Terminology Related to Genetics
Introduction
Genetics is one of the most important branches of Biology and forms a
significant portion of the syllabus. A clear understanding of genetic
terminology is essential before studying topics such as Mendelian
inheritance, chromosomal theory of inheritance, linkage, recombination,
mutations, sex determination, pedigree analysis, molecular genetics,
biotechnology, and evolution.
What Is Genetics?
Genetics
Definition
Genetics is the branch of biology that deals with:
- Heredity
- Inheritance
- Variation
- Genes
- Chromosomes
- DNA
- Genetic disorders
The term Genetics was coined by William Bateson in 1905.
Heredity
Definition
Heredity is the transmission of hereditary characteristics from parents
to offspring through genes.
It ensures continuity of species from one generation to the next.
Inheritance
Definition
Inheritance is the biological process by which hereditary traits are
transmitted from parents to offspring through genes located on chromosomes.
Variation
Definition
Variation refers to differences in characteristics among individuals of
the same species. Variation is essential because it provides the raw material for
evolution.
Examples
- Height
- Blood group
- Eye colour
- Skin colour
Character
Definition
A character is any heritable feature of an organism that can exist in
different forms.
Example
Character: Plant height
Forms (Traits):
- Tall
- Dwarf
Trait
Definition
A trait is a particular expression or form of a character.
Examples Character → Flower colour
Traits:
- Purple
- White
Character → Seed shape
Traits:
- Round
- Wrinkled
Gene
The term Gene was introduced by Wilhelm Johannsen in 1909.
Definition
A gene is the basic structural and functional unit of heredity. It is a segment of DNA that carries
the information required to produce a specific protein or functional RNA and
thereby influences a particular trait.
Functions
- Controls inheritance
- Directs protein synthesis
- Determines phenotype
- Undergoes mutation
- Replicates during cell division
Allele
Definition
Alleles are alternative forms of the same gene occupying the same locus
on homologous chromosomes.
Example
Gene: Plant height
Alleles:
T = Tall
t = Dwarf
Allelomorphs
Alleles were originally called allelomorphs. Today, the term allele is
universally used.
Locus
Definition
The specific position occupied by a gene on a chromosome is called its locus.
Each gene occupies a
fixed position.
Chromosome
Definition
Chromosomes are thread-like DNA-protein structures present in the nucleus
that carry genes and transmit hereditary information.
Human Chromosomes
Total: 46, Organized as:
- 22 pairs of autosomes
- 1 pair of sex chromosomes
Homologous Chromosomes
Definition
A pair of chromosomes having:
- Same size
- Same shape
- Same gene arrangement
- Same loci
One chromosome is inherited from each parent.
Autosomes
Definition
Chromosomes that do not determine Gender.
Humans possess: 22 pairs, Total: 44 autosomes
Sex Chromosomes
Definition
Chromosomes involved in determining sex i,e, gender of offsprings.
Humans possess: One pair, Female: XX, Male: XY
DNA
Definition
Deoxyribonucleic Acid (DNA) is the hereditary material of almost all
living organisms.
Functions
- Stores genetic information
- Replicates before cell division
- Controls protein synthesis
- Transfers hereditary information
Genome
Definition
The complete set of genetic material (DNA) present in an organism. Human genome contains approximately: 3.2
billion base pairs.
Genotype
Definition
The genetic constitution or genetic makeup of an organism.
Examples
TT, Tt, tt
Phenotype
Definition
The observable characteristics of an organism.
Examples
- Tall plant
- Brown eyes
- White flower
Phenotype depends upon:
Genotype + Environment
Dominant Allele
Definition
An allele that expresses itself in both homozygous and heterozygous
conditions.
Represented by: Capital letter, Example: T
Recessive Allele
Definition
An allele expressed only in homozygous condition.
Represented by: Small letter, Example: t
Homozygous
Definition
An individual possessing two identical alleles for a gene.
Examples- TT, tt
Also called: Pure
Heterozygous
Definition
An individual possessing two different alleles for a gene.
Example- Tt
Also called: Hybrid
Pure Line
Definition
A genetically uniform line produced through continuous
self-fertilization. Produces identical offspring generation after generation.
Hybrid
Definition
An offspring produced by crossing genetically different parents.
Example
TT × tt
↓
Tt
Parental Generation (P)
The original parents used in a genetic cross. Represented as: P
Filial Generation
F₁ Generation
First generation offspring obtained from parental cross.
F₂ Generation
Second generation obtained by selfing or intercrossing F₁ individuals.
Monohybrid Cross
Definition
A genetic cross involving only one character.
Example- Tall × Dwarf
Dihybrid Cross
Definition
A cross involving two different characters simultaneously.
Example- Seed shape and seed colour.
Test Cross
Definition
Cross between: Unknown genotype × Homozygous recessive
Purpose: To determine genotype.
Back Cross
Definition
Cross between: F₁ hybrid × Either parent
Test cross is a special type of back cross.
Gamete
Definition
A haploid reproductive cell produced by meiosis.
Examples
- Sperm
- Ovum
- Pollen
- Egg
Contains one allele of every gene.
Zygote
Definition
Diploid cell formed by fusion of two gametes. Undergoes mitosis to develop into
embryo.
Haploid
Definition
Cells containing one complete set of chromosomes.
Represented as: n
Example- Human sperm: 23 chromosomes
Diploid
Definition
Cells containing two complete chromosome sets.
Represented as: 2n
Example- Human somatic cells: 46 chromosomes
Pure Breeding
Individuals that produce offspring identical to themselves after repeated
self-fertilization. Usually homozygous.
Punnett Square
Definition
A diagram used to predict possible genotypes and phenotypes of offspring.
Developed by Reginald
Crundall Punnett.
Probability
The mathematical chance of occurrence of an event.Widely used in
genetics.
Example
Probability of obtaining TT from Tt × Tt = 1/4
Pedigree
Definition
A diagram showing inheritance of traits over several generations of a
family. Used extensively in
medical genetics.
Carrier
Definition
A heterozygous individual carrying one recessive disease allele without
showing symptoms.
Example- Carrier female for Hemophilia.
Mutation
Definition
A sudden, permanent, heritable change in DNA or chromosome structure. Mutation is the ultimate source of new
genetic variation.
Mutagen
Definition
Any agent capable of causing mutations.
Examples
Physical:
- X-rays
- Gamma rays
- UV rays
Chemical:
- Nitrous acid
- Mustard gas
Biological:
- Certain viruses
LINKAGE
Definition
The tendency of genes located on the same chromosome to be inherited
together.
Recombination
Definition
Formation of new gene combinations during meiosis due to crossing over
and independent assortment. Produces genetic diversity.
Crossing Over
Definition
Exchange of chromosome segments between homologous chromosomes.
Occurs during: Pachytene stage of Prophase I of meiosis.
Gene Pool
Definition
The total collection of all genes and alleles present in a population. Evolution changes gene pool
composition.
Allele Frequency
Definition
Proportion of a particular allele present in a population. Evolution is measured by changes in
allele frequencies.
Penetrance
Definition
The proportion of individuals with a particular genotype who actually
express the associated phenotype.
i.
Complete penetrance = all individuals
express the trait.
ii.
Incomplete penetrance = some
individuals do not express the trait despite carrying the genotype.
Expressivity
Definition
The degree or intensity with which a genotype is expressed in different
individuals.
Example: Individuals with the same genetic disorder may show mild or
severe symptoms.
Pleiotropy
Definition
A single gene influencing multiple different phenotypic traits.
Example: The gene responsible for Sickle Cell Disease affects red
blood cell shape, anemia, pain crises, and susceptibility to infections.
Polygenic Inheritance
Definition
Inheritance of a single character controlled by two or more genes.
Examples
- Human skin colour
- Human height
Multiple Alleles
Definition
More than two alternative forms of a gene existing in a population.
Example- ABO blood group system: IA, IB, i
Codominance
Definition
Both alleles express themselves equally in heterozygous condition.
Example- Blood group AB.
Incomplete Dominance
Definition
Neither allele completely dominates the other.
Example- Pink flowers in snapdragon plants.
Epitasis
Definition
Interaction in which one gene masks or modifies the expression of another
non-allelic gene.
Atavism
Definition
Reappearance of an ancestral trait in modern individuals.
Example- Rare occurrence of a tail-like structure in humans.
Hereditary Disease
Definition
Disease transmitted genetically from parents to offspring.
Examples
- Hemophilia
- Sickle Cell Disease
- Thalassemia
Congenital Disorder
Definition
A disorder present at birth.
May be:
- Genetic
- Environmental
- Multifactorial
Genetic Disorder
Definition
Disease caused by abnormalities in genes or chromosomes.
Examples
- Down syndrome
- Turner syndrome
- Klinefelter syndrome
SUMMARY TABLE
|
Term |
Definition |
|
Genetics |
Study of heredity and variation |
|
Heredity |
Transmission of traits |
|
Inheritance |
Mechanism of transmission |
|
Character |
General heritable feature |
|
Trait |
Particular form of a character |
|
Gene |
Basic unit of heredity |
|
Allele |
Alternative form of a gene |
|
Locus |
Position of a gene |
|
Genotype |
Genetic constitution |
|
Phenotype |
Observable expression |
|
Homozygous |
Same alleles |
|
Heterozygous |
Different alleles |
|
Dominant |
Expressed in heterozygous state |
|
Recessive |
Expressed only in homozygous state |
|
Gamete |
Haploid reproductive cell |
|
Zygote |
Diploid cell formed after
fertilization |
|
Mutation |
Heritable change in DNA |
|
Recombination |
New gene combinations |
|
Linkage |
Genes inherited together |
|
Genome |
Complete genetic material |
|
Gene Pool |
Total genes in a population |