Mendel, Gregor Johann (1822-1884), Austrian monk, whose experimental work became the basis of modern hereditary theory.Mendel was born on July 22, 1822, to a peasant family in Heinzendorf (nowHynčice, Czech Republic). He entered the Augustinian monastery at Brünn(now Brno, Czech Republic), which was known as a center of learning andscientific endeavor. He later became a substitute teacher at the technicalschool in Brünn. There Mendel became actively engaged in investigatingvariation, heredity, and evolution in plants at the monasterys experimentalgarden. Between 1856 and 1863 he cultivated and tested at least 28,000pea plants, carefully analyzing seven pairs of seed and plantcharacteristics. His tedious experiments resulted in the enunciation of twogeneralizations that later became known as the laws of heredity. Hisobservations also led him to coin two terms still used in present-daygenetics: dominance, for a trait that shows up in an offspring; andrecessiveness, for a trait masked by a dominant gene.
Mendel published his important work on heredity in 1866. Despite, orperhaps because of, its descriptions of large numbers of experimentalplants, which allowed him to express his results numerically andsubject them to statistical analysis, this work made virtually noimpression for the next 34 years. Only in 1900 was his workrecognized more or less independently by three investigators, one ofwhom was the Dutch botanist Hugo Marie de Vries, and not until thelate 1920s and the early 30s was its full significance realized,particularly in relation to evolutionary theory. As a result of years ofresearch in population genetics, investigators were able todemonstrate that Darwinian evolution can be described in terms of thechange in gene frequency of Mendelian pairs of characteristics in apopulation over successive generations.Mendels later experiments with the hawkweed Hieracium provedinconclusive, and because of the pressure of other duties he ceasedhis experiments on heredity by the 1870s. He died in Brünn onJanuary 6, 1884.
In cases of incomplete dominance, the inheritance of a dominant and arecessive allele results in a blending of traits to produce intermediatecharacteristics. For example, four-o’clock pink plants may have red, white,or pink flowers. Plants with red flowers have two copies of the dominantallele R for red flower color (RR). Plants with white flowers have twocopies of the recessive allele r for white flower color (rr). Pink flowersresult in plants with one copy of each allele (Rr), with each allelecontributing to a blending of colors.
Another exception to Mendelian genetics involves genes with multiplealleles. Certain traits are controlled by multiple alleles that have complexrules of dominance. In humans, for example, the gene for blood type hasthree alleles: IA, IB, and i. With three alternatives for each member of agene pair, there are six possible combinations of these genes (IAIA,IBIB, ii, IAi, IBi, IAIB). Although there are six possible combinations,humans have only four major blood types: A, B, AB, and O. This resultsbecause both IA and IB dominate over i, but not over each other, so aperson with a gene combination of IAIA or IAi has blood type A. Thegene combinations IBIB and IBi both produce blood type B. IAIB resultsin a blood type AB, and ii results in blood type O.
A significant number of human traits, such as eye color, skin color,height, weight, and muscle strength are typically regulated by more thanone allele in a pattern known as polygenic inheritance. Several thousandalleles, for example, may combine to determine a person’s potential forpole-vaulting, and several hundred may play a role in establishing aperson’s normal weight. Certain diseases may result from mutations inone or more alleles involved in polygenic inheritance. Researchers haveidentified nearly a dozen mutated alleles that are associated withdiabetes mellitus, and a similar number are linked to asthma. Heartdisease may be linked to two or three times that number. Some types ofcancer may be correlated with more than 100 different genes. Polygenicinheritance is quite complex, and the ways in which multiple genesinteract to produce traits are not fully understood.
In his experiments, Mendel was careful to study traits in pea plants whereone trait did not appear to influence another, such as the plant’s height orthe pea’s texture. These two phenotypes (height and texture) occurrandomly with respect to one another in a manner known as independentassortment. Today scientists understand that independent assortmentoccurs when the genes affecting the phenotypes are found on differentchromosomes.An exception to independent assortment develops when genes appearnear one another on the same chromosome. When genes occur on thesame chromosome, they are inherited as a single unit. Genes inherited inthis way are said to be linked. For example, in fruit flies the genesaffecting eye color and wing length are inherited together because theyappear on the same chromosome.
But in many cases, genes on the same chromosome that are inheritedtogether produce offspring with unexpected allele combinations. Thisresults from a process called crossing over. Sometimes at the beginning ofmeiosis, a chromosome pair (made up of a chromosome from the motherand a chromosome from the father) may intertwine and exchange sectionsof chromosome. The pair then breaks apart to form two chromosomes witha new combination of genes that differs from the combination supplied bythe parents. Through this process of recombining genes, organisms canproduce offspring with new combinations of maternal and paternal traitsthat may contribute to or enhance survival.
There are two main categories of gene-mapping techniques: linkage, orgenetic, mapping, a method that identifies only the relative order ofgenes along a chromosome; and physical mapping, more precisemethods that can place genes at specific distances from one another ona chromosome. Both types of mapping use markers in the DNAsequence, detectable physical or molecular characteristics that differamong individuals and that are passed from one generation to the next.
Linkage mapping was developed in the early 1900s by Americangeneticist Thomas Hunt Morgan. By observing how frequently certaincharacteristics were inherited in combination in numerous generations of fruit flies, he concluded that traits that were often inherited in combination must be associated with genes that were near one another on the chromosome. From his studies, Morgan was able to create a rough map showing the relative order of these associated genes on the chromosomes, and in 1933 he was awarded the Nobel Prize in physiology or medicine for his work. Human linkage maps are created mainly by following inheritance patterns in large families over many generations. Originally, thesestudies were limited to inherited physical traits that could be observed easily in each family member. Today, however, sophisticated laboratory techniques allow researchers to create more detailed linkage maps by comparing the position of the target gene relative to the order of genetic markers, or specific known segments of DNA.