How is the information that makes us different from one another stored and read in our DNA?
Describe the structure of DNA, the relationship between genes, alleles, genotype and phenotype, and how the genetic code underpins variation
A focused answer to the WACE Year 12 Human Biology Unit 4 dot point on the genetic basis of variation. DNA structure and base pairing, the meaning of gene, allele, locus, genotype and phenotype, and how the genetic code links DNA to proteins and to variation.
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What this dot point is asking
WACE wants you to have the vocabulary and the molecular basis of inheritance secure, because every later topic (variation, selection, evidence, human evolution) rests on it. This page is the foundation: what DNA is, how it is organised into genes and alleles, and how the code links a sequence to a trait.
The structure of DNA
DNA (deoxyribonucleic acid) is the molecule that stores genetic information. It is a double helix: two strands twisted around each other. Each strand is a chain of nucleotides, and each nucleotide has a sugar (deoxyribose), a phosphate group, and one of four nitrogen bases: adenine (A), thymine (T), cytosine (C) and guanine (G). The two strands are held together by complementary base pairing: A always pairs with T, and C always pairs with G. This base pairing means each strand carries the information to rebuild the other, which is the basis of accurate DNA replication.
Genes, alleles and loci
A gene is a length of DNA that codes for a particular protein (or part of one), and proteins build and run the body, so genes ultimately determine traits. The position of a gene on a chromosome is its locus. Most genes come in alternative versions called alleles, which differ slightly in base sequence and so may produce slightly different proteins. For example, a gene may have one allele that produces a working enzyme and another that does not. Because humans are diploid, each person carries two alleles of most genes, one inherited from each parent.
Genotype and phenotype
The genotype is the combination of alleles an individual carries for a trait. The phenotype is the observable characteristic that results, such as eye colour or blood group. An individual may be homozygous (two identical alleles) or heterozygous (two different alleles) for a gene. The phenotype depends on which alleles are present, how they interact (dominant, recessive, codominant), and often on the environment as well.
The genetic code
The genetic code is the set of rules by which the base sequence of a gene is translated into the amino acid sequence of a protein. Bases are read in groups of three, called codons or triplets, and each triplet specifies one amino acid. The order of bases therefore determines the order of amino acids, which determines the protein's shape and function. A change in the base sequence can change an amino acid and so change the protein, which is how a mutation can alter a phenotype.
How this maps to the exam
Expect questions that ask you to define and use the terms gene, allele, locus, genotype and phenotype correctly, describe the base-pairing structure of DNA, or explain how the genetic code links DNA to a protein and so to a trait. This vocabulary is assumed in every later Unit 4 question, so precision here pays off throughout the paper.