SUMMARY IN BIOLOGY:

TAXONOMY AND GENETICS

BY: REACH DENVER A. TAGUIC

- **Mitosis**:

- The process of cell division results in two genetically identical daughter

cells.

- Essential for growth, repair, and asexual reproduction.

- Phases: prophase, metaphase, anaphase, telophase.

- **Meiosis**:

- Specialized cell division producing gametes (sperm and eggs) with half
the chromosome number.

- Essential for sexual reproduction.

- Two rounds of division: meiosis I and meiosis II.

- Involves genetic diversity through crossing over.

- **Mendelian Genetics**:

- Study of heredity based on Gregor Mendel's principles.

- Concepts of dominant and recessive traits.

- Explains heredity through alleles.

- **Monohybrid and Dihybrid Crosses**:

- Monohybrid: Tracks a single trait.

- Dihybrid: Examines two traits simultaneously.

- Illustrates inheritance patterns and offspring phenotypes.

- **Phenotypic and Genotypic Ratios**:

- Phenotypic ratios: Observable traits in offspring.

- Genotypic ratios: Genetic makeup of offspring.

- Example: Monohybrid cross typically results in a 3:1 phenotypic ratio and

a 1:2:1 genotypic ratio.

- **Biodiversity**:

- Variety of life on Earth, including genetic, species, and ecosystem

diversity.

- Crucial for ecosystem resilience and human welfare.

- **Hierarchical Classifications in Taxonomy**:

- System for organizing living organisms into hierarchical groups.

- Ranges from broad categories (domain, kingdom) to specific groups

(genus, species).

- Helps understand evolutionary relationships and communicate about

species.

Taxonomy classifies organisms into hierarchical categories based on

shared characteristics and evolutionary relationships. The main
classifications in taxonomy are:

1. **Domain**: The highest taxonomic rank, which classifies life into

three main categories: Archaea, Bacteria, and Eukarya.

2. **Kingdom**: The second highest rank, which includes groups such as

Animalia (animals), Plantae (plants), Fungi (fungi), and Protista
(mostly unicellular organisms).
3. **Phylum**: A classification that groups organisms based on body
plans and organizational features. For example, Chordata (animals with
a spinal cord) and Arthropoda (invertebrates with exoskeletons).

4. **Class**: A subdivision of phylum that further categorizes organisms.

For example, Mammalia (mammals) and Aves (birds).

5. **Order**: A group within a class. For example, the order Carnivora

includes carnivorous mammals like cats and dogs.

6. **Family**: A smaller group within an order, grouping closely related

organisms. For example, Felidae (the cat family).

7. **Genus**: A way to categorize closely related species. For example,

Panthera includes big cats such as lions, tigers, and leopards.

8. **Species**: The most specific classification, identifying individual

organisms that can interbreed and produce fertile offspring. For
example, Panthera leo refers to the lion.

- **Domain**:

- Highest taxonomic rank, classifying life into three main categories:

Archaea, Bacteria, and Eukarya.

- **Kingdom**:

- Second highest rank, includes groups such as:

- Animalia (animals)

- Plantae (plants)

- Fungi (fungi)

- Protista (mostly unicellular organisms)

- **Phylum**:

- Groups organisms based on body plans and organizational features.

Examples:

- Chordata (animals with a spinal cord)

- Arthropoda (invertebrates with exoskeletons)

- **Class**:

- Subdivision of phylum that further categorizes organisms. Examples:

- Mammalia (mammals)

- Aves (birds)

- **Order**:

- A group within a class. Example:

- Carnivora (carnivorous mammals like cats and dogs)

- **Family**:

- Smaller group within an order, grouping closely related organisms.

Example:

- Felidae (the cat family)

- **Genus**:

- Categorizes closely related species. Example:

- Panthera (big cats like lions, tigers, and leopards)

- **Species**:

- Most specific classification, identifying individual organisms that can

interbreed and produce fertile offspring. Example:
 - Panthera leo (lion)

These classifications help organize biological diversity and provide a

framework for understanding relationships among organisms.

These classifications help organize biological diversity and provide a

framework for understanding the relationships among different organisms.

Important Reminders and notes:

Mendelian Inheritance: Mendelism or Mendelian Genetics

Mendelian inheritance, also known as Mendelism or Mendelian

genetics, is a set of principles that explain how hereditary traits are
passed from parents to their offspring.

These principles were initially developed by Gregor Johann Mendel, an

Austrian monk, and botanist, who is regarded as the father of genetics.
Mendel conducted pioneering experiments with garden peas (Pisum sativum)
in the 19th century and established the fundamental laws of inheritance.
Mendelian Inheritance- Mendelism or Mendelian Genetics

Mendel’s contributions to the field of genetics were initially overlooked but

were rediscovered and recognized in the early 20th century. Despite facing
initial controversy, Mendel’s work laid the foundation for classical genetics
and has since provided a framework for understanding the basic principles of
heredity.

Table of Contents

 Mendel’s Experiment

o Monohybrid cross

o Dihybrid cross

 Mendel’s Laws of Inheritance

o Law of Dominance

o Law of Segregation

o Law of Independent Assortment

Mendel’s Experiment

Gregor Mendel conducted breeding experiments on the pea plant between

1856 and 1863 in order to study the patterns of inheritance. He specifically
chose pea plants for a number of reasons including their availability in
various varieties, self-pollination capabilities, short life cycles, ease of
cultivation, and distinct characteristics. Mendel focused on studying seven
specific traits in pea plants: seed shape, seed color, flower color, pod shape,
pod color, flower position, and stem height.

Mendel conducted two main experiments, monohybrid and dihybrid crosses,

to determine the laws of inheritance.

Monohybrid cross

 In the monohybrid cross, Mendel studied the inheritance of a single

trait. Mendel conducted crosses between pea plants with different
traits of the same character, such as tallness (TT) and dwarfness (tt),
and observed their inheritance patterns.

 The parental generation (P) are the organisms involved in the initial
cross, while the first filial generation (F1) represents the offspring of
this cross.

 In the F1 generation, all the plants showed the dominant trait (tallness),
while the recessive trait (dwarfness) was not present. This pattern of
displaying only the dominant trait in the F 1 generation was the same
across all the traits Mendel studied.

 When the F1 plants were crossed among themselves, resulting in the

second filial generation (F2), some offspring showed the recessive trait,
which was not observed in the F1 generation. F2 generation exhibited a
3:1 ratio of the dominant and recessive traits.

 Mendel observed and found that this pattern was consistent in all the
traits he studied.
Monohybrid-Cross

Dihybrid cross

 In the dihybrid cross, Mendel studied the inheritance of two different

traits. He crossed purebred parental plants with different traits. For
example- plants with yellow, round seeds (YYRR) were crossed with
plants with green, wrinkled seeds (yyrr).

 The resulting F1 generation displayed only the dominant traits of

yellow and round seeds. In the F2 generation, both parental traits
appeared in four types of combination in a phenotypic ratio of
approximately 9:3:3:1, showing the independent assortment of the two
traits.
Dihybrid cross in Pea plants

Mendel’s Laws of Inheritance

Mendel proposed three laws explaining the inheritance of traits.

Law of Dominance
According to the law of dominance, when there are two alternative forms
(alleles) of a particular trait present in an organism, one allele will be
dominant and the other recessive. In the F1 generation, only the dominant
allele is expressed, while the recessive allele remains masked or
unexpressed. This law explains how the traits of the parents are expressed in
the offspring during a monohybrid cross.

Mendel’s Law of Dominance- Pea Plant

Law of Segregation

The Law of Segregation, also known as the Law of Purity of Gametes,

explains how the alleles responsible for a specific trait separate during the
formation of gametes and how they are passed on to the offspring. According
to this law, each individual possesses two alleles for a particular trait, one
inherited from each parent. During gamete formation, these alleles separate
from each other, so that each gamete carries only one allele for each trait.
Since each gamete carries only one allele for a trait, they are considered
pure or homozygous for that particular characteristic.
Law of Segregation- Morgan’s work on Drosophila. Image Source: Study and
Score.

Law of Independent Assortment

According to the Law of Independent Assortment, alleles for different traits

separate and are inherited independently during the formation of gametes.
This means that the alleles for one trait are not linked or influenced by the
alleles for other traits. Mendel’s dihybrid cross provides support for the Law
of Independent Assortment.

Mendel’s Law of Independent Assortment

Modes of Inheritance

Mendelian inheritance patterns can be categorized into three major types:

autosomal dominant, autosomal recessive, and X-linked inheritance.

Autosomal Dominant Inheritance is a type of inheritance where the

presence of a single dominant allele is sufficient to express a trait or disease,
even if the other chromosome carries a normal allele. This means that an
affected individual only needs to inherit one copy of the dominant allele from
either parent to exhibit the trait or disease. An affected individual has a 50%
chance of passing the trait to each independent offspring. Examples of
autosomal dominant diseases are Huntington’s disease and Marfan
syndrome.

Autosomal Recessive Inheritance is the mode of genetic inheritance

where the expression of a trait or disease requires the presence of two
copies of an abnormal recessive allele, one inherited from each parent. In
this inheritance pattern, both alleles must be abnormal for the trait to be
expressed. Carriers of a single copy of the recessive allele do not display the
trait but can pass it on to their children. Couples who are carriers have a 25%
risk of having an affected child. Examples of autosomal recessive diseases
are cystic fibrosis and sickle cell anemia.
X-Linked Inheritance refers to the inheritance of traits or diseases
associated with genes located on the X chromosome. Since males have one
X and one Y chromosome, they typically exhibit the phenotype of X-linked
traits inherited from their mother, as they only inherit one X chromosome.
On the other hand, females have two X chromosomes, so they may be
carriers of X-linked traits without displaying the phenotype. Examples of X-
linked diseases are hemophilia and color blindness.

Deviation from Mendel’s Findings

Mendel’s principles laid the foundation for genetics. However, exceptions and
variations in Mendel’s findings have been discovered that have helped us
develop a more complete understanding of inheritance patterns. Some of
these variations are:

Incomplete Dominance occurs when the offspring’s phenotype is not the

same as either of the parents but is intermediate between the phenotypes of
the parents. It happens when one allele for a trait is not completely dominant
over the other, resulting in a combination of the phenotypes of both alleles.
This goes against Mendel’s law of dominance, which states that one allele is
dominant and masks the expression of the other. For example, in the
snapdragon flower, crossing plants with red and white flowers resulted in
pink flowers.

incomplete dominance
Alleles: Definition, Types, Features, Applications

August 3, 2023 by Nidhi Dewangan

Edited By: Sagar Aryal

A key component of genetics, alleles are crucial for comprehending

genetic variation and inheritance patterns. They are distinct gene
variants that reside at the same chromosomal locus (position). For
each gene, a person receives two alleles, one from each parent.
Variations of the same trait or separate traits might occur from the
existence of different alleles.

Alleles

Table of Contents

 Types and Features of Alleles

 Dominant and Recessive Alleles

 Incomplete Dominance

]Types and Features of Alleles

Alleles are distinct due to a number of characteristics.

  First of all, loci are the particular locations that alleles inhabit on
chromosomes.

 Understanding the physical organization of genes within the genome

depends on knowing where the loci are on the chromosomes.

 Furthermore, each person has two alleles for each gene, one from each
parent.

 This enables the genetic variety and variation that is necessary for
adaptation and evolution within a population.

 Thirdly, additional genetic or environmental factors may have an

impact on how alleles express themselves.

 Even when people share the same genes, this can result in differences
in how features are expressed.

Alleles come in a variety of forms. The most prevalent or typical allele in

nature that encodes the typical or standard form of a trait is known as a wild-
type allele. Alleles that have suffered a change or mutation in their DNA
sequence and produced a different version of the trait are said to be mutant
alleles. Depending on how they interact with the other allele in the pair,
mutant alleles can either be recessive or dominant. Alleles, known as neutral
alleles, have no influence on an organism’s phenotype or its observable
characteristics.

Dominant and Recessive Alleles

 According to Mendelian genetics, dominant alleles can exist in either

one or two copies in a person, whereas recessive alleles can exist in
only two copies.

 This indicates that only the dominant allele will be exhibited in the
phenotype if a person carries one copy of a dominant allele and one
copy of a recessive allele.

 This is so that the effect of the dominant allele, which “dominates”

over the recessive allele, can be seen.

 On the other hand, if a person carries two copies of the recessive gene,
the dominant allele will not be shown in the phenotype, and the
recessive allele will be expressed.

 This is so that the dominant allele’s expression is hidden by the

recessive allele.
  An example of this is the inheritance of the trait for widows’ peak,
which is a type of hairline. The gene for widows’ peak has two alleles,
W and w.

 The W allele is dominant and produces a widow’s peak, while the w

allele is recessive and produces a straight hairline.

 If a person inherits one W allele and one w allele, they will have a
widow’s peak hairline because the W allele is dominant over the w
allele.

Incomplete Dominance

Incomplete dominance is a pattern of inheritance in which neither allele is

completely dominant over the other, resulting in an intermediate phenotype.
This means that the phenotype of a heterozygous individual is different from
both the homozygous dominant and homozygous recessive individuals.

incomplete dominance

A classic example of incomplete dominance is the inheritance of flower color

in snapdragons. The gene for flower color has two alleles: R, which produces
red flowers, and W, which produces white flowers. When a homozygous red-
flowered plant (RR) is crossed with a homozygous white-flowered plant (WW),
the F1 generation has pink flowers (RW), which is an intermediate phenotype
between red and white.
Genotypic Ratio- Definition, Calculation and 2 Examples

August 3, 2023 by Bikash Dwivedi

Edited By: Sagar Aryal

The genotypic ratio is the depiction of the ratio of the resulting

patterns and frequencies of inherited genes after crossing in the
offspring of organisms. In simple language, it is the ratio of the
composition of genes in the progenies of an organism.

Genotype is the genetic constitution or expression of an organism. In simple

language, the genotype is the type or combination of different genes in an
organism resulting in some external appearance.

Genotypic Ratio

Genes of an organism can be expressed (dominant) or remain suppressed

without expression (recessive). The expressed genes result in the phenotypic
characteristics of an organism. The particular phenotype can also have
different genotypes. For eg., Tt and TT are different genotypes but express
the same phenotype of tallness.

Table of Contents

 Calculation of Genotypic Ratio

 Genotypic Ratio in Monohybrid Cross

  Genotypic Ratio in Dihybrid Cross

Calculation of Genotypic Ratio

The calculation of the ratio of genotypes for monohybrid, dihybrid, and

trihybrid crosses is different. The basic steps of calculation are:

 Choosing the characters to be crossed or the characters under study.

 By applying Mendel’s Laws of Inheritance, cross them by forming a

cross diagram or Punnett-square chart.

 Then calculate the number of specific genotypes (not be confused with

the phenotype). The tallness can be denoted by TT and Tt and they
both are different genotypes where one is pure and homozygous
whereas the other is hybrid and heterozygous respectively.

 The type of genotypic outcomes must be listed distinctly and the ratio
can be finally determined by adjusting the characters along with their
specific numbers (as counted in the Punnett chart) respectively.
Genotypes can be looked at diagonally in the Punnett chart which
would make the counting easier.

Genotypic Ratio in Monohybrid Cross

It is the type of cross in which a single trait of an individual is taken and

studied for its outcomes after the cross. Let’s take, for example, height: a
homozygous tall father (TT) who is crossed with a mother with short height
(tt) where tallness is dominant over shortness, then the genotypic-ratio can
be calculated as follows:
Genotypic Ratio in Monohybrid Cross

The genotypes in the second filial generation are:

 Pure Tall (With homozygous allele (TT)) – 1

 Hybrid Tall (With heterozygous allele (Tt)) – 2

 Pure Dwarf/ Short (With homozygous allele (tt)) – 1

So the genotypic ratio will be 1:2:1, respectively. It simply indicates

that if four offspring are born in the above case, then there is a higher
probability of only one of them being pure tall, although two of them will be
phenotypically tall.

Genotypic Ratio in Dihybrid Cross

It is the type of cross in which two characters or traits are taken into
consideration and the combined outcomes of both are calculated. Let’s take,
for example, height and color: a tall and white colored male (TTWW) who is
crossed with a short and black female (ttww) (considering tallness and white
skin are dominant over shortness and black skin), then the cross-ratio can be
calculated as follows:
Genotypic Ratio in Dihybrid Cross

The genotypes in the second filial generation are tall and white.

 One pure tall and pure white (Homozygous alleles: TTWW)

 Two pure tall and hybrid white (TTWw)

 One pure tall and pure black (TTww)

 Two hybrid tall and pure white (TtWW)

 Four hybrid tall and hybrid white (TtWw)

 Two hybrid tall and pure black (Ttww)

 One pure short and pure white (ttWW)

 Two pure short and hybrid white (ttWw)

 One pure short and pure black (ttww)

So the genotypic ratio will be 1:2:1:2:4:2:1:2:1, respectively,

considering the above data and assumptions. It simply indicates that if 16
offspring are born in the above case, then there is a higher probability that
only one will be pure tall and white and only one pure short and black, all
others will be hybrid.

Monohybrid Cross: Definition, Steps & Examples Explained

A Monohybrid cross is a genetic cross between two individuals with

homozygous genotypes of a single character or trait, often resulting in an
opposite phenotype.

 Monohybrid crosses are usually performed to determine the genotypes

of offspring of homozygous individuals. The hybrid produced from this
cross helps in the identification of the dominant genotype in the allele.

 Even though monohybrid crosses are often associated with

homozygous genotypes, these are also used to determine the genetic
mix between individuals with heterozygous genotypes.

 The success of the monohybrid cross is determined by evaluating the

monohybrid ratio of the second-generation offspring.

 Monohybrid crosses are performed to identify the dominant allele for a

particular genetic trait. The cross occurs between the parents where
one parent is homozygous for one allele, and the other is homozygous
for the other allele.

 The result of the cross occurs in the form of heterogeneous hybrid

offsprings expressing the dominant trait.

Table of Contents

 Steps of Monohybrid Cross

 Examples of Monohybrid Cross

o 1. Mendel’s Peas

o 2. Huntington’s Disease

 Monohybrid Cross vs Dihybrid Cross (7 Differences)

 References and Sources

Steps of Monohybrid Cross

Monohybrid crosses are performed to estimate the phenotypic and genotypic

ratios of the crosses and to determine the dominant allele. The following are
the steps that are used to perform a monohybrid cross;

1. A particular character or trait is selected, and the alleles are indicated

with certain alphabet characters. The dominant alleles are indicated
with upper case letters, whereas the recessive alleles are indicated
with lower case letters.

2. The Punnet square is set up by listing the phenotype and genotype of

the parents being crossed. The genotype of the gametes is also
determined as the gametes will be haploid as a result of meiotic
division.

3. The probable combination of the genotypes is written within the

Punnet square as all combinations are possible as the process of
fertilization is random.

4. The phenotypic and genotypic ratios of the offspring are determined

and written down. The resulting combination is called the F1
generation.

Examples of Monohybrid Cross

1. Mendel’s Peas

 George Mendel used the monohybrid cross to determine the dominant

and recessive traits in the case of peas.

 An example of such experiments is the length of the plant. Some peas

are taller while the others are shorter.

 The homozygous allele for the tall pea plant is represented by TT, and
the homozygous allele for the short/dwarf pea plant is represented by
tt.

 A monohybrid cross between the two plants results in the production of

heterozygous genotype (Tt).

 In terms of phenotype, the hybrids are long pea plants, thus indicating
that the tall trait is the dominant trait and the short trait is a recessive
trait.
2. Huntington’s Disease

 Huntington’s disease is a condition resulting from a genetic disorder.

The disease is caused by the Huntingtin gene.

 Studies have been conducted to determine the genotypic condition

responsible for the disease.

 When a homozygous dominant gene of an individual was paired with

the homozygous recessive gene of another individual, all the offspring
carried the dominant allele.

 It was discovered that the dominant allele was responsible for the
disease and that all the children of such individuals would have the
disease.
 Huntington’s Disease (Hyperkinetic Disorder)

Monohybrid Cross vs Dihybrid Cross (7 Differences)

Characteristic
Monohybrid Cross Dihybrid Cross
s

A Dihybrid cross is a type

A Monohybrid cross is a type of
of genetic cross between
genetic cross between two individuals
two individuals with either
Definition with homozygous genotypes of a
homozygous or
single character or trait, often
heterozygous genotypes of
resulting in an opposite phenotype.
two characters or traits.

Dihybrid crosses take place

Monohybrid crosses take place between homozygous or
Occurs
between homozygous individuals with heterozygous individuals
between
different alleles for a single trait. with different alleles for
two distinct traits.

Phenotypic The phenotypic ratio of the offspring The phenotypic ratio of the
ratio in the F2 generation in the case of a offspring in the F2
monohybrid cross is 3:1. generation in the case of
 dihybrid cross is 9:3:3:1.

The genotypic ratio of the

The genotypic ratio of the offspring in offspring in the F2
Genotypic
the F2 generation in the case of a generation in the case of
ratio
monohybrid cross is 1:2:1. dihybrid cross is
1:2:2:4:1:2:1:2:1.

Test cross The test cross-ratio of a monohybrid The test cross-ratio of a

ratio cross is 1:1:1:1. dihybrid cross is 1:1.

Monohybrid crosses are performed to Dihybrid crosses are

Significance determine the dominant allele of a performed to study
character. offspring assortment.

An example of a dihybrid
An example of a monohybrid cross is cross is the cross between
Examples the cross between tall pea plants and pea plants with yellow
dwarf pea plants. round and green wrinkled
seeds.

Recap

Mendel’s Laws

I. Mendel’s Law of Segregation of genes (the “First Law”)

Image Source: Encyclopædia Britannica.

 The Law of Segregation states that every individual organism contains

two alleles for each trait, and that these alleles segregate (separate)
during meiosis such that each gamete contains only one of the alleles.

 An offspring thus receives a pair of alleles for a trait by

inheriting homologous chromosomes from the parent organisms: one
allele for each trait from each parent.

 Hence, according to the law, two members of a gene pair segregate

from each other during meiosis; each gamete has an equal probability
of obtaining either member of the gene.

II. Mendel’s Law of Independent Assortment (the “Second Law”)

Mendel’s Law of Independent Assortment. Created with BioRender.com

 Mendel’s second law. The law of independent assortment; unlinked

or distantly linked segregating genes pairs behave independently.

 The Law of Independent Assortment states that alleles for separate

traits are passed independently of one another.

 That is, the biological selection of an allele for one trait has nothing to
do with the selection of an allele for any other trait.

 Mendel found support for this law in his dihybrid cross experiments. In
his monohybrid crosses, an idealized 3:1 ratio between dominant and
recessive phenotypes resulted. In dihybrid crosses, however, he found
a 9:3:3:1 ratio.

 This shows that each of the two alleles is inherited independently from
the other, with a 3:1 phenotypic ratio for each.

III. Mendel’s Law of Dominance (the “Third Law”)

 The genotype of an individual is made up of the many alleles it
possesses.

 An individual’s physical appearance, or phenotype, is determined by its

alleles as well as by its environment.

 The presence of an allele does not mean that the trait will be
expressed in the individual that possesses it.
 If the two alleles of an inherited pair differ (the heterozygous
condition), then one determines the organism’s appearance and is
called the dominant allele; the other has no noticeable effect on the
organism’s appearance and is called the recessive allele.

 Thus, the dominant allele will hide the phenotypic effects of the
recessive allele.

 This is known as the Law of Dominance but it is not a transmission law:

it concerns the expression of the genotype.

 The upper case letters are used to represent dominant alleles, whereas
the lowercase letters are used to represent recessive alleles