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Observable Patterns of Inheritance Chapter 11 Introduction 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 11.1 Mendel’s Insight Into Inheritance Gregor Mendel • Strong background in plant breeding and mathematics • Using pea plants, found indirect but observable evidence of how parents transmit genes to offspring 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 Genetic Terms A pair of homologous chromosomes Figure 11.4 Page 179 A gene locus A pair of alleles Three pairs of genes Figure 11.4 Page 179 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 11.2 Mendel’s Theory of Segregation Monohybrid Crosses Experimental intercross between two F1 heterozygotes AA X aa Aa (F1 monohybrids) Aa X Aa ? Mendel’s Monohybrid Cross Results F2 plants showed dominant-torecessive ratio that averaged 3:1 Figure 11.5 Page 180 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 Probability The chance that each outcome of a given event will occur is proportional to the number of ways that event can be reached Monohybrid Cross Illustrated True-breeding homozygous recessive parent plant F1 PHENOTYPES aa True-breeding homozygous dominant a parent plant Aa Aa Aa Aa a A Aa Aa A Aa Aa AA An F1 plant self-fertilizes and produces gametes: F2 PHENOTYPES Aa A AA Aa Aa aa a A AA Aa Figure 11.7 Page 181 a Aa aa 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 Test Cross • Individual that shows dominant phenotype is crossed with individual with recessive phenotype • Examining offspring allows you to determine the genotype of the dominant individual Punnett Squares of Test Crosses Homozygous recessive a a Homozygous recessive a a A Aa Aa A Aa Aa a aa aa A Aa Aa Two phenotypes All dominant phenotype 11.3 Independent Assortment Dihybrid Cross Experimental cross between individuals that are homozygous for different versions of two traits Dihybrid Cross: F1 Results purple flowers, tall TRUEBREEDING PARENTS: AABB GAMETES: AB x white flowers, dwarf aabb AB ab ab AaBb F1 HYBRID OFFSPRING: Figure 11.9 (1) Page 183 All purple-flowered, tall Dihybrid Cross: F2 Results AaBb X AaBb 1/4 AB 1/4 Ab 1/4 aB 1/4 AB 1/4 Ab 1/4 aB 1/4 ab 1/4 ab 1/16 AABB 1/16 AABb 1/16 AaBB 1/16 AaBb 1/16 AABb 1/16 AAbb 1/16 AaBb 1/16 Aabb 1/16 AaBB 1/16 AaBb 1/16 aaBB 1/16 aaBb 1/16 AaBb 1/16 Aabb 1/16 aaBb 1/16 aabb 9/16 purple-flowered, tall 3/16 purple-flowered, dwarf 3/16 white-flowered, tall 1/16 white-flowered, dwarf Figure 11.9(2) Page 183 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 Metaphase I: A A a a B B b b OR A A a a b b B B Metaphase II: Gametes: A A a a A A a a B B b b b b B B B A B A 1/4 AB b a b a 1/4 ab b A b A 1/4 Ab B a B a 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 11.4 Dominance Relations Complete dominance Incomplete dominance Codominance Incomplete Dominance X Incomplete Homozygous Homozygous parent parent Dominance All F1 are heterozygous X Figure 11.10 Page 184 F2 shows three phenotypes in 1:2:1 ratio Codominance: ABO Blood Types • 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 Range of genotypes: Blood types: IA IA IB IB or or IA i IA IB IB i ii A AB B O Figure 11.11 Page 184 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 11.5 Multiple Effects of Single Genes Pleiotropy • Alleles at a single locus may have effects on two or more traits • Marfan syndrome - Mutation in gene for fibrillin affects skeleton, cardiovascular system, lungs, eyes, and skin 11.6 Interactions Between Gene Pairs Epistasis • Interaction between the products of gene pairs • Common among genes for hair color in mammals Coat Color in Retrievers BBEE X bbee F1 puppies are all BbEe F2 puppies Figure 11.13 Page 186 BE Be bE be BE BBEE BBEe BbEE BbEe Be BBEe BBee BbEe Bbee bE BbEE BbEe bbEE bbEe be BbEe Bbee bbEe bbee black brown yellow Comb Shape in Poultry P: RRpp (rose comb) X rrPP (pea comb) F1: RrPp (all walnut comb) F2: 9/16 walnut RRPP RRPp RrPP RrPp 3/16 rose RRpp Rrpp 3/16 pea rrPP rrPp 1/16 single rrpp Figure 11.15 Page 187 Campodactyly: Unexpected Phenotypes • Effect of allele varies: – Bent fingers on both hands – Bent fingers on one hand – No effect • Many factors affect gene expression 11.7 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 (line of bell-shaped curve indicates continuous variation in population) Range of values for the trait Number of individuals with some value of the trait Number of individuals with some value of the trait Describing Continuous Variation Range of values for the trait 11.8 Environmental Effects Temperature Effects on Phenotype • Rabbit is homozygous for an allele that specifies a heat-sensitive version of an enzyme in melaninproducing pathway • Melanin is produced in cooler areas of body Figure 11.18 Page 190 Environmental Effects on Plant Phenotype • Hydrangea macrophylla • Action of gene responsible for floral color is influenced by soil acidity • Flower color ranges from pink to blue