Principle of Genetics

1. Principle of Genetics
Pooja Singh

2. Mendel's laws of inheritance
• Mendel's laws of inheritance, also known as
Mendelian inheritance, refer to the fundamental
principles of genetic inheritance discovered by
the pioneering biologist Gregor Mendel in the
19th century.
• Mendel's work laid the foundation for modern
genetics and helped establish the field of
heredity.
• Gregor Mendel conducted groundbreaking
experiments on pea plants in the 19th century,
which laid the foundation for our understanding
of inheritance and the basic principles of
genetics.

3. Mendel’s Experiment
• Selection of Traits: Mendel
chose to study specific traits in
pea plants that exhibited clear-
cut variations.
• He focused on seven distinct
traits, including seed color,
flower color, seed shape, pod
color, pod shape, flower
position, and stem length.
• Each trait had two contrasting
forms, such as yellow vs. green
seeds or round vs. wrinkled
seeds.

4. • Pure Breeding: Mendel ensured that the plants he used for his
experiments were pure-breeding, meaning they were self-fertilized
for several generations and consistently produced offspring with the
same trait.
• Controlled Cross-Pollination: To study the inheritance patterns,
Mendel manually cross-pollinated the plants.
• He removed the male reproductive organs (stamens) from a selected
pure-breeding plant (the parent generation, P) to prevent self-
fertilization.
• Then, he transferred pollen from another pure-breeding plant with
the contrasting trait (e.g., yellow-seeded plant to green-seeded
plant).

5. • First Filial Generation (F1): The cross-pollination resulted in the first
generation of offspring, called the F1 generation.
• Mendel observed that all the F1 plants displayed only one of the two
contrasting traits, completely masking the presence of the other trait.
• For example, in the cross between yellow-seeded and green-seeded plants, all
the F1 plants had yellow seeds.
Parental
F1 Progeny

6. • Second Filial Generation (F2): Mendel allowed the F1 plants to self-fertilize or cross-
pollinated them with other F1 plants.
• This led to the creation of the second generation, known as the F2 generation. Surprisingly,
Mendel observed a 3:1 ratio of the dominant to recessive trait in the F2 generation for
each trait he studied.
• For instance, in the case of seed color, approximately three-fourths of the F2 plants had
yellow seeds, while one-fourth had green seeds.
Parental
F1 Progeny
F1 Selfing
F2 Progeny
Monohybrid
Cross

7. Law of Segregation
• Law of Segregation (or Law of Purity of Gametes): Mendel observed
that for each trait, an organism possesses two copies of a gene, which
are called alleles.
• During gamete formation (the production of reproductive cells like
sperm and eggs), these alleles segregate from each other, so that
each gamete receives only one copy.
• As a result, offspring inherit one allele from each parent, which
determines their genetic makeup for a particular trait.

8. Law of Independent Assortment
• Law of Independent Assortment: Mendel also discovered that the
inheritance of one trait is independent of the inheritance of other
traits.
• The assortment or distribution of alleles for different traits during
gamete formation occurs randomly and is not influenced by the
inheritance of other traits.
• This law holds true as long as the genes for the different traits are
located on separate chromosomes or are far enough apart on the
same chromosome to undergo independent assortment during
meiosis.

9. Law of Dominance
• Law of Dominance: In a heterozygous individual (having two different
alleles for a trait), the dominant allele will be expressed, masking the
presence of the recessive allele
• Mendel's laws provide the basic principles for understanding how
genetic traits are passed from one generation to the next.
• These laws explain how variations arise and are inherited in a
predictable manner, paving the way for the study of genetics and the
understanding of inherited traits in humans, animals, and plants.
• It's important to note that while Mendel's laws accurately describe the
inheritance patterns for many traits, there are exceptions and complexities that
arise due to factors like incomplete dominance, codominance, multiple alleles.
Nonetheless, Mendel's laws remain a fundamental cornerstone of classical
genetics

10. Test Cross
• A test cross is a genetic cross
used to determine the
genotype of an individual
expressing a dominant trait.
• It involves crossing the
individual of unknown
genotype with a homozygous
recessive individual for the
same trait.
• The purpose of a test cross is
to reveal whether the
individual with the dominant
phenotype is homozygous
dominant (two copies of the
dominant allele) or
heterozygous (one copy of
the dominant allele and one
copy of the recessive allele).
Rr Rr
Rr Rr
R- rr
R R
r
r
Rr rr
Rr rr
R r
r
r
Unknown flower
is Homozygous
Unknown flower
is Heterozygous

11. Incomplete dominance
• Incomplete dominance is a genetic phenomenon in which neither
allele in a pair completely masks the expression of the other, resulting
in an intermediate phenotype in the heterozygous condition.
• In incomplete dominance, the offspring display a phenotype that is a
blend or mixture of the phenotypes associated with the two different
alleles.
• In this case, neither allele is completely dominant over the other, and
the heterozygous genotype shows a unique phenotype that is distinct
from the phenotypes of the homozygous genotypes.
• The blending effect occurs because the two alleles interact and
contribute partially to the phenotype.

12. • A classic example of
incomplete dominance is seen
in flower color in certain plant
species.
• Let's consider a hypothetical
example involving a flower
color trait with two alleles:
• "R" for red color (dominant)
and "W" for white color
(recessive).
• In this case, the heterozygous
genotype "RW" does not result
in red or white flowers, but
rather in pink flowers, which is
an intermediate phenotype.
Red Flower (RR) White Flower (WW)
Pink Flower (RW)

13. • It's important to note that incomplete
dominance does not imply a blending
of genetic material, as alleles are still
transmitted intact to the next
generation.
• When two individuals with the
intermediate phenotype are crossed,
their offspring may exhibit a
phenotypic ratio of 1:2:1, with one-
quarter of the offspring showing the
red phenotype, one-half showing the
intermediate (pink) phenotype, and
one-quarter showing the white
phenotype.
• Incomplete dominance demonstrates
that phenotypic expression can be
influenced by the interaction between
alleles, resulting in a range of
phenotypes rather than a simple
dominant-recessive relationship.
1:2:1

14. Codominance
• Codominance is a genetic phenomenon in which both alleles of a gene
are expressed simultaneously and fully in the phenotype of a
heterozygous individual.
• Unlike incomplete dominance, where the phenotype is an intermediate
blend, codominance results in the simultaneous expression of both
alleles without any blending or dominance of one over the other.
• In codominance, the heterozygous genotype displays a phenotype that
shows the traits associated with both alleles distinctly.
• Each allele contributes to the phenotype in a separate and identifiable
manner, without one allele masking or overpowering the other.

15. • A classic example of codominance is observed in the ABO blood group
system in humans.
• In this system, there are three alleles that determine blood type: A, B,
and O.
The A allele codes for the A antigen on red blood cells,
The B allele codes for the B antigen,
The O allele does not code for any antigen.

16. • When an individual
has the heterozygous
genotype IAIB, both
the A and B antigens
are expressed on their
red blood cells,
resulting in the AB
blood type.
• This demonstrates
codominance, as both
alleles are expressed
fully and
simultaneously,
without one
dominating over the
other.
Allele from
Father
Allele from
Mother
Genotype of
Progeny
Blood group of
Progeny
IA IA IAIA A
IA IB IAIB AB
IA i IAi A
IB IB IBIB B
IB I IBi B
IB IA IBIA AB
i i ii O

17. Dihybrid cross
• The inheritance of two genes, also known as a dihybrid cross, involves
the simultaneous consideration of the inheritance patterns of two
different traits controlled by two different genes.
• This type of cross helps us understand how traits are inherited
independently from each other and how they assort during the
formation of gametes.

18. • To explain the inheritance of two
genes, let's consider an example
using two genes: one controlling seed
color and the other controlling seed
shape in pea plants.
• We'll use the letters "Y" and "y" to
represent the alleles for seed color,
with "Y" being the dominant allele
for yellow seeds and "y" the recessive
allele for green seeds.
• Similarly, we'll use the letters "R" and
"r" to represent the alleles for seed
shape, with "R" being the dominant
allele for round seeds and "r" the
recessive allele for wrinkled seeds.
Round and Yellow
Seed (RRYY)
Homozygous
dominant
Wrinkled and
Green seed (rryy)
Homozygous
recessive
Round and Yellow
seed (RrYy)
Heterozygous
dominant

19. • Determine the genotypes
of the parents: One parent
is homozygous dominant
for both traits (YYRR), while
the other parent is
homozygous recessive for
both traits (yyrr).
• Determine the possible
gametes: The homozygous
dominant parent can
produce gametes YR, and
the homozygous recessive
parent can produce
gametes yr.
• Set up the Punnett square:
RRYY RRYy RrYY RrYy
RRYy RRyy RrYy Rryy
RrYY RrYy rrYY rrYy
RrYy Rryy rrYy rryy
RrYy RrYy
Selfing
RY Ry rY ry
RY
Ry
rY
ry

20. • Determine the phenotypes:
Determine the phenotypes
associated with each
genotype using the given
alleles.
• YYRR: Yellow round seeds
• YYRr: Yellow round seeds
• YYrr: Yellow round seeds
• YyRR: Yellow round seeds
• YyRr: Yellow round seeds
• Yyrr: Yellow round seeds
• yyRR: Green round seeds
• yyRr: Green round seeds
• yyrr: Green wrinkled seeds
RRYY RRYy RrYY RrYy
RRYy RRyy RrYy Rryy
RrYY RrYy rrYY rrYy
RrYy Rryy rrYy rryy

21. • Calculate the phenotypic and genotypic ratios: Count the number of
each phenotype and genotype to determine the ratios.
• Phenotypic ratio: 9 Yellow round seeds : 3 Yellow wrinkled seeds : 3 Green round
seeds : 1 Green wrinkled seeds (9:3:3:1)
• Genotypic ratio:
• 1 YYRR : 2 YYRr : 1 YYrr : 2 YyRR : 4 YyRr : 2 Yyrr : 1 yyRR : 2 yyRr : 1 yyrr
• The correct ratio for a dihybrid cross involving independent assortment
of two genes is indeed 9:3:3:1.

22. Chromosomal Theory of Inheritance
• The chromosomal theory of inheritance, also known as the Sutton-
Boveri theory
• It is a fundamental concept in genetics that states that genes are
located on chromosomes and that the behavior of chromosomes
during cell division (meiosis) accounts for the patterns of inheritance
observed in offspring.

23. • The key principles of the chromosomal theory of inheritance are as follows:
• Genes are located on chromosomes: The theory proposed that
chromosomes carry genetic information in the form of genes.
• Genes are segments of DNA that contain instructions for specific traits.
• Chromosomes segregate during meiosis: During the process of meiosis,
which produces gametes (sperm and egg cells), chromosomes separate and
segregate into different daughter cells.
• This segregation accounts for the segregation of alleles (different forms of a
gene) during inheritance.
• Chromosomes assort independently: The theory also proposed that
chromosomes assort independently during meiosis, meaning that the
distribution of one pair of chromosomes into gametes is independent of
the distribution of other pairs of chromosomes.
• This principle helps explain the inheritance of multiple traits and the
occurrence of genetic recombination.

24. Linkage
• Linkage refers to the tendency of genes located on the same
chromosome to be inherited together as a unit, rather than
independently assorting during meiosis.
• Genes that are physically close to each other on the same
chromosome are more likely to be inherited together due to their
close proximity.
• This occurs because, during meiosis, the homologous chromosomes
may not undergo independent assortment, and the linked genes
remain together on the same chromosome.

25. • The degree of linkage between genes is measured by the
recombination frequency, which reflects the likelihood of genetic
recombination occurring between two genes during crossing over in
meiosis.
• If two genes are tightly linked, they have a low recombination
frequency, indicating that they are usually inherited together.
• Conversely, if two genes are farther apart on the chromosome, they
have a higher recombination frequency, suggesting that they are more
likely to be separated by crossing over during meiosis.

26. Pleiotropy
• Pleiotropy is a genetic phenomenon in which a single gene influences multiple, seemingly
unrelated traits or phenotypic effects.
• In other words, a single gene has multiple effects on the phenotype of an organism.
• In pleiotropy, a mutation or variation in a single gene can lead to diverse phenotypic
outcomes in various parts or systems of an organism's body.
• These phenotypic effects can manifest in different tissues, organs, or developmental stages.
• Pleiotropic effects can be beneficial, detrimental, or have a neutral impact on an organism's
fitness.
• Pleiotropy can arise due to the gene's involvement in multiple biological pathways, its
influence on the expression of other genes, or its participation in complex regulatory
networks.
• The underlying molecular mechanisms of pleiotropy can include altered protein functions,
changes in gene expression patterns, or modifications in developmental processes.

27. • A classic example of pleiotropy is seen
in the human genetic disorder known
as phenylketonuria (PKU).
• Individuals with PKU have a mutation
in the gene that encodes the enzyme
phenylalanine hydroxylase, which is
responsible for metabolizing the amino
acid phenylalanine.
• The mutation leads to a loss of enzyme
function, resulting in the accumulation
of phenylalanine in the body.
• This can lead to various phenotypic
effects, such as intellectual disability,
skin disorders, and neurological
problems.
Phenylalnine
Tyrosine
P-Hydroxyphenyl
pyruvate
Acetic Acid
Phenylalanine
hydroxylase
(PHA)
Tyrosine
transaminase
Series of reaction
Phenyl pyruvic acid
accumulation
PHA
absent

28. Types of Gametes
• Male heterogamety is a type of sex determination system found in certain
organisms where males possess different sex chromosomes than females.
• In this system, the sex chromosomes are not equivalent in males and
females.
• The most well-known example of male heterogamety is observed in humans
and many other mammals. In these species, males have one X and one Y
chromosome (XY), while females have two X chromosomes (XX).
• The presence of the Y chromosome determines maleness, as it carries genes
that promote the development of male sexual characteristics.
• During reproduction, males contribute either an X or a Y chromosome to the
offspring, while females always contribute an X chromosome.
• Therefore, the sex of the offspring is determined by the sperm, with those
carrying an X chromosome resulting in female offspring (XX) and those
carrying a Y chromosome resulting in male offspring (XY).

29. • In birds, females have two different sex chromosomes: (ZW), while
males have two Z chromosomes (ZZ).
• The presence of the W chromosome determines femaleness.
• Therefore, females are homogametic (ZW), meaning they possess two
different sex chromosomes, while males are heterogametic (ZZ), with
two of the same sex chromosomes.
• During bird reproduction, it is the female that determines the sex of the
offspring.
• Females can pass either a Z or a W chromosome to their offspring, while
males always contribute a Z chromosome.
• Offspring that receive a Z chromosome from both parents (ZZ) develop
as males, while those that receive a Z chromosome from the male and a
W chromosome from the female (ZW) develop as females.

30. Mutation
• A mutation is a permanent alteration in the DNA sequence of an
organism's genome.
• It can occur naturally or be induced by various factors, such as exposure
to radiation, chemicals, or errors during DNA replication or repair
processes.
• Mutations can take different forms, ranging from small changes in a
single nucleotide (point mutations) to larger scale alterations such as
deletions
 insertions
duplications
rearrangements of DNA segments

31. Types of Mutation
• Substitution: It is the replacement of one nucleotide with another, which
can result in different amino acids being incorporated into a protein or no
change at all, depending on the specific substitution and its effect on the
protein-coding region.
• Insertion: It involves the addition of one or more nucleotides into the DNA
sequence, leading to a shift in the reading frame and potentially altering the
entire protein sequence downstream of the mutation.
C T G G A G
C T G G G G
Original
Mutated
C T G G A G
C T G G T G G A G
Original
Mutated

32. • Deletion: It is the removal of one or more nucleotides from the DNA
sequence, causing a shift in the reading frame and potentially leading to a
truncated or non-functional protein.
• Duplication: It results in the replication of a DNA segment, leading to an
increased number of copies of specific genes or regions in the genome.
C T G G A G
C T X A G
Original
Mutated
C T G G A G
C T G G G A G
Original
Mutated

33. • Inversion: It involves the reversal of the orientation of a DNA segment
within the genome.
• Translocation: It is the movement of a DNA segment from one location
in the genome to another, often between non-homologous
chromosomes.
C T G G A G
C T A G G G
Original
Mutated
Original
Mutated

34. • Mutations can have various effects on an organism's phenotype.
• Some mutations have no apparent impact or may even be beneficial,
providing an advantage in certain environments or leading to new traits.
• However, mutations can also be detrimental, causing genetic disorders,
diseases, or reduced fitness.
• The effects of mutations depend on their location within the genome,
the type of mutation, and the specific genes or regulatory elements
affected.
• It's worth noting that not all mutations occur randomly.
• Some mutations can be induced or deliberately introduced through
processes such as genetic engineering or mutagenesis to study gene
function, develop new traits in crops or animals, or understand the
molecular basis of diseases.

35. • The genes responsible for color vision are located on the X chromosome. In
humans, females have two X chromosomes (XX), while males have one X
and one Y chromosome (XY).
• The genes for red and green photopigments, are closely related and found
on the X chromosomes only.
• In individuals with normal color vision, these genes produce functional
photopigments that allow the perception of a wide range of colors.
• However, in individuals with color blindness, there are mutations or
variations in these genes, leading to abnormal or non-functional
photopigments.
XX Xc Y
Xc X
XY Xc Xc
Normal
Female
Normal
Male
Colorblind
Female
Carrier
Female
Colorblind
Male

36. • Males are more commonly affected by color blindness because they
have only one X chromosome.
• If the X chromosome they inherit from their mother carries a mutated
color vision gene, they will have a higher likelihood of experiencing color
blindness.
• On the other hand, females have two X chromosomes, so even if they
inherit a mutated gene on one X chromosome, they often have a second
normal copy on their other X chromosome, which can compensate for
the mutated gene and maintain normal color vision.
• As a result, females are generally less likely to be color blind, but they
can still be carriers of the mutated gene and pass it on to their offspring.

37. Hemophilia
• Hemophilia is a genetic disorder characterized by impaired blood
clotting, primarily affecting males.
• It is caused by mutations in the genes responsible for producing clotting
factors, most commonly Factor VIII (hemophilia A) or Factor IX
(hemophilia B).
• These mutations lead to a deficiency or dysfunction of the respective
clotting factor, resulting in prolonged bleeding and difficulty in clot
formation.
• Hemophilia is usually inherited in an X-linked recessive pattern, meaning
males are more commonly affected, while females are typically carriers.
XX XY XH XH XH X XH Y
Normal
Female
Normal
Male
Colorblind
Female
Colorblind
Male
Carrier
Female

38. Sickle cell anemia
• Sickle cell anemia is a genetic disorder characterized by the presence of
abnormal hemoglobin molecules in red blood cells.
• It is caused by a mutation in the gene that codes for the beta-globin chain of
hemoglobin, resulting in the production of hemoglobin S (HbS) instead of
normal hemoglobin A.
• The HbS causes red blood cells to become misshapen, rigid, and prone to
clumping, leading to reduced oxygen-carrying capacity, blockage of blood
vessels, and tissue damage.
• Sickle cell anemia is inherited in an autosomal recessive pattern, with
symptoms including chronic anemia, pain crises, organ damage, and
increased susceptibility to infections.
• Management involves supportive care, pain management, and interventions
to prevent complications.

39. Thalassemia
• Thalassemia is a group of genetic blood disorders characterized by abnormal
production of hemoglobin due to mutations or deletions in the genes that
control the synthesis of alpha or beta globin chains.
• Alpha thalassemia results from mutations on the alpha globin genes located
on chromosome 16, while beta thalassemia is caused by mutations on the
beta globin genes on chromosome 11.
• The severity of thalassemia varies depending on the number of affected
genes and specific mutations.
• Reduced or absent synthesis of globin chains leads to ineffective
hemoglobin production, chronic anemia, and potential complications.
• Treatment options include blood transfusions, iron chelation therapy, and, in
severe cases, stem cell transplantation.

40. Chromosomal abnormality
• Aneuploidy is a chromosomal abnormality characterized by an abnormal
number of chromosomes in a cell.
• It can result from errors during cell division, leading to cells with an extra
copy of a chromosome (trisomy) or missing a copy (monosomy).
Non
disjunction

41. • Polyploidy, on the other hand, refers to the presence of multiple
complete sets of chromosomes in an organism. It can occur naturally or
be induced artificially.
• Polyploidy often leads to increased size, vigor, and diversity in plants, and
it can contribute to speciation.
• Polyploid organisms may have triploid (three sets), tetraploid (four sets),
or higher numbers of chromosome sets.
Non disjunction
gametes (2N)
Polyploidy (4N)

42. Down syndrome
• Down syndrome is a genetic disorder caused by the presence of an
extra copy of chromosome 21.
• It is characterized by intellectual disabilities, distinct facial features, and
potential health issues.
• Down syndrome occurs in about 1 in 800 births and varies in severity
among individuals.
Chromosome no. 20 Chromosome no. 21
Trisomy
Chromosome no. 22

43. Klinefelter
• Klinefelter syndrome is a genetic condition that occurs in males, typically
caused by the presence of an extra X chromosome (XXY).
• It leads to various physical, developmental, and hormonal differences.
• Individuals with Klinefelter syndrome may have reduced testosterone
levels, infertility, tall stature, gynecomastia (enlarged breasts), and learning
or behavioral challenges.
• Early diagnosis and management can help address associated health issues
and provide support for the physical and psychological well-being of
individuals with Klinefelter syndrome.

44. Turner's syndrome
• Turner's syndrome is a genetic disorder that affects females,
characterized by the partial or complete absence of one X
chromosome.
• It leads to various physical and developmental features, including short
stature, infertility, hormonal imbalances, and potential heart or kidney
abnormalities.
• Hormone therapy and other interventions can help manage associated
health concerns.