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.

14. THANK YOU