Chromosomal Theory of Inheritance: The Chromosomal Theory of Inheritance was proposed by Boveri and Sutton, which asserts that genes are located at specific positions on chromosomes and that the action of the chromosomes during meiosis helps explain Mendel’s laws of heredity. Chromosomes are thread-like structures of nucleic acids and proteins found within the nucleus of live cells and are primarily involved in gene-shaped genetic information.

Research in Genetics made remarkable advances in the second half of the nineteenth century. Charles Darwin and Gregor Mendel started exploring possible mechanisms of heredity as early as the 1860s. Later, Theodor Boveri, Walther Flemming, and Walter Sutton made strides in understanding what chromosomes look like and how they move during mitosis over the following decades. In addition, they found a role they play in the transmission of genetic characteristics. In this article, let us learn more about the Chromosomal Theory of Inheritance with interesting Examples. 

Chromosomal Theory of Inheritance: Definition

The chromosomal theory of inheritance states that the genes (or Mendelian Factors) are located at specific loci on the chromosomes, and it is the chromosomes that segregate (or separate) and assort (distribute into groups of the same kind) independently during meiosis and recombine at the time of fertilisation in the zygote.

Chromosomal Theory of Inheritance: Who Proposed It?

The chromosomal theory of inheritance was proposed by Sutton and Boveri independently in \(1902.\) It was experimentally proved by Morgan and his colleagues.

Features of Chromosomal Theory of Inheritance

The salient features of the chromosomal theory of inheritance are as follows:

1. The gametes formed during gametogenesis (i.e., sperms and ova) form a bridge between one generation and the next generation. The sperms and ova carry all the hereditary traits.
2. Both the sperm and egg contribute equally to the heredity of the offspring. The sperm provides only the nuclear part of the zygote, which contains the hereditary characters. The sperm and egg nuclei fuse during the fertilisation to form the zygote.
3. The nucleus contains chromosomes, so the chromosomes must carry the hereditary traits.
4. Every chromosome or chromosome pair has a definite role in developing an individual. Loss of a complete or a part of the chromosome produces structural and functional deficiency in the organism.
5. Like hereditary traits, chromosomes retain their number, structure and individuality throughout the life of an organism and from generation to generation.
6. Both chromosomes, as well as the genes, occur in pairs in the somatic or diploid cells.
7. A gamete contains only one chromosome of a type and only one of the two alleles of a character.
8. The paired condition of both chromosomes and genes is restored during fertilisation.
9. Genetic homogeneity, heterogeneity, dominance and recessiveness can be suggested by chromosomal type and behaviour.
10. Homologous chromosomes synapse during meiosis and then segregate independently into different cells, which establish the quantitative basis of segregation and independent assortment of hereditary characters.
11. In many organisms, the sex of an individual is determined by specific chromosomes called sex chromosomes.

Parallelism Between Mendelian Factor and Chromosome

Sutton and Boveri recognised close parallelism between the Mendelian factor and behaviour of chromosomes during sexual reproduction, like:

1. Chromosomes occur in homologous pairs. Therefore, Mendel also assumed that factors exist in pairs.
2. Synapsed chromosomes segregate during gametogenesis, precisely, meiosis-I. Mendel also suggested the segregation of paired factors during gametogenesis.
   (a) This is Mendel’s law of segregation.
3. The chromosomes of the various homologous pairs are assorted at random, and chromosomes of each pair segregate independently. This is the same behaviour of factors, as Mendel suggested.
   (a) This is Mendel’s law of Independent assortment.
4. The homologous chromosomes from two parents fuse during fertilisation, and thus, the zygote has one homologous chromosome from each parent. Mendel held the opinion that paternal and maternal characters mix up in the progeny.
5. The chromosomes retain their structures and individuality throughout the life cycle of an individual. Mendel also suggested that the characters are not lost, even if they are not expressed.

Law of Segregation

Sutton, along with Boveri, studied the behaviour of chromosomes and found that it was parallel to the behaviour of genes. They used chromosome movements to explain Mendel’s laws. Together they proposed a theory that the chromosomes were the carriers of Mendel’s factors. They observed two important things:

(i) The chromosomes, as well as genes, occur in pairs.
(ii) The two alleles of a gene pair are located on homologous sites on homologous chromosomes.

There exist striking similarities between the behaviour of Mendel’s factors and the behaviour of chromosomes during meiosis and fertilisation. Meiosis during gametogenesis leads to the formation of haploid gametes. This segregation of parental alleles during gamete formation and subsequent syngamy restores the diploid, homologous chromosomes in offspring.

Chromosomal Theory of Inheritance: Law of Independent Assortment

Mendel’s Law of Independent Assortment can now be explained very well by the chromosomal theory as follows:

1. Alleles for seed colour (yellow and green) are present on one homologous pair (chromosome \(1\)).
2. Alleles for seed shape (round and wrinkled) are present on another homologous chromosome pair (chromosome \(7\)).
3. Parents homozygous for yellow and round seeds produce gametes that have one chromosome of each type, and therefore, they have one gene for yellow colour \(\left( {\rm{Y}} \right)\) and one gene for round shape \(\left( {\rm{R}} \right).\)
4. Similarly, parents homozygous for green colour \(\left( {\rm{y}} \right)\) and wrinkled shape \(\left( {\rm{r}} \right)\) seeds produce gametes that have one chromosome of each type, one having \(‘{\rm{r}}’\) gene and the other having \(‘{\rm{y}}’\) gene.
5. Cross-pollination between these two types of parents results in the formation of \({{\rm{F}}_{\rm{1}}}\) heterozygotes having genotype \({\rm{YyRr}}{\rm{.}}\)
6. The \({{\rm{F}}_{\rm{1}}}\) heterozygous parents produce four types of gametes having \({\rm{YR,}}\,{\rm{yR,}}\,{\rm{Yr}}\) and \({\rm{yr}}\) genes.
7. These gametes randomly fuse and give rise to \({{\rm{F}}_2}\) generation with four types of pea plants in a dihybrid ratio of \(9:3:3:1.\)

Chromosomal Theory of Linkage

In \(1911,\) Morgan and Castle proposed chromosome theory of linkage which states that:

(i) Linked genes occur in the same chromosome.
(ii) The genes lie in a linear sequence on the chromosome.
(iii) There is a tendency to maintain the parental combination of genes except for occasional crossovers.
(iv) Strength of the linkage between the two genes is inversely proportional to the distance between the two, i.e.; the linked genes show a higher frequency of crossing over if the distance between them is higher and a lower frequency if the distance is small.

Fig: Explaining Mendel’s Law of Assortment of Genes in Terms of Movement of     Chromosomes During Meiosis.

Summary

Thus, chromosomal theory of inheritance explains that the genes (or Mendelian Factors) are located at specific loci on the chromosomes, and it is the chromosomes that segregate (or separate) and assort (distribute into groups of the same kind) independently during meiosis and recombine at the time of fertilisation in the zygote. Since Mendel’s factors or genes are present on the chromosomes, Mendel’s Law of Segregation and Independent Assortment can be explained better by the chromosomal theory.

FAQs About Chromosomal Theory of Inheritance

Let’s look at some of the commonly asked questions about Chromosomal Theory of Inheritance:

Q.1. What are the features of the chromosomal theory of inheritance?
Ans: The features of the chromosomal theory of inheritance are as follows:
1. The gametes formed during gametogenesis (i.e., sperms and ova) form a bridge between one generation and the next generation. The sperms and ova carry all the hereditary traits.
2. Both the sperm and egg contribute equally to the heredity of the offspring. The sperm provides only the nuclear part of the zygote, which contains the hereditary characters. The sperm and egg nuclei fuse during the fertilisation to form the zygote.
3. Nucleus contains chromosomes, so the chromosomes must carry the hereditary traits.
4. Every chromosome or chromosome pair has a definite role in the development of an individual. Loss of a complete or a part of the chromosome produces structural and functional deficiency in the organism.

Q.2. What does the chromosomal theory of inheritance state?
Ans: The chromosomal theory of inheritance states that the genes (or Mendelian Factors) are located at specific loci on the chromosomes, and it is the chromosomes that segregate (or separate) and assort (distribute into groups of the same kind) independently during meiosis and recombine at the time of fertilisation in the zygote.

Q.3. When are two genes situated very close to each other in a chromosome?
Ans: When the two genes are linked to each other, they are situated close to each other.

Q.4. Name the two laws of Mendel which are explained by the chromosomal theory of inheritance?
Ans: The two laws of Mendel which are explained by the chromosomal theory of inheritance are:
(a) Law of Segregation
(b) Law of Independent Assortment

Q.5. Who proposed the chromosomal theory of inheritance?
Ans: The chromosomal theory of inheritance was proposed by Sutton and Boveri independently in \(1902.\) Morgan and his colleagues experimentally proved it.

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