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Lecture 17

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Ryerson University
Biological Sciences
Mark Fitzpatrick

BIOA01H3 – Lecture 17 th Week of Oct. 7 : Peas, Probability, and Pedigree Chapter 11 11.1 Genetic Linkage and Recombination  Mendel found that his observations from crosses were consistent with his hypothesis that each of the genes assorted independently of all of the others  If Mendel extended study to additional characters, would soon found exceptions to this principle  Genes located on different chromosomes assort independently during meiosis b/c two chromosomes behave independently of one another when they line up on metaphase plate  Genes located on same chromosome may be inherited in genetic crosses – not assorting independently – because chromosomes inherited as single physical entity in meiosis  Genes on same chromosome known as linked genes, and phenomenon is called linkage 11.1a The Principles of Linkage and Recombination Were Determined with Drosophila  Fruit fly, Drosophila melanogaster as a model organism to investigate Mendel’s principles in animals  Groups of genes tended to assort together in crosses were believed to be on same chromosome  Undergrad student developed insight that resulted in construction of first genetic map showing relative order of genes on chromosome and estimation of distance separating genes  Used indirect measure of distance because technology wasn’t available to use any other method  Reasoned that genes sitting relatively far apart on chromosome more likely to be separated from one another during meiotic crossing-over than genes lying closer together  Geneticists working w/ fruit flies agreed on “normal” or “wild-type” genotypes; any change in wild type is a mutant  Mutant alleles named based on altered phenotype of organism that expresses them  Names for dominant mutant alleles written w/ first letter in uppercase, those for recessive mutant alleles written with first letter in lowercase i.e. dominant mutant allele transforming antenna to a leg called Antennapedia (Antp), whereas recessive mutant allele altering eye colour called vermillion (v)  Notation for wild-type allele made by adding superscripted + sign to mutant allele notation  You know you understand this when Antp+ is a recessive allele giving a normal phenotype when homozygous  Began breeding program using true-breeding fruit flies w/ normal red eyes & normal wing length (genotype pr pr vg vg ) along w/ true-breeding fly w/ purple eyes & vestigial wings (genotype prpr vgvg)  F1 generation all dihybrid pr pr vg vg & b/c of dominance of wild-type alleles, all had red eyes & normal wings  Selected wild-type females from F1 generation & mated w/ homozygous recessive males (w/ purple eyes & vestigial wings)  If purple & vestigial genes carried on diff. chromosomes, Mendel’s principle of independent assortment predicts four classes of phenotypes in offspring, in approx. 1:1:1:1 ratio of red eyes & normal wings: purple, vestigial: red, vestigial: purple normal  Given over 2800 offspring, 700 should be in each class though observed two types of progeny that were much higher than 700 & two types that were much lower  Morgan’s hypothesis to explain non-Mendelian distribution is that two genes are linked genetically – physically associated on same chromosome  pr and vg are linked genes  Further hypothesized that behavior of linked genes explained by chromosome recombination during meiosis; proposed that frequency of recombination is function of distance between linked genes  pr pr vg vg F1 dihybrid parents produce 4 types of gametes + +  Two parental gametes, pr vg and pr vg generated by simple segregation of chromosomes during meiosis w/o any recombination between genes + +  Two recombinant gametes, pr vg and pr vg result from crossing-over between homologous chromatids when paired in prophase I of meiosis 1  Offspring of cross produced by fusion of each of four gametes w/ pr vg gamete produced by prpr vgvg male parent  Phenotypes of offspring directly reflect genotypes of gametes produced by dihybrid parent 11.1b Recombination Frequency Can Be Used to Map Chromosomes  Recombinant offspring frequencies can be used to make a linkage map of a chromosome showing the relative locations of genes.  I.e. assume 3 three genes a, b, and c are carried together in same chromosome  Crosses reveal 9.6% frequency of recombinants for a and b, an 8% frequency for a and c, and a 2% frequency for b and c  allow genes to be arranged in only one sequence on chromosomes  Unit of a linkage map, called a map unit (abbrev. to mu) is equivalent to recombinant offspring frequency of 1%  also called centimorgan  Mu not absolute physical distances such as nm, rather they’re relative, showing positions of genes w/ respect to each other 11.1c Widely Separated Linked Genes Assort Independently  Genes can be widely separated on chromosome that recombination almost certain to occur between them  When this is the case, genes assort independently even though on same chromosome  Map distance separating them will be 50mu 11.2 Sex-Linked Genes  Genes located on sex chromosomes are called sex-linked genes; inherited differently in males and females  Chromosomes other than sex chromosomes are called autosomes 11.2a Females Are XX and Males Are XY in Both Humans and Fruit Flies  Females have two copies of X chromosomes that are completely homologous w/ each other  Males have one copy of X and one shorter chromosome called Y, which is homologous with X only in a short region 11.2b Human Sex Determination Depends on the SRY Gene  Gene carried on Y chromosome, SRY (sex determining region of Y) is master switch that directs development of maleness early in embryonic development  ~First month embryonic development, rudimentary structures that give rise to reproductive organs are same in XX or XY embryos  After 6-8 weeks, SRY gene active, producing protein that regulates expression of other genes, thereby stimulating part of these structures to develop as testes  Development of testes causes tissues to degenerate that would’ve become female structures i.e. vagina  Remaining structures develop into penis and scrotum 11.2c Sex-Linked Genes Were First Discovered in Drosophila  B/c males & females have diff. sets of sex chromosomes, genes carried can be inherited in distinctly non- Mendelian pattern called sex-linkage Sex linkage arises from two differences between male and females: 1) Males have one X chromosome, therefore one allele for each gene on this chromosome (males are hemizygous for X-linked genes) 2) Males have one copy of Y chromosome & one allele for each gene on this chromosome  Morgan discovered sex-linked genes using flies 2  Started when male fly had white eyes instead of normal red eyes  bred white-eyed male w/ true- breeding female w/ red eyes & F1 flies all had red eyes
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