CHROMOSOMAL THEORY OF INHERITANCE

Chromosomal theory of inheritance
Key points:
Origin of chromosomal theory
Contribution of different scientists
Postulates of Chromosomal theory

1. CHROMOSOMAL THEORY OF INHERITANCE Submitted by: Humaira Yahya

2. 01 Key points 02 Contribution of different scientists 03 Postulates of Chromosomal theory Origin of chromosomal theory

3. TIMELINE 1882 1902 Discovered chromosomes in cells of Salamander larvae. Name was given by Waldeyer . Walther Flemming Proposed  chromosome theory of inheritance. Proposing that genes are located on chromosomes. Walter Sutton and Theodor Boveri Three scientist rediscovered Mendel’s work. Carl Correns described relation between hereditary units with chromosomes. Rediscovery of Mendel’s work Mendel published his work on pea plant. Gregor Mendel Experiments on Drosophila discovered gene linkage. Thomas Hunt Morgan 1910 1900 1865

4. Contribution of Scientists Gregor Mendel Walther Flemming Waldeyer Carl Correns Gregor Mendel gave the concept of “hereditary factors” but it was abstract concept as no cellular structure was known that could house for these imaginary units which caused rejection of his work. Another reason of rejection was use of Mathematical calculations in Biology. The emergence of chromosomal theory is linked with the discovery of chromosome by German embryologist Walther Flemming in 1882, while he was examining the rapidly dividing cells of salamander larvae. Waldeyer proposed the term “chromosome” which means colored bodies. In 1900 Mendel’s work was rediscovered by three European scientists, Hugo de Vries, Carl Correns and Erich von Tschermak. For the first time, the relationship of heredity units with chromosomes was put forward by Carl Correns but he had no supportive evidences for this idea.

5. Contribution of Scientists Walter Sutton Theodor Boveri The actual credit of this theory goes to both Walter Sutton and Theodor Boveri these scientist independently proposed hypotheses which then became theory in 1902. Sutton, who was American, studied chromosomes and meiosis in grasshoppers. Boveri, who was German, studied the same things in sea urchins. This theory states that individual genes are found at specific locations on particular chromosomes,  that the behavior of Mendel’s factors (genes) is parallel to the behavior of chromosomes at meiosis. This observation indicates that genes would be present in chromosomes.

6. MENDEL'S WORK Alleles of different genes assort independently during gamete formation (Law of Independent Assortment) Factors (alleles) separate into gametes (Law of Segregation) Factors (genes) come in pairs. One from mother and one from father. CHROMOSOMAL THEORY OF INHERITANCE Different chromosomes assort independently in the gametes Chromosomes from a pair separate during meiosis. One chromosome in each gamete. Chromosomes come in pairs (Homologous pairs) One chromosome from mother and one from father. Parallel behavior of genes and chromosomes during meiosis

7. Contribution of Scientists Many investigators seriously objected on Sutton’s theory. According to that, if we accept that genes are located on chromosome and that genes segregate and assort independently due to segregation and independent assortment of chromosomes in meiosis, why is it that number of genes that assort independently of one another in an organism is much greater than the number of chromosome. In 1910 the objection was cleared after the discovery of linkage by the historical experimentation of T.H. Morgan on Drosophila. T.H. Morgan

8. Experiments on Drosophila Morgan's, chromosome theory-verifying experiments began when he found a mutation in a gene affecting fly eye color. This mutation made a fly's eyes white, rather than their normal red. Morgan found that the eye color gene was inherited in different patterns by male and female flies. Male flies have an X and a Y chromosome (XY), while female flies have two X chromosomes (XX). It didn't take Morgan long to realize that the eye color gene was being inherited in the same pattern as the X chromosome.

9. Experiments on Drosophila F1 all the flies were red eyed irrespective of gender. F2 all the females were red eyed while the male flies were red as well as white eyed.

10. Experiments on Drosophila F2 all the females were red eyed while the male flies were red as well as white eyed. As there were no white eyed females in F2 Morgan concluded that eye color gene is linked to X chromosome. As the inheritance pattern of eye color gene and X chromosome is same in drosophila. Morgan performed several other crosses confirming his discovery. Discovery of linkage was a major breakthrough, because of this discovery theory of Walter and Sutton was accepted which is now known as chromosomal theory of inheritance.

11. Postulates Every cell contains two copies of each kind of chromosome, and there are two copies of each kind of gene. The chromosome complement, like Mendel's genes, appears unchanged as it is transmitted from parents to offspring through generations. During meiosis, homologous chromosomes pair and then separate to different gametes, just as the alternative alleles of each gene segregate to different gametes. Maternal and paternal copies of each chromosome pair move to opposite spindle poles without regard to the assortment of any other homologous chromosome pair, just as the alternative alleles of unrelated genes assort independently. At fertilization, an egg's set of chromosomes unites with a randomly encountered sperm's set of chromosomes, just as alleles obtained from one parent unite at random with those from the other parent. In all cells derived from the fertilized egg, one half of the chromosomes and one-half of the genes are of maternal origin, the other half of paternal origin.

12. The puzzle of inheritance finally got solved.

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