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Chapter 11

Biology 1201A Chapter 11 Notes.docx

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Biology 1201A
Richard Gardiner

Chapter 11: Genes, Chromosomes, and Human Genetics 11.1 Genetic Linkage and Recombination -chromosomes contain many genes, with each gene at a particular location, or locus -genes located on different chromosomes assort independently during meiosis because the two chromosomes behave independently of one another as they line up on the metaphase plate -genes located on the same chromosome may be inherited together in genetic crosses that is, they do not assort independently because the chromosome is inherited as a single physical entity in meiosis -genes on the same chromosome are known as linked genes, and the phenomenon is called linkage 11.1a The Principles of Linkage and Recombination Were Determined with Drosophilia -Thomas H. Morgan used the fruit fly, Drosophilia melanogaster as a model organism to investigate Mendels principles in animals -groups of genes that tended to assort together in crosses were believed to be carried on the same chromosome -genes sitting relatively far apart on a chromosome would be more likely to be separated from one another during meiotic crossing-over than genes lying closer together if recombination is to be used as a measure of the distance separating genes, it must be detectable -which chromosome, recombinant or not, is carried in a given gamete is most clearly revealed only in offspring resulting from fertilization with a homozygous recessive gamete -Morgan began a specific breeding chromoso+e +sin+ t+ue-breeding fruit flies with normal red eyes and normal wing length (genotype pr pr vg vg ) along with a true-breeding fly with the recessive traits of purple eyes and vestigial (short and crumpled) wings (genotype prpr vgvg) F g1neration were all dihybrid pr pr vg vg and due to dominance all had red eyes and normal wings Morgan then mated F fem1les with homozygous recessive males o if the purple and vestigial genes were carried on different chromosomes, Mendels principle of independent assortment would predict four classes of phenotypes, in a 1:1:1:1 ratio of red eyes, normal wings: purple, vestigial: red, vestigial: purple, normal o counts were much higher than the 1:1:1:1 ratio for two types of offspring; the red, normal and the purple, vestigial Morgans hypothesis to explain this was that the two genes are linked genetically physically associated on the same chromosome o linked genes o the behaviour of these linked genes is explained by chromosome recombination during meiosis the relative frequency of recombinant progeny became Morgans measure of the distance separating genes 11.1b Recombination Frequency Can Be Used to Map Chromosomes -a student in Morgans lab, Alfred Sturtevant, realized that the variation in recombinant offspring frequencies could be used as a means of mapping genes on chromosomes -recombinany offspring frequencies can be used to make a linkage map of a chromosome showing the relative locations of genes -genes farther apart on a chromosome are more likely to have more than one crossover occur between them whereas a single crossover between two genes gives recombinant chromatids, a double crossover between two genes gives the parental arrangement of alleles and is thus undetectable -the unit of a linkage map, called a map unit (mu) is equivalent to a recombinant offspring frequency of 1% map unit also called the centimorgan (cM) -map units are not absolute physical distances they are relative, showing the positions of the genes with respect to each other the frequency of crossing over giving rise to recombinant offspring varies to some extent from one position to another along chromosomes -in recent years, DNA sequencing of whole genomes has supplemented the linkage maps of a number of species shows the precise physical locations of genes right down to the number of base pairs separating them 11.1c Widely Separated Linked Genes Assort Independently -genes can be so widely separated on a chromosome that recombination is almost certain to occur at some point between them in every cell undergoing meiosis -when this is the case, the genes assort independently even though they are on the same chromosome -the map distance separating them will be 50mu (reflecting 50% recombinant offspring) this is the same proportion of recombinant offspring observed when genes are on different chromosomes -linkage between such widely separated genes can still be detected, however, by testing their linkage to one or more genes that lie between them -we now know that some of the genes Mendel studied are actually on the same chromosome ex., although the genes for flower colour and seed colour are actually located on the same chromosome, they are so far apart that frequent recombination between them made them assort independently 11.2 Sex-Linked Genes -in many organisms, one or more pairs of chromosomes are different in males from those in females genes located on these chromosomes, the sex chromosomes, are called sex-linked genes o they are inherited differently in males and females -chromosomes other than the sex chromosomes are called autosomes genes on these chromosomes have the same patterns of inheritance in both sexes 11.2a Females Are XX and Males Are XY in Both Humans and Fruit Flies -in most species with sex chromosomes, females have two copies of a chromosome known as the X chromosome forming a fully homologous pair -males have only one X chromosome -another chromosome, the Y chromosome, occurs in males but not in females has a short region of homology with the X chromosome that allows them to pair during meiosis -each normal gamete produced by an XX female carries an X chromosome -half the gametes produced by an XY male carry an X chromosome and half carry a Y when a sperm cell carrying an X chromosome fertilizes an X-bearing cell, the new individual develops into an XX female when a sperm cell carrying a Y chromosome fertilizes an X-bearing cell, the combination produces an XY male -fertilization is expected to produce males and females with an equal probability of 1/2 11.2b Human Sex Determination Depends on the SRY Gene -one gene carried on the Y chromosome, SRY (for sex-determining region of the Y), appears to be the master switch that directs development toward maleness at an early point in embryonic development -for the first month or so of embryonic development, the rudimentary structures that give rise to reproductive organs and tissues are the same in XX or XY embryos -after 6-8 weeks, the SRY gene becomes active in XY embryos 11.2c Sex-Linked Genes Were First Discovered in Drosophilia -since males and females have different sets of sex chromosomes, the genes carried on these chromosomes can be inherited in a distinctly non-Mendelian pattern called sex linkage arises from two differences between males and females o males have one X chromosome and therefore one allele for each gene on this chromosome (males are hemizygous for X-linked genes, hemi=half), whereas
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