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Chapter 15

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  1. Chapter 15 The Chromosomal Behavior of Inheritance

  2. Relating Mendelaism to Chromosomes • Relating Mendelasim to Chromosomes • Chromosome theory of Inheritance • Based on observations from Sutton, Boveri, de Vries, and von Seysenegg between mendel’s principles and chromosome behavior • Mendelian factors or genes are located on chromosomes • It is chromosomes that segregate and assort independently

  3. Morgan’s Experimental Design • Thomas Hunt Morgan performed experiments at Columbia University on Drosphila Melanogaster in the 1900’s which provided evidence that the inherited factors are located on chromosomes • Wildtype – normal or most frequently observed phenotype (Cy+= allele for straight wings) • Mutantphenotypes – alternative phenotypes to the wild-type due to mutation (recessive= w); (dominant= Cy)

  4. Discovery of Sex Linkage • Found single white eye male instead of wild type red eye; all females where red eyed • Morgan deduced that eye color is located on the X chromosome because females where able to mask the mutant allele. • Sex-linkedgenes – genes located on sex chromosomes

  5. Genetic Recombination • GeneticRecombination – production of offspring with new combinations of traits different from those found in the parents; results from independent assortment of chromosomes during meiosis • Parentaltypes – progeny whose phenotypes have the same as on or the other of the parents • Recombinants – progeny whose phenotypes differ from either parent

  6. Crossing Over • CrossingOver – the appearance of combinations that should not appear during normal independent assortment • Morgan’s results did not produce linked or unlinked phenotypic ratios

  7. Recombination Data Map • RecombinationDataMap – sequence of genes related to there frequency of occurrence • The farther apart genes are on a chromosome, the higher the probability a crossover will be between them increasing the recombination frequency (Sturtevant) • Centimorgan – a map unit defined as 1% recombinant frequency • LinkageMap – genetic map based on genetic frequencies

  8. Sex Chromosomes • Chromosomal basis of sex in humans • Mammals have the X-Y mechanism (male = XY) (female = XX) • Heterogametic – produces two kinds of gametes; male • Homogametic – sex that produces one kind of gamete; females • SRY – sex-determining region of Y chromosome that triggers testicular development; in the absence of SRY, ovaries form

  9. Sex-Linked Genes • In humans, usually refers to X-liked traits; since the X chromosome is larger than the Y-chromosome. More traits are carried on the X chromosome • Y chromosome traits code only for traits found in males • Fathers pass X-linked alleles to daughters only • Males receive X-alleles from their mother • Fathers cannot pass X-linked traits to their sons • Mothers pass sex-linked alleles to both sons and daughters • Any male receiving a mutant allele from his mother will express the trait

  10. X-inactivation in Female Mammals • In females, most diploid cells have only one fully functioning X chromosome • Barr body – located inside the nuclear envelope. A densely staining object that is an inactivated X-chromosome in female mammalian cells • Chromosome in activation is determined randomly

  11. Errors and Exceptions in Chromosomal Inheritance • Nondisjunction – failure of homologues to separate during meiosis or mitosis • Meiosis errors: • Occur during meiosis I when pairs do not separate • Occur during meiosis II when chromatids do not separate • Meiotic errors: • Occur during embryonic divisions; will pass on to all successive cells • Aneuploidy – having an abnormal number of chromosomes • Polyploidy – a chromosome number that is more than two complete sets • Triploidy – 3n • Tetraploidy – 4n

  12. Errors and Exceptions in Chromosomal Inheritance (continued) • Alteration of chromosome structure • Deletion – loss of a fragment or centromere • Translocation – joining of a fragment chromosome to another nonhomologous chromosome • Inversion – reattach fragment to original homologue except in reverse position

  13. Errors and Exceptions in Chromosomal Inheritance (continued) • Human chromosome disorders due to alteration • Downsyndrome – Trisomy 21 • Patausyndrome – Trisomy 13 • Edwardssyndrome – Trisomy 18 • Klinefeltersyndrome – XXY • Male sex organs with small testes, sterile, feminine body contours, breast enlargement. • Klinefelter syndrome – XYY • Normal small, above average height • TripleXsyndrome – XXX • Fertile, normal phenotype • Turnersyndrome – XO • Short, secondary sex characteristics do not develop, sterile, internal sex organs immature • Cri Du Chat syndrome – deletion on chromosome number 5 • Mental retardation

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