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Objectives Topic 7: Heredity Inheritance Describe basic genetic inheritance terminology. Complete monohybrid and dihybrid genetic crosses. SCI141 Earlobe Variation Whether a person has attached or detached earlobes depends on a single gene Attached earlobes: two copies of the recessive allele for this gene Detached earlobes: either one or two copies of the dominant allele Early Ideas about Heredity People knew that sperm and eggs transmitted information about traits Blending theory Problem: Would expect variation to disappear in traits persists Variation Gregor Mendel Strong background in plant breeding and mathematics Using pea plants, found indirect but observable evidence of how parents transmit genes to offspring 1 Chromosome A DNA molecule & attached proteins Duplicated in preparation for mitosis one chromosome (unduplicated) one chromatid its sister chromatid one chromosome (unduplicated) one chromosome (duplicated) one chromosome (duplicated) Chromosome Number Sum total of chromosomes in a cell Somatic cells Chromosome Two number is diploid (2n) Chromosome Units of information about specific traits Passed from parents to offspring Each has a specific location (locus) on a chromosome number is haploid (n) of each chromosome type Alleles of each type of chromosome Gametes One Genes Different molecular forms of a gene Arise by mutation Dominant allele masks a recessive allele that is paired with it Allele Combinations Homozygous having AA two identical alleles at a locus or aa Heterozygous having two different alleles at a locus Aa 2 Genetic Terms Genotype & Phenotype A pair of homologous chromosomes A gene locus Genotype refers to particular genes an individual carries Phenotype refers to an individual’s observable traits Cannot always determine genotype by observing phenotype A pair of alleles Three pairs of genes Dominance Tracking Generations Parental generation mates to produce First-generation offspring mate to produce Second-generation offspring P Dominant Alleles Recessive Alleles Codominance F1 F2 Monohybrid Crosses Experimental intercross between two F1 heterozygotes AA X aa Aa (F1 monohybrids) Aa X Aa ? Steps in Solving Crosses 1. 2. 3. 4. Assign a symbol for each allele Determine parental Genotypes Determine parental Gamete possibilities Determine cross possibilities 3 Tourette’s syndrome Assign a symbol for each allele Tourette’s allele is dominant T = Dominant t = recessive Let’s say that both parents are heterozygous for Tourette’s syndrome Parental Genotype Determine possible gametes Both parent or heterozygous Parents are Tt What does that mean? Possible gametes are? Use Punnett Square T T t Mendel’s Monohybrid Cross Results F2 plants showed dominant-torecessive ratio that averaged 3:1 5,474 round 1,850 wrinkled 6,022 yellow 2,001 green 882 inflated 299 wrinkled 428 green 152 yellow 705 purple 224 white 651 long stem 207 at tip 787 tall 277 dwarf t 4 Multiple alleles More than two alleles are possible Blood More than one gene is responsible for a trait Skin Color type Three alleles IA Æ A type IB Æ B type i Æ recessive Polygenic Inheritance Let’s try some crosses 5