Introduction

Chapter 2: What's a GENE

Concepts. | Linkage and Single Gene Crossover. | Linkage and double crossover..

Concepts

The genetic material replicates, stores, and expresses information and varies by mutation
Until 1944, observations favored protein as the genetic material
Evidence favoring DNA as the genetic material was first obtained during the study of bacteria and bacteriophages
Indirect and direct evidence supports the concept that DNA is the genetic material in eukaryotes
RNA serves as the genetic material in some viruses
Knowledge of nucleic acid chemistry is essential to the understanding of DNA structure
The structure of DNA holds the key to understanding its function
Alternative forms of DNA exist
The structure of RNA is chemically similar to DNA but is single-stranded
Many analytical techniques have been useful during the investigation of DNA and RNA

Although Mendel's Second Law (or Fourth Postulate) of independent assortment states that genes assort independently, it is more correct to say that genes on different chromosomes assort independently. Genes on the same chromosome do not assort independently; they are said to be linked. In this case, a non-random assortment is observed. Linked genes tend to be inherited together. If crossing over occurs during prophase I of meiosis, even linked genes may assort independently of one another, producing new combinations of genes.

The different ways that two genes can segregate into the gametes are illustrated in the animations below.

Recombination was first observed by Thomas Hunt Morgan and his student Alfred H. Sturtevant. Recombination between chromosomes generates a much greater number of new gene combinations than independent assortment alone.

TOP

Linkage and Double Crossover

The probability of a crossover event occurring between two genes increases with increasing distance between them. The farther apart two genes are, the more DNA there is between them, and thus there is more opportunity for crossing over. It is also possible for multiple crossover events to occur simultaneously. When a double crossover (DCO) occurs, the result is potentially misleading: DCO progeny have a parental phenotype that belies the fact that they are recombinant progeny.

The probability of a double crossover event in a three factor cross can be predicted from map distances. Therefore, the individual probabilities for a crossover event between each gene can be multiplied together to arrive at an expected frequency for the double crossover class. Frequently the number or proportion of observed double crossovers is less than the predicted number of double crossovers. The ratio of observed double crossovers to expected double crossovers (ODCO/EDCO) is termed the coefficient of coincidence. The interference is equal to one minus the coefficient of coincidence (I=1-coc or I=1-ODCO/EDCO). Interference is usually a positive number since the observed number of double crossovers is most frequently lower than expected.

TOP