Tetrad analysis can provide insights into recombination beyond those possible by analysis of products of random gametes. If parental ditype (PD) is much more common than nonparental ditype (NPD), this indicates that the genes are on the same chromosome. If the genes are on the same chromosome, NPD requires a 4 strand double crossover. Tetratype (T) results from a crossover between the two genes if they are on the same chromosome, or between at least one of the genes and the centromere if they are on different chromosomes. Map distance for linked genes is 1/2 X %tetratype asci, if they are close together.
In Neurospora, the phenomenon of ordered tetrads allows genes to be mapped relative to their centromeres. Second division segregation (sds), with a deviation from 4:4 segregation in the asci, results if there is a crossover between the gene and the centromere. Gene-centromere map distance = 1/2 X %sds for genes relatively close to the centromere.
The surprising observation of deviation s from 2:2 segregation in tetrads (e.g., 3:1 ratio) led to discovery of the phenomenon of "gene conversion". Genes on either side of the site of gene conversion were often recombined. The event was thought to reflect what went on during the actual recombination process at the molecular level. Studies at the molecular level have shown that the recombination intermediate ("Chi structure") apparently involves formation of a heteroduplex molecule that may be mispaired at the site of the recombination if alleles differ in that region. Mismatch repair of the heteroduplexes may lead to conversion of an allele into the alternate allele, resulting in a 3:1 ratio.
Changes in chromosome number and structure are important in medicine, agricultural research and evolution. Terms defined: polyloidy, autoployploidy, allopolyploidy, aneuploidy, trisomy, monosomy, duplication, deletion, inversion, translocation.
Chromosomes are studied by blocking dividing cells at metaphase with colchicine or a related chemical and spreading the cells on a microscope slide. Chromosome shapes are metacentric (centromere in middle), submetacentric (centromere nearer one end), acrocentric (centromere close to end). Robertsonian rearrangements are a common type of rearrangement, especially in animals. They involve joining acrocentric chromosomes to form a metacentric chromosome. This changes the chromosome number while keeping arm number constant. An example is the change from gorilla and chimp to human, in which two acrocentric chromosomes joined to form human chromosome 2.
Polyploidy involves change in the number of chromosome sets. This is common in plants. Polyploid plants sometimes are larger or have bigger flowers. They can be induced with colchicine treatments. Polyploidy may occur naturally by failure of cell division in mitosis or meiosis. Monoploid refers to the basic chromosome number in a group in which polyploidy is present. Haploid refers to the number in gametes in a species. Examples of polyploidy include tetraploid (4n), hexaploid (6n) and decaploid (10n). Chromosome segregation is usually good from polyploids with even numbers of chromosome sets, giving haploid numbers in gametes that are one-half of the somatic number. Polyploids with odd numbers (e.g. triploids), however, produce gametes with unbalanced sets. In plants, this can result in sterility (e.g., commercial bananas, seedless watermelons), which may be desirable.