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PRINCIPLES OF CROP PRODUCTION
ABT-320
(3 CREDIT HOURS))
LECTURE 2
LECTURE-WISE COURSE BREAKUP
HEREDITY & VARIATIONS
INHERITANCE OF CHARACTERS
CONTROL OF CHARACTERS IN ORGANISMS
MECHANISM OF INHERITANCE OF CHARACTERS
HEREDITY & VARIATIONS
• Heredity is the phenomenon of inheritance of biological
characters of organisms from parental generations to its
progeny. In this way, characters of organisms including
cultivated plants show a continuum from generation to
generation. This continuum of characters is maintained through
the transmission of hereditary factors called genes. However,
variations that are heritable constitute the raw material for the
origin and development of genetic diversity in any population.
Heritable variations are caused by changes at the level of genes
through mutations and the redistributions and combinations of
such changes.
HEREDITY & VARIATIONS
• Examples of heritable characters: Height of stem, Flower
position, Seed coat color, Seed shape, cotyledon color
• Mendel conducted monohybrid and dihybrid crosses in
Pisum sativum. In monohybrid crosses, he selected one
character pair at a time. From monohybrid crosses, he
observed that a biological character is controlled by a pair of
factors (alleles). He further observed that alleles exist in two
forms - the dominant form and the recessive form.
Dominant forms are capable of controlling the expression of
characters by way of controlling the pathways of particular
biochemical reactions. Whereas, recessive forms are usually
inactive forms of genes, thus resulting in the expression of
the character in the absence of the biochemical reaction.
HEREDITY & VARIATIONS
Mendel observed that since the alleles are seen in pairs in
organisms, three combinations are possible: homozygous
dominant, heterozygous and homozygous recessive.
INHERITANCE OF CHARACTERS
(Different Types of Character Expression)
• Genes-Chromosomes
• Mitosis: Equational Division; Meiosis (Reduction Division)
• Dominance: Two alleles present on the homologous loci of a
pair of chromosomes control the expression of characters.
The alleles exist in three form: AA, Aa, aa thus 3 genotypes
and 2 phenotypes for the character.
CODOMINANCE
This is the condition in which a character is controlled by a
pair of alleles and both the alleles are equally dominant. As
a result, a third phenotype which is different from the other
two is shown by the heterozygous organism. ABO blood
group.
INCOMPLETE DOMINANCE
The dominant allele is only incompletely dominant over the
recessive allele. As a result, the heterozygous genotype
produces a third phenotype in which the dominant form of
the character is only incompletely expressed. Red, Pink and
White.
DIFFERENT TYPES OF CONTROL OF
CHARACTERS IN ORGANISMS
Most of the biological characters are controlled by
hereditary factors present inside the nucleus of the cells.
Some of them are controlled by hereditary factors present
inside the cytoplasm. Characters controlled by factors
present inside the nuclei show Mendelian pattern of
inheritance, whereas cytoplasmic characters show nonMendelian pattern of inheritance. Mendelian characters can
be classified into two, based on their nature and genetic
control- discontinuous and continuous characters.
DISCONTINUOUS CHARACTERS
These are also called qualitative characters. They can be
observed by the visual or subjective assessment of quality.
E.g., flower color. No intermediate forms are usually found.
The character is expressed as a result of the occurrence or
non-occurrence of a biochemical reaction. The character and
the factors controlling it show discrete distribution. Usually,
only presence or absence of the hereditary factor
determines the phenotype of the organism.
CONTINUOUS CHARACTERS
These are also called quantitative characters. They are usually
observed by measurement (for example, plant height, fruit
weight etc). Intermediate forms can be observed. The character
is controlled by a number of hereditary factors, each factor
contributing its share towards the expression of the character.
The character shows continuous distribution. It is not the
presence or absence of alleles, but the number of dominant
alleles present, which determines the nature of the characters.
Plant height in most of the crop plants, fruit size and number in
crop plants etc are the examples of quantitative characters.
CYTOPLASMIC CHARACTERS
These are the characters controlled by hereditary factors
present in the cytoplasm. This type of characters do not show
Mendelian pattern of inheritance. They are inherited from the
maternal parent to the progeny through the cytoplasm.
Cytoplasmic characters are usually inherited maternally, i.e.,
from the mother to the progeny. Usually, the cytoplasmic DNA
responsible for the character is transferred from the mother to
the progeny through the egg cytoplasm (e.g., inheritance of
plastid color in plants).
MECHANISM OF INHERITANCE OF
CHARACTERS
INDEPENDENT ASSORTMENT
There are two types of inheritance for chromosomal genesinheritance of genes located on different chromosomes and
inheritance of genes located on the same chromosome.
Genes located on different chromosomes show independent
assortment and genes located on the same chromosome show
joint segregation.
Independent assortment is the phenomenon in which a character
is inherited from the parents to the progeny independent of all
other characters. This type of inheritance is possible only when
the genes for the characters are located on different
chromosomes.
The phenomenon in which there is non-preferential and random
union of alleles of different characters since they are located on
different chromosomes is called independent assortment.
JOINT SEGREGATION
The genes located on a particular chromosome get inherited
together to the progeny. This type of inheritance is called
Joint Segregation. However, a few recombinants are
produced as a result of the genetic recombination that takes
place at the time of crossing over.
MATERNAL INHERITANCE
Maternal inheritance is the inheritance of factors from
mother to the progeny. This takes place in the case of
characters controlled by plasmagenes (cytoplasmic genes).
CHARACTER VARIATIONS
Even though the mechanism of inheritance ensures the
inheritance of characters from parents to the progeny
without change through hereditary factors, very often
variations are shown by the progeny. The reasons for the
occurrence of such variations are many. The most important
reason is the occurrence of mutations. Sexual reproduction
is capable of inducing variations in the progeny by the
reorientation of genes through crossing over resulting in
genetic recombination and also through the union of
gametes originating from different parents.
NATURE AND TYPE OF MUTATIONS
• Mutations are usually classified into two types based on the
reason for occurrence. Mutations occur in nature in a low
frequency without any evident reason. Such mutations are
known as spontaneous mutations.
• Mutations can be induced with the help of certain agents
known as mutagenic agents or mutagens. Mutagens may be
physical or chemical in nature. The major physical mutagens
include different types of non-ionizing and ionizing
radiations whereas the major chemical mutagens include
chemicals like alkylating agents, base analogs, certain dyes
etc. Gamma rays and X rays are the examples of ionizing
radiations whereas UV rays are the examples of non-ionizing
radiations.
THE ROLE OF ENVIRONMENT
Hereditary factors are responsible for the inheritance of
characters from parents to progeny. Mutations and sexual
recombination are responsible for the occurrence of variations.
Environment has also got a very significant role in the expression
and inheritance of characters. The heritability of characters is
highly controlled by environmental factors. Organisms express
optimum levels of characters under ideal environmental
conditions.
THE END