Download THEORY

Course Name:
PHYSICAL ANTHROPOLOGY
Paper No. & Title:
B.A. / B.Sc. (Honours)
5th Semester
(THEORY)
Topic No. & Title:
(2/13)
Theories of inheritance: Man as an object of Genetical study, Laws
of Heredity- Mendel's principles
Introduction:
All the characters of a living being may be grouped under one or other of two headingseither they are inborn or else they are acquired. Inborn characters may be defined as those which
take origin in the germ cell. Among the higher animals at least offspring invariably differ
inherently from their parents that is, they are born different from what their parents were born
with. They make a different start in life. These congenital differences are technically termed as
"variation." Thus a sixth digit on its first appearance in a family is a variation. It follows, since a
variation takes origin in the germ, that it is not an acquirement, but an inborn trait.
Theories of Inheritance:
Many theories of inheritance have been proposed, and some dates back as far as ancient
Greece.
1. Aristotle proposed the theory of pangenesis which held that particles (called pangenes)
from all parts of the body come together to form the eggs and sperm.
i.
ii.
iii.
Changes that occurred in the various body parts during an organism's life could be
passed on to the next generation.
Pangenesis was accepted by Lamarck and Darwin and was the prevailing theory
into the nineteenth century.
Pangenesis is incorrect because reproductive cells are not composed of
contributions from body cells and changes in body cells do not influence egg and
sperm cells.
2. In the 17th century, Aton van Leeuwenhoek observed the homunculus, a miniature human
being, in human sperm cells. He and his followers (spermists) believed that:
i.
ii.
The mother serves only as an incubator for the homunculus.
All characteristics are inherited from the father.
3. Also during the 17th century, Regnier de Graaf and his followers (ovists) proposed that:
i.
ii.
The egg contains an entire human in miniature and that semen only stimulates its
growth.
All characteristics are thus inherited from the mother. Regnier de Graff was the
first person to describe the ovarian follicle in which human egg cells are produced.
4. Based upon their observations with ornamental plant breeding, scientists in the 19th
century realized that both parents contribute to the characteristics of offspring. The
"blending" theory then became the favoured explanation of inheritance. According to this
theory:
i.
ii.
Hereditary materials from male and female parents mix to form the offspring, and
once blended, the hereditary material is inseparable.
Since the hereditary material is inseparable, the population should reach a uniform
appearance after many generations.
5. This theory was inconsistent with the observations that:
i.
ii.
Populations do not reach a uniform appearance.
Some traits are absent in one generation and present in the next.
Mendelian inheritance or Mendelian genetics or Mendelism:
It is a scientific description of how hereditary characteristics are passed from parent
organisms to their offspring; it underlies much of genetics. Modern genetics began in the 1860's
with the experiments of Gregor Mendel a 19th century Austrian Priest who discovered the
fundamental principles of inheritance. He is often called the "father of genetics." The laws of
inheritance were derived by Gregor Mendel, conducting hybridization experiments in garden
peas (Pisum sativum) (Marantz, 2009). Between 1856 and 1863, he cultivated and tested some
29,000 pea plants. From these experiments he deduced two generalizations which later became
known as Mendel's Principles of Heredity or Mendelian inheritance. He described these
principles in a two part paper, Experiments on Plant Hybridization that he read to the Natural
History Society of Brno on February 8 and March 8, 1865, and which was published in 1866. His
studies showed that there was particulate inheritance of traits according to basic laws of
inheritance. The significance of Mendel's work was not recognized until the turn of the twentieth
century.
Mendel's conclusions were largely ignored. Although they were not completely unknown
to biologists of the time, they were not seen as generally applicable, even by Mendel himself,
who thought they only applied to certain categories of species or traits. A major block to
understanding their significance was the importance attached by 19th century biologists to the
apparent blending of inherited traits in the overall appearance of the progeny, now known to be
due to multigene interactions, in contrast to the organ-specific binary characters studied by
Mendel. In 1900, however, his work was "re-discovered" by three European scientists, Hugo de
Vries, Carl Correns, and Erich von Tschermak. The exact nature of the "re-discovery" has been
somewhat debated: De Vries published first on the subject, mentioning Mendel in a footnote,
while Correns pointed out Mendel's priority after having read De Vries's paper and realizing that
he himself did not have priority. De Vries may not have acknowledged truthfully how much of
his knowledge of the laws came from his own work, or came only after reading Mendel's paper.
Later scholars have blamed Von Tschermak of not truly understanding the results at all.
Mendel's findings allowed other scientists to predict the expression of traits on the basis
of mathematical probabilities. A large contribution to Mendel's success can be traced to his
decision to start his crosses only with plants he demonstrated were true-breeding. He also only
measured absolute (binary) characteristics, such as colour, shape, and position of the offspring,
rather than quantitative characteristics. He expressed his results numerically and subjected them
to statistical analysis. His method of data analysis and his large sample size gave credibility to
his data. He also had the foresight to follow several successive generations (f2, f3) of his pea
plants and record their variations. Finally, he performed "test crosses" (back-crossing
descendants of the initial hybridization to the initial true-breeding lines) to reveal the presence
and proportion of recessive characters. Without his hard work and careful attention to procedure
and detail, Mendel's work could not have had the impact it made on the world of genetics.
Mendel's Laws of Inheritance:
Mendel discovered that when crossing white flower and purple flower plants, the result is
not a blend. Rather than being a mix of the two, the offspring was purple flowered. He then
conceived the idea of heredity units, which he called "factors", one of which is a recessive
characteristic and the other dominant. Mendel said that factors, later called genes, normally occur
in pairs in ordinary body cells, yet segregate during the formation of sex cells. Each member of
the pair becomes part of the separate sex cell. The dominant gene, such as the purple flower in
Mendel's plants, will hide the recessive gene, the white flower. After Mendel self-fertilized the
F1 generation and obtained the 3:1 ratio, he correctly theorized that genes can be paired in three
different ways for each trait: AA, aa, and Aa. The capital "A" represents the dominant factor and
lowercase "a" represents the recessive.
Mendel stated that each individual has two factors for each trait, one from each parent.
The two factors may or may not contain the same information. If the two factors are identical,
the individual is called homozygous for the trait. If the two factors have different information,
the individual is called heterozygous. The alternative forms of a factor are called alleles. The
genotype of an individual is made up of the many alleles it possesses. An individual's physical
appearance, or phenotype, is determined by its alleles as well as by its environment. An
individual possesses two alleles for each trait; one allele is given by the female parent and the
other by the male parent. They are passed on when an individual matures and produces gametes:
egg and sperm. When gametes form, the paired alleles separate randomly so that each gamete
receives a copy of one of the two alleles. The presence of an allele doesn't promise that the trait
will be expressed in the individual that possesses it. In heterozygous individuals the only allele
that is expressed is the dominant. The recessive allele is present but its expression is hidden.
Mendel summarized his findings in three laws; the Law of Segregation, the Law of Independent
Assortment and the law of dominance.
First Law: The Law of Segregation
The Law of Segregation states that every individual possesses a pair of alleles for any
particular trait and that each parent passes a randomly selected copy (allele) of only one of these
to its offspring. The offspring then receives its own pair of alleles for that trait. Whichever of the
two alleles in the offspring is dominant determines how the offspring expresses that trait (e.g. the
colour of a person's eyes).
Mendel's second law: Law of independent assortment
The Law of Independent Assortment, also known as "Inheritance Law" states that
separate genes for separate traits are passed independently of one another from parents to
offspring. That is, the biological selection of a particular gene in the gene pair for one trait to be
passed to the offspring has nothing to do with the selection of the gene for any other trait.
Mendel’s third Law: Law of Dominance
Inheritance of many features (e.g. eye colour, flower colour, seed shape etc.) is controlled
by one pair of genes. When the two genes are of the same kind (e.g. brown colour of eyes, red
colour of flower) the condition is termed homozygous. When a pair of chromosomes has the
gene controlling the same feature (flower colour) in two different forms (red flower gene on one
chromosome and white flower gene on its pair, the condition is termed heterozygous.
Out of a pair of contrasting characters present together, only one is able to express itself while
the other remains suppressed.
His most important principle was the "principle of dominance", that the trait from the dominant
allele is always expressed over the recessive allele.
Conclusion
From the present discussion, it can be said that human genetics is a science that examines
the gene as a unit of storage, transmission, and realization of hereditary information for
development and function of human beings. Many theories of inheritance were reported before
Mendel’s law of inheritance. Mendel's findings allowed other scientists to predict the expression
of traits on the basis of mathematical probabilities and can be applied on human beings. Further,
many scientists have been worked out on genetic mechanisms step by step to molecular level.
The basic principles of human genetics are derived originally from the genetic principles of
plants and animals, as many of these are also valid for man.