Chromosome theory of inheritance: a chromosome is a long, continuous strand of DNA plus several types of associated proteins and RNA. A specie has a characteristic chromosome number and chromosomes are distinguished by size, centromere position and DNA probe patterns. Chromosome stability and integrity are essential to trait transmission. Chromosomes also include protein specific to all cell types
Gene theory: genes carried on the same chromosome do not follow Mendel’s second law because they do not independently assort in meiosis. Maps are constructed based on how often crossovers occur between two linked genes. The more the crossovers the farther apart the genes in humans, X-linked recessive traits pass from carrier mothers to affected sons and more severe in males (e.g. haemophilia, colour blindness, etc.) because they have only an X chromosome to a female’s two  X-linked dominant traits are rare. In female mammals, one X chromosome is inactivated in each somatic cell early in development.

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