Genetics is the study of heredity and the variation of inherited characteristics. To understand the principles of genetics, it’s crucial to familiarize yourself with key terms used in the field. Below is an in-depth look at essential Genetic Terminology, important for Class 12th students illustrated with examples.

1. Gene

A gene is a segment of DNA that contains the instructions for making a specific protein or set of proteins. These proteins, in turn, play critical roles in the function and development of organisms.

Example: The gene responsible for eye color in humans is located on chromosome 15. This gene influences whether a person has blue, green, or brown eyes.

2. Allele

An allele is a variant form of a gene. Different alleles can produce variations in the expression of a trait.

Example: For the eye color gene, one allele might code for brown eyes (B), while another allele codes for blue eyes (b).

3. Genotype

Genotype refers to the genetic makeup of an organism, specifically the combination of alleles it possesses for a particular gene.

Example: If a person has one allele for brown eyes and one for blue eyes, their genotype could be written as Bb. If they have two alleles for brown eyes, their genotype would be BB.

4. Phenotype

Phenotype is the observable physical or biochemical characteristics of an organism, determined by both its genotype and environmental influences.

Example: The phenotype for the Bb genotype (one brown eye allele and one blue eye allele) would be brown eyes, as the brown allele is dominant.

5. Dominant Allele

A dominant allele is one that expresses its trait even when only one copy is present in the genotype. It masks the effect of a recessive allele when both are present.

Example: In the eye color example, the brown eye allele (B) is dominant, so a person with the genotype Bb will have brown eyes.

6. Recessive Allele

A recessive allele is one that only expresses its trait when two copies are present in the genotype, meaning it is masked by a dominant allele when both are present.

Example: The blue eye allele (b) is recessive. A person will have blue eyes only if their genotype is bb (i.e., they have two copies of the recessive allele).

7. Homozygous

An organism is homozygous for a particular gene when it has two identical alleles for that gene.

Example: A person with the genotype BB (two brown eye alleles) or bb (two blue eye alleles) is homozygous for the eye color gene.

8. Heterozygous

An organism is heterozygous for a particular gene when it has two different alleles for that gene.

Example: A person with the genotype Bb (one brown eye allele and one blue eye allele) is heterozygous for the eye color gene.

9. Locus

A locus is the specific physical location of a gene on a chromosome.

Example: The locus for the CFTR gene, which is associated with cystic fibrosis, is on the long arm of chromosome 7.

10. Chromosome

A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In eukaryotic cells, chromosomes are located in the nucleus.

Example: Humans have 23 pairs of chromosomes, for a total of 46 chromosomes. Chromosome 1 is the largest, while chromosome 21 is the smallest.

11. Autosome

An autosome is any chromosome that is not a sex chromosome. Autosomes carry the majority of an organism’s genetic information.

Example: Of the 23 pairs of chromosomes in humans, 22 pairs are autosomes, and one pair is the sex chromosomes (X and Y).

12. Sex Chromosome

Sex chromosomes determine the sex of an organism and carry genes related to sexual development. In humans, these are the X and Y chromosomes.

Example: Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).

13. Codominance

Codominance occurs when two different alleles at a locus are both fully expressed in the phenotype.

Example: In human blood types, the A and B alleles are codominant. A person with one A allele and one B allele (genotype AB) will have AB blood type, with both A and B antigens present on their red blood cells.

14. Incomplete Dominance

Incomplete dominance is a situation in which neither allele is completely dominant, resulting in an intermediate phenotype.

Example: In snapdragons, a flower with a red allele (R) and a white allele (r) will have pink flowers (Rr), an intermediate color between red and white.

15. Polygenic Inheritance

Polygenic inheritance refers to the inheritance of traits that are controlled by two or more genes, often resulting in a continuous range of phenotypes.

Example: Human skin color is a polygenic trait, influenced by several different genes, leading to a wide range of skin tones.

16. Pleiotropy

Pleiotropy occurs when one gene influences multiple, seemingly unrelated phenotypic traits.

Example: The gene that causes Marfan syndrome affects connective tissues, leading to symptoms such as long limbs, heart defects, and eye problems.

17. Epistasis

Epistasis is the interaction between genes in which the expression of one gene is affected by one or more other genes.

Example: In Labrador retrievers, coat color is determined by two genes. One gene determines the pigment color (B for black, b for brown), while another gene (E) determines whether the pigment is deposited. Dogs with the genotype ee will be yellow regardless of their BB, Bb, or bb genotype.

18. Linkage

Linkage refers to the tendency of genes located close to each other on the same chromosome to be inherited together.

Example: In fruit flies, the genes for body color and wing shape are linked, so certain combinations of these traits are more likely to be inherited together.

19. Crossing Over

Crossing over is the exchange of genetic material between homologous chromosomes during meiosis, leading to new combinations of alleles in the offspring.

Example: During meiosis in humans, crossing over between homologous chromosomes can result in a child inheriting a unique combination of traits from their parents.

20. Mutation

A mutation is a change in the DNA sequence of a gene, which can result in a new allele.

Example: A mutation in the hemoglobin gene can lead to sickle cell anemia, a condition where red blood cells become misshapen and can cause various health problems.

21. Pedigree

A pedigree is a diagram that shows the occurrence of a genetic trait in several generations of a family.

Example: A pedigree chart can be used to track the inheritance of a recessive genetic disorder like cystic fibrosis within a family.

22. Punnett Square

A Punnett square is a grid used to predict the genotypes and phenotypes of offspring from a particular genetic cross.

Example: If a pea plant with genotype Tt (tall) is crossed with another Tt plant, a Punnett square can show that the offspring have a 25% chance of being TT (tall), 50% chance of being Tt (tall), and 25% chance of being tt (dwarf).