in suc linkage cessive generations two point test cross figure out genotype of offspring from phenotype abundance of parental types suggests linked genes alleles of two loci are involved the more recombinants there are the further away the genes are on the chromosomes and the less likely they are to be linked Crossing Over how recombinant Different from parental comes about non sister chromatids exchange segments of chromosomal material break and rejoin catalyzed by enzymes Sex Chromosomes female XX y chromosomes bear only a few active genes male XY Abnormal Sex Chromosomes XXY Kleinfelters male underdeveloped X Turner female defects underdeveloped Y does not survive XYY male normal the fact that males only have one X chromosome makes them more prone to genetic dis eases they express all alleles on X chromosomes dominant or recessive always hemizygous only one copy of X linked genes Epigenic Inheritance inheriting way genes are expressed produces sta ble gene inactivation Incomplete Dominance Cross between organisms with two different phenotypes produce off springs with a third phenotype that is a blend of the parental traits Co Dominance cross between organisms with two different phenotypes produce offspring with a third phenotype where dominant parental traits are equally expressed in the same offspring Epistasis presence of certain alleles of one locus can prevent mask the expression of alleles of a different locus and express their own instead