Chromosome theory of inheritance
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The chromosome theory of inheritance states that chromosomes contain genes and are responsible for Mendel's principles of segregation and independent assortment during meiosis. Thomas Hunt Morgan's experiments with fruit flies led to the discovery of sex linkage, where genes on the X chromosome show different inheritance patterns between males and females. Nettie Stevens' analysis of beetle karyotypes revealed that females have two X chromosomes while males have one X and one Y chromosome, establishing the sex chromosome system. Morgan then used this information to propose X-linked inheritance for white eye color in fruit flies, providing support for the chromosome theory of inheritance.
Chromosome theory of inheritance
- 1. Chromosome Theory of Inheritance Presented by Hargovind laxkar 161103007 M.Sc. Biotechnology
- 2. The Chromosome Theory of Inheritance • The chromosome theory of inheritance arose out of Sutton and Boveri’s careful observations of meiosis. It states that chromosomes are composed of Mendel’s hereditary determinants, or what we now call genes. • The physical separation of alleles during anaphase of meiosis I is responsible for Mendel’s principle of segregation.
- 4. • The genes for different traits assort independently of one another at meiosis I because they are located on different nonhomologous chromosomes, which themselves assort independently. • This phenomenon explains Mendel’s principle of independent assortment.
- 6. Thomas Hunt Morgan’s Experiments • Early in the 20th century, Thomas Hunt Morgan adopted fruit flies (Drosophila melanogaster) as a model organism for genetic research. • Morgan’s first goal was to identify different phenotypes. • He called the most common phenotype for each trait wild-type. • He then inferred that phenotypes that differed from the wild-type resulted from a mutation, or a change in a gene. Individuals with traits attributable to mutation are known as mutants.
- 7. • Morgan identified red eyes as the wild-type for eye color, and white eyes as a mutation. • When he mated a wild-type female fly with a mutant male fly, all of the F1 progeny had red eyes. • However, when Morgan did the reciprocal cross, the F1 females had red eyes but the F1 males had white eyes. • These experiments suggest a relationship between the sex of the progeny and the inheritance of eye color in Drosophila.
- 8. The Discovery of Sex Chromosomes • Nettie Stevens analyzed beetle karyotypes and found that female’s diploid cells contain 20 large chromosomes; but male’s diploid cells have 19 large and 1 small (Y) chromosomes. • X and Y chromosomes are now called sex chromosomes, they determine the sex of the offspring. • In beetles, females have two X chromosomes while males have an X and Y. • Other species have other systems.
- 9. Sex Linkage and the Chromosome Theory • Sex chromosomes pair during meiosis I and then segregate during meiosis II. This results in gametes with either an X or a Y chromosome. • Females produce all X gametes. • Males produce half X gametes and half Y gametes.
- 11. X-Linked Inheritance • Morgan put together his experimental results with Steven’s observations on sex chromosomes, and proposed that the gene for white eye color in fruit flies is located on the X chromosome and that the Y chromosome does not carry an allele of this gene. • Morgan's hypothesis is called X-linked inheritance (or X-linkage). Females (XX) would then have two copies of the gene and males (XY) would have only one.
- 13. X-Linked Inheritance and the Chromosome Theory • The various inheritance patterns that can occur when genes are carried on the sex chromosomes, such that females and males have different numbers of alleles of that gene, is termed sex-linked inheritance or sex-linkage. • Non-sex chromosomes are called autosomes. Genes on autosomes are said to show autosomal inheritance.
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