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Chapter 15

Chapter 15. The Chromosomal Behavior of Inheritance. 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

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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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