introduction of genetics by DR. Hajare Balaji genetic basis of inheritance moleculae basis of inheritance chromosomal basis of inheritance

1. INTRODUCTION OF
GENETICS

2. DEFINITIONS
 Genetics is the study of genes, and
tries to explain what they are and
how they work. Genes are how
living organisms inherit features
or traits from their ancestors; for
example, children usually look like
their parents because they have
inherited their parents' genes.
Genetics tries to identify which
traits are inherited, and explain how
these traits are passed from
generation to generation.

3. GENETIC BASIS OF
INHERITANCE
 Some traits are part of an organisms' physical
appearance; such as a person's eye-color, height or
weight. Other sorts of traits are not easily seen
and include blood types or resistance to diseases.
Some traits are inherited through our genes, so tall
and thin people tend to have tall and thin children.
 Other traits come from interactions between our
genes and the environment, so a child might inherit
the tendency to be tall, but if they are poorly
nourished, they will still be short. The way our
genes and environment interact to produce a trait
can be complicated. For example, the chances of
somebody dying of cancer or heart disease seems
to depend on both their genes and their lifestyle.

4. MOLECULAR BASIS OF
INHERITANCE
• Genes are made from a long molecule called DNA, which is copied
and inherited across generations.
• DNA is made of simple units that line up in a particular order within
this large molecule. The order of these units carries genetic
information, similar to how the order of letters on a page carry
information.
• The language used by DNA is called the genetic code, which lets
organisms read the information in the genes. This information is the
instructions for constructing and operating a living organism
• The information within a particular gene is not always exactly the
same between one organism and another, so different copies of a
gene do not always give exactly the same instructions.
• Each unique form of a single gene is called an allele. As an example,
one allele for the gene for hair color could instruct the body to
produce a lot of pigment, producing black hair, while a different
allele of the same gene might give garbled instructions that fail to
produce any pigment, giving white hair.
• Mutations are random changes in genes, and can create new
alleles. Mutations can also produce new traits, such as when
mutations to an allele for black hair produce a new allele for white
hair. This appearance of new traits is important in evolution.

5. CHROMOSOMAL BASIS OF INHERITANCE
 The chromosome theory of inheritance is based on afew fundamental principles
1. Chromosomes contain the genetic material 1. Chromosomes contain the
genetic material
2. Chromosomes are replicated and passed along from 2. Chromosomes are
replicated and passed along from parent to offspring parent to offspring
3. The nuclei of most eukaryotic cells contain chromosomes 3. The nuclei of
most eukaryotic cells contain chromosomes that are found in homologous pairs that
are found in homologous pairs
During meiosis, each homologue segregates into one of the two daughter nuclei
4. During the formation of gametes, different types of 4. During the formation of
gametes, different types of (nonhomologous) chromosomes segregate independently
chromosomes segregate independently
5. Each parent contributes one set of chromosomes to
its offspring
The sets are functionally equivalent
Each carries a full complement of genes

6. MENDELIAN WORKJohann Gregor Mendel (1822-1884)
Father of Genetics
Gregor Mendel, through his work on pea plants, discovered the fundamental laws of
inheritance. He deduced that genes come in pairs and are inherited as distinct units, one from
each parent. Mendel tracked the segregation of parental genes and their appearance in the
offspring as dominant or recessive traits. He recognized the mathematical patterns of inheritance
from one generation to the next. Mendel's Laws of Heredity are usually stated as:
1. The Law of Segregation: Each inherited trait is defined by a gene pair. Parental genes are
randomly separated to the sex cells so that sex cells contain only one gene of the pair.
Offspring therefore inherit one genetic allele from each parent when sex cells unite in
fertilization.
2. The Law of Independent Assortment: Genes for different traits are sorted separately from
one another so that the inheritance of one trait is not dependent on the inheritance of another.
3. The Law of Dominance: An organism with alternate forms of a gene will express the form that
is dominant.
The genetic experiments Mendel did with pea plants took him eight years (1856-1863) and he
published his results in 1865. During this time, Mendel grew over 10,000 pea plants, keeping track
of progeny number and type. Mendel's work and his Laws of Inheritance were not appreciated in
his time. It wasn't until 1900, after the rediscovery of his Laws, that his experimental results were
understood.

7. LAW OF DOMINANCE
“In a cross of parents that are
pure for contrasting traits, only
one form of the trait will appear in
the next generation. Offspring that
are hybrid for a trait will have only
the dominant trait in the
phenotype.”

8. LAW OF SEGREGATION
One of these principles, now called Mendel's Law of Segregation, states that allele pairs separate or
segregate during gamete formation, and randomly unite at fertilization.
There are four main concepts related to this principle. They are as follows:
• A gene can exist in more than one form or allele.
• Organisms inherit two alleles for each trait.
• When sex cells are produced (by meiosis), allele pairs separate leaving each cell with a
single allele for each trait.
• When the two alleles of a pair are different, one is dominant, and the other is recessive.
For example, the gene for seed color in pea plants exists in two forms. There is one form or allele for
yellow seed color (Y) and another for green seed color (y). In this example, the allele for yellow seed
color is dominant, and the allele for green seed color is recessive. When the alleles of a pair are
different (heterozygous), the dominant allele trait is expressed, and the recessive allele trait is
masked. Seeds with the genotype of (YY) or (Yy) are yellow, while seeds that are (yy) are green.

9. LAW OF INDEPENDENT
ASSORTMENT
• This law says inheriting an allele has nothing to do with
inheriting an allele for any other trait. The alleles from
parents are passed on independently to the offspring. After
fertilization, the resulting zygote(s) can end up with any
combination of chromosomes from the parents and all the
possible combinations occur with equal frequency.
• Like segregation, independent assortment occurs during
meiosis, specifically in prophase I when the chromosomes
line up in random orientation along the metaphase plate.
Crossing over, the exchange and recombination of genetic
information between chromosomes also occurs in
prophase I and adds to the genetic diversity of the offspring.

10. NEO-MENDELISM
Codominant alleles
Codominant alleles occur when rather than expressing an intermediate phenotype, the
heterozygotes express both homozygous phenotypes.
An example is in human ABO blood types, the heterozygote AB type manufactures antibodies to
both A and B types. Blood Type A people manufacture only anti-B antibodies, while type B people
make only anti-A antibodies.
Codominant alleles are both expressed. Heterozygotes for codominant alleles fully express both
alleles. Blood type AB individuals produce both A and B antigens. Since neither A nor B is
dominant over the other and they are both dominant over O they are said to be codominant.

11. NEO-MENDELISM
Incomplete dominance
Incomplete dominance is a condition when neither allele is
dominant over the other. The condition is recognized by the
heterozygotes expressing an intermediate phenotype relative to the
parental phenotypes. If a red flowered plant is crossed with a white
flowered one, the progeny will all be pink. When pink is crossed with
pink, the progeny are 1 red, 2 pink, and 1 white.
Flower color in snapdragons is an example of this pattern. Cross a
true-breeding red strain with a true-breeding white strain and the F1
are all pink (heterozygotes). Self-fertilize the F1 and you get an F2
ratio of 1 red: 2 pink: 1 white. This would not happen if true blending
had occurred (blending cannot explain traits such as red or white
skipping a generation and pink flowers crossed with pink flowers
should produce ONLY pink flowers).

12. NEO-MENDELISM
Multiple alleles
Many genes have more than two alleles (even though any one diploid
individual can only have at most two alleles for any gene), such as the ABO
blood groups in humans, which are an example of multiple alleles.
Multiple alleles result from different mutations of the same gene. Coat
color in rabbits is determined by four alleles. Human ABO blood types are
determined by alleles A, B, and O. A and B are codominants which are both
dominant over O. The only possible genotype for a type O person is OO. Type
A people have either AA or AO genotypes. Type B people have either BB or BO
genotypes. Type AB have only the AB (heterozygous) genotype. The A and B
alleles of gene I produce slightly different glycoproteins (antigens) that are
on the surface of each cell. Homozygous A individuals have only the A
antigen, homozygous B individuals have only the B antigen, homozygous O
individuals produce neither antigen, while a fourth phenotype (AB)
produces both A and B antigens.

13. EPISTASIS
• Epistasis is the term applied when one gene interferes
with the expression of another (as in the baldness/widow's
peak mentioned earlier). Bateson reported a different
phenotypic ratio in sweet pea than could be explained by
simple Mendelian inheritance.
• This ratio is 9:7 instead of the 9:3:3:1 one would expect of a
dihybrid cross between heterozygotes. Of the two genes (C
and P), when either is homozygous recessive (cc or pp)
that gene is epistatic to (or hides) the other. To get purple
flowers one must have both C and P alleles present.

14. POLYGENIC INHERITANCE
Polygenic inheritance is a pattern responsible for many features that seem simple on the surface. Many traits such
as height, shape, weight, color, and metabolic rate are governed by the cumulative effects of many genes. Polygenic
traits are not expressed as absolute or discrete characters, as was the case with Mendel's pea plant traits. Instead,
polygenic traits are recognizable by their expression as a gradation of small differences (a continuous variation).
The results form a bell shaped curve, with a mean value and extremes in either direction.

15. MOLECULAR STRUCTURE OF DNA
Deoxyribonucleic acid, or DNA, is a molecule that contains the instructions an organism needs to develop, live and reproduce. These
instructions are found inside every cell, and are passed down from parents to their children.
DNA structure
 DNA is made up of molecules called nucleotides. Each nucleotide contains a phosphate group, a sugar group and a nitrogen base. The
four types of nitrogen bases are adenine (A), thymine (T), guanine (G) and cytosine (C). The order of these bases is what determines DNA's
instructions, or genetic code. Human DNA has around 3 billion bases, and more than 99 percent of those bases are the same in all people,
according to the U.S. National Library of Medicine (NLM).
 Similar to the way the order of letters in the alphabet can be used to form a word, the order of nitrogen bases in a DNA sequence forms
genes, which in the language of the cell, tells cells how to make proteins. Another type of nucleic acid, ribonucleic acid, or RNA,
translates genetic information from DNA into proteins.
 Nucleotides are attached together to form two long strands that spiral to create a structure called a double helix. If you think of the
double helix structure as a ladder, the phosphate and sugar molecules would be the sides, while the bases would be the rungs. The
bases on one strand pair with the bases on another strand: adenine pairs with thymine, and guanine pairs with cytosine.
 DNA molecules are long — so long, in fact, that they can't fit into cells without the right packaging. To fit inside cells, DNA is coiled tightly
to form structures we call chromosomes. Each chromosome contains a single DNA molecule. Humans have 23 pairs of chromosomes,
which are found inside the cell's nucleus.

16. WATSON
AND CRICK
MODEL OF
DNA

17. PACKAGING OF DNA IN
EUKARYOTS
DNA, Histones, and Chromatin
 Certain proteins compact chromosomal DNA into the microscopic space of the
eukaryotic nucleus.
 These proteins are called histones, and the resulting DNA-protein complex is called
chromatin.
 It may seem paradoxical that proteins are added to DNA to make it more compact.
 However, if you have ever tried to store a garden hose, you know that it is much easier to
do so if you begin by coiling the hose. Of course, coiling requires work, and energy is
needed to perform work.
 Thus, within the nucleus, histones provide the energy (mainly in the form of
electrostatic interactions) to fold DNA. As a result, chromatin can be packaged into a
much smaller volume than DNA alone.

18. PACKAGING
OF DNA

19. THANK YOU
Dr. Hajare Balaji