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Observing Patterns in Inherited Traits Chapter 10 Early Ideas about Heredity • People knew that sperm and eggs transmitted information about traits • Blending theory • Problem: – Would expect variation to disappear – Variation in traits persists Gregor Mendel • Strong background in plant breeding and mathematics • Using pea plants, found indirect but observable evidence of how parents transmit genes to offspring The Garden Pea Plant • Self-pollinating • True breeding (different alleles not normally introduced) • Can be experimentally crosspollinated Genetic Terms A pair of homologous chromosomes A gene locus A pair of alleles Three pairs of genes Genes • Units of information about specific traits • Passed from parents to offspring • Each has a specific location (locus) on a chromosome Alleles • Different molecular forms of a gene • Arise by mutation • Dominant allele masks a recessive allele that is paired with it Allele Combinations • Homozygous – having two identical alleles at a locus – AA or aa • Heterozygous – having two different alleles at a locus – Aa Genotype & Phenotype • Genotype refers to particular genes an individual carries • Phenotype refers to an individual’s observable traits • Cannot always determine genotype by observing phenotype Tracking Generations • Parental generation mates to produce P • First-generation offspring mate to produce F1 • Second-generation offspring F2 Monohybrid Crosses • Use F1 offspring of parents that breed true for different forms of a trait: (AA x aa = Aa) • The experiment itself is a cross between two identical F1 heterozygotes, which are the “monohybrids” (Aa x Aa) Monohybrid Crosses Homozygous dominant parent Homozygous recessive parent (chromosomes duplicated before meiosis) meiosis I meiosis II (gametes) (gametes) fertilization produces heterozygous offspring Mendel’s Monohybrid Cross Results F2 plants showed dominanttorecessive ratio that averaged 3:1 Probability The chance that each outcome of a given event will occur is proportional to the number of ways that event can be reached Punnett Square of a Monohybrid Cross Female gametes A Male gametes A AA a Aa a Aa aa Dominant phenotype can arise 3 ways, recessive only 1 F1 Results of One Monohybrid Cross F2 Results of Monohybrid Cross Testcross • Individual that shows dominant phenotype is crossed with individual with recessive phenotype • Examining offspring allows you to determine the genotype of the dominant individual Mendel’s Theory of Segregation • An individual inherits a unit of information (allele) about a trait from each parent • During gamete formation, the alleles segregate from each other Dihybrid Cross Experimental cross between individuals that are homozygous for different versions of two traits A Dihybrid Cross - F1 Results AB 1 AABB homozygous dominant parent plant (purple flowers, tall stem) X ab 2 aabb homozygous recessive parent plant (white flowers, short stem) 3 F1 OUTCOME All F1 plants are AaBb heterozygotes (purple flowers, tall stems) F1 Results of Mendel’s Dihybrid Crosses • All plants displayed the dominant form of both traits • We now know: – All plants inherited one allele for each trait from each parent – All plants were heterozygous (AaBb) Phenotypic Ratios in F2 AaBb X AaBb Four Phenotypes: – Tall, purple-flowered (9/16) – Tall, white-flowered (3/16) – Dwarf, purple-flowered (3/16) – Dwarf, white-flowered (1/16) Explanation of Mendel’s Dihybrid Results If the two traits are coded for by genes on separate chromosome s, sixteen gamete combinations are possible Independent Assortment • Mendel concluded that the two “units” for the first trait were to be assorted into gametes independently of the two “units” for the other trait • Members of each pair of homologous chromosomes are sorted into gametes at random during meiosis Independent Assortment Nucleus of a diploid (2n) reproductive cell with two pairs of homologous chromosomes Possible alignments of the two homologous chromosomes during metaphase I of meiosis The resulting alignments at metaphase II Allelic combinations possible in gametes 1/4 AB 1/4 ab 1/4 Ab 1/4 aB Tremendous Variation Number of genotypes possible in offspring as a result of independent assortment and hybrid crossing is 3n (n is the number of gene loci at which the parents differ) Impact of Mendel’s Work • Mendel presented his results in 1865 • Paper received little notice • Mendel discontinued his experiments in 1871 • Paper rediscovered in 1900 and finally appreciated Dominance Relations • Complete dominance • Incomplete dominance – Heterozygote phenotype is somewhere between that of two homozyotes • Codominance – Non-identical alleles specify two phenotypes that are both expressed in heterozygotes Genetics of ABO Blood Types: Three Alleles • Gene that controls ABO type codes for enzyme that dictates structure of a glycolipid on blood cells • Two alleles (IA and IB) are codominant when paired • Third allele (i) is recessive to others ABO Blood Type: Allele Combinations • Type A - IAIA or IAi • Type B - IBIB or IBi • Type AB - IAIB • Type O - ii ABO Blood Type: Glycolipids on Red Cells • Type A - Glycolipid A on cell surface • Type B - Glycolipid B on cell surface • Type AB - Both glyocolipids A & B • Type O - Neither glyocolipid A nor B ABO and Transfusions • Recipient’s immune system will attack blood cells that have an unfamiliar glycolipid on surface • Type O is universal donor because it has neither type A nor type B glycolipid Flower Color in Snapdragons: Incomplete Dominance Red-flowered plant X White-flowered plant (homozygote) (homozygote) Pink-flowered F1 plants (heterozygotes) Flower Color in Snapdragons: Incomplete Dominance Pink-flowered plant X Pink-flowered plant (heterozygote) (heterozygote) White-, pink-, and red-flowered plants in a 1:2:1 ratio Flower Color in Snapdragons: Incomplete Dominance • Red flowers - two alleles allow them to make a red pigment • White flowers - two mutant alleles; can’t make red pigment • Pink flowers have one normal and one mutant allele; make a smaller amount of red pigment Comb Shape in Poultry Alleles at two loci (R and P) interact • • • • Walnut comb - RRPP, RRPp, RrPP, RrPp Rose comb - RRpp, Rrpp Pea comb - rrPP, rrPp Single comb - rrpp Pleiotropy • Alleles at a single locus may have effects on two or more traits • Classic example is the effects of the mutant allele at the beta-globin locus that gives rise to sickle-cell anemia Genetics of Sickle-Cell Anemia • Two alleles 1) HbA Encodes normal beta hemoglobin chain 2) HbS Mutant allele encodes defective chain • HbS homozygotes produce only the defective hemoglobin; suffer from sickle-cell anemia Pleiotropic Effects of HbS/HbS • At low oxygen levels, cells with only HbS hemoglobin “sickle” and stick together • This impedes oxygen delivery and blood flow • Over time, it causes damage throughout the body Genetics of Coat Color in Labrador Retrievers • Two genes involved - One gene influences melanin production • Two alleles - B (black) is dominant over b (brown) - Other gene influences melanin deposition • Two alleles - E promotes pigment deposition and is dominant over e Allele Combinations and Coat Color • Black coat - Must have at least one dominant allele at both loci – BBEE, BbEe, BBEe, or BbEE • Brown coat - bbEE, bbEe • Yellow coat - Bbee, BbEE, bbee Albinism • Phenotype results when pathway for melanin production is completely blocked • Genotype - Homozygous recessive at the gene locus that codes for tyrosinase, an enzyme in the melaninsynthesizing pathway Campodactyly: Unexpected Phenotypes • Effect of allele varies: – Bent fingers on both hands – Bent fingers on one hand – No effect • Many factors affect gene expression Continuous Variation • A more or less continuous range of small differences in a given trait among individuals • The greater the number of genes and environmental factors that affect a trait, the more continuous the variation in versions of that trait Human Variation • Some human traits occur as a few discrete types – Attached or detached earlobes – Many genetic disorders • Other traits show continuous variation – Height – Weight – Eye color Number of individuals with some value of the trait (line of bell-shaped curve indicates continuous variation in population) Number of individuals with some value of the trait Describing Continuous Variation Range of values for the trait Range of values for the trait Temperature Effects on Phenotype • Himalayan rabbits are Homozygous for an allele that specifies a heatsensitive version of an enzyme in melaninproducing pathway • Melanin is produced in cooler areas of body Environmental Effects on Yarrow Plants