Presentation on theme: "NOTES: Ch 15 - Chromosomes, Sex Determination & Sex Linkage."— Presentation transcript:
NOTES: Ch 15 - Chromosomes, Sex Determination & Sex Linkage
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
● Mendelian genes have specific loci (positions) on chromosomes ● It is the chromosomes that undergo segregation and independent assortment! The Chromosome Theory of Inheritance states that:
P Generation Gametes Meiosis Yellow-round seeds (YYRR) Fertilization Green-wrinkled seeds (yyrr) All F 1 plants produce yellow-round seeds (YyRr) Meiosis Two equally probable arrangements of chromosomes at metaphase I Anaphase I LAW OF INDEPENDENT ASSORTMENT Metaphase II Fertilization among the F 1 plants F 2 Generation Gametes F 1 Generation LAW OF SEGREGATION
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
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
● Morgan noted WILD TYPE, or normal, phenotypes that were common in the fly populations ● Traits alternative to the wild type are called mutant phenotypes
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 F 1 generation all had red eyes -The F 2 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!
P Generation F 1 Generation F 2 Generation P Generation F 1 Generation Ova (eggs) Sperm F 2 Generation Ova (eggs) Sperm
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?
Thomas Morgan’s Research ● Morgan identified more than 50 genes on Drosophila’s 4 chromosomes. ● He discovered that many seemed to be “linked” together –They are almost always inherited together & only rarely become separated ● Grouped genes into 4 linkage groups
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.
So… ● If 2 genes are found on the same chromosome are they linked forever? –NO!! ● CROSSING OVER during Meiosis can separate linked genes
Testcross parents Parental-type offspring Recombinant offspring Ova Sperm 965 Wild type (gray-normal) 944 Black- vestigial 206 Gray- vestigial 185 Black- normal Gray body, normal wings (F 1 dihybrid) Sperm Testcross offspring Recombination frequency 391 recombinants 2,300 total offspring 100 = 17% = Ova Gametes Replication of chromosomes Black body, vestigial wings (double mutant) Replication of chromosomes Meiosis I and II: No new allele combinations are produced. Meiosis I: Crossing over between b and vg loci produces new allele combinations. Recombinant chromosomes Meiosis II: Separation of chromatids produces recombinant gametes with the new allele combinations.
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 genes Alfred Sturtevant
Gene Maps ● This map shows the relative locations of each known gene on a chromosome
● 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 Linkage Maps
Short aristae Black body Cinnabar eyes Mutant phenotypes Vestigial wings Brown eyes Wild-type phenotypes Red eyes Normal wings Red eyes Long aristae (appendages on head) Gray body III I IV X Y II
Sex-linked genes exhibit unique patterns of inheritance ● In humans and other animals, there is a chromosomal basis of sex determination
● 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 XY
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
Sperm Ova Sperm Ova Sperm Ova
● Some disorders caused by recessive alleles on the X chromosome in humans: -Color blindness -Duchenne muscular dystrophy -Hemophilia
● When a gene is located on the X chromosome, females receive 2 copies of the gene, and males receive only 1 copy –Example: Color-blindness (c) is recessive to normal vision (C), and it is located on the X chromosome; hemophilia
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: X C X c X c Y
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? X C X c XcYXcY X c X c YX C Y a)1/4 (25%) b)1/4 (25%) c)1/2 (50%) d)1/2 (50%)
EXAMPLE PROBLEM: ● 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… X H X h X H Y x
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? X H X h XHYXHY X H X H X h X h YX H Y a)0/4 (0%) b)1/4 (25%) c)1/2 (50%) d)1/2 (50%)
Queen Victoria’s Legacy in Royal Families of Europe
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
Early embryo: X chromosomes Allele for orange fur Cell division and X chromosome inactivation Allele for black fur Active X Inactive X Black fur Orange fur Active X Two cell populations in adult cat:
Tortoise-shell cats! (a.k.a. “Torties”) X B X b
So, what about the Y chromosome?
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
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
Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n + 1 Number of chromosomes Nondisjunction of homologous chromosomes in meiosis I n + 1n – 1 n + 1n – 1nn Nondisjunction of sister chromatids in meiosis I
● Aneuploidy results from the fertilization of gametes in which nondisjunction occurred ● Offspring with this condition have an abnormal number of a particular chromosome
● a TRISOMIC zygote has three copies of a particular chromosome ● a MONOSOMIC zygote has only one copy of a particular chromosome ● Polyploidy is a condition in which an organism has more than two complete sets of chromosomes
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
Deletion Duplication Inversion Reciprocal translocation A deletion removes a chromosomal segment. A duplication repeats a segment. An inversion reverses a segment within a chromosome. A translocation moves a segment from one chromosome to another, nonhomologous one.
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
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
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
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
Reciprocal translocation Normal chromosome 9 Normal chromosome 22 Translocated chromosome 9 Translocated chromosome 22 Philadelphia chromosome