2. 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 (now
Hynčice, Czech Republic). He entered the Augustinian monastery at Brünn
(now Brno, Czech Republic), which was known as a center of learning and
scientific endeavor. He later became a substitute teacher at the technical
school in Brünn. There Mendel became actively engaged in investigating
variation, heredity, and evolution in plants at the monastery's experimental
garden. Between 1856 and 1863 he cultivated and tested at least 28,000
pea plants, carefully analyzing seven pairs of seed and plant
characteristics. His tedious experiments resulted in the enunciation of two
generalizations that later became known as the laws of heredity. His
observations also led him to coin two terms still used in present-day
genetics: dominance, for a trait that shows up in an offspring; and
recessiveness, for a trait masked by a dominant gene.

3. Mendel published his important work on heredity in 1866. Despite, or
perhaps because of, its descriptions of large numbers of experimental
plants, which allowed him to express his results numerically and
subject them to statistical analysis, this work made virtually no
impression for the next 34 years. Only in 1900 was his work
recognized more or less independently by three investigators, one of
whom was the Dutch botanist Hugo Marie de Vries, and not until the
late 1920s and the early '30s was its full significance realized,
particularly in relation to evolutionary theory. As a result of years of
research in population genetics, investigators were able to
demonstrate that Darwinian evolution can be described in terms of the
change in gene frequency of Mendelian pairs of characteristics in a
population over successive generations.
Mendel's later experiments with the hawkweed Hieracium proved
inconclusive, and because of the pressure of other duties he ceased
his experiments on heredity by the 1870s. He died in Brünn on
January 6, 1884.

5. In cases of incomplete dominance, the inheritance of a dominant and a
recessive allele results in a blending of traits to produce intermediate
characteristics. For example, four-o’clock pink plants may have red, white,
or pink flowers. Plants with red flowers have two copies of the dominant
allele R for red flower color (RR). Plants with white flowers have two
copies of the recessive allele r for white flower color (rr). Pink flowers
result in plants with one copy of each allele (Rr), with each allele
contributing to a blending of colors.

6. Another exception to Mendelian genetics involves genes with multiple
alleles. Certain traits are controlled by multiple alleles that have complex
rules of dominance. In humans, for example, the gene for blood type has
three alleles: IA, IB, and i. With three alternatives for each member of a
gene 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 results
because both IA and IB dominate over i, but not over each other, so a
person with a gene combination of IAIA or IAi has blood type A. The
gene combinations IBIB and IBi both produce blood type B. IAIB results
in a blood type AB, and ii results in blood type O.

7. A significant number of human traits, such as eye color, skin color,
height, weight, and muscle strength are typically regulated by more than
one allele in a pattern known as polygenic inheritance. Several thousand
alleles, for example, may combine to determine a person’s potential for
pole-vaulting, and several hundred may play a role in establishing a
person’s normal weight. Certain diseases may result from mutations in
one or more alleles involved in polygenic inheritance. Researchers have
identified nearly a dozen mutated alleles that are associated with
diabetes mellitus, and a similar number are linked to asthma. Heart
disease may be linked to two or three times that number. Some types of
cancer may be correlated with more than 100 different genes. Polygenic
inheritance is quite complex, and the ways in which multiple genes
interact to produce traits are not fully understood.

8. In his experiments, Mendel was careful to study traits in pea plants where
one trait did not appear to influence another, such as the plant’s height or
the pea’s texture. These two phenotypes (height and texture) occur
randomly with respect to one another in a manner known as independent
assortment. Today scientists understand that independent assortment
occurs when the genes affecting the phenotypes are found on different
chromosomes.
An exception to independent assortment develops when genes appear
near one another on the same chromosome. When genes occur on the
same chromosome, they are inherited as a single unit. Genes inherited in
this way are said to be linked. For example, in fruit flies the genes
affecting eye color and wing length are inherited together because they
appear on the same chromosome.

9. But in many cases, genes on the same chromosome that are inherited
together produce offspring with unexpected allele combinations. This
results from a process called crossing over. Sometimes at the beginning of
meiosis, a chromosome pair (made up of a chromosome from the mother
and a chromosome from the father) may intertwine and exchange sections
of chromosome. The pair then breaks apart to form two chromosomes with
a new combination of genes that differs from the combination supplied by
the parents. Through this process of recombining genes, organisms can
produce offspring with new combinations of maternal and paternal traits
that may contribute to or enhance survival.

10. There are two main categories of gene-mapping techniques: linkage, or
genetic, mapping, a method that identifies only the relative order of
genes along a chromosome; and physical mapping, more precise
methods that can place genes at specific distances from one another on
a chromosome. Both types of mapping use markers in the DNA
sequence, detectable physical or molecular characteristics that differ
among individuals and that are passed from one generation to the next.

11. Linkage mapping was developed in the early 1900s by American
geneticist Thomas Hunt Morgan. By observing how frequently certain
characteristics 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, these
studies 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.