Chapter 11 Genes.docx

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Biological Sciences
Effiette Sauer

Chapter 11 Genes, Chromosomes and Human Genetics 11.1 Genetic Linkages and Recombination  Linked Genes: genes on the same chromosome, phenomenon known as linkage 11.1a Linkage and Recombination  Normal or wild type is a gene with a mutation  The notation for wild-type allele is adding the superscripted + sign 11.1b Recombination Frequency Can be Used to Map Chromosomes  Recombinant offspring frequency can be used to make linkage map of chromsomes showing relative location of genes o i.e. assume all three genes a,b,c are on the same chromosome, crosses reveal a and b recombinants are 9.6% and a and c are 8% recombinants proving that the chromsomes are ordered a -8%- c – 2%- b, total distance from a-b is 9.6% o this is because genes father apart on a chromosomes are more likely to have more than one crossover occur between them  the unit of a linkage map is called a map unit (mu) = to frequency of 1% also = to 1 centimorgan (cM)  they are relative units showing the positions of genes with respect to each other 11.1c Widely Separated 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  For example the gene a and c are located so far apart that they assort independently and show no linkage ab are 23mu apart and bc are 34mu apart thus ac must be 57mu apart, ac recombinant is shown as 50%, thus it is above 11.2 Sex Linked Genes  Sex-linked Genes: genes located on the sex chromosomes, genes are inherited differently on males and females  Autosomes: chromosomes that re not sex chromosomes 1-22 11.2a Female XX and Male XY  Females have two copies of the X chromosomes, forming fully homologous XX pair  Males have one copy of the X chromosome and on Y chromosome, that only appears in male, making them males  Y chromosome has a small amount of genes creating enough homology for the X and Y to pair up during meiosis 11.2b Human Sex determination depends on SRY Gene  One gene on the Y chromosome determines the development towards maleness in embroyonic development (SRY GENE)  Females have two copies of the X chromosome thus having two alleles for a gene  Males only have one copy of the X chromosome  FLIES o P Generation: XRXR x XrY F1: XRXr or XRY F2: XRXR or XRXr or XRY or XrY o P Generation: XrXr x XRY F1: XRXr or XrY F2: XRXr or XrXr or XRY or XrY 11.2d Sex Linked Genes in Humans  Pedigree: chart that shoes all parents and offsprings for as many generations as possible, the sex of the individual in different genes and the presence or absence of a trait o Male – square, female – circle, carrier- dot, colored in – present gene  Carrier: means a person that has a mutant gene that they are heterozygous for but do not phenotypically show the trait 11.2e Inactivation of One X Chromosome Evens out Gene Effects  Barr Body: condensed X chromosome can be seen within the nucleus in cells of females as a dense mass of chromatin  Inactivation occurs in embryonic stage, thus all other cells derive from this cell causing inactivation in all other cells 11.3a Deletion, Duplications, Translocations and Inversions  Deletion: occurs if a broken segment is lost from a chromosome  Duplication: occurs if a segment is broken from one chromosome and inserted into its homologue, added to the al
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