by123 chapter 15 - CHAPTER 15 THE CHOMOSOMAL BASIS OF INHERITANCE Chromosome theory of inheritance a basic principle in biology stating that gens are

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 males to pair and behave like homologous chromosomes during meiosis in the tests.