by123 chapter 15

by123 chapter 15 - CHAPTER 15: THE CHOMOSOMAL BASIS OF...

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CHAPTER 15: THE CHOMOSOMAL BASIS OF INHERITANCE Chromosome theory of inheritance- a basic principle in biology stating that gens are located on chromosomes and that the behavior of chromosomes during meiosis accounts for inheritance patterns Thomas Hunt Morgan did an experiment with fruit flies ( Drosophilia melanosaster ). This species only has 4 chromosomes. He did experiments by breeding them, and looking for mutations. He found that the white eyed mutations occurred on the Y chromosome. Wild type - the phenotype for a character most commonly observed in natural populations, such as red eyes in fruit flies Mutant phenotypes - due to alleles assumed to have originated as mutations in the wild type Linked genes - genes located on the same chromosome that tend to be inherited together in genetic crosses Genes are not assorting independently in real life Crossing over unlinks linked genes Genetic recombination - general term for the production of offspring with combinations of traits that differ from those found in either parent Parental types - offspring that have the same phenotype as one of the parents Recombinant types - offspring that have a different phenotype than the parents When 50% of all offspring are recombinants, there is 50% frequency of recombination Recombination frequencies depend on the distances between genes on a chromosome. The farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency. Distances between genes are expressed in map units. 1 map unit = 1% recombination frequency Chromosomal systems of sex determination o Humans adhere to the X-Y system : In mammals, the sex of an offspring depends on whether the sperm cell contains an X chromosome or a Y. Short segments at either end of the Y chromosome are the only regions that are homologous with the corresponding regions of the X. These homologous regions allow the X and Y chromosomes in
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by123 chapter 15 - CHAPTER 15: THE CHOMOSOMAL BASIS OF...

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