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Chapter 9: Patterns of Inheritance 1. Genetic Concepts 2. Single Gene Inheritance 3. Multiple Gene Inheritance 4. Sex-linked Inheritance 1. Basic Genetic Concepts The Nature of Inheritance (Genetics) Genetics is the study of how genes are inherited AND how they influence the physical characteristics of each individual. Genetics relates to 2 basic processes: 1) the distribution of genes into haploid gametes • i.e., by meiosis 2) how the interaction of gene pairs in a diploid individual influences physical characteristics haploid diploid gene 1 gene 2 gene 3 gene 4 Genes, Alleles & Chromosomes • diploid organisms (e.g., humans) have 2 copies of ea chromosome, hence 2 copies of each gene • genes can come in different versions called alleles • all genes have a characteristic chromosomal location or locus Important Genetic Terms Trait (or character) • physical characteristic (e.g., eye color) Phenotype • individual’s observable traits (e.g., brown eyes) Allele • different forms or versions of the same gene Genotype • the alleles an individual has for a given gene (e.g., AA or Aa or aa) Homozygous • the 2 alleles for a gene are identical (AA or aa) Heterozygous • the 2 alleles for a gene are different (Aa) Dominant allele (designated by capital letter: A) • 1 copy of the allele determines phenotype (A-) Recessive allele (designated by lower-case letter: a) • affects phenotype only when homozygous (aa) Homozygous dominant = AA Homozygous recessive = aa Heterozygous = Aa 2. Single Gene Inheritance Gregor Mendel Deduced the basics of inheritance by analyzing pea plant crosses: • examined a number of traits exhibiting dominant & recessive phenotypes The Inheritance of Flower Color P generation (true-breeding parents) × Purple flowers White flowers All plants have purple flowers F1 generation Fertilization among F1 plants (F1 × F1) F2 generation 3 4 1 of plants of plants 4 have purple flowers have white flowers Based on results such as this for various traits, Mendel correctly deduced that: • offspring inherit 2 “alleles”, 1 from ea parent • one “allele” is dominant over the other (e.g., purple over white) Basis of Mendel’s results In Mendel’s F1 crosses ½ of the gametes from each parent contained “P”, ½ contained “p”: Fertilization results in 4 possible combinations, 3 of which result in purple flowers The Punnett Square …is useful for determining expected results of a cross: 1) write out the cross (i.e., the genotypes of each parent) 2) indicate all possible gametes for each parent on either axis of a Punnet square 3) fill in all possible fertilizations (i.e., combinations of gametes) 4) determine the expected proportions of ea phenotype Pp x P p P p PP Pp Pp pp Pp Expected Genotypic Ratio: 1 PP : 2 Pp : 1 pp Expected Phenotypic Ratio: 3 purple : 1 white Pedigrees & Human Genetics Pedigrees are extremely useful for deducing patterns of human genetic inheritance… D? Dd D? Dd Joshua Lambert John Eddy Abigail Linnell Parents Hepzibah Daggett Sperm D dd Jonathan Lambert D? Abigail Lambert Dd Elizabeth Eddy D Offspring Dd Dd dd Dd **Inheritance pattern shows this form of deafness to be recessive** Dd Dd DD Normal d Dd Normal (carrier) Eggs dd Female Male Deaf Hearing d Dd Normal (carrier) dd Deaf Test Cross Test crosses are used to reveal an unknown genotype • cross the unknown with a homozygous recessive: P? x pp • if all offspring display dominant phenotype: homozygous dominant (PP) • if ½ recessive, ½ dominant phenotype: heterozygous (Pp) Incomplete Dominance Dominant allele is expressed to lesser degree when only 1 allele is present (heterozygous): • heterozygotes have an intermediate phenotype Codominance & Blood Type • “A” & “B” alleles are codominant (both alleles expressed) • “O” allele is recessive Blood Group (Phenotype) Genotypes Antibodies Present in Blood O ii Anti-A Anti-B A IAIA or IAi Anti-B B IBIB or IBi Anti-A AB IAIB — Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left O A B AB IAi x IB IA i IAI B I Bi IB i i IAi ii Expected Phenotypic Ratio: ¼ type AB ¼ type A ¼ type B ¼ type O 3. Multiple Gene Inheritance Inheritance of 2 Traits The results of a cross involving 2 genes can also be predicted using a Punnet square. First the possible gametes produced by each parent with regard to both genes must be determined: e.g. Aa Bb x Aa Bb • the # of possible gametes is the product of the # of different alleles for each gene 2 “A” alleles (A & a) x 2 “B” alleles (B & b) = 4 The 4 possible gametes for each parent in this cross (Aa Bb x Aa Bb) are: Aa Bb AB Ab AB AABB Ab aB ab AABb AaBB AaBb AABb AAbb AaBb Aabb Expected Ratios: 9 A-B3 A-bb aB ab AaBB AaBb aaBB aaBb 3 aaB1 aabb AaBb Aabb aaBb aabb One of Mendel’s “Dihybrid” Crosses • crossed plants that breed true for 2 different traits, then crossed F1 plants: e.g., plants with smooth, yellow seeds (SSYY) X wrinkled, green seeds (ssyy) SSYY x ssyy = all SsYy SsYy x SsYy = 9:3:3:1 (dihybrid cross) Gamete Production by Meiosis • all possible gametes will be produced in equal proportions if each gene is on a different chromosome F1 generation All round yellow seeds (RrYy) R r y Y R Y R r Y y Metaphase I of meiosis (alternative arrangements) r R Y y r Anaphase I of meiosis y r R r R Y y r R Y y Metaphase II of meiosis y Y Y y Y Gametes R R 1 y r r 1 RY 4 Y r r 1 ry 4 y y Y R R 1 rY 4 Ry 4 4. Sex-linked Inheritance Human Sex Determination XX = female XY = male Sex-linked Inheritance Sex-linked inheritance involves genes on the X (or Y) chromosome: • human females have 2 X chromosomes, hence 2 alleles for each X-linked gene • human males have 1 X chromosome, hence 1 allele for each X-linked gene X-linked Genes Genes on the X chromosome have a unique inheritance pattern in males: • only 1 allele, so no masking of recessive alleles H h X X x H X XH H X Y Y H H X X H X Y H h X X h X Y • X-linked alleles are always inherited from the mother (carrier) e.g., Hemophilia is caused by a recessive X-linked allele (h): Xh An “X-linked” Pedigree The inheritance pattern shows that color-blindness is recessive & X-linked Key Terms for Chapter 9 • trait, phenotype, genotype, allele • homozygous, heterozygous • dominant, recessive, codominant • true-breeding, monohybrid, dihybrid • test cross, Punnet square • sex-linked, X-linked • pedigree Relevant Review Questions: 1-7, 10-12, 15-17