Chromosomal Theory of Inheritance

Chromosome Theory of Inheritance Definition

The chromosomal theory of inheritance states that genetic material is passed from parents to offspring through chromosomes.

What is a Chromosome?

Genes code for every physical and biochemical trait a living organism possesses. Genes are segments of DNA that code for heritable traits. DNA is very long and located in the nucleus of every cell. If we stretch out all the DNA in one cell, it is estimated that it will be over 6 feet long. The cell itself is very small, so to fit 6 feet of DNA into the nucleus of one cell, we need to wrap and coil it. Once the DNA is wrapped, the structure is called a chromosome.

Importantly, humans don't wrap all their DNA into one giant chromosome. Instead, we segment our DNA into multiple smaller chromosomes. Typically, humans have 46 chromosomes in all somatic cells of the body. Each human possesses two copies of each chromosome, one from our mother and one from our father, except for male sex chromosomes (in which a male has one X and one Y chromosome). These two copies are called homologous chromosomes. Therefore, it can be said that a human carries 23 pairs of homologous chromosomes and two copies of every gene.

How do Chromosomes Determine Traits?

While homologous chromosomes carry the same gene, each chromosome may have alternative forms of that gene called alleles. For example, one allele of the dimples gene may code for cheek dimples, while another allele may code for no cheek dimples. Whether or not this child has dimples, the observable trait is called a phenotype, while the genetic combination of alleles for a gene is called a genotype.

Since the chromosomes are responsible for carrying the genetic information, the Chromosomal Theory of Inheritance definition is that genetic material is passed from parents to offspring through chromosomes.

Boveri and Sutton's Chromosome Theory of Inheritance

The idea that chromosomes carry genetic information is a fairly new discovery made in the early 1900s. Although Mendel's laws of Independent Assortment and Segregation were generally accepted and the existence of chromosomes was known, it wasn't until 1902 that Theodore Boveri and Walter Sutton identified chromosomes, not proteins, as the carriers of genetic information from one generation to the next.

Before 1902, it was widely accepted that proteins carried genetic information. The reasoning was that proteins performed all the functions of the cell, therefore, the proteins would be passed on to perform the functions of the cell in the next generation. We now know that is not true, but that was a widely accepted belief. In 1902, however, Boveri observed that an embryo could not develop unless chromosomes were present. In the same year, Sutton observed the separation of chromosomes into daughter cells during meiosis. These two independent observations led to the development of the Chromosome Theory of Inheritance, which pinned chromosomes as the carriers of genetic information from parent to offspring.

Principles of the Chromosome Theory of Inheritance

The Chromosome Theory of Inheritance follows three core principles:

1. Fertilization
2. The Law of Segregation
3. The Law of Independent Assortment

In this section, we will try to explain the results of Mendel's experiments according to chromosome behavior.

Fertilization

Chromosomes from each parent combine during a process known as fertilization. Sperm contains 23 chromosomes from the father, and the egg contains 23 chromosomes from the mother. During fertilization, the chromosomes give rise to the 46 chromosomes in humans. Therefore, just like Mendel's genes were made up of two alleles, chromosomes come in pairs known as homologous chromosomes, with each allele coming from one parent.

The Law of Segregation

The Law of Segregation states that only one of the two alleles is passed onto the gamete randomly. Mendel saw that when he crossed parent peas that were both Yy, he produced offspring with a 3:1 ratio of green to yellow seeds. This meant that each parent pea only passed on only one allele, either Y or y (Fig. 1).

Similarly, most human cells have 46 chromosomes, but a gamete only has 23 chromosomes following meiosis. Since each cell has pairs of each chromosome, the Law of Segregation states that only one of two chromosomes in the pair is passed onto the gamete.

Figure 1. A Punnett square demonstrating the law of segregation.

The Law of Independent Assortment

The Law of Independent Assortment states that the alleles for different genes are sorted into gametes independently from each other. For example, having an allele for dimples does not influence the sorting of alleles for eye color. Mendel's dihybrid cross experiment showed that when he crossed parent peas with an AaBb genotype, the offspring had AB, Ab, aB, and ab alleles in equal ratios suggesting that pea color did not affect the pea-size. Similarly, when genes are located on different chromosomes, each chromosome pair will be selected independently of other chromosome pairs. For example, the segregation of chromosome 1 during meiosis will not influence the segregation of chromosome 2.

Chromosomal Theory of Inheritance Summary

A summary of how Mendel's experiments support the Chromosomal Theory of Inheritance is shown below (Table 1):

Table 1: Summary of how Mendel's experiments are supported in the chromosomal theory of inheritance.

Ultimately, these three principles of the Chromosome Theory of Inheritance explain how genes are passed from one generation to another. Moreover, the theory helps explain the increase in genetic variation from parent to offspring. Without the three principles of the Chromosome Theory of Inheritance, we would be identical clones of our parents.

Chromosomal Theory of Inheritance Examples

While the Chromosomal Theory of Inheritance can help explain Mendelian genetics, it can also explain some unique phenomena of heredity like mutations, sex-linked genes, linked genes, and chromosomal disorders.

Mutations

Mutations are changes in the DNA sequence. Some mutations are silent, meaning that the change in sequence does not cause any phenotypic change. But other mutations are not silent, meaning that they cause an alteration in a trait. While a change in DNA sequence sounds like an impairment, it may actually be beneficial, such as the mutation causing sickle-cell anemia conferring malaria resistance. Regardless, mutations in gametes can be inherited in offspring. Recall that, during meiosis, the law of segregation states that only one of the two alleles is passed onto the gamete at random. If the chromosome passed onto the gamete contains a mutation in a specific gene, that mutation will also be passed onto the offspring. Conversely, the chromosome that does not contain the mutation can also be passed on as well.

Sex-linked Genes

As we discussed, genes are found on chromosomes, including sex chromosomes, responsible for determining our biological sex. Biological females have two X-chromosomes (XX) in humans, while biological males have one X and one Y chromosome (XY). Genes found on the Y chromosome will be inherited only in male offspring, while genes found on the X-chromosome will be inherited by both male and female offspring. Ultimately, if genes are located on the sex chromosome, it changes how genes are inherited between males and females.

Linked Genes

The Law of Independent Assortment states that alleles for different genes are sorted into gametes independently from each other. However, that is not always the case. The Law of Independent Assortment is true for genes on different pairs of chromosomes, but what if two genes are located very close together on the same chromosome?

We know that one chromosome in a chromosome pair is passed onto the gamete during meiosis. However, if the two genes are located close to each other, both of those genes (and their alleles) from the same chromosome will be inherited together. Therefore, if two genes are close together, we should expect to see two genes inherited together much more frequently than expected.

Chromosomal Disorders

Rather than mutations of single genes, there are diseases that are caused by mutations in large regions of the chromosome itself. For example, if a pair of chromosomes is not separated properly during meiosis, it can lead to an event called non-disjunction. Non-disjunction causes a chromosome to be missing or an extra chromosome present in the offspring. These mutations can lead to diseases like trisomy 21, also known as Down's syndrome, in which a person has three copies of chromosome 21.