CLINICAL GENETICS

A SHORT INTRODUCTION

BY

DR.C.GANESAN M.D.,

PROFESSOR OF MEDICINE

Preface

Genetics is the foundation of modern biology and medicine, explaining how hereditary information is transmitted, expressed, and altered in living organisms. This glossary presents essential genetics terms in simple two-line definitions, providing a quick and reliable reference for school students, medical and paramedical students, teachers, researchers, and candidates preparing for competitive examinations.

The aim of this preparatio is to simplify complex genetic concepts into clear, concise, and easy-to-understand explanations, making genetics accessible for rapid learning, revision, and everyday reference.  

CLINICAL GENETICS -A SHORT INTRODUCTION

PART 02

101. DIFFERENTIAL CONTRACTION THEORY
An early hypothesis explaining chiasma formation by unequal contraction.
Largely replaced by modern recombination models.

102. DIHYBRID
An organism heterozygous for two different genes.
Used in Mendel’s dihybrid crosses.

103. DIOECIOUS
Organisms having separate male and female individuals.
Example: human beings.

104. DISCONTINUOUS VARIATION
Variation with distinct categories and no intermediates.
Usually controlled by single genes.

105. DIZYGOTIC TWINS
Twins formed from two different zygotes.
They are genetically different like siblings.

106. DNA FINGERPRINTING
Technique used to identify individuals based on DNA patterns.
It analyzes variable regions of DNA.

107. DNA IS THE GENETIC MATERIAL
Concept proven by experiments like Hershey-Chase.
DNA carries hereditary information in organisms.

108. DOMINANCE
Ability of one allele to mask expression of another allele.
It determines phenotype in heterozygotes.

109. DOUBLE CROSSING OVER
Two crossing over events occurring between homologous chromosomes.
It affects recombination frequency.

110. DOUBLE STRANDED DNA
DNA consisting of two complementary strands forming a helix.
Strands are held together by hydrogen bonds.

111. DOWN’S SYNDROME
A genetic disorder caused by trisomy of chromosome 21.
It leads to intellectual disability and characteristic features.

112. DRIFT OR RANDOM GENETIC DRIFT
Random change in allele frequency in a population.
More significant in small populations.

113. DRIVER MUTATION
Mutation that contributes directly to cancer development.
It provides growth advantage to cells.

114. DUPLICATE GENES
Genes present in multiple copies within a genome.
They may evolve new functions.

115. ELONGATION OF POLYPEPTIDE CHAIN
Stage of protein synthesis where amino acids are added to growing chain.
Occurs at ribosome during translation.

116. ENCODE PROJECT
A research project to identify functional elements in the genome.
It aims to map all regulatory regions.

117. ENDONUCLEASES
Enzymes that cut DNA at internal sites.
Used in genetic engineering.

118. ENVIRONMENTAL DETERMINATION OF SEX
Sex determination influenced by environmental factors like temperature.
Common in reptiles.

119. ENVIRONMENTAL FACTORS
External influences affecting gene expression and phenotype.
Examples include nutrition, climate, and toxins.

120. ENZYME INDUCTION
Process where enzyme synthesis is increased in response to substrate.
It regulates metabolic pathways.

121. ENZYME REPRESSION
Process where enzyme synthesis is decreased due to end-product accumulation.
It helps regulate metabolic pathways efficiently.

122. EPIGENETIC
Refers to heritable changes in gene expression without DNA sequence alteration.
Includes mechanisms like DNA methylation and histone modification.

123. EPIGENETIC CODE
Pattern of chemical modifications on DNA and histones regulating gene activity.
It controls when and where genes are expressed.

124. EPISTASIS
Interaction where one gene masks or modifies expression of another gene.
It results in modified phenotypic ratios.

125. ERYTHROBLASTOSIS FOETALIS
A hemolytic disease of newborn due to Rh incompatibility.
Maternal antibodies destroy fetal red blood cells.

126. EUCHROMATIN
Loosely packed chromatin that is transcriptionally active.
It contains genes that are frequently expressed.

127. EUGENICS
Study aimed at improving genetic quality of human populations.
It involves selective breeding practices.

128. EUPHENICS
Improvement of phenotype by environmental modification.
Does not alter the genetic makeup.

129. EUPLOIDY
Condition where chromosome number is an exact multiple of haploid set.
Includes diploidy and polyploidy.

130. EXCINUCLEASE
Enzyme involved in DNA repair by removing damaged DNA segments.
Plays role in nucleotide excision repair.

131. EXOME
The portion of genome consisting of all exons.
It represents protein-coding regions.

132. EXON
Coding region of a gene that remains in mature mRNA.
It is translated into protein.

133. EXONUCLEASE
Enzyme that removes nucleotides from ends of DNA or RNA.
It functions in DNA repair and replication.

134. EXPRESSIVITY
Degree to which a gene is expressed in an individual.
It varies among individuals with same genotype.

135. FAS RECEPTOR
A cell surface receptor involved in apoptosis signaling.
Activation leads to programmed cell death.

136. FEEDBACK INHIBITION
Regulation where end product inhibits enzyme activity.
Maintains metabolic balance.

137. FINGERPRINTING
Identification technique using unique biological patterns.
In genetics, it specifically refers to DNA fingerprinting.

138. FISH (FLUORESCENT IN SITU HYBRIDIZATION)
A technique using fluorescent probes to detect DNA sequences.
Used for chromosome analysis.

139. FISHER’S THEORY
Theory explaining evolution of sex ratio towards 1:1.
Based on natural selection principles.

140. FOOTPRINTING
Technique used to identify DNA-binding sites of proteins.
Protects bound DNA regions from enzymatic cleavage.

141. FOUR STRAND DOUBLE CROSSING OVERS
Crossing over involving all four chromatids of homologous chromosomes.
Leads to maximum recombination.

142. FRAMESHIFT MUTATION
Mutation caused by insertion or deletion of nucleotides.
It alters reading frame of genetic code.

143. FRATERNAL TWINS
Another term for dizygotic twins.
They arise from two separate fertilizations.

144. FUNCTIONS OF DNA
DNA stores, replicates, and transmits genetic information.
It also directs protein synthesis.

145. GATEKEEPER
Tumor suppressor genes that regulate cell proliferation (e.g., RB gene).
Their loss leads to uncontrolled cell division.

146. GENE
Basic unit of heredity composed of DNA.
It codes for a functional product like protein.

147. GENE

A gene is the basic unit of heredity made of DNA that carries information to produce a specific protein or functional RNA.
It is located on chromosomes and determines inherited traits by controlling cellular structure and function.

148. GENE CONCEPT

The gene concept refers to the idea that genes are units of heredity responsible for transmitting traits from parents to offspring.
It has evolved from a simple inheritance unit to a molecular definition as a DNA sequence coding for functional products like proteins or RNA.

149. GENE EXPRESSION
Process by which genetic information is converted into functional products.
Includes transcription and translation.

150. GENE FLOW
Movement of genes between populations.
It reduces genetic differences.

151. GENE FREQUENCY
Proportion of a specific allele in a population.
It is a key concept in population genetics.

152. GENE POOL
Total collection of genes present in a population.
It includes all alleles of all individuals.

153. GENE THERAPY
Technique of treating diseases by modifying genes.
It involves insertion, deletion, or correction of DNA.

154. GENETIC BLOCK
A mutation that blocks a metabolic pathway.
It leads to accumulation of intermediates.

155. GENETIC CODE
Sequence of codons in mRNA specifying amino acids.
It is universal, degenerate, and non-overlapping.

156. GENETIC COUNSELING
Process of advising individuals about genetic risks.
It helps in understanding inherited disorders.

157. GENETIC DRIFT
Random fluctuation of allele frequencies in populations.
It is more prominent in small populations.

158. GENETIC ENGINEERING
Direct manipulation of DNA to alter genetic makeup.
Used in medicine, agriculture, and research.

159. GENETIC EQUILIBRIUM
State where allele frequencies remain constant over generations.
Described by Hardy–Weinberg law.

160. GENETICS
Study of heredity and variation in organisms.
It explains transmission of traits.

161. GENIC BALANCE THEORY
Theory stating sex is determined by ratio of X chromosomes to autosomes.
Proposed in Drosophila studies.

162. GENIC INTERACTION
Interaction between genes affecting phenotype expression.
Includes epistasis and complementary genes.

163. GENOME
Complete set of genetic material in an organism.
Includes all genes and non-coding sequences.

164. GENOME INSTABILITY
Increased tendency of genome to acquire mutations.
It is associated with cancer development.

165. GENOTYPE
Genetic constitution of an organism.
It determines inherited traits.

166. GENOTYPE AND PHENOTYPE
Genotype is genetic makeup, while phenotype is observable trait.
Phenotype results from gene-environment interaction.

167. GERMPLASM THEORY
Theory by Weismann stating heredity is transmitted via germ cells.
Somatic changes are not inherited.

168. GIANT CHROMOSOMES
Large chromosomes visible under light microscope.
Examples include polytene and lampbrush chromosomes.

169. GLYCOMICS
Study of structure and function of carbohydrates in cells.
It focuses on glycans and glycoproteins.

170. GROWTH FACTORS
Proteins that regulate cell growth and division.
They act through specific receptors.

171. GYNANDROMORPHS
Organisms with both male and female characteristics.
Result from abnormal chromosome distribution.

172. HAEMOPHILIA (BLEEDER’S DISEASE)
An X-linked recessive disorder affecting blood clotting.
It leads to prolonged bleeding.

173. HAIRPIN
A secondary structure formed by folding of single-stranded nucleic acids.
It involves base pairing within the strand.

174. HAPLOID
A cell containing one complete set of chromosomes.
Gametes are haploid.

175. HARDY–WEINBERG LAW
Principle stating allele frequencies remain constant in absence of evolution.
Applies under ideal population conditions.

176. HEMIZYGOUS
Presence of only one allele for a gene.
Common in X-linked genes in males.

177. HERMAPHRODITE
An organism possessing both male and female reproductive organs.
It can produce both gametes.

178. HETEROCHROMATIN
Densely packed chromatin that is transcriptionally inactive.
It contains repetitive DNA sequences.

179. HETEROCHROMATIN SEX
Sex determination involving heteromorphic sex chromosomes.
Example: XY in males.

180. HETEROZYGOTE
An individual having two different alleles of a gene.
Shows variation in traits.

181. HETEROZYGOUS
Condition of having two different alleles for a gene.
It results in variation of traits.

182. HOLANDRIC GENES
Genes located on the Y chromosome.
They are transmitted only from father to son.

183. HOMOGAMETIC SEX
Sex that produces only one type of gamete regarding sex chromosomes.
Example: females (XX) in humans.

184. HOMOGENTISIC ACID
An intermediate in tyrosine metabolism.
Its accumulation leads to alkaptonuria.

185. HOMOLOGOUS CHROMOSOMES
Pairs of chromosomes with similar structure and gene sequence.
They carry corresponding genes from each parent.

186. HOMOZYGOTE
An individual having identical alleles for a gene.
It may be dominant or recessive.

187. HUMAN CHROMOSOMES
Humans have 46 chromosomes arranged in 23 pairs.
Includes 22 autosomal pairs and 1 pair of sex chromosomes.

188. HUMAN GENETICS
Study of inheritance patterns in humans.
It includes genetic disorders and variation.

189. HUMAN GENOME PROJECT
An international project to map the entire human genome.
It identified all human genes and sequences.

190. HYBRID
Offspring produced from genetically different parents.
It shows traits of both parents.

191. HYBRIDIZATION
Process of crossing two genetically different individuals.
Used to combine desirable traits.

192. HYPOXIA-INDUCIBLE FACTORS (HIFS)
Proteins activated under low oxygen conditions.
They regulate genes for adaptation to hypoxia.

193. IDENTICAL TWINS
Twins derived from a single zygote splitting into two.
They are genetically identical.

194. IDEOGRAM
A diagrammatic representation of chromosomes showing size and banding.
Used in karyotype analysis.

195. IDEOGRAM OF HUMAN MALE
Graphical representation of male karyotype (46, XY).
Shows chromosome structure and banding patterns.

196. IMMUNE REACTION
Response of immune system to foreign substances.
It involves antigen recognition and antibody production.

197. INBORN ERROR OF METABOLISM
Genetic disorders due to enzyme deficiencies.
They disrupt normal metabolic pathways.

198. INBREEDING
Mating between closely related individuals.
It increases homozygosity.

199. INBREEDING AND OUTBREEDING
Inbreeding increases similarity, while outbreeding increases diversity.
Both affect genetic variation in populations.

200. INCOMPLETE DOMINANCE
Condition where heterozygote shows intermediate phenotype.
Example: pink flowers from red and white.

END OF PART 02