Chromosomal Theory of Inheritance: The speculation that chromosomes might be the key to understanding heredity led several scientists to examine Mendel’s publications and re-evaluate his model in terms of the behavior of chromosomes during mitosis and meiosis. In 1902, Theodor Boveri observed that proper embryonic development of sea urchins does not occur unless chromosomes are present. That same year, Walter Sutton observed the separation of chromosomes into daughter cells during meiosis. Together, these observations led to the development of the Chromosomal Theory of Inheritance, which identified chromosomes as the genetic material responsible for Mendelian inheritance.

Chromosomal Theory of Inheritance

The Chromosomal Theory of Inheritance was consistent with Mendel’s laws and was supported by the following observations:

• During meiosis, homologous chromosome pairs migrate as discrete structures that are independent of other chromosome pairs.
• The sorting of chromosomes from each homologous pair into pre-gametes appears to be random.
• Each parent synthesizes gametes that contain only half of their chromosomal complement.
• Even though male and female gametes (sperm and egg) differ in size and morphology, they have the same number of chromosomes, suggesting equal genetic contributions from each parent.
• The gametic chromosomes combine during fertilization to produce offspring with the same chromosome number as their parents.

Chromosomal Theory: Linkage and Genetic Recombination

Morgan observed that while crossing a set of characteristics, two genes did not segregate as per Mendel’s law. If two genes were present on the same chromosome, the probability of getting a parental combination was much higher in the next generation compared to the non-parental combination. This physical association of genes was termed as linkage.

The term genetic recombination described the non-parental gene combinations in a dihybrid cross. Once, linked genes were discovered, the frequency of linked genes also influenced the appearance of traits in the next generation. A student of Morgan, Sturtevant discovered the position of linked genes on a chromosome by calculating their frequency of genetic recombination by the process of gene mapping. This method of generating a link map was extensively used during the Human Genome Project.

Important Points on Chromosomal Theory of Inheritance

• Homologous chromosome pairs are independent of other chromosome pairs.
• Chromosomes from each homologous pair are sorted randomly into pre- gametes.
• Parents synthesize gametes that contain only half of their chromosomes; eggs and sperm have the same number of chromosomes.
• Gametic chromosomes combine during fertilization to produce offspring with the same chromosome number as their parents.
• Eye color in fruit flies was the first X-linked trait to be discovered; thus, Morgan’s experiments with fruit flies solidified the Chromosomal Theory of Inheritance.

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By Team Learning Mantras