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Genes on Chromosomes

chromosomal theory of inheritance - similar chromosomes pair w/ each other during meiosis 

  • segregation/independent assortment shown by meiosis I
  • number of traits for each organism always more than number of chromosomes
  • Thomas Hunt Morgan - performed experiments w/ red/white eyed flies
    • found evidence of sex-linked inheritance
    • proved that genes for traits are found on chromosomes
  • males - X/Y chromosomes
  • females - X/X chromosomes
  • sex-linked trait - found on the X chromosome

genetic recombination - crossing over mixed up genes further 

  • physical exchange of chromosomal arms
  • crossing over can occur anywhere along the chromosome
  • more likely that crossing over occurs between chromosomes that are far apart
  • genetic map - measures distance between genes in terms of recombination frequency
    • centimorgan - map unit, distance within which a crossover is expected to happen in 1% of gametes
  • three-point cross - involves 3 linked genes (genes so close that they don’t assort independently)
    • wild type - most common allele of a gene, designated by "+"

human genetic map - uses pedigrees and statistics 

  • LOD (log of odds ratio) - ratio of probability that 2 genes are linked to probability that they aren’t linked
  • anonymous markers - genetic markers that can be detected easily but doesn’t cause a phenotype
    • allows for chromosomes to be mapped
  • polymorphisms - differences between individuals in populations

human chromosomes - 46 chromosomes, 23 pairs; divided into 7 groups 

  • autosomes - 22 of 23 pairs, perfectly matched in males and females
  • sex chromosomes - similar in females, dissimilar in males
  • SRY gene - on Y chromosome, plays important role in male sexual development
    • environmental factors can cause changes in sex of adult fishes/reptiles
  • Barr body - inactive X chromosome in females (deactivated after sex is determined)
  • gap conversion - copies over DNA sequence from other chromosome
    • DNA sections that don’t match are completely discarded
    • Y chromosome consists of palindromes, able to fix itself w/o another DNA sequence (reason why males aren’t extinct)

chromosome number - can cause abnormalities and diseases 

  • nondisjunction - failure of sister chromatids to separate correctly
  • aneuploidy - loss/gain of a chromosome
  • monosomics - humans who lost 1 copy of an autosome, don’t survive development
  • trisomics - humans w/ extra copy of autosome, most don’t survive
    • possible to survive w/ 3 copies of chromosome 13, 15, 18, 21, or 22
  • Down Syndrome - trisomy 21, occurs in 1/750 children
    • translocation - process that adds a small part of chromosome 21
    • cancer more common in people w/ Down Syndrome
    • possible link to cancer/Alzheimer’s on chromosome 21
    • chances of giving birth to a child w/ Down Syndrome increases as the mother gets older (eggs have more time to mutate)
  • Klinefelter syndrome - XXY zygote
    • sterile male w/ many female body characteristics
  • Turner syndrome - XO zygote (O means absence of a chromosome)
    • sterile female w/ webbed neck and sex organs that never mature
  • Jacob syndrome - XYY zygote
    • fertile males w/ normal appearances
    • chances for disease 20x higher in mental/penal institutions

genetic counseling - determining if parents are at risk of producing children w/ defects 

  • checks the genetic state of early embryos
  • pedigree analysis - makes it possible to estimate the probability of someone being a carrier from some disease
  • amniocentesis - amniotic fluid withdrawn from uterus during 4th month of pregnancy
    • cells then grown in cultures in the lab
    • ultrasound used to determine position of needle/fetus so that the fetus won’t be harmed
  • chorionic villi sampling - cells removed from chorion (membrane part of placenta)
    • can be used in 8th week of pregnancy
    • faster than amniocentesis
  • karyotype analysis - checks for aneuploidy
  • lack of normal enzyme activity >> disorder present
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