The members of a species give birth to their own kind.
The offsprings show some basic similarities and some
dissimilarities with their parents and with each other. The
sexually reproducing organisms produce sex cells or
gametes. Two gametes usually distinguishable as male and
female fuse to form a zygote which gradually develops into
a young one. The gametes constitute the link between one
generation and the next and pass on the paternal and
maternal characteristics to the offspring. This relationship
that continues to exist between successive generations is
referred to as heredity. Although the offsprings inherits the
characteristics of the parents and resembles them very
closely, the resemblance is not complete in all respects. The
differences are referred to as variations, which are necessary
for organic evolution. The significance of variations shows
up only if they continue to be inherited by the progeny for
several generations. Thus both heredity and variations are
fundamental factors in the process of organic evolution. The
study of the mode of transmission of characters from one
generation to the next, is known as Genetics.
Mendel’s Laws of Inheritance
The Austrian monk Gregor Mendel is considered as the
pioneer of modern genetics and appropriately called father of
genetics. His experiments with Pisum sativum were spread
over 9 years between 1856 and 1864. The approach of Mendel
was simple, logical, scientific, mathematical and analytical.
Mendel decided to experiment with Pisum sativum because it
is an annual and its different varieties possess contrasting
character and hybridization is easy in it. Its flower is complete
and the structure of flower ensures self fertilization. Anthers
can be easily removed before dehiscing and flowers can be
artificially cross fertilised with desired pollen.
Actually Mendel himself did not postulate any genetical
principles or laws. He simply gave conclusive theoretical
and statistical explanation for his hybridization experiments
in his research paper. However, it was Correns who
postulated three laws on the basis of the result of Mendel’s
work:
1.Law of Dominance: “When two plants with contrasting
characters are crossed together only one of these
characters is expressed in the first generation” eg. when
a true breeding tall plant is crossed with a true breeding
dwarfone, all the plants of F1generation have the same height
as the tall parent. Reciprocal cross gives the same result.
2. Law of Segregation: It is also known as “law of purity
of gametes”. The law states that the hybrids or
heterozygous of F1 generation have two contrasting
characters or allelomorphs of dominant and recessive
nature. They do not mix with each other and are
segregated at the time of gametogenesis, so that each
gamete receives only one character either dominant or
recessive.
3.Law of Independent Assortment: When the parents differ
from each other in two or more than two pairs of
contrasting characters or factors then the inheritance of
one pair of factors is independent of the other factors”.
If we crossed a round and yellow seeded plant with a
wrinkled and green seeded one, in F1 hybrids all progenies
have yellow round seeds when the F1 hybrids are allowed to
cross among themselves they produced 4 types of seeds in
the ratio of 9 : 3 : 3 : 1. Where ‘R’ (round) is dominant over
r (wrinkled) alleles and Y (hellow) is dominant over ‘y’
(green) alleles in dihybrid cross.
?
Gene: An inherited factor that determines a biological
character of an organism is called a genes this is a
functional unit of hereditary materials.
?
Allelomorphs or Alleles, Homozygous and
Heterozygous: Alleles the abbreviated form of the terms
‘allelomorphs’ indicates alternative same gene for
example, ‘R’ and ‘r’ are two allelomorphs of the same
gene for flower colour. In pure red or pure white flower
same allele is duplicated (RRandrr) while in hybrid red
flower both the alleles will be present (Rr). An individual
having only one allele or two identical alleles is known
as homozygous (RR or rr). Similarly an individual having
two different alleles will be called heterozygous or
Hybrid (Dd).
?
Monohybrid, Dihybrid and Trihybrid Cross: When a
single character each controlled by a single pair of genes
or alleles are considered while crossing it is known as
monohybrid cross and the F1 ratio of 3:1 is known as the
monohybrid ratio. Similary crosses can be considered
when two or three pairs of genes or alleles are involved.
Such crosses will be called dihybrid and trihybrid crosses
and the respective ratio (9:3:3:1 and 27:9:9:9:3:3:1
respectively) as dihybrid and trihybrid ratios.
?
Genotype and Phenotype: The genotype of an
individual represents sum total of heredity whereas
phenotype represents features which are produced by
interaction between genotype and environment. A
genotype can thus exhibit different phenotypes under
different conditions. Therefore, similar genotypes may
not have the same phenotype. Conversely similar,
phenotypes do not necessarily mean same genotype. In
order to study the interaction of environment and
heredity for the study of effect of different environments
on a genotype would be to have individuals which have
same genotype. This can be done by using clones, pure
lines or inbred lines.
Sex-linked Inheritance
In dioecious individuals there can be two kinds of
characters namely:
1.
Characters which do not show any difference in
reciprocal crosses.
2.
Characters which show a difference in reciprocal crosses.
The former type of characters are located on autosomes
and the latter are either located on specialised chromosomes
known as sex-chromosomes or if located on autosomes are
influenced by or depend on sex of the individual, who carries
it. Traits which are carried on sex chromosomes are known
as sex-linked traits. Other characters whose expression in a
particular genotype depends on whether the individual is
male or female are known as sex linked traits; another class
of traits called sex-limited traits are distinguished by the
fact that one form of this trait be expressed in one sex only
and not in the other.
Sex Lnkage In Human Beings: In human beings, there
are 46 chromosomes (23 pairs) present in each somatic cell.
In female individual there are 22 pairs of autosomes and
one pair of X-chromosomes (22 pairs + XX) and in male
individuals there are 22 pairs of autosomes and one ‘X’ and
one ‘Y’ chromosome (22 pairs + XY). Since female will
produce only one type of gametes, gametes from male
individual will determine the sex of the progeny. In man
about fifty X-linked diseases have been reported. The most
important and common X linked disease of man are:
1.
Colour blindness
2.
Haemophilia
3.
Anhidrotic ectoderma (non-functional sweat glands)
4.
Night blindness
5.
Myopia (short sightedness)
6.
Juvenile glaucoma (hardening of eye ball).
Turner’s Syndrome: In man there are 46 chromosomes
of which 22 pairs are autosomes one pair is sex
Chromosome (XY in male, XX in female). Turner’s
syndrome is characterised by monosomy of XO type i.e.
one X chromosome of sex chromosome (XX) is missing,
the Turner’s syndrome individuals are phenotypically a
female and can be characterised by short stature, webbed
neck, underdeveloped breasts and small uterus.
Klinefelter’s Syndrome: Klinefelter’s syndromes are
characterised by trisomy (XXY) with total number of
chromosomes 47. These are male individuals who are
phenotypically fairly normal but have a very low sperm
count and are therefore sterile. They have female like
breast development, small testes and sparse body hairs.
Colour Blindness: It is sex-linked inheritance found
more often among men than women. It is regulated by
a recessive gene. Individuals suffering from this tail to
differentiate between colours, mainly between red and
green colours.
Down’s Syndrome (Mongolism): It develops due to the
trisomy of the 21st chromosome i.e. representation of the
chromosome thrice instead of twice. It occurs once in
every 500 to 600 child births. The individuals is mentally
retarded and there is no cure for the abnormality.
Haemophilia: This is a rare hereditary blood disorder
marked by a tendency towards excessive bleeding. It is
a sexlinked abnormality and is entirely restricted to
males.
Albinism: This affects skin preventing development of
skin pigments. It happens due to an autosomal recessive
gene Individuals suffering from this abnormality are
found to lack pigmentation of skin, iris, retina, choroid
and hairs. It has been proved that albinism results from failure on the part of the amino acid. Tysosine, to become metabolised to melanin - a black pigment.
The offsprings show some basic similarities and some
dissimilarities with their parents and with each other. The
sexually reproducing organisms produce sex cells or
gametes. Two gametes usually distinguishable as male and
female fuse to form a zygote which gradually develops into
a young one. The gametes constitute the link between one
generation and the next and pass on the paternal and
maternal characteristics to the offspring. This relationship
that continues to exist between successive generations is
referred to as heredity. Although the offsprings inherits the
characteristics of the parents and resembles them very
closely, the resemblance is not complete in all respects. The
differences are referred to as variations, which are necessary
for organic evolution. The significance of variations shows
up only if they continue to be inherited by the progeny for
several generations. Thus both heredity and variations are
fundamental factors in the process of organic evolution. The
study of the mode of transmission of characters from one
generation to the next, is known as Genetics.
Mendel’s Laws of Inheritance
The Austrian monk Gregor Mendel is considered as the
pioneer of modern genetics and appropriately called father of
genetics. His experiments with Pisum sativum were spread
over 9 years between 1856 and 1864. The approach of Mendel
was simple, logical, scientific, mathematical and analytical.
Mendel decided to experiment with Pisum sativum because it
is an annual and its different varieties possess contrasting
character and hybridization is easy in it. Its flower is complete
and the structure of flower ensures self fertilization. Anthers
can be easily removed before dehiscing and flowers can be
artificially cross fertilised with desired pollen.
Actually Mendel himself did not postulate any genetical
principles or laws. He simply gave conclusive theoretical
and statistical explanation for his hybridization experiments
in his research paper. However, it was Correns who
postulated three laws on the basis of the result of Mendel’s
work:
1.Law of Dominance: “When two plants with contrasting
characters are crossed together only one of these
characters is expressed in the first generation” eg. when
a true breeding tall plant is crossed with a true breeding
dwarfone, all the plants of F1generation have the same height
as the tall parent. Reciprocal cross gives the same result.
2. Law of Segregation: It is also known as “law of purity
of gametes”. The law states that the hybrids or
heterozygous of F1 generation have two contrasting
characters or allelomorphs of dominant and recessive
nature. They do not mix with each other and are
segregated at the time of gametogenesis, so that each
gamete receives only one character either dominant or
recessive.
3.Law of Independent Assortment: When the parents differ
from each other in two or more than two pairs of
contrasting characters or factors then the inheritance of
one pair of factors is independent of the other factors”.
If we crossed a round and yellow seeded plant with a
wrinkled and green seeded one, in F1 hybrids all progenies
have yellow round seeds when the F1 hybrids are allowed to
cross among themselves they produced 4 types of seeds in
the ratio of 9 : 3 : 3 : 1. Where ‘R’ (round) is dominant over
r (wrinkled) alleles and Y (hellow) is dominant over ‘y’
(green) alleles in dihybrid cross.
?
Gene: An inherited factor that determines a biological
character of an organism is called a genes this is a
functional unit of hereditary materials.
?
Allelomorphs or Alleles, Homozygous and
Heterozygous: Alleles the abbreviated form of the terms
‘allelomorphs’ indicates alternative same gene for
example, ‘R’ and ‘r’ are two allelomorphs of the same
gene for flower colour. In pure red or pure white flower
same allele is duplicated (RRandrr) while in hybrid red
flower both the alleles will be present (Rr). An individual
having only one allele or two identical alleles is known
as homozygous (RR or rr). Similarly an individual having
two different alleles will be called heterozygous or
Hybrid (Dd).
?
Monohybrid, Dihybrid and Trihybrid Cross: When a
single character each controlled by a single pair of genes
or alleles are considered while crossing it is known as
monohybrid cross and the F1 ratio of 3:1 is known as the
monohybrid ratio. Similary crosses can be considered
when two or three pairs of genes or alleles are involved.
Such crosses will be called dihybrid and trihybrid crosses
and the respective ratio (9:3:3:1 and 27:9:9:9:3:3:1
respectively) as dihybrid and trihybrid ratios.
?
Genotype and Phenotype: The genotype of an
individual represents sum total of heredity whereas
phenotype represents features which are produced by
interaction between genotype and environment. A
genotype can thus exhibit different phenotypes under
different conditions. Therefore, similar genotypes may
not have the same phenotype. Conversely similar,
phenotypes do not necessarily mean same genotype. In
order to study the interaction of environment and
heredity for the study of effect of different environments
on a genotype would be to have individuals which have
same genotype. This can be done by using clones, pure
lines or inbred lines.
Sex-linked Inheritance
In dioecious individuals there can be two kinds of
characters namely:
1.
Characters which do not show any difference in
reciprocal crosses.
2.
Characters which show a difference in reciprocal crosses.
The former type of characters are located on autosomes
and the latter are either located on specialised chromosomes
known as sex-chromosomes or if located on autosomes are
influenced by or depend on sex of the individual, who carries
it. Traits which are carried on sex chromosomes are known
as sex-linked traits. Other characters whose expression in a
particular genotype depends on whether the individual is
male or female are known as sex linked traits; another class
of traits called sex-limited traits are distinguished by the
fact that one form of this trait be expressed in one sex only
and not in the other.
Sex Lnkage In Human Beings: In human beings, there
are 46 chromosomes (23 pairs) present in each somatic cell.
In female individual there are 22 pairs of autosomes and
one pair of X-chromosomes (22 pairs + XX) and in male
individuals there are 22 pairs of autosomes and one ‘X’ and
one ‘Y’ chromosome (22 pairs + XY). Since female will
produce only one type of gametes, gametes from male
individual will determine the sex of the progeny. In man
about fifty X-linked diseases have been reported. The most
important and common X linked disease of man are:
1.
Colour blindness
2.
Haemophilia
3.
Anhidrotic ectoderma (non-functional sweat glands)
4.
Night blindness
5.
Myopia (short sightedness)
6.
Juvenile glaucoma (hardening of eye ball).
Turner’s Syndrome: In man there are 46 chromosomes
of which 22 pairs are autosomes one pair is sex
Chromosome (XY in male, XX in female). Turner’s
syndrome is characterised by monosomy of XO type i.e.
one X chromosome of sex chromosome (XX) is missing,
the Turner’s syndrome individuals are phenotypically a
female and can be characterised by short stature, webbed
neck, underdeveloped breasts and small uterus.
Klinefelter’s Syndrome: Klinefelter’s syndromes are
characterised by trisomy (XXY) with total number of
chromosomes 47. These are male individuals who are
phenotypically fairly normal but have a very low sperm
count and are therefore sterile. They have female like
breast development, small testes and sparse body hairs.
Colour Blindness: It is sex-linked inheritance found
more often among men than women. It is regulated by
a recessive gene. Individuals suffering from this tail to
differentiate between colours, mainly between red and
green colours.
Down’s Syndrome (Mongolism): It develops due to the
trisomy of the 21st chromosome i.e. representation of the
chromosome thrice instead of twice. It occurs once in
every 500 to 600 child births. The individuals is mentally
retarded and there is no cure for the abnormality.
Haemophilia: This is a rare hereditary blood disorder
marked by a tendency towards excessive bleeding. It is
a sexlinked abnormality and is entirely restricted to
males.
Albinism: This affects skin preventing development of
skin pigments. It happens due to an autosomal recessive
gene Individuals suffering from this abnormality are
found to lack pigmentation of skin, iris, retina, choroid
and hairs. It has been proved that albinism results from failure on the part of the amino acid. Tysosine, to become metabolised to melanin - a black pigment.
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