Question

Several of the traits examined by Mendel in his dihybrid crosses are now known to be encoded by genes that occur on the same chromosome. Given the outcome of his experiments, what can you infer about the distance between these genes? How would his results have been different if the traits examined had included plant height and pod shape, which are encoded by genes that are very close together on the same chromosome? Use the following terms in your response: alleles; loci; recombination; expected ratio.

Answer 1

Mendel's first law is concerned with the transmission of individual genes in isolation from each other, his second law was formulated in an attempt to codify the manner in which different genes are transmitted relative to each other. In modern terms, Mendel's second law states that the segregation of alleles from any one locus will have no influence on the segregation of alleles from any other locus. In the language of probability, this means that each segregation event is independent of all others and this provides the name for Mendel's second law: "the law of independent assortment."

Mendel’s work suggested that traits are inherited independently of each other. Morgan identified a 1:1 correspondence between a segregating trait and the X chromosome, suggesting that the random segregation of chromosomes was the physical basis of Mendel’s model. This also demonstrated that linked genes disrupt Mendel’s predicted outcomes. The fact that each chromosome can carry many linked genes explains how individuals can have many more traits than they have chromosomes. However, observations in Morgan’s laboratory suggested that alleles positioned on the same chromosome were not always inherited together. During meiosis, linked genes somehow became unlinked.

Homologous recombination is a common genetic process, yet Mendel never observed it. Had he investigated both linked and unlinked genes, it would have been much more difficult for him to create a unified model of his data on the basis of probabilistic calculations. Researchers who have since mapped the seven traits investigated by Mendel onto the seven chromosomes of the pea plant genome have confirmed that all of the genes he examined are either on separate chromosomes or are sufficiently far apart as to be statistically unlinked. Some have suggested that Mendel was enormously lucky to select only unlinked genes, whereas others question whether Mendel discarded any data suggesting linkage. In any case, Mendel consistently observed independent assortment because he examined genes that were effectively unlinked.

The Chromosomal Theory of inheritance, proposed by Sutton and Boveri, states that chromosomes are the vehicles of genetic heredity. Neither Mendelian genetics nor gene linkage is perfectly accurate; instead, chromosome behavior involves segregation, independent assortment, and occasionally, linkage. Sturtevant devised a method to assess recombination frequency and infer the relative positions and distances of linked genes on a chromosome on the basis of the average number of crossovers in the intervening region between the genes. Sturtevant correctly presumed that genes are arranged in serial order on chromosomes and that recombination between homologs can occur anywhere on a chromosome with equal likelihood. Whereas linkage causes alleles on the same chromosome to be inherited together, homologous recombination biases alleles toward an inheritance pattern of independent assortment.

In fact, Mendel's formulation of the basic principles of heredity was not even dependent on an understanding of the fact that genes existed within chromosomes. Rather, the existence of genes was inferred solely from the expression in offspring of visible traits at predicted frequencies based on the traits present in the parental and grandparental generations.

( the frequency of recombination provides a relative estimate of genetic distance. Genetic distances are measured in centimorgans (cM) with one centimorgan defined as the distance between two loci that recombine with a frequency of 1%. Thus, as a further example, if two loci recombine with a frequency of 2.5%, this would represent an approximate genetic distance of 2.5 cM. In the mouse, correlations between genetic and physical distances have demonstrated that one centimorgan is, on average, equivalent to 2,000 kilobases. It is important to be aware, however, that the rate of equivalence can vary greatly due to numerous factors.)

if the two traits are located close to one another on the same chromosome—in other words, if they are linked—the observed ratio will be quite different from that seen for unlinked traits. Allele combinations that began together will tend to stay together, and the offspring will show a skewed ratio reflecting the original combinations.