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Get Ready for A & P! Genetics, Genetic Abnormalities & Bioethics Early Ideas about Heredity People knew that sperm and eggs transmitted information about traits Blending theory - traits blended Problem: Would expect variation to disappear However, variation in traits persists Gregor Mendel The founder of modern genetics Fig. 11-2, p.170 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 Each from parents to offspring has a specific location (locus) on a chromosome Alleles Different Arise molecular forms of a gene 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 A pair of homologous chromosomes, each in the unduplicated state (most often, one from a male parent and its partner from a female parent) A gene locus (plural, loci), the location for a specific gene on a chromosome. Alleles are at corresponding loci on a pair of homologous chromosomes A pair of alleles may be identical or nonidentical. They are represented in the text by letters such as D or d Three pairs of genes (at three loci on this pair of homologous chromosomes); same thing as three pairs of alleles Fig. 11-4, p.171 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 3 major genotypes : homozygous dominant (AA) homozygous recessive (aa) heterozygous (Aa) Tracking Generations Parental generation mates to produce First-generation offspring mate to produce Second-generation offspring P F1 F2 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 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 Fig. 11-6, p. 172 Trait Studied Dominant Form Recessive Form F2 Dominant-toRecessive Ratio SEED SHAPE 5,474 round 1,850 wrinkled 2.96:1 SEED COLOR 6,022 yellow 2,001 green 3.01:1 POD SHAPE 882 inflated 299 wrinkled 2.95:1 POD COLOR 428 green 152 yellow 2.82:1 FLOWER COLOR 705 purple 224 white 3.15:1 651 long stem 207 at tip 3.14:1 787 tall 277 dwarf 2.84:1 FLOWER POSITION STEM LENGTH Fig. 11-6, p.172 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 173 a Aa aa 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 True-breeding homozygous recessive parent plant F1 PHENOTYPES aa True-breeding homozygous dominant parent plant a a A Aa Aa A Aa Aa Aa Aa Aa Aa AA Fig. 11-7b1, p.173 Punnett Squares of Test Crosses An F1 plant self-fertilizes and produces gametes: F2 PHENOTYPES Aa A a A AA Aa a Aa aa AA Aa Aa aa Fig. 11-7b2, p.173 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: 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 Dominance Relations Complete dominance Incomplete dominance Codominance Incomplete Dominance X Incomplete Homozygous Homozygous parent parent Dominance All F1 are heterozygous X F2 shows three phenotypes in 1:2:1 ratio Incomplete Dominance homozygous parent X homozygous parent All F1 offspring heterozygous for flower color: Cross two of the F1 plants and the F2 offspring will show three phenotypes in a 1:2:1 ratio: Fig. 11-11, p.176 Codominance: ABO Blood Types Gene that controls ABO type codes for enzyme that dictates structure of a glycolipid on blood cells alleles (IA and IB) are codominant when paired Two Third allele (i) is recessive to others ABO Blood Type Range of genotypes: IAIA IBIB or or IAi Blood Types: A IAIB AB IBi ii B O Fig. 11-10a, p.176 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 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 11.16 Page 179 Autosomal Recessive Inheritance Patterns If parents are both heterozygous, child will have a 25% chance of being affected Fig. 12-10b, p. 191 Fig. 11-21, p.183 Autosomal Dominant Inheritance Trait typically appears in every generation Fig. 12-10a, p. 190 Huntington Disorder Autosomal dominant allele Causes involuntary movements, nervous system deterioration, death Symptoms don’t usually show up until person is past age 30 People often pass allele on before they know they have it Achondroplasia Autosomal dominant allele In homozygous form usually leads to stillbirth Heterozygotes Have display a type of dwarfism short arms and legs relative to other body parts Autosomal Dominant Inheritance Fig. 12-5, p.190 Sex Chromosomes Discovered Mammals, In in late 1800s fruit flies XX is female, XY is male other groups XX is male, XY female Human X and Y chromosomes function as homologues during meiosis The X Chromosome Carries Most more than 2,300 genes genes deal with nonsexual traits Genes on X chromosome can be expressed in both males and females The Y Chromosome Fewer than two dozen genes identified One is the master gene for male sex determination SRY gene (sex-determining region of Y) SRY present, testes form SRY absent, ovaries form Crossing Over •Each chromosome becomes zippered to its homologue •All four chromatids are closely aligned •Nonsister chromosomes exchange segments Effect of Crossing Over After crossing over, each chromosome contains both maternal and paternal segments Creates new allele combinations in offspring Crossover Frequency Proportional to the distance that separates genes A B C D Crossing over will disrupt linkage between A and B more often than C and D In-text figure Page 178 X-Linked Recessive Inheritance Males show disorder more than females Son cannot inherit disorder from his father Fig. 12-10, p.194 Examples of X-Linked Traits Color blindness Inability to distinguish among some of all colors Hemophilia Blood-clotting disorder 1/7,000 males has allele for hemophilia A Was common in European royal families Color Blindness Fig. 12-12, p.195 male Pedigree Symbols female marriage/mating offspring in order of birth, from left to right Individual showing trait being studied sex not specified I, II, III, IV... generation Fig. 12-19a, p.200 Duplication Gene sequence that is repeated several to hundreds of times Duplications occur in normal chromosomes May have adaptive advantage Useful mutations may occur in copy Duplication normal chromosome one segment repeated three repeats Deletion Loss of some segment of a chromosome Most are lethal or cause serious disorder Deletion Cri-du-chat Fig. 12-13, p.196 Inversion A linear stretch of DNA is reversed within the chromosome segments G, H, I become inverted In-text figure Page 196 Translocation A piece of one chromosome becomes attached to another nonhomologous chromosome Most are reciprocal Philadelphia chromosome arose from a reciprocal translocation between chromosomes 9 and 22 Translocation one chromosome a nonhomologous chromosome nonreciprocal translocation In-text figure Page 206 In-text figure Aneuploidy Individuals have one extra or less chromosome (2n + 1 or 2n - 1) Major cause of human reproductive failure Most human miscarriages are aneuploids Polyploidy Individuals have three or more of each type of chromosome (3n, 4n) Common Lethal in flowering plants for humans 99% die before birth Newborns die soon after birth Nondisjunction n+1 n+1 n-1 chromosome alignments at metaphase I n-1 nondisjunction alignments at at anaphase I metaphase II anaphase II Figure 12.16 Page 198 Nondisjunction Fig. 12-16a, p.198 Down Syndrome Trisomy of chromosome 21 Mental impairment and a variety of additional defects Can Risk be detected before birth of Down syndrome increases dramatically in mothers over age 35 Down Syndrome Fig. 12-17, p.199 Turner Syndrome Inheritance 98% of only one X (XO) spontaneously aborted Survivors are short, infertile females No functional ovaries Secondary sexual traits reduced May be treated with hormones, surgery Klinefelter Syndrome XXY condition Results mainly from nondisjunction in mother (67%) Phenotype is tall males Sterile or nearly so Feminized traits (sparse facial hair, somewhat enlarged breasts) Treated with testosterone injections XYY Condition Taller Most otherwise phenotypically normal Some Once than average males mentally impaired thought to be predisposed to criminal behavior, but studies now discredit Genetic Abnormality A rare, uncommon version of a trait Polydactyly Unusual number of toes or fingers Does not cause any health problems View of trait as disfiguring is subjective Genetic Disorder Inherited conditions that cause mild to severe medical problems Why don’t they disappear? Mutation introduces new rare alleles In heterozygotes, harmful allele is masked, so it can still be passed on to offspring Genetic Disorders and Genetic Abnormalities Gene Mutations Base-Pair Substitutions Insertions Deletions Base-Pair Substitution a base substitution within the triplet (red) original base triplet in a DNA strand During replication, proofreading enzymes make a substitution possible outcomes: or original, unmutated sequence a gene mutation Frameshift Mutations Insertion Extra base added into gene region Deletion Base removed from gene region Both shift the reading frame Result in many wrong amino acids Frameshift Mutation part of DNA template mRNA transcribed from DNA THREONINE PROLINE GLUTAMATE GLUTAMATE LYSINE resulting amino acid sequence base substitution in DNA altered mRNA THREONINE PROLINE VALINE GLUTAMATE LYSINE altered amino acid sequence deletion in DNA altered mRNA THREONINE PROLINE GLYCINE ARGININE altered amino acid sequence Fig. 14-10, p.226 Mutation Rates Each gene has a characteristic mutation rate Average rate for eukaryotes is between 10-4 and 10-6 per gene per generation Only mutations that arise in germ cells can be passed on to next generation Mutagens Ionizing radiation (X rays) Nonionizing Natural radiation (UV) and synthetic chemicals