Genetics questions in general science are almost entirely definitional and numerical — which ratio comes from a monohybrid cross, how many chromosomes a human cell has, which disorder is sex-linked, who discovered the structure of DNA. The facts do not change from year to year. This post covers the vocabulary of heredity, Mendel's crosses and their ratios, DNA and chromosomes, sex determination and genetic disorders, followed by 15 previous year questions.
Key Points (Quick Revision)
- Gregor Johann Mendel is the Father of Genetics; he worked on the garden pea
- Monohybrid cross gives a 3:1 phenotypic ratio; dihybrid gives 9:3:3:1
- Human cells have 46 chromosomes — 22 pairs of autosomes plus 1 pair of sex chromosomes
- XX is female, XY is male — the father determines the sex of the child
- Haemophilia and colour blindness are X-linked recessive; Watson and Crick gave the DNA double helix in 1953
1. The vocabulary of heredity
Half the marks in this topic come from knowing which word means what. Papers test the terms directly, and they also use one term as a distractor for another.
| Term | What it means |
|---|---|
| Gene | A segment of DNA that carries the instruction for one character |
| Allele | The alternative forms of the same gene, such as tall and dwarf |
| Genotype | The genetic make-up, written as TT, Tt or tt |
| Phenotype | The visible character that results, such as tall or dwarf |
| Homozygous | Both alleles are the same — TT or tt, also called pure |
| Heterozygous | The two alleles differ — Tt, also called hybrid |
| Dominant | The allele that shows in the phenotype even when present once |
| Recessive | The allele that shows only when present in both copies |
Two points are worth fixing here. Genotype is written, phenotype is seen — a plant with the genotype Tt has the phenotype tall, because T is dominant. And the unit of heredity is the gene, while the gene itself sits on a chromosome, so a question asking for the "carrier of hereditary characters" wants chromosome, not gene.
2. Mendel's experiments and the ratios that follow
Gregor Johann Mendel, an Austrian monk, worked on the garden pea (Pisum sativum) and published his findings in 1866. He is called the Father of Genetics. He chose seven pairs of contrasting characters — among them tall and dwarf plants, round and wrinkled seeds, and yellow and green seed colour.
A monohybrid cross follows one character. Crossing a pure tall plant (TT) with a pure dwarf plant (tt) gives an F1 generation that is entirely tall (Tt), because T is dominant. Self-pollinating that F1 gives the F2 generation, and this is where the ratios appear:
- Phenotypic ratio 3:1 — three tall to one dwarf
- Genotypic ratio 1:2:1 — one TT, two Tt, one tt
A dihybrid cross follows two characters together, and its F2 phenotypic ratio is 9:3:3:1. This number is asked directly and is worth memorising as it stands.
Mendel's conclusions are stated as three laws:
| Law | What it says |
|---|---|
| Law of Dominance | In a heterozygous pair, only the dominant allele expresses itself |
| Law of Segregation | The two alleles separate during gamete formation, so each gamete carries only one — also called the law of purity of gametes |
| Law of Independent Assortment | Two different characters are inherited independently of each other |
Papers regularly ask which law is not a Mendelian law, or which law the dihybrid cross demonstrates — the answer to the second is independent assortment, since it needs two characters. A test cross crosses an individual with a homozygous recessive partner to find out whether it is TT or Tt.
3. DNA, RNA and chromosomes
DNA — deoxyribonucleic acid — carries the genetic information. Its double-helix structure was given by James Watson and Francis Crick in 1953, work for which they shared the Nobel Prize with Maurice Wilkins in 1962. The name most often used as a distractor here is Rosalind Franklin, whose X-ray diffraction images were central to the discovery.
The structural facts exams ask about are few and fixed:
- DNA has two strands wound as a double helix; RNA normally has one strand
- The sugar in DNA is deoxyribose; in RNA it is ribose
- DNA bases are adenine, guanine, cytosine and thymine; in RNA, uracil replaces thymine
- Base pairing is A with T and G with C, held by hydrogen bonds
- Adenine and guanine are purines; cytosine, thymine and uracil are pyrimidines
A chromosome is the thread-like structure in the nucleus that carries the genes, made of DNA and protein. The term was coined by Waldeyer, and chromosomes are visible during cell division.
The human numbers are asked constantly. A human body cell has 46 chromosomes, that is 23 pairs — 22 pairs of autosomes and one pair of sex chromosomes. A gamete, being haploid, has 23. The complete set of human genetic material was mapped by the Human Genome Project, which ran from 1990 and was completed in 2003.
4. Sex determination and genetic disorders
In humans, a female is XX and a male is XY. The mother's egg always carries an X chromosome, while the father's sperm may carry either an X or a Y. This gives the single most asked conclusion in the chapter: the father determines the sex of the child.
Genetic disorders fall into three groups, and the grouping is what questions test.
Sex-linked (X-linked recessive) disorders appear far more often in males, because a male has only one X chromosome and so has no second copy to mask the defective allele.
- Haemophilia — blood fails to clot normally; also called the royal disease
- Colour blindness — usually an inability to distinguish red and green, tested using Ishihara charts
- Duchenne muscular dystrophy — progressive weakening of muscles
Autosomal disorders are carried on the non-sex chromosomes and affect both sexes equally.
- Sickle cell anaemia — red blood cells take a sickle shape; a defect in haemoglobin
- Thalassemia — reduced production of normal haemoglobin
- Albinism — absence of the pigment melanin
- Phenylketonuria — a metabolic disorder involving phenylalanine
Chromosomal disorders come from a wrong number of chromosomes rather than a faulty gene.
| Disorder | Chromosome position | Affects |
|---|---|---|
| Down syndrome | Trisomy of chromosome 21 — 47 in all | Both sexes |
| Turner syndrome | Single X, written 45 XO | Females |
| Klinefelter syndrome | An extra X, written 47 XXY | Males |
The distinction to hold on to is this: haemophilia and colour blindness are sex-linked, sickle cell anaemia and thalassemia are not. Options swap these two groups almost every time the topic appears.
5. Previous year questions
- Who is known as the Father of Genetics? — Gregor Johann Mendel
- On which plant did Mendel conduct his experiments? — Garden pea (Pisum sativum)
- What is the phenotypic ratio of a monohybrid cross in the F2 generation? — 3:1
- What is the genotypic ratio of a monohybrid cross? — 1:2:1
- What is the phenotypic ratio of a dihybrid cross? — 9:3:3:1
- How many chromosomes are present in a human body cell? — 46, that is 23 pairs
- How many pairs of autosomes does a human cell have? — 22 pairs
- Which parent determines the sex of a child? — The father
- Who discovered the double-helix structure of DNA? — Watson and Crick, in 1953
- Which nitrogen base is found in RNA but not in DNA? — Uracil
- Which disease is known as the royal disease? — Haemophilia
- Down syndrome is caused by an extra copy of which chromosome? — Chromosome 21
- Turner syndrome is represented by which chromosome pattern? — 45 XO
- What is the term for the genetic make-up of an organism? — Genotype
- In which year was the Human Genome Project completed? — 2003
6. How to revise this topic
Begin with the term table, because the vocabulary is what the rest of the chapter is built on. Say each term with an example attached — genotype Tt, phenotype tall — so that the pair never separates in your memory.
Then fix the three ratios as plain numbers: 3:1, 1:2:1 and 9:3:3:1. Attach each to its cross rather than learning them as a list, because a question will give you the cross and ask for the ratio, or the reverse.
For the disorders, revise by group rather than by name. Keep the three sex-linked ones together, the autosomal ones together and the three chromosomal ones together with their numbers. When the options arrive mixed, you will be sorting them by group rather than trying to recall each disease separately.
Finally, keep the numbers of this chapter in one line of your notes — 46, 23 pairs, 22 autosomes, 1953, 2003, chromosome 21. These are the facts that appear year after year, and none of them will change.
7. Frequently Asked Questions
Why do more males suffer from colour blindness than females?
Because colour blindness is X-linked recessive. A male has only one X chromosome, so a single defective allele expresses itself. A female has two X chromosomes and needs the defective allele on both, which is far less likely, so she is usually only a carrier.
What is the difference between genotype and phenotype?
Genotype is the genetic make-up, written with letters such as TT, Tt or tt. Phenotype is the visible character that results, such as tall or dwarf. A plant with genotype Tt has the phenotype tall, because T is dominant.
Which parent determines the sex of a child and why?
The father. The mother's egg always carries an X chromosome, while the father's sperm may carry an X or a Y. An X-bearing sperm gives XX, a female, and a Y-bearing sperm gives XY, a male.
Is sickle cell anaemia a sex-linked disorder?
No. Sickle cell anaemia is an autosomal disorder, carried on a non-sex chromosome, so it affects males and females equally. Haemophilia and colour blindness are the sex-linked ones, and options frequently swap the two groups.
What is the difference between a gene and a chromosome?
A gene is a segment of DNA carrying the instruction for one character, and it is the unit of heredity. A chromosome is the thread-like structure in the nucleus, made of DNA and protein, that carries many genes along its length.
For more General Science topics in simple language, see our Cell Structure and Functions and Blood Groups and Human Blood posts, or browse the General Science section. Preparing for a specific exam? Start with the SSC CGL guide or join the 100 Hour GS Course.