Heredity
Learning time: 45 minutes
Does it look like science fiction?
What is
genetics?
Genetics is the branch of biology that
deals with heredity and variation.
Look at yourself?Do you have basic
design of your parents?
Offsprings have similar body design
Characters and traits
Eye colour
black/blue
Height
tall/short
Hair colour
grey/black
Shape of ear
Attached ear lobe or free
What is
heredity?
Heredity is defined as transmission of
character or traits from parents to
offspring.
What is
variation?
The differences in characters of
parents and offspringʼs is known as
variation.
Accumulation of variations(1)
Why do fruits, for example, bananas look alike?
This is because most of the fruits reproduce asexually, which typically involves only one
parent. During asexual reproduction, the genetic makeup of only a single parent is
transferred, which is why we do not see much of variation in their physical
characteristics. However, we do see slight changes produced because of minor
inaccuracies while copying of DNA.
Accumulation of variations(2)
But in sexual reproduction, we see a huge variety amongst organisms because it
involves the fusion of genes from both parents. This fusion gives rise to innumerable
combinations to the very same gene set resulting in organisms looking different from
their parent. The variation that is accumulated, is passed on over the years, which
results in noticeable changes from the past few generations.
The basis for evolutionary processes is, however, dependant on the selection of variants
by environmental factors.
Variation, as we have seen, is significant in the field of science. It results in the formation
of new species that can adapt far more efficiently than their predecessors.
DNA -The Blueprint
DNA makes us to look like our parents.DNA is the genetic material that transferred
from our parent to offspring.It occurs in the majority of the living organisms such as
viruses,prokaryotes and eukaryotes.
Prokaryotes have circular DNA called nucleoid where as viruses contain single
stranded,circular or double stranded DNA.Prokaryotes also have extra chromosomal DNA
called plasmids.
Eukaryotes have their DNA in their nucleus and some organelles.DNA combined with
histone proteins to form chromatin.Chromatin condense into chromosomes during the cell
division.DNA is also present in some organelles like chloroplast,mitochondria.
All these DNA molecule differs in occurrence in various organisms but they are similar in
structure.
DNA Structure
Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA) are the two types of nucleic acid
found in living systems. Nucleic acids are polymers of nucleotides.
(i) A nucleotide has three parts, i.e. a nitrogenous base, a pentose sugar (deoxyribose in
DNA and ribose in RNA) and a phosphate group.
(ii) Nitrogenous bases are purines, i.e. adenine, guanine and pyrimidines, i.e. cytosine,
uracil and thymine.
(iii) Cytosine is common for both DNA and RNA and thymine is present in DNA. Uracil is
present in RNA at the place of thymine.
(iv) A nitrogenous base is linked to the pentose sugar through a N-glycosidic linkage to form
a nucleoside, i.e. adenosine and guanosine, etc.
DNA Structure
(v) When a phosphate group is linked to 5′ —OH of a
nucleoside through phosphodiester linkage, a
corresponding nucleotide is formed.
(vi) Two nucleotides are linked through 3′ -> 5′
phosphodiester linkage to form a dinucleotide.
(vii) Several nucleotides can be joined to form a
polynucleotide chain.
(viii) The backbone in a polynucleotide chain is formed
due to sugar and
phosphates.
DNA replication
DNA replication is the biological process in which two identical copies of
DNA are produced from one original DNA molecule.
Asexual reproduction usually results in the production of genetically identical offspringʼs;
the only genetic variation arises as a result of occasional inaccuracies in DNA replication (or
DNA copying) at the time of cell division.
Since it is a biochemical process, DNA replication sometimes results in variations in the
new molecule. These changes may be viable or non-viable.
Viable changes are seen as changes in the offspring, while non-viable changes result in
death of the new cell..
This inbuilt tendency for variation is the basis for heredity and evolution
Hereditary material in Nucleus
The chromatin is a thin, thread-like intermingled mass of chromosome material
composed of genetic substance DNA (deoxyribonucleic acid) and proteins (i.e.,
histones)
The chromosomes is made up of DNA and proteins. The chromatin is condensed into
two or more thick ribbon-like chromosomes during the division of cell.
Functional segments of DNA are called genes. Genes carry information for protein
synthesis, which in turn control the development of characters. Since genes are present
on chromosomes and chromosomes pass from one generation to the next through,
genes are called hereditary units.
Packaging of DNA
DNA - Hereditary unit
Histones - Packaging proteins.
Nucleosomes - DNA and proteins
after packaging.
Chromatin - Thin entangled network
of DNA and proteins.
Chromosome - Condensed form of chromatin.
Terms
1. Contrasting characters: A pair of visible characters such as tall and dwarf, white and
violet flowers, round and wrinkled seeds, green and yellow seeds etc.
2. Dominant trait: The character which expresses itself in a (F1) generation is dominant
trait. Example : Tallness is a dominant character in pea plant.
3. Recessive trait: The character which does not express itself but is present in a
generation is recessive trait. Ex. dwarfism in the pea plant.
4. Homozygous: A condition in which both the genes of same type are present for
example; an organism has both the genes for tallness it is expressed as TT and genes for
dwarfness are written as tt.
Terms
5. Heterozygous: A condition in which both the genes are of different types for example;
an organism has genes Tt it means it has a gene for tallness and the other for dwarfness
only tall character is expressed.
6. Genotype: It is genetic make up of an individual for example; A pure tall plant is
expressed as TT and hybrid tall as Tt.
7. Phenotype: It is external appearance of the organism for example; a plant having Tt
composition will appear tall although it has gene for dwarfness.
Mendel and his contribution
G.J. Mendel started his work on Pisum sativum (garden pea). He was known as Father of
genetics. He had chosen seven pair of contrasting character-
Pisum sativum
The reason of choosing garden pea for
experiment was●
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Short life cycle
Large number of seeds produced
Self-pollination
Several contrasting characters
can be found
Monohybrid cross
When one pair of contrasting characters was taken to cross two pea plants, it is known
as monohybrid cross.
He took pure tall (genotype TT) and
pure dwarf (genotype tt) pea plants and
cross pollinated them to obtain first
generation or first filial generation. In
this figuration (F1 generation)
he obtained only tall plants. This meant that only one of the parental traits was seen,
not the mixture of the two.
Monohybrid cross (2)
The plants of F1 generation or progeny are then self pollinated to obtain F2 generation
or progeny. Now all plants were not tall. He obtained 75% tall plants and 25% dwarf
plants i.ee. the phenotypic ratio was 3:1.
Mendel noted that the dwarf trait of the
parent pea plant which had seemingly
disappeared in the first generation progeny
reappeared in the second generation. In
this way, Mendelʼs experiments with tall and
dwarf pea plants showed that the traits may
be dominant and recessive.
Monohybrid cross(3)
Mendel noted that all the pea plants produced in the F2 generation were either tall or
dwarf. There were no plants with intermediate height (or medium height) in-between
the tall and dwarf plants. In this way, Mendelʼs experiment showed that the traits (like
tallness and dwarfness) are inherited independently. This is because if the traits of
tallness and dwarfness had blended (or mixed up), then medium sized pea plants would
have been produced.
Result of monohybrid cross
Monohybrid cross
This indicates that in the F, generation both tall and dwarf traits were inherited but
tallness expressed it self. Tallness is a dominant trait and dwarfness is a recessive trait.
F2 generation has a genotypic ratio of 1 : 2 : 1 of three types of plants represented by TT,
Tt and tt as shown in the cross.
Conclusion: Phenotypic ratio—Tall : Dwarf 3 : 1
Genotype ratio—Pure Tall : Hybrid Tall : Pure Dwarf 1 : 2 : 1
Law of Dominance: When parents having pure contrasting characters are crossed then
only one character expresses itself in the F1 generation. This character is the dominant
character and the character/factor which cannot express itself is called the recessive
character.
Dihybrid cross
Mendel also carried out experiments to observe inheritance of two pairs of contrasting
characters, which is called dihybrid cross.
He cross breed pea plants bearing round green seed with plants bearing wrinkled and
yellow seeds. In the F1 generation he obtained all round and yellow seeds it means
round and yellow traits of seeds are dominant features while wrinkled and green are
recessive.
He self-pollinated the plants of F1 generation to obtain F2 generation, he obtained four
different types of seeds round yellow, round green, wrinkled yellow and wrinkled green
in the ratio of 9 : 3 : 3 : 1. He concluded that traits are independently inherited.
Dihybrid cross
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Round and yellow seeds-9.
Round and green seeds-3.
Wrinkled and yellow seeds-3.
Wrinkled and green seeds-1
How are traits transmitted to progeny?
The characteristics or traits of parents are transmitted to their progeny (offsprings)
through genes present on their chromosomes during the process of sexual
reproduction.
As genes work in pairs one is dominant and other one is recessive and each parent
possesses a pair of genes for each characteristic on a pair of chromosomes.
Thus, the male and female gamete carries one gene for each characteristic from the
gene pairs of parents. But when male and female gamete fuses during fertilization
zygote is formed, which grows and develops to form a new organism having
characteristics from both parents which it has inherited through genes.
How do genes control the Traits?
The progeny inherits two genes or a pair of genes for each trait from its parents but
the trait shown by the progeny depends on which inherited gene is dominant of the
two.
How do genes control the characteristics or Traits?
A gene is the section of DNA on a chromosome which codes for the formation of a
protein controlling a specific characteristic of the organism. Suppose a plant progeny
has gene for the characteristic called ʻtallnessʼ. Now, the gene for tallness will give
instructions to the plant cells to make a lot of plant growth hormones. And due to this,
the plant will grow too much and hence become tall and if the plant has a set of gene
for dwarfness, then less plant growth hormone will be produced and remain short
and become dwarf. Just like plants the characteristics in animals are also transmitted
from the parents through genes by the process of sexual reproduction.
Mendel’s Law
Law of Dominance: If the two alleles at a locus differ, then one, the dominant allele,
determines the organismʼs appearance; the other, the recessive allele, has no noticeable
effect on the organismʼs appearance.
Law of Segregation: The two alleles for a heritable character separate (segregate) during
gamete formation and end up in different gametes.
Law of Independent Assortment: Each pair of alleles segregates independently of other
pairs of alleles during gamete formation
Sex determination
Autosomes: Those chromosomes which do not play any role in sex determination.
Sex chromosomes: Those chromosomes which play a role in determining sex of the
newborn.
In human beings, all chromosomes are not paired. 22 chromosomes are paired but one
pair called sex chromosome is odd in not having a perfect pair in males.
Females have a perfect pair both represented by XX. On the other hand males have a
normal sized X but the other is short called Y so it is shown as XY.
All gametes or ova formed by the female are similar i.e. have X chromosome.
Males form two types of sperms i.e. half with X chromosome and the other half with Y
chromosome.
Sex determination
If the sperm having X chromosome fertilizes
with ovum with X chromosome then the
baby will have XX chromosome and it will
be female.
If the sperm having Y chromosome fertilizes
with ovum with X chromosome then the
baby will have XY chromosomes and it will
be male.
Thank you….