Genetics nasics

1. Genetics
Dr. Sachin S. Bhagat
Dept. of Botany
Patkar-Varde College

2. Genetics
• Phenotype
• Genotype
• Mendelian Genetics – monohybrid, dihybrid
• Test cross
• Back cross ratios
• Epistatics and non epistatic interaction
• multiple alleles

3. Genetics
• The study of genes and heredity
• Branch of biology that deals with the heredity and
variation of organisms
• Branch of biology that deals with the study of genetic
variation genes, and heredity.
• Genetics is the study of how genes and how traits are
passed down from one generation to the next
• Gene is a unit of information inside a cell which controls
what a living thing will be like. Genes are passed from
parents to children
• A gene is the basic physical and functional unit of heredity.
Genes are made up of DNA.
• heredity, the sum of all biological processes by which
particular characteristics are transmitted from parents to
their offspring.

4. Phenotype
• The genotype is a set of genes in DNA responsible for
unique trait or characteristics while the phenotype is
the physical appearance or characteristic of an
organism.
• Experiment with garden pea plants (Pisum sativum) as
did Austrian monk Gregor Mendel (1822-1884). Mendel
chose to experiment with peas because they possessed
four important qualities:
• Peas had been shown to be true-breeding (all offspring
will have the same characteristic generation after
generation).
Peas exhibit a variety of contrasting traits (purple vs.
white flowers; round vs. wrinkled seeds).
• The shape of the pea flower protected it from foreign
pollen. Peas usually reproduce by self-pollination, in
which pollen produced by a flower fertilizes eggs in the
same flower.
• Pea plants grow quickly and do not require much space.

5. Phenotype characters
• The term "phenotype" refers to the observable
physical properties of an organism; these include
the organism's appearance, development, and
behavior.
• An organism's phenotype is determined by its
genotype, which is the set of genes the organism
carries, as well as by environmental influences
upon these genes.
• Impact on phenotype
• Trait
• Variation
• Environmental factors
• Behavioral aspects

6. Phenotypic variations
• Variations in organisms occur by different
mechanisms of genetic recombination and
genetic mutations. These variations make
natural selection possible. And natural selection
acts over many generations. It can increase the
proportion of mutants in the population.
• Most of the variations are produced by the
environment. Thus, environment has a great
role in affecting gene expression. In gametes,
crossing over plays the role for recombinants.
These produce variations.
• If variations are positive, they are supported by
nature. And if variations or mutations are
harmful, they are eliminated. Hence, variation
leads to evolution.

7. Pea plants traits
The traits that Mendel studied are listed below:
• Form of ripe seed (R) – smooth or wrinkled
• Color of seed albumen (Y) – yellow or green
• Form of ripe pods (I) – inflated or constricted
• Color of unripe pods (G) – green or yellow
• Color of flower (P) – purple or white
• Position of flowers (A) – axial or terminal
• Length of stem (T) – tall or dwarf

8. Genotype
• The genotype of an organism is its complete set of genetic
material
• The genotype is an organism's genetic makeup, or its genetic
blueprint.
• Genotype or alleles or variants an individual carries in a
particular gene or genetic location.
• The number of alleles an individual can have in a specific gene
depends on the number of copies of each chromosome found
in that species, also referred to as ploidy.
• In diploid species like humans, two full sets of chromosomes
are present, meaning each individual has two alleles for any
given gene.
• If both alleles are the same, the genotype is referred to
as homozygous. If the alleles are different, the genotype is
referred to as heterozygous.

9. Allele
• An allele is a variant form of a gene. Some genes have a
variety of different forms, which are located at the same
position, or genetic locus, on a chromosome.
• A gene is a portion of DNA that determines a certain
trait. An allele is a specific form of a gene. Genes are
responsible for the expression of traits. Alleles are
responsible for the variations in which a given trait can
be expressed.
• An allele is a variation of the same sequence of
nucleotides at the same place on a long DNA molecule,
• The chromosomal or genomic location of a gene or any
other genetic element is called a locus and alternative
DNA sequences at a locus are called alleles.

10. Mendelian Genetics – monohybrid, dihybrid
• Mendel believed that heredity is the result of
discrete units of inheritance, and every single
unit (or gene) was independent in its actions in
an individual's genome.
• According to this Mendelian concept, inheritance
of a trait depends on the passing-on of these
units.
• Based on his pea plant studies, Mendel proposed
that traits are always controlled by single genes.
• The key principles of Mendelian inheritance are
summed up by Mendel's three laws: the Law of
Independent Assortment, Law of Dominance,
and Law of Segregation.

11. Mendel’s Laws of Inheritance
• Mendel's Law of dominance When parents with
pure, contrasting traits are crossed together, only
one form of trait appears in the next generation.
The hybrid offsprings will exhibit only the
dominant trait in the phenotype
• Law of Segregation: The Law of Segregation states
that for any trait, each parent’s pairing of genes
(alleles) split and one gene passes from each
parent to an offspring. Which particular gene in a
pair gets passed on is completely up to chance.
• Law of Independent Assortment: The Law of
Independent Assortment states that different pairs
of alleles are passed onto the offspring
independently of each other. Therefore,
inheritance of genes at one location in a genome
does not influence the inheritance of genes at
another location.

12. Monohybrid Cross
• A monohybrid cross is a breeding experiment
between (parental generation) organisms that
differ in a single given trait.
• A monohybrid cross is hybrid of two individuals
with homozygous genotypes which result in the
opposite phenotype for a certain genetic trait.”
• The cross between two monohybrid traits (BB &
bb) is called a Monohybrid Cross. Monohybrid
cross is responsible for the inheritance of one
gene.
• A monohybrid cross is a cross between two
organisms with different variations at one
genetic locus of interest.
• The character being studied in a monohybrid
cross are governed by two or multiple variations
for a single location of a gene.

13. Monohybrid Cross Experiment
• Experiment with self-pollination of F1 progeny
plants. Surprisingly, he observed that one out of
four plants were white while the other three were
purple.
• The Purple and the white plants were in the ratio
of 3:1.
• He also noted that no progeny was in intermediate
colour, i.e., no blending was observed.
• The result was the same for other traits of plants
too, and he called them second hybrid generation
and the offspring were called Filial2 or F2 progeny.
• Mendel observed that traits which were absent in
F1 generation had reappeared in the F2 generation.

14. Monohybrid Cross
• He called such suppressed traits as recessive traits and
expressed traits as dominant traits. He also concluded that
some ‘factors’ are inherited by offspring from their parent
over successive generations.
• Later, these ‘factors’ were called genes. Genes are
responsible for the inheritance of traits from one
generation to another. Genes consist of a pair of alleles
which code for different traits.
• If a pair of alleles is the same, i.e., PP or pp, such alleles
are called homozygous pair while those that are different
or non-identical (e.g. Pp) are called heterozygous pair.
• In the first step of a monohybrid cross, the homozygous
traits of an individual are crossed. In the next step, when
the heterozygous traits are crossed, it is confirmed
whether the trait is dominant or recessive.

15. Dihybrid cross
• A dihybrid cross describes a mating experiment between
two organisms that are identically hybrid for two traits.
• A hybrid organism is one that is heterozygous, which means
that is carries two different alleles at a particular genetic
position, or locus.
• Therefore, a dihybrid organism is one that is heterozygous at
two different genetic loci.
• In 1865, Gregor Mendel performed dihybrid crosses on pea
plants and discovered a fundamental law of genetics called
the Law of Independent Assortment.
• Mendel began his experiments by first crossing two
homozygous parental organisms that differed with respect
to two traits.
• An organism that is homozygous for a specific trait carries
two identical alleles at a particular genetic locus.
• From his experiment, Mendel observed that the pairs of
traits in the parental generation sorted independently from
one another, from one generation to the next.

16. Dihybrid cross-Experiment
• Mendel chose to cross a pea plant that was homozygous
and dominant for round (RR), yellow (YY) seeds with a
pea plant that was homozygous and recessive for
wrinkled (rr), green (yy) seeds, represented by the
following notation:
• RRYY x rryy
• Organisms in this initial cross are called the parental, or P
generation. The offspring of the RRYY x rryy cross, which
is called the F1 generation, were all heterozygous plants
with round, yellow seeds and the genotype RrYy.
• Next, Mendel crossed two plants from the F1 generation.
This step is the dihybrid cross, and it is represented as:
• RrYy x RrYy
• Mendel observed that the F2 progeny of his dihybrid
cross had a 9:3:3:1 ratio and produced nine plants with
round, yellow seeds, three plants with round, green
seeds, three plants with wrinkled, yellow seeds and one
plant with wrinkled, green seeds.

17. Dihybrid Cross
• Two types of breeding processes to know the mechanism of genes
and examine the inheritance of traits from parents & grandparents,
one is monohybrid cross and the other is dihybrid cross.
• The latter occurs when the F1 generation offspring differ in two
traits.
• A cross between two entities that are heterozygous for two different
traits.
• Mendel carried out the following experiment for this cross:
• For crossing, he took a pair of contradicting characteristics or traits
• Mendel crossed round-yellow seed and wrinkled-green seed
• In the F1 generation, the outcome was seeds that were round and
yellow
• The F1 generation indicated that the round and yellow traits are
dominant while the green colour and the wrinkled shape were
recessive traits.
• Self-pollination of F1 progeny resulted in four varying combinations
of seeds in the subsequent generation, the F2 generation.
• The outcome and the dihybrid cross-ratio were – round-yellow,
wrinkled-yellow, wrinkled-green, round-green and the ratio was –
9:3:3:1.