GENETICS ESSENTIALS Concepts and Connections

Transcription

1 Benjamin A. Pierce GENETICS ESSENTIALS Concepts and Connections THIRD EDITION Sex Is Determined by a Number of Different Mechanisms There are several different mechanisms of sex determination The X and Y chromosomes pair during meiosis, even though they are not homologous (the genes located on each are different) CHAPTER 4 Extensions and Modifications of Basic Principles 2014 W. H. Freeman and Company Sex Determination Sexual reproduction: alternates between haploid and diploid states Most organisms have two sexual phenotypes, male and female Chromosomal Sex-Determination Systems: Sex Chromosomes and Non-Sex Chromosomes (Autosomes) XX-XO system: XX female XO male grasshoppers XX-XY system: XX female XY male mammals Chromosomal Sex-Determination Systems ZZ-ZW system: ZZ male ZW female Birds, snakes, butterflies, some amphibians, and fishes 1

2 Genic Sex-Determining System No sex chromosomes, only the sex-determining genes Found in some plants, fungi, protozoans, and fish Environmental Sex Determination Environmental factors Slipper shell: position in the stack Crepidula fornicata Sex Determination in Drosophila melanogaster Genic balance system X : A ratio (X, number of X chromosomes; A, number of haploid sets of autosomes) Temperature in reptiles Turtles: Warmer = more females Crocodiles: Warmer = more males Sex Determination in Humans XX-XY SRY gene on the Y chromosome determines maleness Turner syndrome: XO; 1/3000 female births Klinefelter syndrome: XXY, or XXXY, or XXXXY, or XXYY; 1/1000 male births Poly-X females: 1/1000 female births 2

3 4.2 Sex-Linked Characteristics Are Determined by Genes on the Sex Chromosomes X-Linked characteristics X-linked white eye in Drosophila X-linked color blindness in humans 3

4 Concept Check 4 Hemophilia (reduced blood clotting) is an X- linked recessive disease in humans. A woman with hemophilia mates with a man who exhibits normal blood clotting. What is the probability that their child will have hemophilia? All of their male offspring will have hemophilia, and none of their female offspring will have hemophilia, so the overall probability of hemophilia in their offspring is ½. Calico and Tortoiseshell cats Orange is X-linked X + = black X o = orange Random inactivation Mean some cells produce orange some cells produce black - Tortoiseshell Concept Check 5 How many Barr bodies will a male with XXXYY chromosomes have in each of his cells? What are those Barr bodies? 4

5 Y-Linked Characteristics Only present in males All male offspring will exhibit the trait Y chromosome lost DNA over time Important for sex determination in SRY Additional Factors at a Single Locus Can Affect the Results of Genetic Crosses Genes at the same locus two versions of the same gene; each version of the same gene is defined as allele. Types of dominance Complete dominance Incomplete dominance Codominance Additional Factors at a Single Locus Can Affect the Results of Genetic Crosses Penetrance: the percentage of individuals having a particular genotype that express the expected phenotype. Expressivity: the degree to which a character is expressed. 5

6 Lethal Alleles Additional Factors at a Single Locus Can Affect the Results of Genetic Crosses If before birth genotypes do not appear among offspring Ratio skewed 2:1 Multiple alleles: for a given locus, more than two alleles are present within a group of individuals. ABO blood group Gene Interaction Takes Place When Genes At Multiple Loci Determine a Single Phenotype Gene interaction: Effects of genes at one locus depend on the presence of genes at other loci. Gene interaction that produces novel phenotypes Gene interaction with epistasis: one gene masks the effect of another gene 6

7 Gene Interaction Takes Place When Genes At Multiple Loci Determine a Single Phenotype Epistasis: one gene masks the effect of another gene. Can look similar to dominance/recessive, but not the same gene Recessive epistasis Dominant epistasis Dominant epistasis in squash Duplicate recessive epistasis Recessive Epistasis BBEE X bbee Black Lab Yellow Lab Gene Interaction Takes Place When Genes At Multiple Loci Determine a Single Phenotype BbEe Black Lab X BbEe Black Lab ee is epistatic to B and b 9/16 B_E_ Black 3/16 bbe_ Brown 3/16 B_ee Yellow 1/16 bbee Blue eye color hypothesis: Duplicate recessive epistasis. Could two blue eyed parents have a brown eyed child? Concept Check 8 A number of all-white cats are crossed, and they produce the following types of progeny. 12/16 all-white, 3/16 black, and 1/16 gray. What is the genotype of the black progeny? a. Aa b. Aa Bb c. A_ B_ d. A_ bb Sex Influences the Inheritance and Expression of Genes in a Variety of Ways Sex-influenced and sex-limited characteristics Sex-influenced characteristics Sex-limited characteristics Both sex-influenced and sex-limited characteristics are encoded by autosomal genes whose expression is affected by the sex of the individual who possesses the gene. Sexlinked characteristics are encoded by genes on the sex chromosomes. 7

8 Cytoplasmic Inheritance Some DNA is found in the mitochodria (or in plants the chloroplasts). Inheritance of mitochodria is almost always just from the mother. Different phenotypes exists mostly due to the uneven distribution of the DNA (no mieosis). Position Effect Offspring s phenotype determined by phenotype of branch where the pollen germinated. Maternal Effect Offspring s phenotype determined by mother s genotype. Sex Influences the Inheritance and Expression of Genes in a Variety of Ways Sex-influenced and sex-limited characteristics Genetic maternal effect Genomic imprinting: differential expression of genetic material depending on whether it is inherited from the male or female parent Genomic Imprinting Gene expressed only if given by male parent Epigenetics: phenomena due to alterations to DNA that do not include changes in the base sequence; often affect the way in which the DNA sequences are expressed 8

9 The Expression of a Genotype May Be Influenced By Environmental Effects Temperature-sensitive allele: an allele whose product is functional only at certain temperature. The Inheritance of Continuous Characteristics Discontinuous characteristics: relatively few phenotypes Continuous characteristics: continuous distribution of phenotypes; occurs when genes at many loci interact Polygenic characteristics: characteristics encoded by genes at many loci Pleiotropy: one gene affects multiple characteristics Benjamin A. Pierce GENETICS ESSENTIALS Concepts and Connections THIRD EDITION CHAPTER 5 Linkage, Recombination, and Eukaryotic Gene Mapping Linked Genes Do Not Assort Independently Principle of segregation: alleles separate during meiosis Independent assortment: alleles at one locus sort independently from alleles at another locus Recombination: alleles sort into new combinations 2014 W. H. Freeman and Company 9

10 Linked Genes Segregate Together and Crossing Over Produces Recombination Between Them Complete linkage leads to nonrecombinant gametes and nonrecombinant progeny Crossing over with linked genes leads to recombinant gametes and recombinant progeny Concept Check For single crossovers, the frequency of recombinant gametes is half the frequency of crossing over because a. a testcross between a homozygote and heterozygote produces ½ heterozygous and ½ homozygous progeny. b. the frequency of recombination is always 50%. c. each crossover takes place between only two of the four chromatids of a homologous pair. d. crossovers occur in about 50% of meiosis. 10

11 Linked Genes Segregate Together and Crossing Over Produces Recombination Between Them Calculating Recombination Frequency Recombination frequency = (No. recombinant progeny/total no. of progeny) x 100% Coupling and Repulsion Configuration of Linked Genes Coupling (cis configuration): wild-type alleles are found on one chromosome; mutant alleles are found on the other chromosome a+ b+ a b Coupling or cis a+ b a b+ Repulsion or trans Testing for independent assortment Our Friend Chi Square Linked Genes Segregate Together and Crossing Over Produces Recombination Between Them Gene Mapping with Recombination Frequencies Genetics maps are determined by recombinant frequency Map unit and centimorgans Constructing a Genetic Map with Two- Point Testcrosses? Genetic vs. Physical Maps Genetic maps are based on rates of recombination; physical maps are based on physical distances. Two-point crosses issues 1.Cannot distinguish between genes far apart on one chromosome & on different chromosomes. Both may exhibit 50% recombination. 2. Testcross for two genes that are linked but not close to each other will underestimate the true distance because of double recombination 3. Double crossover will produce non-recombined phenotype 11

12 A Three-Point Testcross Can Be Used to Map Three Linked Genes Constructing a Genetic Map with the Three-Point Testcross Determining the gene order Determining the location of crossovers Recombination Freq 50% On separate chromosomes Or very far apart Start of worked problem: Genetic Mapping Linkage groups Recombination Freq less than 50% In same linkage group Basic idea % recombinants = distance a and b, 10% recomb. 10m.u. apart linkage group 1 a and c, 50% recomb. assorted independently, c in another linkage group a and d, 14% recomb. In linkage group 1, b and d, 4% recomb, so b is closer to d than to a Map so far is a 10 b 4_d Three-point cross Concept Check Write the genotypes of all recombinant and nonrecombinant progeny expected from the following three-point cross: Concept Check A three-point testcross is carried out between three linked genes. The resulting nonrecombinant progeny are s + r + c + and s r c, and the doublecrossover progeny are s r c + and s + r + c. Which is the middle locus? The c locus 12

13 Three-point cross 1. Determine gene order. Based on double recombinants Least number of progeny 2. Determine location of crossovers. Identify number and location of single crossovers. Identify non-recombinant progeny. 3. Calculate recombination frequencies. single recombinants + double recombinants total number of offspring 4. Coefficient of coincidence = number of observed double crossovers number of expected double crossovers Number of expected double crossovers = probability of single crossovers X probability of single crossovers X total number between genes 1 & 2 between genes 2 & 3 progeny 5. Interference the degree to which one crossover interferes with additional crossovers. Interference = 1 coefficient of coincidence When no double crossovers occur, interference is complete (interference = 1). When it is between 0 and 1, one crossover is interfering with additional crossovers. When it is 0, one crossover does not interfere with additional crossovers. When it is negative, a crossover increases the probability of another crossover. A Three-Point Testcross Can Be Used to Map Three Linked Genes Calculating the Recombination Frequencies Sum of all single and double crossovers/total progeny Interference and Coefficient of Coincidence Coefficient of coincidence = Number of observed double crossovers/number of expected double crossovers Interference = 1n Coefficient of coincidence P

14 Mapping Techniques New Techniques more closely look at DNA variations Mapping with Molecular Markers RFLPs Genome-wide Association Studies Associations within populations Haplotype Bananas have multiple sets of chromosomes. Autopolyploid: 3n 33 chromosomes CHAPTER 6 Chromosome Variation Chromosome Mutations Include Rearrangements, Aneuploids, and Polyploids Chromosome morphology (position of the centromere on the chromosome): Metacentric Submetacentric Acrocentric Telocentric 14

15 Chromosome Mutations Include Rearrangements, Aneuploids, and Polyploids Types of Chromosome Mutations: Rearrangements: Aneuploidy, change in # of individual chromosome: 2n +1 Polyploidy, addition of whole set: 3n Chromosome Rearrangements Alter Chromosome Structure Four types of chromosomal rearrangements: Duplication, Deletion, Inversion, Translocation Unbalanced gene dosage: Specific Duplications Tandem Duplication directly next to duplicated region Displaced Duplication away from duplicated region Reverse Duplication ABCDEFFEDCBA 15

16 Bar phenotype Fewer eye facets Bar shape X-linked duplication Unbalanced gene dosage Developmental abnormalities 16

17 Chromosome Rearrangements Alter Chromosome Structure Deletions: loss of a chromosomal segment Large deletions easily detected; during pairing, normal chromosome loops out Effects of deletions Imbalances in gene product Pseudodominance: Expression of a normally recessive gene in a heterozygous due to a deletion on one chromosome Haploinsufficiency When two copies are needed to produce wild type effect. A deletion results in only one normal copy of the gene; Notch: Drosophila needs two copies of gene to form on wings Chromosome Rearrangements Alter Chromosome Structure Inversion (depends on the involvement of the centromere in the inversion): Paracentric inversion Pericentric inversion Translocations Nonreciprocal translocation Reciprocal translocation Robertsonian translocation 17

18 Aneuploidy Is an Increase or Decrease in the Number of Individual Chromosomes Variations in copy number: aneuploidy and polyploidy Causes of aneuploidy Deletion of centromere during mitosis and meiosis Robertsonian translocation Nondisjunction during meiosis and mitosis Types of Aneuploidy Nullisomy: loss of both members of a homologous pair of chromosomes. 2n 2 Monosomy: loss of a single chromosome. 2n 1 Trisomy: gain of a single chromosome. 2n + 1 Tetrasomy: gain of two homologous chromosomes. 2n

19 Concept Check 5 A diploid organism has 2n = 36 chromosomes. How many chromosomes will be found in a trisomic member of this species? 2n + 1 = = 37 Aneuploidy Is an Increase or Decrease in the Number of Individual Chromosomes Effects of Aneuploidy In humans Sex-chromosome aneuploids Turner syndrome. XO Klinefelter syndrome. XXY Aneuploidy Is an Increase or Decrease in the Number of Individual Chromosomes Effects of Aneuploidy: In plants: mutants could actually be trisomics In humans Autosomal aneuploids: Trisomy 21: Down syndrome» Primary Down syndrome, 75% random nondisjunction in egg formation» Familial Down syndrome, Robertsonian translocation between chromosomes 14 and 21 Aneuploidy Is an Increase or Decrease in the Number of Individual Chromosomes Effects of Aneuploidy: In humans Autosomal aneuploids Trisomy 18: Edward syndrome, 1/8000 live births Trisomy 13: Patau syndrome, 1/15,000 live births Trisomy 8: 1/25,000 ~ 1/50,000 live births 19

20 Aneuploidy Is an Increase or Decrease in the Number of Individual Chromosomes Effects of Aneuploidy: In humans Autosomal aneuploids Aneuploidy and maternal age: Possible interpretation of this connection? Uniparental disomy: both chromosomes are inherited from the same parent. Mosaicism and nondisjunction in mitotic division Polyploidy is the Presence of More Than Two Sets of Chromosomes Autopolyploidy From single species Allopolyploidy From two species 20

21 Allopolyploid Allopolyploid Allopolyploid Allopolyploid Polyploidy is the Presence of More Than Two Sets of Chromosomes Polyploidy and Sterility No seeds - Bananas & Watermelon The significance of polyploidy Increase in cell size Larger plant attributes Evolution: may give rise to new species Wheat is hexploid Originally 3 species Allopolyploidy between species Autopolyploidy same species 21

22 Concept Check Species A has 2n = 16 chromosomes and species B has 2n = 14. How many chromosomes would be found in an allotriploid of these two species? a. 21 or 24 b. 42 or 48 c. 22 or 23 d. 45 Symbols 2n = diploid 3n, 4n = polyploid 2n + 1 = trisomy (type of aneuploidy),+2 tetrasomy 2n 1 = monosomy (X_ unk or XO indicates no second sex chromosome 22