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 01

1. A-DNA
A right-handed double helical form of DNA seen under dehydrated conditions.
It is shorter and wider than B-DNA with about 11 base pairs per turn.

2. ABO BLOOD GROUP
A human blood group system based on A, B, and O alleles determining RBC antigens.
It follows codominance and multiple allele inheritance.

3. ACQUIRED CHARACTER
A trait developed during an organism’s lifetime due to environmental influence.
It is not inherited genetically to the next generation.

4. ADENOMATOUS POLYP
A benign glandular tumor arising in epithelial tissue, especially in the colon.
It is considered a precancerous lesion with malignant potential.

5. AGOUTI
A gene that controls coat color pattern in animals by regulating pigment distribution.
It produces banded hair coloration instead of uniform color.

6. ALBINISM
A genetic disorder characterized by absence of melanin pigment in skin, hair, and eyes.
It is usually inherited as an autosomal recessive trait.

7. ALKAPTON
It is a traditional terms that refers to the  metabolic intermediate related to tyrosine degradation.
Its accumulation is associated with metabolic disorders.

8. ALKAPTONURIA
An inherited metabolic disorder caused by deficiency of homogentisic acid oxidase.
It leads to dark urine and ochronosis.

9. ALLELES OR ALLELOMORPHS
Alternative forms of a gene located at the same locus on homologous chromosomes.
They determine variations of a particular trait.

10. ALLOPOLYPLOID
An organism containing multiple chromosome sets derived from different species.
It results from hybridization followed by chromosome doubling.

11. ALTERNATIVE SPLICING
A process by which a single gene produces multiple mRNA variants.
It increases protein diversity without increasing gene number.

12. AMES ASSAY
A test used to detect mutagenic potential of chemical compounds using bacteria.
It is widely used for screening carcinogens.

13. AMNIOCENTESIS
A prenatal diagnostic procedure involving sampling of amniotic fluid.
It is used to detect genetic abnormalities in the fetus.

14. ANEUPLOID OR HETEROPLOID
A condition where chromosome number is not an exact multiple of haploid set.
Examples include monosomy and trisomy.

15. ANGIOGENESIS
The formation of new blood vessels from pre-existing vessels.
It plays a key role in growth, healing, and tumor development.

16. ANTIBODY
A protein produced by B-lymphocytes in response to antigens.
It specifically binds to and neutralizes foreign substances.

17. ANTICODON
A triplet of nucleotides on tRNA complementary to an mRNA codon.
It ensures correct amino acid incorporation during translation.

18. ANTIGEN
A foreign substance that triggers an immune response in the body.
It is recognized specifically by antibodies or immune cells.

19. ANTI-SENSE RNA
A strand of RNA complementary to messenger RNA.
It regulates gene expression by blocking translation.

20. APOPTOSIS
Programmed cell death characterized by controlled cellular breakdown.
It is essential for development and tissue homeostasis.

21. ARS (AUTOSOMAL RECESSIVE SYNDROME )

A genetic disorder that occurs when an individual inherits two copies of a mutant gene, one from each parent.
Carriers are usually asymptomatic, and the condition typically appears only when both parents transmit the recessive allele.

22. ATAVISM
Reappearance of ancestral traits in an organism.
It occurs due to expression of previously suppressed genes.

23. AUTOPOLYPLOID
An organism with multiple chromosome sets from the same species.
It arises due to duplication of its own genome.

24. AUTORADIOGRAPHY
A technique that uses radioactive isotopes to visualize biological molecules.
It helps in studying DNA, RNA, and protein synthesis.

25. AUTOSOME
Any chromosome that is not involved in sex determination.
Humans have 22 pairs of autosomes.

26. BACK MUTATION
A mutation that restores the original phenotype from a mutant form.
It is also called reverse mutation.

27. BACKCROSS
A cross between a hybrid organism and one of its parents.
Used to study inheritance patterns.

28. BACTERIOPHAGE
A virus that infects and replicates within bacteria.
It is widely used in molecular genetics research.

29. BALANCED LETHAL SYSTEMS
Genetic system where lethal alleles are maintained in heterozygous condition.
Homozygous individuals die, maintaining genetic balance.

30. BARR BODY OR SEX CHROMATIN
An inactivated X chromosome in female somatic cells.
It appears as a dense chromatin mass in the nucleus.

31. BASE PAIRS
Pairs of nitrogenous bases held together by hydrogen bonds in DNA.
A pairs with T, and G pairs with C ensuring genetic stability.

32. BASE–ANALOGUE MUTATION
Mutation caused by incorporation of base analogues during DNA replication.
These analogues mimic normal bases but pair incorrectly.

33. B-DNA
The most common form of DNA under physiological conditions.
It is a right-handed helix with 10 base pairs per turn.

34. BENIGN TUMOR
A non-cancerous growth that does not invade surrounding tissues.
It usually grows slowly and remains localized.

35. BETA-THALASSEMIA
A genetic disorder caused by reduced or absent beta-globin chain synthesis.
It leads to anemia and defective hemoglobin formation.

36. BIDIRECTIONAL REPLICATION
DNA replication proceeding in both directions from the origin.
It increases the speed of DNA synthesis.

37. BIOCHEMICAL GENETICS
Study of genetic control of biochemical pathways in organisms.
It links genes with enzyme functions.

38. BIOENGINEERING
Application of engineering principles to biological systems.
Used in medical devices, tissue engineering, and biotechnology.

39. BIOETHICS
Study of ethical issues arising from biological and medical research.
It deals with topics like cloning, gene therapy, and consent.

40. BIOINFORMATICS
Use of computational tools to analyze biological data.
It is essential for genome sequencing and analysis.

41. BIOLOGIC RESPONSE MODIFIERS
Substances that modify immune system responses(eg cytokines and interferons).
They are used in treatment of cancer and immune disorders.

42. BIOPHYSICS
Application of physical principles to study biological systems.
It explains molecular structure and function.

43. BIOTECHNOLOGY
Use of living organisms to develop useful products.
Includes genetic engineering, fermentation, and cloning.

44. BLOOD TRANSFUSION
Transfer of blood or blood components from donor to recipient.
Compatibility of blood groups is essential to avoid reactions.

45. BLOOM SYNDROME
A genetic disorder characterized by short stature and genomic instability.
It increases susceptibility to cancers.

46. BLUNT-ENDED DNA
DNA fragments with no overhanging ends after cleavage.
They are joined by DNA ligase without base pairing.

47. BURKITT LYMPHOMA
A highly aggressive B-cell cancer associated with chromosomal translocation.
Often linked with Epstein-Barr virus infection.

48. CAGE (CAP ANALYSIS OF GENE EXPRESSION)
A technique to identify transcription start sites in genes.
It analyzes the 5′ end of mRNA.

49. CANCER
A group of diseases involving uncontrolled cell growth and division.
It may invade tissues and spread to other parts of the body.

50. CARCINOGEN
A substance capable of causing cancer.
It induces mutations or promotes tumor formation.

51. CARCINOMA
A malignant tumor arising from epithelial cells.
It is the most common type of cancer.

52. CARRIER
An individual who carries a recessive allele without showing symptoms.
They can transmit the trait to offspring.

53. CASPASES
Proteolytic enzymes involved in apoptosis.
They play a key role in programmed cell death.

54. CDNA (COMPLEMENTARY DNA)
DNA synthesized from mRNA using reverse transcriptase.
It represents expressed genes without introns.

55. CELL CYCLE
The sequence of events leading to cell division.
It includes interphase and mitotic phase.

56. CENTRIOLE
A cylindrical organelle involved in spindle formation during cell division.
Found in animal cells.

57. CENTROMERE
The region of chromosome where sister chromatids are joined.
It is the attachment site for spindle fibers.

58. CENTROSOME
A microtubule-organizing center containing centrioles.
It regulates spindle formation during mitosis.

59. CHANGES IN THE STRUCTURE OF CHROMOSOME
Alterations such as deletion, duplication, inversion, or translocation.
These changes may lead to genetic disorders.

60. CHIASMA TYPE THEORY
Theory explaining crossing over through chiasmata formation.
It occurs during meiosis between homologous chromosomes.

61. CHIMERIC MOLECULE
A molecule composed of genetic material from different sources.
It is commonly produced using recombinant DNA technology.

62. CHIP (CHROMATIN IMMUNOPRECIPITATION)
A technique used to study protein-DNA interactions in cells.
It identifies binding sites of DNA-associated proteins.

63. CHIP-CHIP
A method combining ChIP with microarray analysis.
It is used to detect genome-wide protein-DNA interactions.

64. CHIP-EXO
An advanced ChIP technique with exonuclease trimming.
It provides high-resolution mapping of protein-DNA binding sites.

65. CHIP-SEQ
A method combining ChIP with DNA sequencing.
It identifies protein binding regions across the genome.

66. CHI-SQUARE (Χ²)
A statistical test used to compare observed and expected results.
It is widely used in genetic inheritance studies.

67. CHROMATID
One of the two identical strands of a duplicated chromosome.
They are joined together at the centromere.

68. CHROMATIN REMODELING
Dynamic modification of chromatin structure to regulate gene expression.
It allows access of transcription machinery to DNA.

69. CHROMOSOMAL ABERRATIONS
Structural or numerical changes in chromosomes.
They may lead to genetic disorders or cancer.

70. CHROMOSOMAL INSTABILITY (CIN)
A condition with increased rate of chromosomal changes.
It is commonly seen in cancer cells.

71. CHROMOSOMAL THEORY OF SEX DETERMINATION
Theory stating sex is determined by sex chromosomes (X and Y).
Proposed by Wilson and Stevens based on insect studies.

72. CHROMOSOMAL TRANSLOCATION
Exchange of segments between non-homologous chromosomes.
It may be balanced or unbalanced.

73. CHROMOSOME MAP
A diagram showing the relative positions of genes on a chromosome.
Based on recombination frequencies.

74. CHROMOSOME MAPPING
The process of determining gene locations on chromosomes.
It helps in studying linkage and inheritance.

75. CIRCULAR DNA
DNA molecule forming a closed loop structure.
Commonly found in bacteria and plasmids.

76. CISTRON
A functional unit of DNA coding for a polypeptide.
Equivalent to a gene in classical genetics.

77. CLIP (CROSS-LINKING IMMUNOPRECIPITATION)
A technique to study RNA-protein interactions.
It identifies binding sites on RNA molecules.

78. CLONE
A genetically identical copy of a cell or organism.
Produced through asexual reproduction or cloning techniques.

79. CLONING THE NIF GENE
Transfer of nitrogen fixation gene into another organism.
Used to improve nitrogen-fixing ability in crops.

80. CODOMINANCE
A genetic condition where both alleles express equally.
Example: AB blood group.

81. CODON
A triplet of nucleotides in mRNA coding for an amino acid.
It determines the sequence of proteins.

82. COEFFICIENT OF COINCIDENCE
Ratio of observed to expected double crossovers.
Used to measure interference.

83. COLOUR BLINDNESS
A genetic disorder affecting color perception.
Often X-linked and affects males more frequently.

84. COMPARATIVE GENOMICS
Study of similarities and differences between genomes of organisms.
Helps understand evolution and gene function.

85. COMPLEMENTARY GENES
Genes that interact to produce a phenotype only when both are present.
They follow non-Mendelian inheritance patterns.

86. COMPLETE DOMINANCE
A condition where one allele completely masks the effect of another.
Only the dominant phenotype is expressed.

87. CONJUGATION
Transfer of genetic material between bacteria through direct contact.
Mediated by sex pili.

88. CONTINUOUS VARIATION
Variation showing a range of phenotypes without distinct categories.
Controlled by multiple genes and environment.

89. COPY NUMBER VARIATION (CNV)
Variation in the number of copies of a DNA segment.
It contributes to genetic diversity and disease.

90. COSMID
A hybrid vector combining plasmid and phage DNA.
Used for cloning large DNA fragments. 

91. COUPLING
The inheritance of linked genes together on the same chromosome.
It reduces recombination between closely located genes.

92. CRIS-CROSS OR ZIG-ZAG INHERITANCE
Pattern of inheritance where traits pass from father to daughter to grandson.
Commonly seen in X-linked traits.

93. CRISPR/CAS
A gene-editing system derived from bacterial immune defense.
It allows precise modification of DNA sequences.

94. CROSSING OVER
Exchange of genetic material between homologous chromosomes during meiosis.
It increases genetic variation.

95. CYTOPLASMIC INHERITANCE
Transmission of traits through cytoplasmic organelles like mitochondria.
It is usually maternally inherited.

96. DEFICIENCY
Loss of a segment of a chromosome.
It may lead to genetic disorders.

97. DEGENERATE CODE
Property of genetic code where multiple codons code for the same amino acid.
It provides protection against mutations.

98. DELETION
Removal of a DNA segment from a chromosome.
It may cause severe genetic effects.

99. DEOXYRIBONUCLEIC ACID (DNA)
The hereditary material carrying genetic information in cells.
It consists of double-stranded helix of nucleotides.

100. DIFFERENCES BETWEEN GENOTYPE AND PHENOTYPE
Genotype refers to genetic makeup of an organism.
Phenotype refers to observable characteristics influenced by genes and environment.

END OF PART 01