Continued breeding experiments, better microscopes, and more scientists working in the field have advanced the knowledge of inheritance in organisms and, at the same time, complicated the simple patterns discovered by Mendel. This article covers some of the intricacies.
Shortly after the genetic community accepted Mendel's Law of Independent Assortment, several exceptions to its operation were found. Most of these exceptions were the result of linkage of the genes being studied on one chromosome. When the usual crosses were made (P 1: parents pure‐line dominant for two traits × pure‐line recessive for two traits), the F 1 individuals of the cross were all dominant and presumably heterozygous. Selfing (transferring pollen from the anthers to the stigma of the same flower) of the F 1 resulted in no predictable ratios, and never the expected 9:3:3:1. Two phenotypes, those of the original P 1 parents, were in high frequency in the F 2 and two other phenotypes, in low frequency, combined the phenotypes of the two original parents. In searching for explanations for the phenomena, the scientists followed the principle of parsimony—that is, they looked first for the simplest explanation that fits all the facts. In this instance, the simplest interpretation—and the correct one—is that the genes for the traits lie close together on the same chromosome.
Linkage and crossing over
Linkage might properly explain the high frequencies of two phenotypes, but what of the low frequency, other combinations? The most logical explanation is that during Prophase I of meiosis when the four chromatids of two homologous chromosomes lie close together, crossing over occurs; that is, there is a physical exchange of material between non‐sister chromatids and a genetic recombination. Thus, unexpected gene frequencies occur because genes no longer travel in their previous sequences. The X‐shaped location of the crossover is called the chiasma (plural, chiasmata), and there may be several in each pair.
If genes lie close together on a chromosome, there is less chance of crossing over taking place than if they lie farther apart. After tabulating the frequency of crossing over for known genes, it is possible to construct linkage maps of the chromosomes and determine approximate locations of genes.
Incomplete dominance occurs when both alleles in a heterozygous individual are expressed, producing a phenotype different from either single allele. For example, red snapdragons crossed with white snapdragons produce pink snapdragons. A cross of two pinks restores the red and white in a 1 red:2 pink:1 white ratio. The dominant allele does not completely mask the recessive in this case. Although the phenotype is changed, the alleles themselves are unaltered, as can be shown by a backcross in which they segregate and express their original trait in the homozygous condition.
A mutation is defined as any change in the DNA of an organism—a sufficiently broad definition to include all manner of changes: deletions (a piece of the chromosome breaks off and is lost), translocations (pieces of material are exchanged between two nonhomologous chromosomes), inversions (two breaks occur and the segment in between rotates and reattaches with its gene sequence in opposite direction to the original), base substitutions (a different base is substituted for the original), duplications (gene sequences are repeated and added to the chromosome), and other changes. Point mutations (gene mutations) are changes in DNA that are limited to one base pair; the gene changes and becomes different from its allele. Chromosome mutations occur when parts of a chromosome, or whole chromosomes, change.
A cell or an organism containing more than two sets of chromosomes is called a polyploid, which most often forms when homologs do not separate at anaphase I in meiosis ( nondisjunction). Gametes produced in this fashion will be diploid (2 n) rather than haploid (1 n). If two of the diploid gametes unite, the resulting individual will be tetraploid (4 n). Tetraploids are able to reproduce because there is an even number of chromosomes to pair at meiosis—there's simply one set too many. If an odd number (triploid, pentaploid, and so forth) results through only partial disjunction or some other deviation, the individual is usually sterile because the extra set of chromosomes lacks a partner (homolog) during meiotic division. Polyploidy in animals is rare because of this. Because plants commonly reproduce vegetatively, however, polyploidy is common in many plant families (and is especially prevalent in the arctic flora). A particular kind of asexual reproduction termed apomixis permits transmission of polyploids through seeds. Apomictic plants form embryos and seeds without fertilization. (Dandelions are apomictic, as are many grass taxa.) Polyploids that form within individuals of the same species are called autopolyploids. Those that are produced when two different species cross are allopolyploids and interspecific hybrids.
Numerous other varieties of interactions occur. Epistasis (epi = upon), for example, results when the action of one gene masks the expression of a different gene. Some plants have multiple alleles for a specific gene. Others have polygenic inheritance in which many genes combine to express a trait. The differences in the trait show a continuous variation because none of the genes have a clear dominance over the others. Genes that influence several phenotypic characteristics are termed pleiotropic genes.