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Chromosomes, Genes, Alleles, & Mutations (3.1/3.2)

Gene WebQuest:

3.1 Genes

Nature of science:

Developments in scientific research follow improvements in technology—gene sequencers are used for the sequencing of genes. (1.8)


A gene is a heritable factor that consists of a length of DNA and influences a specific characteristic.

A gene occupies a specific position on a chromosome.

The various specific forms of a gene are alleles.

Alleles differ from each other by one or only a few bases.

New alleles are formed by mutation.

The genome is the whole of the genetic information of an organism.

The entire base sequence of human genes was sequenced in the Human Genome Project.

Applications and skills:

Application: The causes of sickle cell anemia, including a base substitution mutation, a change to the base sequence of mRNA transcribed from it and a change to the sequence of a polypeptide in hemoglobin.

Application: Comparison of the number of genes in humans with other species.

Skill: Use of a database to determine differences in the base sequence of a gene in two species.


Students should be able to recall one specific base substitution that causes glutamic acid to be substituted by valine as the sixth amino acid in the hemoglobin polypeptide.

The number of genes in a species should not be referred to as genome size as this term is used for the total amount of DNA. At least one plant and one bacterium should be included in the comparison and at least one species with more genes and one with fewer genes than a human.

The Genbank® database can be used to search for DNA base sequences. The cytochrome C gene sequence is available for many different organisms and is of particular interest because of its use in reclassifying organisms into three domains.

Deletions, insertions and frame shift mutations do not need to be included.


Sequencing of the human genome shows that all humans share the vast majority of their base sequences but also that there are many single nucleotide polymorphisms that contribute to human diversity.

Theory of knowledge:

There is a link between sickle cell anemia and prevalence of malaria. How can we know whether there is a causal link in such cases or simply a correlation?


Aim 7: The use of a database to compare DNA base sequences.

Aim 8: Ethics of patenting human genes.

Essential idea: Chromosomes carry genes in a linear sequence that is shared by members of a species.

3.2 Chromosomes

Nature of science:

Developments in research follow improvements in techniques—autoradiography was used to establish the length of DNA molecules in chromosomes. (1.8)


Prokaryotes have one chromosome consisting of a circular DNA molecule.

Some prokaryotes also have plasmids but eukaryotes do not.

Eukaryote chromosomes are linear DNA molecules associated with histone proteins.

In a eukaryote species there are different chromosomes that carry different genes.

Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes.

Diploid nuclei have pairs of homologous chromosomes.

Haploid nuclei have one chromosome of each pair.

The number of chromosomes is a characteristic feature of members of a species.

A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length.

Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex.

Applications and skills:

Application: Cairns’ technique for measuring the length of DNA molecules by autoradiography.

Application: Comparison of genome size in T2 phage, Escherichia coli, Drosophila melanogaster, Homo sapiens and Paris japonica.

Application: Comparison of diploid chromosome numbers of Homo sapiens, Pan troglodytes, Canis familiaris, Oryza sativa, Parascaris equorum.

Application: Use of karyograms to deduce sex and diagnose Down syndrome in humans.

Skill: Use of databases to identify the locus of a human gene and its polypeptide product.


The terms karyotype and karyogram have different meanings. Karyotype is a property of a cell—the number and type of chromosomes present in the nucleus, not a photograph or diagram of them.

Genome size is the total length of DNA in an organism. The examples of genome and chromosome number have been selected to allow points of interest to be raised.

The two DNA molecules formed by DNA replication prior to cell division are considered to be sister chromatids until the splitting of the centromere at the start of anaphase. After this, they are individual chromosomes.


Sequencing of the rice genome involved cooperation between biologists in 10 countries.


Aim 6: Staining root tip squashes and microscope examination of chromosomes is recommended but not obligatory.

Aim 7: Use of databases to identify gene loci and protein products of genes.

Essential idea: Alleles segregate during meiosis allowing new combinations to be formed by the fusion of gametes.