The Chromosome Theory of Inheritance

Walter Sutton, an American geneticist, studied chromosomes and meiosis in grasshoppers. Theodor Boveri, a German biologist, studied the same in sea urchins. Both achieved great success and helped understand the chromosome theory. 

The Phylogenetic theory proposes that chromosomes, like Mendel’s genes, are homologous pairs in an organism. Genes are homologous pairs too, but they differ in that the members of a pair separate during meiosis. Each sperm or egg receives only one chromosome from each pair, and they are sorted into the individual gametes independently of one another during meiosis. The chromosome theory of inheritance was put forward long before there was any evidence that traits were carried on chromosomes. It was a controversial topic. 

Main Objectives

• The chromosome theory of inheritance means genes are located on chromosomes.
• Phenotypic characters are used to determine whether genes are sex-linked.
• You can use pedigree analysis to differentiate between dominant, recessive, and sex-linked traits.
• You can easily predict the phenotypic ratio and offspring types.

Morgan’s Law 

Thomas Hunt Morgan did many other experiments to confirm an X chromosome location for the colour of the eye gene. Morgan based his analysis of his observations on the assumption that the gene must reside on or be closely associated with the X chromosome. Clinical confirmation of Morgan’s theory came in the form of Bridges’ demonstration that male and female flies with unusual eye colours can be produced when sex chromosomes are not disjoined during meiosis – an exception that proved the rule. 

Linkage 

Genetic linkage takes place when genes, which control two different traits, are situated close to each other on the same chromosome, whereas sex-linkage is a special case of linkage where traits are linked to sex chromosomes. In crossing over between the locations of two genes on a chromosome, alleles for the genes occur in new combinations – a crossover event between two genes on chromosome results in recombination or new combinations of genes. 

Some Basic Aberrations

• Autosomal recessive

  • It affects both males and females equally

  • Alleles must be present in both parents

  • Parents may not display the trait 

  • One-fourth of children are affected

• Autosomal dominant

  • Males and females are equally affected by it

  • Only one parent must carry the allele

  • One parent must also display the trait if the child shows the same

  • One-half of children are affected 

• X-linked recessive

  • Typically affects only males

  • The affected male passes the allele to the daughter and not to the son

  • Trait skips a generation

Location of Gene on Chromosome

The chromosome theory of inheritance was repeatedly tested by scientists before it was even declared true. Thomas Hunt Morgan discovered that chromosomes contain genes based on experiments on fruit flies, Drosophila melanogaster. They resemble human individuals with two X chromosomes for females, whereas individuals with one X and one Y chromosome represent male. 

For Drosophila, red colour of eye in normal flies,represents the dominant trait. It makes perfect sense with a dominant or recessive pattern that Morgan found a recessive mutation (allele) causing red eyes in all pregnancies of a red-eyed female who mated with a white-eyed male.

When Morgan crossed white-eyed females with red-eyed males–he was surprised by the results. It was believed that he would get all red-eyed progeny, but instead, he got all white-eyed females and red-eyed males. This seems to be violating Mendel’s Law of independent assortment. 

The chromosome theory of inheritance was supported by molecular genetic studies because they can only explain how this trait is inherited when the eye colour gene is found on the X chromosome. This proves linkage or gene inheritance on the sex chromosomes (X and Y). As females share the same X chromosome with a male sibling, sex-linked traits show interesting inheritance patterns. The inheritance pattern for males with recessive alleles means that they will always show recessive traits because only one copy of each allele is present in each male. Unlike most genes, however, most autosomal genes are encoded on non-sex chromosomes, where all genes are present in the same number in men and women.

Conclusion

Walter Sutton and Theodor Boveri proposed the Chromosome Theory of Inheritance, stating that chromosomes are the vehicles of genetic heredity. Alfred Sturtevant devised a method for assessing recombination frequency and determining the relative locations and distances of linked genes on a chromosome based on the average number of crossovers in the intervening regions. Mendelian genetics and gene linkage are, however, not exact. Instead, the behaviour of chromosomes is characterised by segregation, independent assortment, and, occasionally, linkage. As Sturtevant correctly hypothesised, genes are arranged in serial order on chromosomes, as well as recombination between homologs having an equal probability anywhere along the chromosome.