The genes controlling the seven pea characters studied by Mendel are now known to be located on how many different chromosomes?

Question

Accepted Answer

The correct option is 1: Four

The question asks about the modern genetic understanding of Gregor Mendel's seminal experiments with pea plants (*Pisum sativum*). Mendel investigated the inheritance patterns of seven distinct character pairs, such as seed shape (round/wrinkled) and flower color (purple/white). At the time, he formulated the Law of Independent Assortment, which suggested that the inheritance of one trait was independent of another. Although Mendel had no knowledge of chromosomes, modern research confirms that the genes controlling these seven traits are surprisingly clustered onto only **four** of the pea plant's seven homologous pairs of chromosomes. Specifically, the seven genes are found on Chromosome 1, Chromosome 4, Chromosome 5, and Chromosome 7. Mendel's successful observation of independent assortment, despite some genes being on the same chromosome (linked genes), is largely attributed to a stroke of luck: either the linked genes he selected were very far apart on the chromosome, making crossing over highly probable, or he simply did not publish results that showed linkage.

The incorrect option is 2: Two

The option of two chromosomes is factually incorrect as a grouping of two would imply that five of the seven genes were genetically linked and would have resulted in non-Mendelian inheritance patterns. For independent assortment to occur, the genes must either reside on different chromosomes or be very distant from each other on the same chromosome. Placing the seven genes on only two chromosomes would have led to pervasive gene linkage, which Mendel did not report and which would contradict his famous 9:3:3:1 dihybrid cross ratio. Mendel's experiments would have failed to establish his laws of inheritance if the linkage was this extensive, requiring a higher number of chromosomes to explain the independent segregation of his chosen traits.

The incorrect option is 3: Seven

This option, which matches the number of traits Mendel studied, represents a common but incorrect assumption. The pea plant actually has seven pairs of homologous chromosomes in total. For all seven genes to have truly followed the Law of Independent Assortment with no exceptions for linkage, they would ideally have been located one gene per chromosome pair, which is what this option suggests. However, modern genetic mapping revealed that the genes for the seven characters were not distributed across all seven chromosomes; instead, they were concentrated on only four. Therefore, while the pea plant has seven chromosomes, only four of them carry the genes for the characters Mendel analyzed.

The incorrect option is 4: Five

Although there is some historical confusion and early mapping suggested five chromosomes, the most current and accepted genetic analysis confirms that the seven famous genes are located on four chromosomes: 1, 4, 5, and 7. The misconception of five chromosomes sometimes arises from studies attempting to map a broader range of traits, or due to placing one of the closely linked genes on a separate chromosome for simplicity in older diagrams. Therefore, while five is closer to the correct figure than two or seven, it is not the accurate number determined by modern molecular biology for the canonical set of seven traits. Mendel’s key finding of independent assortment can be explained by the four-chromosome arrangement because, even for the genes located on the same chromosome (like Chromosome 1), the physical distance between them was great enough to prevent noticeable linkage.