Thirty years after the work of Gregor Mendel in the nineteenth century, several rediscoveries of his work in genetics brought his theories to the fore. At about the same time, the discovery of chromosomes, coupled with the earlier knowledge, took genetics in a new direction.
Gregor Mendel (1822-1884) is often considered the founder of the science of genetics. Though his experiments with pea plants became the basis for understanding genetics in all plants and animals, he died unknown. In 1900 three simultaneous “rediscoveries” of Mendel’s studies put his name at the forefront of biology.
With the reintroduction to the world of Mendel’s genetic laws, biologists began to look more closely at genetic phenomena. These researchers used Mendel’s laws as the basis for more in-depth studies of genetics, which led to the modern understanding of genes, chromosomes, and their inheritance.
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Rediscovery of Mendel
In 1900, working independently of one another, biologists Erich Tschermak, Hugo de Vries, and Karl Erich Correns each published data that reasserted Mendel’s historic principles of heredity. Each scientist came to this rediscovery from a slightly different perspective.
Tschermak, an Austrian botanist, coincidentally started pea plant breeding experiments in 1898. He performed these experiments for two years before he accidentally discovered a reference to Mendel’s work from thirty years earlier.When he read Mendel’s papers, Tschermak found that he had duplicated many of Mendel’s breeding experiments, and, embarrassingly, his own work was not as thorough.
The Austrian published his own findings and gave credit to Mendel for performing the original breeding work. Tschermak is known for applying the genetic principles he helped rediscover to developing wheat-rye hybrids and a disease-resistant oat hybrid.
Hugo de Vries, whose primary concern was understanding how evolution worked, was a professor at the University of Amsterdam. De Vries wanted to find a genetic basis for Charles Darwin’s theory of natural selection to understand how species could change over time.
De Vries studied the evening primrose and found that, after cultivating the plant for years, several varieties arose through abrupt, unexplained genetic changes. Based on these changes, he came up with a theory of mutation in which he hypothesized that rapid alterations in organisms could explain how evolution could quickly produce newspecies.
For eight years, starting in 1892, de Vries conducted breeding experiments that led him to the same laws of heredity that Mendel had discovered.When he reported his own work, de Vries was very careful to attribute his concepts to Mendel.
Karl Correns was a German botanist at the University of Tübingen in the 1890’s. By coincidence, Correns conducted breeding experiments with peas that reproduced Mendel’s experiments. In a survey of the literature, Correns found Mendel’s papers, published many years earlier. Much of Correns’s life work was spent in developing additional evidence to support Mendel’s hypotheses.
Correns was the first researcher to suggest that if certain genes were physically close to each other, they might be “coupled” in some way and be consistently inherited in offspring. His concept explained why some traits did not seem to follow Mendel’s law of independent assortment,which stated that all traits separated independently of one another when inherited by offspring.
Chromosomes
Chromosomes were not discovered until the end of the nineteenth century, so Mendel was never able to suggest any physical basis for his genetic theories. It was not until the science of cytology (the study of cells)was founded that scientists started to examine cells and their replication more closely.
They discovered that somatic cells (that is, nonreproductive cells) consistently went through a pattern of division in which chromosomes were duplicated and separated between two new daughter cells.
Walter Sutton and Theodore Boveri, working with grasshopper cells, were the first scientists to notice that chromosomes in somatic cells occur in pairs.Sutton and Boveri suggested a connection between the pairs of chromosomes and Mendelian genetics.
They believed that chromosomes carried the units of inheritance and that the way chromosomes divided accounted for how Mendel’s laws functioned. Their work formed the basis for the chromosomal theory of inheritance.
The chromosomal theory of inheritance suggested that Mendel’s genes reside on chromosomes and that when plants and animals reproduce, half their genetic material comes from each parent, forming sets of chromosomes.
For example, barley has fourteen chromosomes in each somatic cell. Seven of those chromosomes are contributed from the “mother” plant and seven from the “father” plant, to make a total of fourteen chromosomes in the offspring. Therefore, half of the genes from all organisms come from each parent to determine the progeny’s genetic makeup.
Each chromosome is essentially one long, linear strand of deoxyribonucleic acid (DNA), wrapped up and compacted for easy duplication and transport by the cell. There are two copies of each chromosome (called homologous pairs or homologs) in every somatic cell of an organism, each with the same physical appearance.
Such a cell with two copies of each chromosome type is called a diploid cell. Reproductive cells, known as gametes, have half the number of chromosomes and are known as haploid cells. It is these haploid cells from each parent that comprise the new diploid cells of the offspring. Half the chromosomes in each diploid cell come from each parental haploid cell.
Copies of the same gene on each chromosome pair are found at the same location (called a locus) and control traits of the organism. The copies of the same gene at a locus are called alleles.
For example, one copy of chromosome #1 might be from the male parent and have a dominant allele (symbolized by A), and the other copy of chromosome #1 might be from the female parent and have a recessive allele (symbolized by a). These two alleles together (Aa), each on a separate chromosome, would constitute the genotype, and the expression of these alleles produces the phenotype (physical traits of an organism).
Linkage
In 1905 William Bateson and Reginald Punnet were the first to show clear evidence that Correns’s theory of “genetic coupling” was correct. They crossed sweet peas having purple flowers and long pollen grains with sweet peas having red flowers and round pollen grains.
According to Mendel’s rules, the offspring in the second sweet pea generation should have segregated (genetically separated) into four phenotype combinations (purple/long, purple/short, red/long, and red/short) in a ratio of 9:3:3:1, because the two genes controlling these traits should have separated independently of each other. Bateson and Punnet did not obtain a 9:3:3:1 ratio.
Instead, the parental traits stayed together in the offspring more often than expected, and more offspring looked like the parents than expected: purple/long or red/short. Bateson and Punnet called this phenomenon linkage, and any genetic traits that followed this pattern were said to be linked to each other.
In 1910 American geneticist Thomas Hunt Morgan explained the physical basis for linkage. Through his experiments with fruit flies, Morgan found that alleles for different traits only followed Mendel’s law of independent assortment if they were on different chromosomes or if they were very far apart when they were on the same chromosome.
If the genes for two traits were on the same chromosome, they were often passed down to the next generation jointly and stayed together consistently from generation to generation. Morgan further found that the physically closer that two alleles were to each other on a chromosome, the more closely “linked” they were to each other, staying together in off-spring a greater percentage of the time.
Incomplete Dominance
Linkage was one of the first phenomena to break the Mendelian laws, but there were many additional conditions that Mendel would have puzzled over, such as incomplete dominance.
Usually in a heterozygous organism (one with a dominant allele and a recessive allele at the same locus), the phenotype is controlled by the dominant allele, and the trait from the recessive allele will be masked. When incomplete dominance occurs, the dominant trait is weakened, and the heterozygotes look as though they have a trait partway between the recessive and dominant traits.
For example, if a red-flowered snapdragon, RR, is crossed with a white-flowered snapdragon, rr, all the first-generation offspring are heterozygous, Rr. If the trait were dominant, then all the flowers in the offspring would be red. However, the trait displays incomplete dominance, and all the flowers are pink.
Multiple Alleles
Although an individual can have only up to two alleles at a locus,more than two alleles can exist in a population. For example, some populations of red clover are estimated to have hundreds of alleles at a locus for self-sterility.
As a result, most individuals have alleles that are different from those of other members of the population, thus preventing self-pollination and making out-crossing with other plants successful. Some plants can also have more than two alleles at a locus if they are polyploid.
A polyploid plant has more than two homologous chromosomes of each type. The most common type of polyploid is a tetraploid, which has four homologous chromosomes of each type. With four chromosomes of each type, a locus has four alleles instead of just two. Other levels of polyploidy exist in plants, even as much as cases with ten, twenty, or more homologous chromosomes of each type.
Gene Interactions
Gene interactions occur when two or more different loci (gene locations) affect the outcome of a single trait. The most common type of gene interaction is known as epistasis.
Epistasis describes a situation where an allele at one locus masks the phenotypic effects of a different locus. The gene being masked is called the hypostatic gene, while the gene doing the masking is called the epistatic gene.
Bateson and Punnet discovered this phenomenon during their sweet pea breeding experiments. They crossed purple-flowered plants with white-flowered plants. In the first generation, they got all purple-flowered offspring—so they concluded that purple was the dominant gene.
In the next generation, they did not get a 3:1 ratio of purple-flowered to white-flowered plants, as would be expected if purple was dominant. They got a ratio of 9:7 purple to white-flowered plants.
It turned out that there were two different loci involved in the control of petal color: at the first locus C (purple) was dominant to c (white), and at the second locus P (purple) was dominant to p (white). When either locus was homozygous recessive, either cc or pp, the flowers were white, regardless of the genotype of the other locus. The recessive alleles were epistatically affecting (or masking) the dominant alleles.
Polygenic Inheritance
Certain traits are too complex to be controlled by a single locus. These traits are controlled by a complex of two or more loci, a phenomenon known as polygenic inheritance.
In humans, multiple loci control height, intelligence, and skin coloration. These multiple genes lead to continuous variation, meaning that one observes a wide range of phenotypic variation. The first experiment demonstrating continuous variation was conducted by Swedish scientist Herman Nilsson-Ehle in 1910.
He studied the inheritance of the red pigment on the hulls of wheat. He found that red-hulled wheat crossed with white-hulled wheat for several generations gave him plants ranging in pigment from white, light-pink, and pink to medium, basic, and dark-red. Nilsson-Ehle found that three loci control this color variation in the wheat.
Pleiotropy
Pleiotropy also breaks Mendel’s laws. Usually, one locus controls a single trait. A pleiotropic gene is a single locus that controls multiple traits. If there is a loss of function mutation in a pleiotropic gene, the organism is affected inmultiple ways.
One example of such a trait can be found in the plant Arabidopsis thaliana. This plant has a mutant allele known as tu8. This gene was originally isolated as a mutant in the biochemical pathway that makes glucosinolate, a chemical used against pathogens.
This tu8 mutation also causes the plant to be dwarfed, late flowering, and heat-sensitive. Genetic experiments by researchers James Campanella and Jutta Ludwig Mueller have shown that all these traits are controlled by a mutation in a single gene.
Cytoplasmic Inheritance
Finally, cytoplasmic inheritance, often known as maternal inheritance or extra nuclear inheritance, isa phenomenon referring to any genetic traits not inherited from nuclear genes. For example, both chloroplasts and mitochondria contain their own genetic information that is inherited by every generation. This information is inherited in a different fashion from that of nuclear genes.
Nuclear genes, in the form of chromosomes, are donated equally by each parent. The genetic information from chloroplasts and mitochondria is not donated equally. Offspring come from the joining of the male and female gametes, but the size of these gametes differs drastically.
Male gametes, both animal sperm and plant pollen, are often one-one hundredth the volume of an egg cell. Because of their small size, male gametes often have little cytoplasm. Because the chloroplast and mitochondria reside in the cytoplasm, it is usually the case that none of the organellar DNA of the male gamete is included in the offspring.
The female parent alone donates the chloroplast and mitochondrial alleles. Although maternal inheritance is the rule in most plants, a few groups, such as some members of the evening primrose family (Onagraceae) have displayed biparental inheritance of organellar DNA.
