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Chapter 11

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University of Toronto Scarborough
Biological Sciences

 Chapter 11  Genes, Chromosomes, and Human Genetics  11.1 Genetic Linkage and Recombination  Genes located on the same chromosome may be inherited together in genetic crosses – they do not assort independently – because the chromosome is inherited in meiosis.  Linked genes – genes on the same chromosome  Linkage – the phenomenon of genes being located on the same chromosome  11.1a The Principles of Linkage and Recombination Were Determined with Drosophila  Thomas H. Morgan: used the fruit fly (Drosophila melanogaster) to investigate Mendel’s principles in animals.  Alfred Sturtevant: developed insight that resulted in the construction of the first genetic map showing the relative order of genes on a chromosome and estimated the distance separating the genes.  Genetic maps:  Made using the process of recombination (detectable) occurring on the space separating 2 genes on the chromosomes paired during meiosis  far apart = more likely to be separated during meiotic crossing-over  Morgan’s Genetic Symbolism used to understand the crosses.  Basic Principles:  Normal or Wild-type genotype – a mutant  mutant alleles – based on the altered phenotype of the organism that expresses them  wild-type allele – superscripted + sing (Antp )  dominant mutant – first letter in uppercase (Antp)  recessive mutant – first letter in lowercase (v)  Morgan – true breeding fruit flies with normal red eyes and normal wing length (genotype pr , pr , vg , vg ) + a true breeding fly with recessive trait of purple eyes and vestigial (short and crumpled) wings (genotype p+pr vgvg+  F1(wild-type female + double mutant male) – dihybrid (pr , pr, vg , vg) and because of the dominant wild-type alleles they all ha red eyes and normal wings  F2(F 1ild-type dihybrid females + homozygous recessive males (test cross parent)) – 2 types of progeny were higher and 2 were lower than 700/2800 for the 1:1:1:1 ratio  NO Mendel’s principle of independent assortment, which predicts 4 classes of phenotypes in the offspring due to the fact that purple and vestigial genes were carried on different chromosomes.  Non-Mendelian distribution because Morgan:  2 genes are linked genetically – physically associated on the same chromosome – pr and vg are linked genes.  Chromosome recombination during meiosis -> behaviour of linked genes  Frequency of recombination = distance between linked genes  2 parental gametes pr vg and pr vg generated by simple segregation of the chromosomes during meiosis without recombination (crossing over) between the genes + +  2 recombinant gametes pr vg and pr vg generated by recombination between the homologous chromatids when they are paired in prophase I of meiosis  offspring of cross: produced by fusion of each of these 4 gametes with a pr vg gamete produced by the prpr vgvg male parent.  Phenotypes of offspring = genotypes of gametes produced by dihybrid parents  “parental” – refers to genotype not phenotype – these are the ones that inherit chromosome that were NOT involved in recombination in the dihybrid parent  Morgan: relative frequency of recombinant progeny = measure of distance separating genes.  Eg. Purple eyes and vestigial wings on the same chromosome and are separated by a recombinant offspring frequency distance of 10.7%  11.b Recombination Frequency Can Be Used to Map Chromosomes  Morgan:  Recombinant offspring frequency was characteristic of the 2 particular genes involved and varied from less than 1% up to a maximum of 50%  Alfred Sturtevant:  variation in recombinant offspring frequencies used to map genes on chromosomes.  Linkage map – map of a chromosome showing the relative locations of genes based on recombination frequencies  Eg. Genes a-b 9.6%, a-c 8%, c-b 2%   it doesn’t sum up because genes farther apart on a chromosome = more likely to have more than one crossover between them.  single crossover - between 2 genes gives recombinant chromatids.  double crossover (2 single crossovers occurring in the same meiosis) - between 2 genes give the parental arrangement of alleles and is undetectable  a-b --------------8%---------------- --- 2%-- A C B ---------------------9.6%---------------- -----  Created the first linkage map: arrangement of 6 genes on the Drosophila X chromosome.  Map unit (mu) AKA centimorgan– unit of a linkage map = to a recombinant offspring frequency of 1%  NOT absolute physical distances, IT IS relative, showing positions of genes with respect to each other because the frequency of crossing-over giving rise to recombinant offspring varies from one position to another along chromosomes  11.1c Widely Separated Linked Genes Assort Independently  Genes can be widely separated on a chromosome that recombination is almost certain to occur at some point between them in every cell undergoing meiosis.  Then, genes assort independently even though they are on the same chromosome.  The map distance separating them will be 50 mu (50% recombinant offspring and 50% of recombinant offspring observed when genes are on different chromosomes).  Recombination event in a cell creates 2 recombinant and 2 nonrecombinant chromatids  Eg. 100 meiocytes going through meiosis = 400 gametes yield.  2 genes in 10 of the cells = 20 recombinant chromatids produced in prophase I  20 gametes would receive recombinant chromosomes and 20/400 =5% of the total testcross progeny would be recombinant.  They would be 5 mu apart  Assume recombination event occurs along the chromosome in the space separating the 2 genes in every one of the 100 cells going through meiosis  Result: 200 /400 = 50% recombinants = 50 mu separating the genes  Linkage between widely separated genes can still be detected by testing their linkage to genes that lie between them.  Eg. A-b 23 mu, b-c 34%, a-c 57%  A-C genes that are located so far apart that they assort independently and show no linkage since a cross over occur.  57 mu and 57% recombinant offspring frequency would not be seen because the maximum frequency of recombinant chromatids is 50%.  Eg. Mendel’s flower colour and seed colour  11.2 Sex-Linked Genes  Sex-linked genes – gene located on a sex chromosome  pairs of chromosomes are different in both sexes because sex-linked genes are inherited differently.  Autosomes – chromosome other than the sex chromosomes  Genes on autosomes are the same patterns of inheritance for both sexes.  11.2a Females are XX and Males are XY in Both Humans and Fruit Flies  Species with sex chromosomes:  Females: 2 X chromosomes  Gametes = X chromosome  Sperm cell carries X chromosome and fertilizes an X bearing egg cell = XX.  Males: one X and Y chromosome  Y chromosome has short region of homology with the X chromosome  pair during meiosis  Gametes = half X and Y chromosome  sperm cell carries Y chromosome and fertilizes an X bearing egg cell = XY.  Punnett Square shows that fertilization is expected to produce females and males with an equal probability of ½. SAME as drosophila.  Other sex combinations been found: insects XO males (no Y chromosomes), birds/butterflies/reptiles males have ZZ instead of XX and females are heterozygous ZX  11.2b Human Sex Determination Depends of the SRY Gene  One gene carried on the Y chromosome; SRY (sex determining region of the Y) appears to be the master switch that leads to male at an early point in embryonic dstelopment.  1 month: rudimentary structures  reproductive organs and tissues are the same in XX and XY embryos  After 6-8 weeks:  XY embryos: SRY gene becomes active, producing a protein that regulates the expression of other genes, stimulating part of these structures to develop the testes. Tissues degenerate that would otherwise form female genitals; the remaining form the male genitals.  XX embryos: NO SRY gene, development proceeds toward female reproductive structures. Rudimentary male structures degenerate in XX embryos because the hormones released by the developing testes in XY embryo are not present.  X and Y chromosomes are sex chromosomes, but only a few genes they carry have any influence on sex determination or sexual function.  Most of the 2400 known genes on X chromosome code for phenotypes needed for both sexes.  Genes governing structures needed by one sex are coded on autosomes.  Eg. Males inherit genes needed for uterine development and pass them onto the offspring to be used by daughters – genes NOT expressed. Females have genes for penis structure, by do not express them.  11.2c Sex-Linked Genes Were First Discovered in Drosophila  Males and females have different sets of sex chromosomes; the genes carried on these chromosomes can be inherited in a non-Mendelian pattern called sex linkage.  Se Linkage arises from 2 differences between males and females: - Males have 1 X chromosome, females have 2 X chromosomes - Males have 1 Y chromosome, females have no Y chromosomes  Morgan discovered sex-linked genes and their pattern of sex linkage  Cross of white-eyed male with true-breeding red-eyed female  F : all red-eyed 1  White-eye is recessive  F2(F1interbreed): all 2 females had red eyes, half of2F males had red eyes and half had white eyes  NOT Mendel’s principles  expectation that both male and female F flies would 2 show a 3:1 ratio of red-eyed flies to white-eyed flies  Hypothesized: the alleles segregating in the cross were of a gene located on the X chromosome –sex-linked gene  White-eyed male parent in the cross had the genotype X Y W  X – X chromosome with a white allele and no allele on W+ W  Red-eyed female parent in the cross had the genotype X X  Phenotype is red – dominance of XW+ W+ W  F 2 Males are halW+X Y (red eyes) and W+lf X W (white eyes). Females are all red eyed; father X and mother either X or X  Reciprocal cross – phenotypes were switched between parents W W W+  White eyed female (X X ) anW red eyW+ male (X Y)  F 1 males had white eyes (X Y) – X from mother. Females had red eyes all heterozygous (X W+X )  F 2 male and females showed a 1:1 ratio of red and white eyes  Key indicator of sex linkage is when all male offspring of a cross between a true- breeding mutant female and a wild-type male have the mutant phenotype – since male receives X chromosome from the female parent  11.d Sex Linked Genes in Humans Are Inherited as They Are in Drosophila  Pedigree – a summarized chart showing all parents and offspring for as many generations as possible, the sex of individuals in the different generation, and the presence or absence of the trait of interest.  Female =  Male =  Solid circle / square = presence of the trait  Humans & fruit flies  Sex-linked recessive traits appear more frequently among males than females because males need to receive only one of the allele on the X chromosome inherited from their mothers to develop the trait. Females need two copies of the recessive allele, one from each parent, to express the trait.  Hemophilia  Males are bleeders if they receive an X chromosome that carries the recessive allele.  Develops in females with the recessive allele on both of their X chromosomes- rare  11.2e Inactivation of One X Chromosome Evens out Gene Effects  Mammalian females have twice as many copies of genes carried on the X chromosomes as males, BUT they don’t require twice as much of the products of the genes.  Products from genes on the X chromosome could be equalized in both sexes if:  One X of female is “turned off”  this one is USED in body cells in nature  Inactivation occurs by a condensation process – folds and packs the chromatin of an X chromosome into a tightly coiled state  seen as Barr body – dense mass of chromatin  Inactivation occurs during
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