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Biology
Cells, Genes & Evolution
From the inside of a cell to the logic of natural selection.
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What is generally considered the basic unit of life?
Genes, Alleles & Inheritance
This one goes past the school diagram of a double helix. Expect questions on how the code is read, how it is copied into the next generation, and what the numbers actually are.
About this topic
Genetics started as accountancy. Gregor Mendel counted pea plants, noticed that traits appeared in tidy ratios, and concluded that inheritance works through discrete units rather than a blending of parents. He had no idea what those units were made of, and his paper sat ignored for thirty-five years. The chemistry came later: the structure of DNA in 1953, the genetic code through the 1960s, the human genome sequence in the early 2000s. Each step turned Mendel's abstract units into something physical you can read off a machine. These ten questions run across that whole span, from the base pairing rules to the surprisingly modest number of genes a human turns out to have.
Questions & answers
Genes, Alleles & Inheritance: all 10 questions and answers
Every question in this quiz is listed below with its correct answer and the reasoning behind it. Play first if you would rather not see the answers — or read through as a study sheet.
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1In DNA, which base always pairs with adenine?
- A Uracil
- B Thymine Correct
- C Guanine
- D Cytosine
Answer: B. Thymine
Why: Adenine and thymine are held together by two hydrogen bonds, while guanine and cytosine share three, so stretches rich in G and C are harder to pull apart. In RNA, uracil takes thymine's place.
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2Whose experiments on pea plants established the basic rules of inheritance?
- A Gregor Mendel Correct
- B Carl Linnaeus
- C Hugo de Vries
- D Thomas Hunt Morgan
Answer: A. Gregor Mendel
Why: Mendel tracked seven traits through thousands of plants in a monastery garden in the 1860s and worked out that traits pass on as separate units. His results were rediscovered independently by three botanists around 1900, long after his death.
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3What is an allele?
- A A single strand of messenger RNA
- B A chromosome carrying no active genes
- C The protein that a gene produces
- D One of the alternative versions of a gene Correct
Answer: D. One of the alternative versions of a gene
Why: Alleles differ by small changes in the DNA sequence at the same position, and you inherit one from each parent. Some genes have only two common alleles, while others, such as those controlling immune recognition, have hundreds.
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4If both parents carry one copy of a recessive allele, what is the chance a child inherits two copies?
- A 75 per cent
- B 100 per cent
- C 25 per cent Correct
- D 50 per cent
Answer: C. 25 per cent
Why: Each parent passes on either of their two alleles with equal probability, so one of the four possible combinations gives a double dose. This is why a recessive condition can appear in a child whose parents show no sign of it.
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5What is the process of copying a gene's DNA into messenger RNA called?
- A Recombination
- B Transcription Correct
- C Translation
- D Replication
Answer: B. Transcription
Why: An enzyme called RNA polymerase runs along one strand and builds a matching RNA copy, which then carries the instruction out to a ribosome. Translation is the separate step where that message is turned into a protein.
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6How many bases make up a codon, the unit specifying one amino acid?
- A Three Correct
- B Two
- C Four
- D Six
Answer: A. Three
Why: Three bases give 64 possible codons, more than enough for 20 amino acids plus start and stop signals, so most amino acids are spelled by several codons. The code is very nearly identical in bacteria, plants and people.
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7Which pair of sex chromosomes is typical in a human male?
- A XX
- B YY
- C XXY
- D XY Correct
Answer: D. XY
Why: The Y carries the SRY gene, which switches on the development of testes early in the embryo. It is far smaller than the X and has shed most of its genes over evolutionary time.
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8How does meiosis differ from ordinary cell division?
- A It produces cells without a nucleus
- B It produces exact copies of the parent cell
- C It produces cells with half the usual number of chromosomes Correct
- D It produces cells with twice the usual number
Answer: C. It produces cells with half the usual number of chromosomes
Why: Halving the chromosome count in eggs and sperm means fertilisation restores a full set rather than doubling it each generation. Meiosis also swaps segments between paired chromosomes, which is a large part of why siblings differ.
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9Roughly how many protein-coding genes does the human genome contain?
- A About 500,000
- B About 20,000 Correct
- C About 2,000
- D About 100,000
Answer: B. About 20,000
Why: Estimates before sequencing ran as high as 100,000, so the final figure came as a shock; a nematode worm has a comparable number. Only around 2 per cent of human DNA codes for protein at all.
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10What does the term phenotype describe?
- A The observable characteristics of an organism Correct
- B The full set of genes an organism carries
- C A mutation in a single gene
- D The number of chromosomes in a cell
Answer: A. The observable characteristics of an organism
Why: Phenotype covers anything you can see or measure, from height to blood group, and it depends on the environment as well as the genes. Two plants with identical genotypes will end up different sizes if one is grown in shade.
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