Chromosome Theory of Inheritance & Linkage
Chromosome Theory of Inheritance
Early Research
In 1902, two researchers, Walter Sutton in the United States and Theodor Boveri in Germany, independently observed parallels between the inheritance of Mendelian hereditary factors and the behavior of chromosomes during meiosis and fertilization. They proposed that hereditary factors (genes) were located on chromosomes. This is known as the chromosome theory of inheritance, or the Sutton-Boveri theory.
In 1909, William Bateson introduced the term “genetics” to describe the study of biological inheritance. Wilhelm Johannsen proposed the term “gene” as a substitute for Mendel’s hereditary factor.
Further evidence for the chromosome theory came from the work of McClung, Stevens, and Wilson, who investigated the number of chromosomes in insect cells. They found that males had one pair of chromosomes that were different (heterochromosomes), called the X and Y chromosomes. These were also called sex chromosomes, while the other chromosomes were termed autosomes.
Confirmation of the Chromosome Theory
In 1910, Thomas Hunt Morgan began his work with the fruit fly (Drosophila melanogaster). In 1911, he found that there were four groups of linked characters, leading him to assume that genes were located on chromosomes. Genes on the same chromosome tend to be inherited together, a phenomenon he called linkage. According to Morgan, genes are arranged linearly on chromosomes, and homologous recombination occurs through the crossing over of chromatids.
Linkage and Chromosomal Maps
Linkage is the tendency of genes on the same chromosome to be inherited together. When studying the phenotypic ratios of offspring, it was observed that certain characters appeared together more frequently than expected by chance if they were inherited independently. Linkage is defined as the deviation from the independent segregation ratios observed in a test cross.
The higher the frequency of recombinant individuals (i.e., the higher the recombination frequency), the greater the distance between the linked genes. The order of genes on chromosomes and the distances between them are represented in a chromosome map.
Inheritance of Sex
The Concept of Sex
Sexual organisms are capable of producing sexual cells (gametes), which are cells with half the number of chromosomes that can fuse to form a new individual. Individuals that produce motile gametes are called males, and those that produce sessile gametes are called females.
Sexual organisms have two types of sexual characters: primary sexual characters (essential for reproduction) and secondary sexual characters (aspects that differentiate males from females but are not essential for reproduction).
The sex of individuals can be determined in various ways depending on the organism. In diploid organisms, the most common mechanism is through a pair of chromosomes called sex chromosomes (heterochromosomes).