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LEQ: How do the events of meiosis account for Mendel ’ s laws?. 9.18. Chromosome Theory of Inheritance. Genes occupy a specific location on a chromosome Chromosomes undergo segregation and independent assortment.

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chromosome theory of inheritance
Chromosome Theory of Inheritance

Genes occupy a specific location on a chromosome

Chromosomes undergo segregation and independent assortment

slide3

* Segregation occurs when homologous chromosomes are separated during anaphase I.* Independent Assortment occurs when homologous chromosomes line up together during metaphase I.

linked genes
Linked Genes
  • Genes found on the same chromosome
  • Linked genes do NOT independently assort
  • Look and behave like a monohybrid Punnett square
thomas hunt morgan
Thomas Hunt Morgan
  • American embryologist – early 1900’s, studied fruit flies, identified the process of “crossing over” by studying linked genes
  • Why is Drosophila melanogaster a good organism to study?
    • Few chromosomes
    • Small
    • Easy to care for
    • Lots of offspring
    • Quick generation time
how do you get genetic diversity with linked genes
How do you get genetic diversity with linked genes?

Crossing over occurs. Chromatids with new gene combinations are referred to as recombinant chromatids.

recombination frequency
Recombination Frequency

The percentage of recombinants from a cross. In this example Morgan observed 17% recombination frequency.

sex determination systems
Sex Determination Systems
  • X-Y: humans & fruit flies; XX = female; XY = male; sperm determines sex
  • X-O: insects (grasshoppers & roaches) O = absence of a sex chromosome; XX = female; X- = male; sperm determines sex by having an X or having no sex chromosome
  • Z-W: fish, butterflies, & birds; ZZ = male; ZW = female; egg determines sex
  • Number: ants & bees; females are diploid – fertilized eggs; males are haploid – unfertilized eggs
sex linked genes
Sex Linked Genes
  • Genes that are located on sex chromosomes
  • Thomas Hunt Morgan identified sex linked traits by studying eye color in fruit flies
  • He noticed that white eye color is more common in males than females
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