Genetics: Heredity, Mendel's Laws, DNA, Genes and Genetic Disorders
Genetics is the branch of biology that studies heredity, that is, how characters are passed from parents to their offspring, and how the information for those characters is stored and expressed inside the cell. Every living organism inherits a set of instructions from its parents, and these instructions decide whether a pea plant will be tall or short, whether a child will have brown eyes or blue, and whether a particular disease will run in a family. Genetics explains all of this in a precise, scientific way.
For students preparing for APSC, ADRE, DHS, DME, SSC, Railways, NEET and similar examinations in Assam, genetics is one of the most frequently asked topics in the biology and general science papers. Questions are asked on Mendel's experiments, the laws of inheritance, the structure of DNA, the difference between DNA and RNA, sex determination, blood groups, and common genetic disorders. This article gives a complete deep-study revision of every important concept with the reasoning behind it, so that the facts stay in memory.
Basic Terms
- Heredity: the transmission of characters from parents to offspring.
- Variation: the differences in characters among individuals of the same species.
- Gene: the basic unit of heredity, a segment of DNA that carries the information for a particular trait.
- Allele: the alternative form of a gene, for example the tall and dwarf forms of the height gene.
- Genotype: the genetic makeup of an organism, the actual combination of alleles it carries.
- Phenotype: the observable physical or biochemical character of an organism.
- Homozygous: an organism that carries two identical alleles for a gene, such as TT or tt.
- Heterozygous: an organism that carries two different alleles for a gene, such as Tt.
- Dominant allele: an allele that expresses itself even when only one copy is present.
- Recessive allele: an allele that expresses itself only when both copies are present.
- Chromosome: a thread-like structure of DNA and protein that carries the genes.
- Mutation: a sudden, heritable change in the sequence of DNA.
Core Concepts and Formulas
- Monohybrid cross: a cross involving one pair of contrasting characters. The F2 phenotypic ratio is 3:1 and the genotypic ratio is 1:2:1.
- Dihybrid cross: a cross involving two pairs of contrasting characters. The F2 phenotypic ratio is 9:3:3:1.
- Test cross: crossing an individual of unknown genotype with the recessive parent to find out its genotype.
- Back cross: crossing the F1 offspring with either of its parents.
- Number of gamete types = 2n, where n is the number of heterozygous gene pairs.
- Number of F2 phenotypes in a full heterozygote cross = 2n, and genotypic combinations = 3n.
- Blood group inheritance: the ABO system has three alleles, IA, IB and i. IA and IB are codominant, and both are dominant over i.
Mendel and His Experiments
Gregor Johann Mendel, an Austrian monk, is called the father of genetics. He worked on the garden pea, Pisum sativum, from 1856 to 1863 and published his results in 1866. He chose the pea plant because it is self-pollinating, has a short life cycle, and shows several pairs of clearly contrasting characters such as tall and dwarf, round and wrinkled seeds, yellow and green seeds, and axial and terminal flowers. Mendel studied seven such pairs of characters one at a time and kept careful numerical records. His success came from using pure-breeding parents and from counting large numbers of offspring, which allowed the ratios to emerge clearly.
The Laws of Inheritance
From his experiments, Mendel proposed three laws. The Law of Dominance states that in a heterozygous condition, the dominant allele expresses itself and hides the recessive allele. The Law of Segregation states that the two alleles of a gene separate during gamete formation and each gamete receives only one allele. The Law of Independent Assortment states that the alleles of different genes are distributed independently of one another during gamete formation. During a monohybrid cross, the F1 generation shows only the dominant character, and the recessive character reappears in the F2 generation in the ratio 3:1. This reappearance proves that the recessive factor is not destroyed but only hidden.
DNA, RNA and the Central Dogma
DNA, or deoxyribonucleic acid, is a double-stranded helix made of nucleotides. Each nucleotide has a deoxyribose sugar, a phosphate group and a nitrogenous base. The four bases are adenine, guanine, cytosine and thymine. Adenine always pairs with thymine by two hydrogen bonds, and guanine pairs with cytosine by three hydrogen bonds. This complementary pairing, described by Watson and Crick in 1953, explains how DNA can copy itself accurately. RNA is usually single-stranded and has ribose sugar; in RNA, uracil replaces thymine. The central dogma of molecular biology states that information flows from DNA to RNA to protein. The process of forming RNA from DNA is called transcription, and the process of forming protein from RNA is called translation.
Chromosomes and Sex Determination
Chromosomes are present in the nucleus and carry the genes in a linear order. In humans there are 46 chromosomes, arranged in 23 pairs, of which 22 pairs are autosomes and one pair is the sex chromosomes. In females the sex chromosomes are XX and in males they are XY. Hence the human male is heterogametic, producing two types of sperms, one with X and one with Y, while the female is homogametic and produces only X-bearing eggs. The sex of the child is therefore determined by the father. The union of an X-bearing sperm with an egg gives a girl (XX), and the union of a Y-bearing sperm with an egg gives a boy (XY). This is called the XX-XY system of sex determination.
Mutations and Genetic Disorders
A mutation is a change in the base sequence of DNA that may or may not produce a visible effect. Mutations can occur spontaneously or may be induced by physical agents such as ultraviolet rays and X-rays, or by chemical agents. Many genetic disorders are caused by mutations or by an abnormal number of chromosomes. Colour blindness and haemophilia are X-linked recessive disorders, so they appear more often in males. Thalassaemia and sickle-cell anaemia are caused by defective haemoglobin genes. Down syndrome is caused by trisomy of chromosome 21, giving 47 chromosomes, and it is a classic example of a chromosomal disorder in which the risk increases with the age of the mother.
How Numerical Problems Are Solved
Most genetics numericals are solved by writing the parental genotypes, identifying the gametes, and then drawing a Punnett square. For a monohybrid cross between TT and tt, the F1 is all Tt and the F2 gives TT:Tt:tt in 1:2:1, that is a phenotypic ratio of 3:1. For a dihybrid cross between RRYY and rryy, the F2 phenotypic ratio is 9:3:3:1. When a question asks how many types of gametes are formed, use 2n. Always write the dominant allele in capital letters and the recessive in small letters, and keep the trait symbols consistent. For blood group problems, remember that IA and IB are codominant, so AB blood group shows both antigens.
Examination Trends
In competitive examinations, the most repeated areas are Mendel's laws, the 3:1 and 9:3:3:1 ratios, the structure of DNA and its base pairing, the difference between DNA and RNA, sex determination, and the names of common genetic disorders. Objective questions frequently ask the father of genetics, the organism used by Mendel, the number of chromosomes in humans, and the type of inheritance in haemophilia and colour blindness. A smaller number of questions test pedigree analysis and the concept of codominance as seen in the AB blood group.
Common Mistakes to Avoid
- Do not confuse dominance with the mere presence of an allele; dominance is about expression.
- Do not interchange genotype (letters) with phenotype (appearance).
- Remember that the 9:3:3:1 ratio applies only to a dihybrid cross with independent assortment.
- Do not write uracil in DNA or thymine in RNA; the base differs between the two.
- For sex-linked disorders, remember they are more common in males, not females.
How to Prepare This Topic
Begin with the vocabulary, because most questions are framed using terms like allele, genotype and phenotype. Practise drawing Punnett squares by hand until the monohybrid and dihybrid ratios become automatic. Make a small table that compares DNA and RNA, and another table that lists genetic disorders with their cause and inheritance pattern. Revise the lives and contributions of the scientists mentioned, such as Mendel, Watson and Crick. Finally, attempt previous years' question papers so that you become familiar with the exact wording used in examinations.
Frequently Asked Questions
Who is called the father of genetics and why?
Gregor Johann Mendel is called the father of genetics because he was the first to work out the rules of inheritance through careful experiments on the garden pea and to explain them using the laws of dominance, segregation and independent assortment.
What is the difference between a gene and an allele?
A gene is a segment of DNA that controls a particular trait, while an allele is one of the alternative forms of that gene. A gene occupies a fixed position on a chromosome, and the different alleles of that gene can occupy that same position.
Why do colour blindness and haemophilia appear more in males?
Both are X-linked recessive disorders. A male has only one X chromosome, so a single recessive allele on that X is enough to express the disorder. A female needs the recessive allele on both her X chromosomes, which is far less likely.
What is the cause of Down syndrome?
Down syndrome is caused by the presence of an extra copy of chromosome 21, a condition called trisomy 21. This gives the affected person 47 chromosomes instead of the normal 46 and leads to characteristic physical and intellectual features.
What is the central dogma of molecular biology?
The central dogma states that genetic information flows from DNA to RNA to protein. DNA is transcribed into messenger RNA, and the messenger RNA is translated into a chain of amino acids that folds into a protein.