krit.club logo

Principles of Inheritance and Variation - Chromosomal Theory of Inheritance

Grade 12CBSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

•

The Chromosomal Theory of Inheritance was proposed by Walter Sutton and Theodore Boveri in 19021902. They argued that the pairing and separation of a pair of chromosomes would lead to the segregation of a pair of factors (genes) they carried.

•

Parallelism between Chromosomes and Genes: Both occur in pairs in diploid cells. The two alleles of a gene pair are located on homologous sites on homologous chromosomes.

•

Segregation: Homologous chromosomes separate during meiosis (Anaphase IAnaphase\ I) such that each gamete receives only one chromosome of a pair (either maternal or paternal).

•

Independent Assortment: One pair of chromosomes segregates independently of another pair. This occurs during Metaphase IMetaphase\ I and Anaphase IAnaphase\ I of meiosis.

•

Thomas Hunt Morgan provided experimental verification of this theory using Drosophila melanogasterDrosophila\ melanogaster (fruit flies). DrosophilaDrosophila was chosen because they can be grown on simple synthetic medium, have a short life cycle of about 22 weeks, and produce many offspring.

•

Linkage: The physical association of two or more genes on a chromosome. If genes are tightly linked on the same chromosome, they show very low recombination.

•

Recombination: The process of generating non-parental gene combinations during meiosis due to crossing over between homologous chromosomes.

•

Alfred Sturtevant used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes and 'mapped' their position on the chromosome.

📐Formulae

Recombination Frequency (RF)=Number of RecombinantsTotal Number of Progeny×100\text{Recombination Frequency (RF)} = \frac{\text{Number of Recombinants}}{\text{Total Number of Progeny}} \times 100

1% Recombination=1 centiMorgan (cM)=1 Map Unit1\%\ \text{Recombination} = 1\ \text{centiMorgan (cM)} = 1\ \text{Map Unit}

Coefficient of Coincidence=Observed Double CrossoversExpected Double Crossovers\text{Coefficient of Coincidence} = \frac{\text{Observed Double Crossovers}}{\text{Expected Double Crossovers}}

💡Examples

Problem 1:

In a cross between two genes pp and qq on the same chromosome, the following offspring were obtained: Parental types = 760760, Recombinant types = 240240. Calculate the recombination frequency and the map distance between these two genes.

Solution:

First, calculate the total number of offspring: 760+2401000\begin{array}{r} 760 \\ + 240 \\ \hline 1000 \end{array} Now, apply the recombination frequency formula: RF=2401000×100=24%\text{RF} = \frac{240}{1000} \times 100 = 24\% Since 1% recombination=1 cM1\%\ \text{recombination} = 1\ \text{cM}, the distance is 24 cM24\ \text{cM}.

Explanation:

The recombination frequency represents the percentage of offspring that show non-parental phenotypes. This percentage directly correlates to the physical distance between the genes on the chromosome.

Problem 2:

Contrast the behavior of genes and chromosomes during the cell cycle.

Solution:

  1. Genes occur in pairs, as do chromosomes.
  2. Segregation: Both alleles of a gene pair segregate during gamete formation such that only one is transmitted to a gamete. Similarly, homologous chromosomes segregate during meiosis.
  3. Independent Assortment: Different gene pairs (if on different chromosomes) assort independently. Similarly, different chromosome pairs assort independently of each other.

Explanation:

This parallelism was the basis for Sutton and Boveri's theory, linking the Mendelian 'factors' to the physical structures seen under the microscope (chromosomes).