Genes on Chromosomes

Section 2: Genes on Chromosomes

Sutton's Hypothesis

In 1903, American geneticist W.S. Sutton (1877-1916) studied the formation process of sperm and egg cells using grasshopper cells. He discovered that the separation of a pair of genetic factors, or alleles, as hypothesized by Mendel, closely resembles the separation of homologous chromosomes during meiosis.

Sutton's Hypothesis

Sutton observed that in a certain species of grasshopper, somatic cells contain 24 chromosomes, while reproductive cells have only 12 chromosomes. The zygote formed by the fusion of sperm and egg cells has 24 chromosomes again. The number of chromosomes in the somatic cells of the offspring is the same as that in the somatic cells of the parents. These 24 chromosomes in the offspring's somatic cells are paired, with a total of 12 pairs. Each pair consists of one chromosome from the father and one from the mother.

From this, Sutton inferred that genes (genetic factors) are carried by chromosomes and transmitted from parent to offspring. In other words, genes are located on chromosomes because the behavior of genes and chromosomes shows a clear parallel relationship:

1. Genes maintain integrity and independence during hybridization. Chromosomes also have relatively stable morphology during the formation of gametes and fertilization.
2. Genes exist in pairs in somatic cells, just as chromosomes do. In gametes, only one of the paired genes is present, just as only one of the paired chromosomes is present.
3. One gene in a pair in somatic cells comes from the father and the other from the mother, as is the case with homologous chromosomes.
4. Non-allelic genes assort independently during gamete formation, similar to the independent assortment of non-homologous chromosomes during the late stage of Meiosis I.

Experimental Evidence for Genes on Chromosomes

American biologist T.H. Morgan (1866-1945) initially expressed skepticism toward Mendel's theory of inheritance and was even more doubtful of Sutton's hypothesis that genes are located on chromosomes, considering it subjective speculation lacking experimental evidence. He was determined to design an experiment to investigate the relationship between genetics and chromosomes, and to understand the nature of genes.

The key question was, what should be used as experimental material? The fruit flies buzzing around decaying fruit caught his attention, and after observation, he realized that fruit flies were an ideal experimental material.

Beginning in 1909, Morgan devoted himself to studying the inheritance of fruit flies. One day, he discovered a male fruit fly with white eyes among a group of red-eyed fruit flies. How was this white-eye trait inherited? He conducted the experiment shown in Figure 2-8.

From the experiment, it was easy to see that, in terms of the contrasting traits of red eyes and white eyes in fruit flies, all F1 flies had red eyes, indicating that white eyes were recessive to red eyes. In the F2 generation, the ratio of red-eyed to white-eyed flies was 3:1, consistent with the law of segregation, suggesting that the red and white eye colors in fruit flies are controlled by a pair of alleles. The difference, however, was that the expression of the white-eye trait was always linked to sex. How could this phenomenon be explained?

During the same period, some biologists had already discovered sex chromosomes in the cells of certain insects. In fruit flies, there are four pairs of chromosomes in somatic cells: three pairs of autosomes and one pair of sex chromosomes (Figure 2-9). In female fruit flies, the pair of sex chromosomes is homologous, denoted as XX, while in male fruit flies, the sex chromosomes are heterologous, denoted as XY.

Since the inheritance of white eyes was linked to sex and similar to X-linked inheritance, Morgan and his colleagues hypothesized that if the gene controlling white eyes (denoted as w) was located on the X chromosome, and if the Y chromosome did not carry an allele for this gene, the observed inheritance pattern could be reasonably explained (Figure 2-10).

Morgan and his colleagues later used test crosses and other methods to further verify these explanations. Their work successfully linked a specific gene to a specific chromosome—the X chromosome—thus experimentally proving that genes are located on chromosomes. From then on, Morgan became a staunch supporter of Mendel's theory.

We know that the number of genes in any organism far exceeds the number of chromosomes. For example, fruit flies have four pairs of chromosomes in their somatic cells, carrying over 13,000 genes, while humans have 23 pairs of chromosomes in their somatic cells, with approximately 26,000 genes. Clearly, there must be many genes on each chromosome. After more than a decade of work, Morgan and his students invented a method for determining the relative positions of genes on chromosomes and produced the first genetic map showing the relative positions of various genes on fruit fly chromosomes. They also demonstrated that genes are linearly arranged on chromosomes (Figure 2-11).

Modern Explanation of Mendel's Laws of Inheritance

Cytogenetic studies have shown that Mendel's concept of a pair of genetic factors corresponds to alleles located on a pair of homologous chromosomes, while different pairs of genetic factors correspond to non-allelic genes located on non-homologous chromosomes.

The essence of the law of segregation is that in the cells of a heterozygote, alleles located on a pair of homologous chromosomes have a certain degree of independence. During meiosis, alleles separate with the separation of homologous chromosomes, entering different gametes and being inherited independently by the offspring.

The essence of the law of independent assortment is that the separation or combination of non-allelic genes located on non-homologous chromosomes is independent of each other. During meiosis, while alleles on homologous chromosomes separate, non-allelic genes on non-homologous chromosomes assort independently.