Genetics Questions and Answers
Genetics explains how characters pass from parents to offspring. It begins with Mendel's experiments on the garden pea and his laws, then moves to the chromosome as the carrier of heredity, the structure and replication of DNA, how the sex of a child is determined, and finally mutation.
Study this chapter
About Genetics
Medium ~100 min study
Children resemble their parents, but never exactly. That ordinary observation is the whole subject of this chapter, and it was turned into a science by one man working in a monastery garden. Gregor Johann Mendel chose the garden pea, picked seven pairs of contrasting traits, and counted the offspring carefully enough that the numbers themselves revealed the rules. His conclusions held long before anyone knew what a gene was made of.
The chapter then supplies what Mendel could not. His factors turned out to sit on chromosomes, thread like structures inside the nucleus, and the chemical that carries the information is DNA. So the material moves from ratios counted in a garden, to the shape of a chromosome and the positions its centromere may take, to the double helix itself and the way it copies before a cell divides.
Two shorter topics close the chapter, and both matter beyond the examination. Sex determination explains that the sperm decides whether a child is a boy or a girl, which corrects a belief still widely held. Mutation explains how the genetic material itself can change, which links this chapter to evolution and to several inherited conditions.
What you'll learn
- Describe Mendel's experiments and explain why the garden pea suited them.
- State the laws of segregation and independent assortment and link each to its ratio.
- Work out the phenotypic ratios of monohybrid and dihybrid crosses.
- Label the parts of a chromosome and name the four types by centromere position.
- State the human chromosome number and separate autosomes from allosomes.
- Describe the structure of DNA and the base pairing rule.
- Name the enzymes of DNA replication and state what each one does.
- Explain how the sex of a child is determined and define mutation.
Before you start
- The structure of a cell and the position of the nucleus within it.
- An understanding of cell division and the formation of gametes.
- Familiarity with the idea that characters are passed from parents to offspring.
Topics covered in this chapter
Genetics explained
What this chapter covers
Mendel and his experiments
Mendel worked with the garden pea because it breeds true, is easy to cross and produces many offspring quickly. He chose seven pairs of distinguishing traits: flower colour and position, seed colour and shape, pod colour and shape, and stem length. Every pea plant carries two factors for a given character, and that pair is called an allele.
Monohybrid and dihybrid crosses
A cross following one pair of contrasting traits is a monohybrid cross, and it gives three dominant to one recessive in the second generation. A cross following two pairs at once is a dihybrid cross, and it gives the ratio nine to three to three to one. Each ratio is evidence for a different rule.
Laws of inheritance
Segregation states that the two alleles of a pair separate during gamete formation, so each gamete carries only one allele of a gene. Independent assortment states that different pairs separate independently of one another, and it is precisely this independence that produces the dihybrid ratio.
Chromosomes and their structure
Chromosomes are the thin thread like structures in the nucleus that carry the hereditary material. A chromosome consists of a primary constriction, the centromere, a secondary constriction, the telomere at its terminal end and sometimes a satellite, which is an elongated knob like appendage. The centromere is where spindle fibres attach during cell division.
Types of chromosomes and karyotype
The position of the centromere gives four types. It lies at one end in a telocentric chromosome, near one end in an acrocentric, away from the centre in a sub metacentric, and exactly at the centre in a metacentric. The karyotype is the number and appearance of the complete set of chromosomes of a cell. Each human cell holds 23 pairs, of which 22 pairs are autosomes and one pair are allosomes.
Structure and replication of DNA
Each nucleotide of DNA is built from a deoxyribose sugar, a nitrogenous base and a phosphate group. Alternating sugar and phosphate units form the backbone, while the bases pair across the middle, always a purine with a pyrimidine, which keeps the helix a constant width. During replication helicase unwinds the strands, DNA polymerase builds new complementary strands, and DNA ligase joins the short Okazaki fragments formed on one of them.
Sex determination and mutation
The mother contributes an X chromosome to every ovum, while the father's sperm may carry either an X or a Y. The sperm therefore determines the sex of the child, and the mother is not responsible for it. Mutation is an inheritable change in the amount or structure of a gene or a chromosome. Losing or gaining individual chromosomes is called aneuploidy, in contrast to changes involving whole sets.
Common mistakes to avoid
- Confusing the centromere with the centrosome. The centromere is part of the chromosome; the centrosome lies outside the nucleus.
- Giving the backbone of DNA as phosphate alone or sugar alone. It is the alternating sugar phosphate units together.
- Attributing the joining of Okazaki fragments to DNA polymerase. Polymerase adds nucleotides; ligase seals the gaps between fragments.
- Stating the human chromosome number as 46 pairs. There are 23 pairs, which is 46 chromosomes in total.
- Holding the mother responsible for the sex of a child. The sperm carries either X or Y and so decides it.
- Using aneuploidy and polyploidy interchangeably. Aneuploidy involves single chromosomes; polyploidy involves whole sets.
Test yourself on these with the practice test, then check the worked reasoning in the solved MCQs.
Frequently asked questions
What is an allele according to Mendel?
Every pea plant carries two factors that are responsible for producing a particular character or trait, and that pair of genes is called an allele. The important point for the examination is that the word names a pair, not a single factor, which is what distinguishes it from the other options usually offered.
Why does a dihybrid cross give a 9:3:3:1 ratio?
A dihybrid cross follows two pairs of contrasting traits at the same time. Because the two pairs of alleles separate independently of each other during gamete formation, every combination of the two characters appears among the offspring in predictable proportions. That independence is Mendel's law of independent assortment, and the ratio nine to three to three to one is its evidence.
What is the difference between a centromere and a centrosome?
The centromere is the primary constriction of a chromosome and is the region to which spindle fibres attach during cell division. The centrosome is an entirely separate organelle lying outside the nucleus which organises the spindle itself. The names are similar and are frequently offered together as options, so the distinction is worth fixing firmly.
How are chromosomes classified by centromere position?
There are four types. In a telocentric chromosome the centromere lies at one end. In an acrocentric it lies very near one end, giving one very short arm. In a sub metacentric it lies away from the middle, giving arms of unequal length. In a metacentric it lies exactly at the centre, giving two arms of equal length.
How many chromosomes do human beings have?
Each human cell contains 23 pairs of chromosomes, which is 46 in total. Of these, 22 pairs are autosomes and the remaining single pair are the allosomes, also called the sex chromosomes. A common error in the examination is to write 46 pairs instead of 46 chromosomes.
What forms the backbone of a DNA molecule?
Alternating deoxyribose sugar and phosphate units form the two strands that run along the outside of the molecule, and together they are called the sugar phosphate backbone. The nitrogenous bases project inward from these strands and pair across the middle, always a purine with a pyrimidine.
What are Okazaki fragments and how are they joined?
During DNA replication one strand cannot be copied continuously, so it is built as a series of short pieces called Okazaki fragments. DNA polymerase adds the nucleotides that make up each fragment, and the enzyme DNA ligase then seals the gaps between neighbouring fragments to produce one continuous strand.
Who determines the sex of a child?
The sperm produced by the father determines the sex of the child. Every ovum carries an X chromosome, while a sperm may carry either an X or a Y. If an X bearing sperm fertilizes the ovum the child is female, and if a Y bearing sperm does so the child is male. The mother is not responsible for determining the sex.
Last updated 21 September 2026