Presentation on theme: "NOTES: Ch 15 - Chromosomes, Sex Determination & Sex Linkage"— Presentation transcript:
1 NOTES: Ch 15 - Chromosomes, Sex Determination & Sex Linkage
2 Overview: Locating Genes on Chromosomes ● A century ago the relationship between genes and chromosomes was not obvious● Today we can show that genes are located on chromosomes● The location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene
3 The Chromosome Theory of Inheritance states that: ● Mendelian genes have specific loci (positions) on chromosomes● It is the chromosomes that undergo segregation and independent assortment!
4 P GenerationYellow-roundseeds (YYRR)Green-wrinkledseeds (yyrr)MeiosisFertilizationGametesAll F1 plants produceyellow-round seeds (YyRr)F1 GenerationMeiosisLAW OF SEGREGATIONLAW OF INDEPENDENT ASSORTMENTTwo equallyprobablearrangementsof chromosomesat metaphase IAnaphase IMetaphase IIGametesF2 GenerationFertilization among the F1 plants
5 Morgan’s Experimental Evidence: Scientific Inquiry ● The first solid evidence associating a specific gene with a a specific chromosome came from Thomas Hunt Morgan, an embryologist
6 Morgan’s Choice of Experimental Organism: Fruit Flies! ● Characteristics that make fruit flies a convenient organism for genetic studies:-They breed at a high rate-A generation can be bred every two weeks-They have only four pairs of chromosomes
10 Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair ● In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type)-The F1 generation all had red eyes-The F2 generation showed the 3:1 red:white eye ratio, but only males had white eyes● Morgan determined that the white-eye mutant allele must be located on the X chromosome● Morgan’s finding supported the chromosome theory of inheritance!
13 The Big Question…● It may be easy to see that genes located on DIFFERENT chromosomes assort independently but what about genes located on the SAME chromosome?
14 Thomas Morgan’s Research ● Morgan identified more than 50 genes on Drosophila’s 4 chromosomes.● He discovered that many seemed to be “linked” togetherThey are almost always inherited together & only rarely become separated● Grouped genes into 4 linkage groups
16 Morgan’s Conclusion:● Each chromosome is actually a group of linked genes● BUT Mendel’s principle of independent assortment still holds true● It is the chromosomes that assort independently!!Mendel missed this because 6 of the 7 traits he studied were on different chromosomes.
17 So…● If 2 genes are found on the same chromosome are they linked forever?NO!!● CROSSING OVER during Meiosis can separate linked genes
18 Parental-type offspring Recombinant offspring TestcrossparentsGray body,normal wings(F1 dihybrid)Black body,vestigial wings(double mutant)Replication ofchromosomesReplication ofchromosomesMeiosis I: Crossingover between b and vgloci produces new allelecombinations.Meiosis I and II:No new allelecombinations areproduced.Meiosis II: Separationof chromatids producesrecombinant gameteswith the new allelecombinations.RecombinantchromosomesOvaSpermGametesOvaTestcrossoffspring965Wild type(gray-normal)944Black-vestigial206Gray-vestigial185Black-normalSpermRecombinationfrequency=391 recombinants 100 = 17%2,300 total offspringParental-type offspringRecombinant offspring
19 Gene Maps● Alfred Sturtevant was a graduate student working in Morgan’s lab part-time in 1911● He hypothesized that the farther apart 2 genes are on a chromosome the more likely they are to be separated by crossing-over● The rate of at which linked genes are separated can be used to produce a “map” of distances between genesAlfred Sturtevant
21 Gene Maps● This map shows the relative locations of each known gene on a chromosome
22 Linkage Maps● A linkage map is a genetic map of a chromosome based on recombination frequencies● Distances between genes can be expressed as map units; one map unit, or centimorgan, represents a 1% recombination frequency● Map units indicate relative distance and order, not precise locations of genes
25 Sex-linked genes exhibit unique patterns of inheritance ● In humans and other animals, there is a chromosomal basis of sex determination
26 XX XY ● Human somatic cells contain 23 pairs of chromosomes -22 pairs of autosomes (same in males & females)-1 pair of sex chromosomes (XX or XY)-Females have 2 matching sex chromosomes: XX-Males are XYXXXY
27 Inheritance of Sex-Linked Genes ● The sex chromosomes have genes for many characters unrelated to sex● A gene located on either sex chromosome is called a SEX-LINKED gene● Sex-linked genes follow specific patterns of inheritance
31 ● When a gene is located on the X chromosome, females receive 2 copies of the gene, and males receive only 1 copyExample: Color-blindness (c) is recessive to normal vision (C), and it is located on the X chromosome; hemophilia
33 XC Xc Xc Y EXAMPLE PROBLEM: ● A female heterozygous for normal vision: (we say she has normal vision,but is a carrier of the colorblindness allele)● A male who is colorblind:XC XcXc Y
34 What is the probability that: a) they will have a son who is colorblind?b) they will have a daughter who is colorblind?c) their first son will be colorblind?d) their first daughter will be carrier?XC Xc1/4 (25%)1/2 (50%)XcYXC XcXc XcXC YXc Y
35 XH Y XH Xh EXAMPLE PROBLEM: x ● Hemophilia is a hereditary disease in which the blood clotting process if defective. Classic hemophilia results from an abnormal or missing clotting factor VIII; it is inherited as an X-linked recessive disorder (h).● If a man without hemophilia and a woman who is a carrier of the hemophilia allele have children, what is the probability that…XH YXH Xhx
36 what is the probability that: a) they will have a daughter with hemophilia?b) they will have a son with hemophilia?c) their first son will have hemophilia?d) their first daughter will be a carrier?XH Xh0/4 (0%)1/4 (25%)1/2 (50%)XHYXH XHXH XhXH YXh Y
40 X-inactivation in Female Mammals ● In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development● If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character
41 Two cell populationsin adult cat:Active XEarly embryo:OrangefurX chromosomesCell divisionand XchromosomeinactivationInactive XInactive XBlackfurAllele fororange furAllele forblack furActive X
47 Alterations of chromosome number or structure cause some genetic disorders ● Large-scale chromosomal alterations often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders
48 Abnormal Chromosome Number ● In NONDISJUNCTION, pairs of homologous chromosomes do not separate normally during meiosis● As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy
49 Meiosis INondisjunctionMeiosis IINondisjunctionGametesn + 1n + 1n – 1n – 1n + 1n – 1nnNumber of chromosomesNondisjunction of homologouschromosomes in meiosis INondisjunction of sisterchromatids in meiosis I
50 ● Aneuploidy results from the fertilization of gametes in which nondisjunction occurred ● Offspring with this condition have an abnormal number of a particular chromosome
51 in which an organism has more than two complete sets of chromosomes ● a TRISOMIC zygote has three copies of a particular chromosome● a MONOSOMIC zygote has only one copy of a particular chromosome● Polyploidy is a conditionin which an organism hasmore than two completesets of chromosomes
52 Alterations of Chromosome Structure ● Breakage of a chromosome can lead to four types of changes in chromosome structure:-Deletion removes a chromosomal segment-Duplication repeats a segment-Inversion reverses a segment within a chromosome-Translocation moves a segment from one chromosome to another
53 DeletionA deletion removes a chromosomalsegment.DuplicationA duplication repeats a segment.InversionAn inversion reverses a segmentwithin a chromosome.A translocation moves a segmentfrom one chromosome to another,nonhomologous one.Reciprocaltranslocation
54 Human Disorders Due to Chromosomal Alterations ● Alterations of chromosome number and structure are associated with some serious disorders● Some types of aneuploidy appear to upset the genetic balance less than others, resulting in individuals surviving to birth and beyond● These surviving individuals have a set of symptoms, or syndrome, characteristic of the type of aneuploidy
55 Down Syndrome:● Down Syndrome is an aneuploid condition that results from three copies of chromosome 21● It affects about one out of every 700 children born in the United States● The frequency of Down Syndrome increases with the age of the mother
57 Aneuploidy of Sex Chromosomes ● Nondisjunction of sex chromosomes produces a variety of aneuploid conditions● Klinefelter syndrome is the result of an extra chromosome in a male, producing XXY individuals● Monosomy X, called Turner syndrome, produces X0 females, who are sterile; it is the only known viable monosomy in humans
58 Disorders Caused by Structurally Altered Chromosomes: ● One syndrome, cri du chat (“cry of the cat”), results from a specific deletion in chromosome 5● A child born with this syndrome is mentally retarded and has a catlike cry; individuals usually die in infancy or early childhood● Certain cancers, including chronic myelogenous leukemia (CML), are caused by translocations of chromosomes
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