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GENETICS: Observing Patterns in Inherited Traits 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 found on homologous chromosomes • Arise by mutation • Dominant allele masks a recessive allele that is paired with it Allele Combinations • Homozygous – having two identical alleles – Homozygous dominant, AA – Homozygous recessive, aa • Heterozygous – having two different alleles – Aa Genotype & Phenotype • Genotype refers to particular genes an individual carries (RR or Rr or rr) • Phenotype refers to an individual’s observable traits (flower color, seed shape, etc) Other Definitions • Dominant allele – in a heterozygous individual, a trait that is fully expressed in the phenotype • Recessive allele – in a heterozygous individual, a trait that is completely masked by the expression of the dominant allele • Pure (true) breeding – a population with only one type of allele for a given trait • Self cross – when individuals of a generation fertilize themselves (e.g., self-fertilized flower). Chromosomes 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 specific type of chromosome A pair of alleles (each being a certain molecular form of a gene) at corresponding loci on a pair of homologous chromosomes Three pairs of genes (at three loci on this pair of homologous chromosomes); same thing as three pairs of alleles Fig. 8-1, p.113 Gregor Mendel 1822-1884 • Father of Genetics • Austrian Monk • Strong background in mathematics • observed evidence of how parents transmit genes to offspring • Unaware of cells, chromosomes or genes Fig. 10-2, p.152 Mendel studied the Garden Pea • Mendel began by examining varieties of peas suitable for study – Character- an observable feature, such as flower color – Trait – actual flower color, such as purple or white – Heritable trait – is this character passed on to progeny ? • Experimentally cross-pollinated Mendel’s methods Mendel’s Monohybrid Cross Results Mendel crossed round x wrinkle seeded plants • P (parental generation) round x wrinkled • F1 (1st filial generation offspring) round • F2 (2nd filial generation offspring) round & wrinkled round x wrinkled Credit: © Wally Eberhart Dominant / Recessive Traits • Mendel observed each parent carried two “units” for a given trait • We know these “units” are genes on chromosomes • Dominant traits – show up each generation • Recessive traits – may be masked by dominant traits F1 Results of One Monohybrid Cross 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. 10-7b1, p.155 Monohybrid Cross Experimental intercross between two F1 heterozygotes AA X aa Aa (F1 monohybrids) Aa X Aa ? A Monohybrid Cross 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 a Aa aa F2 Results of Monohybrid Cross An F1 plant self-fertilizes and produces gametes: F2 PHENOTYPES Aa A a A AA Aa a Aa aa AA Aa Aa aa Fig. 10-7b2, p.155 Dominant Form Recessive Form FLOWER COLOR 705 purple 224 white 3.15:1 FLOWER POSITION 651 along stem 207 at tip 3.14:1 STEM LENGTH 787 tall 227 dwarf Average F2 dominant-to-recessive ratio for all of the traits studied: 2.84:1 3:1 Fig. 8-5, p.115 Probability and the Punnett Square male gametes female gametes A a A a A A A aa a A a Aa aa a Aa a A a Aa A AA Aa aa a Aa aa Fig. 8-6a, p.115 POSSIBLE EVENT sperm A sperm A sperm a sperm a meets egg A meets egg a meets egg A meets egg a PROBABLE OUTCOME 1/4 AA offspring 1/4 Aa 1/4 Aa 1/4 aa p.115 Mendel’s Theory of Segregation • Individual inherits a unit of information (allele) for a trait from each parent • During gamete formation, the alleles segregate from each other homozygous dominant parent homozygous recessive parent (chromosomes duplicated before meiosis) meiosis I meiosis II (gametes) (gametes) fertilization produces heterozygous offspring Fig. 8-4, p.114 Dihybrid Cross AB X ab 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 1 AABB purpleflowered, tall parent (homozygous dominant) AB X ab 2 aabb whiteflowered, dwarf parent (homozygous recessive) 3 F1 OUTCOME: All of the F1 plants are purple-flowered, tall (AaBb heterozygotes) Fig. 8-7, p.116 AaBb meiosis, gamete formation AaBb meiosis, gamete formation Fig. 8-7, p.116 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 Independent Assortment • “Units” for one trait were assorted into gametes independently of the “units” for the other trait • Members of each pair of homologous chromosomes are randomly sorted into gametes 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) Dominance Relations Complete dominance Incomplete dominance Codominance Codominance: ABO Blood Types • Gene that controls ABO type codes for enzyme that determines 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: A Multiple Allele System Range of genotypes: Blood types: IA IA IB IB or or IA i I A IB IB i ii A AB B O Fig. 14.10 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings ABO and Transfusions • Type O is universal donor – neither type A nor type B antigens produced • Type AB is universal receiver – no immune response to A or B antigens Incomplete Dominance X Incomplete homozygous homozygous parent parent Dominance All F1 are heterozygous F2 shows three phenotypes in 1:2:1 ratio X 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. 8-10, p.118 Dominance Relations Complete dominance Incomplete dominance Codominance Pleiotropy • Alleles at a single locus may affect two or more traits – Marfan syndrome • Protein: fibrillin-1 – Cystic fibrosis • Protein: cystic fibrosis transmembrane conductance regulator (CFTR) – Color and crossed eyes in Siamese cats – Sickle Cell Anemia Gene interactions and phenotypic expression • Genes may interact with each other: one gene influences phenotypic expression of others • Complex variations: phenotype influenced by gene interactions and/or environmental conditions Interactions among Gene Pairs • Common among genes for hair color in mammals BLACK LABRADOR YELLOW LABRADOR CHOCOLATE LABRADOR Genetics of Coat Color in Labrador Retrievers Epistasis: Phenotypic expression of one gene is governed by another • 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 • Black color – dominant B & E must be present • Yellow color – recessive e & either B or b • Chocolate color – dominant E & recessive b Continuous Variation • A 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 that trait Controlled by more than one gene • Two genes A or a and B or b dark brown 4 dominants medium brown 3 dominants light brown or hazel 2 dominants dark blue, grey or green 1 dominants light blue 0 dominants Fig. 10-14, p.160 X AB Ab aB ab AB AABB AABb AaBB AaBb dark brown med. brown med. brown hazel or lt br. AABb AAbb AaBb Aabb med. brown hazel or lt br. hazel or lt br. gray, grn, dk. bl. AaBB AaBb aaBB aaBb med. brown hazel or lt br. hazel or lt br. gray, grn, dk. bl. AaBb Aabb aaBb aabb hazel or lt br. gray, grn, dk. bl. gray, grn, dk. bl. pale blue Ab aB ab Number of individuals with some value of the trait Plotting Variation The line of a bell-shaped curve reveals continuous variation in the population Range of values for the trait Fig. 8-14a, p.120 Range of values for the trait Fig. 8-14b, p.120 Number of individuals with some value of the trait Fig. 8-15, p.121 Continuous Variation • Skin Color in humans: three genes with multiple alleles Fig. 14.12 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Environmental Effects on Phenotype • Genotype and environment can interact to affect phenotype – Himalayan rabbit ice pack experiment – Transplantation of plant cuttings to different elevations – Human depression Environmental Effects on Phenotype Hydrangeas and Soil Soil pH or trace elements? Phenotypic Plasticity • Phenotype change in response to the environment. • Examples: – Humans tan in response to sun exposure; increased melanin protects cells from harmful solar radiation Phenotypic Plasticity • Phenotype change in response to the environment. • Examples: – Mussels exposed to seastar “scents” develop stronger adductor muscles – Mussels exposed to dog whelk “scent” develop thicker shells Phenotypic Plasticity • Phenotype change in response to the environment. • Predator mediated Human Genetics and Linkages • Autosome Linkages • Sex chromosome linkages • Linkage group; all of the genes along the length of a chromosome • Full linkages stay together after crossover • Incomplete linkages separate at crossover Sex Determination female (XX) male (XY) eggs sperm X x Y X x X X X X XX XX Y XY XY Embryonic Development At seven weeks, appearance of “uncommitted” duct system of embryo At seven weeks, appearance of structures that will give rise to external genitalia Y chromosome Y chromosome Y chromosome Y chromosome present absent present absent testes ovaries 10 weeks ovary penis testis penis 10 weeks vaginal opening uterus vagina birth approaching The Y Chromosome • Small, with few genes • Master gene for male sex determination – SRY gene (sex-determining region of Y) • SRY present, testes form • SRY absent, ovaries form The X Chromosome • Carries more than 2,000 genes • Most genes deal with nonsexual traits • Genes on X chromosome can be expressed in both males and females Crossover Frequency Proportional to the distance between genes A B C D Crossing over will disrupt linkage between A and B more often than C and D Full Linkage Parents: AB ab B A B b x A a b F1 offspring: a All AaBb meiosis, gamete formation Equal ratios of two types of gametes: 50% AB B A b a 50% ab Fig. 8-20a, p.125 Incomplete Linkage AC A Parents: ac a C A x c a c C All AaCc F1 offspring: meiosis, gamete formation A Unequal ratios of four types of gametes: C a c Most gametes have parental genotypes A c a C A smaller number have recombinant genotypes Fig. 8-20b, p.125 Genetic Abnormality • A rare, uncommon version of a trait • Polydactyly – Unusual number of toes or fingers – Does not cause health problems – View of trait as disfiguring is subjective Pedigree for Polydactyly 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 Autosomal Dominant Inheritance Trait typically appears in every generation Achondroplasia • Autosomal dominant inheritance • Homozygous form usually leads to stillbirth • Heterozygotes display a type of dwarfism Autosomal Recessive Inheritance Patterns • If parents are both heterozygous, child will have a 25% chance of being affected Autosomal Recessive Galactosemia X-Linked Recessive Inheritance • Males show disorder more than females • Son cannot inherit disorder from his father Examples of X-Linked Traits • Color blindness – Inability to distinguish among some or all colors • Hemophilia – Blood-clotting disorder – 1/7,000 males has allele for hemophilia A – Was common in European royal families Color Blindness Fig. 8-27, p.128 Hemophilia Structural Changes in Chromosomes • Duplication • Deletion • Inversion • Translocation Duplication normal chromosome one segment repeated three repeats Deletion • Loss of some segment of a chromosome • Most are lethal or cause serious disorder Inversion A linear stretch of DNA is reversed within the chromosome segments G, H, I become inverted Translocation one chromosome a nonhomologous chromosome nonreciprocal translocation In-text figure Page 206 Changes in Chromosome Number • Aneuploidy • Polyploidy • Most changes in chromosome number are due to nondisjuction Aneuploidy • Individuals have one extra or one 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 in flowering plants • Lethal 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 Down Syndrome • Trisomy of chromosome 21 • Mental impairment and a variety of additional defects • Can be detected before birth • Risk of Down syndrome increases dramatically when mothers are over age 35 Down Syndrome • Trisomy of chromosome 21 Down Syndrome Turner Syndrome • Inheritance of only one X (XO) • 98% 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 than average males • Most otherwise phenotypically normal • Some mentally impaired • Once mistakenly associated with criminal behavior X Chromosome Inactivation • One X inactivated in each cell of female • Creates a “mosaic” for X chromosomes • Dosage compensation Mosaic Expression