Every human gene has an address. Once you can read that address you can look up any gene in a public database, find the protein it codes for, and say something useful about whether two genes are likely to be inherited together.
📚 What you need to know
The Human Genome Project gave us the exact locus of every gene across the 23 chromosome pairs.
Public databases (such as the EMBL database) let anyone look up a gene, its locus, its base sequence and its polypeptide product.
A locus is written like 7q31.2: the chromosome number, the arm, then the band position.
p is the short arm, q is the long arm. A smaller band number means closer to the centromere.
Genes on different chromosomes are unlinked. Genes on the same chromosome, especially close together on the same arm, are likely to be linked.
Knowing a locus matters medically — it is the first step towards testing for a faulty allele or replacing it.
How a locus is written
Take 7q31.2, the locus of the CFTR gene. Break it into three pieces:
7 — the gene is on chromosome 7
q — it is on the long arm (p would mean the short arm)
31.2 — the band, counting outwards from the centromere
The centromere splits every chromosome into a short arm and a long arm. Band numbers start at the centromere and rise as you move towards the tip, so 7q11 is much closer to the centromere than 7q31.
Band numbers are counted outwards from the centromere, so two genes with similar band numbers on the same arm sit physically close together.
You are not expected to memorise loci. You are expected to read one and reason from it — usually to say whether two genes could be linked. Practise saying that reasoning out loud in one sentence.
What the databases hold
Once a genome has been sequenced, the data goes into public databases that anyone can search. For a single gene you can pull up:
its locus and the surrounding genes
its DNA base sequence
the mRNA transcribed from it
the polypeptide it codes for, and often the protein’s 3D structure
known alleles and the SNPs that distinguish them
You can also compare sequences between species. Tools such as BLAST search a database for sequences that match the one you paste in, and the closer the match, the more recently the two species shared an ancestor. The same tools work at the protein level.
Gene
Locus
Polypeptide product
What a faulty allele causes
CFTR
7q31.2
CFTR chloride channel protein
Cystic fibrosis
HBB
11p15.4
Beta-globin, part of haemoglobin
Sickle-cell anaemia
F10
13q34
Coagulation factor X
A clotting disorder
Why medicine cares about the locus
Knowing exactly where a gene sits makes several things possible:
Testing. A known locus can be sequenced directly to check which alleles someone carries.
Recombinant DNA technology. A specific sequence can be cut out or inserted only if you know where it is.
Gene therapy. Delivering a working copy of a faulty allele starts with identifying the allele and its position.
Cancer research. Databases of cancer-related genes let researchers and doctors compare a patient’s tumour sequence with known mutations.
Worked examples
WORKED EXAMPLE 1
A gene has the locus 11p15.4. State what each part of this address tells you.
Step 1: the number11 = chromosome 11Step 2: the letterp = the short armThe other option is q, the long arm.Step 3: the band15.4 = the band position on that armChromosome 11, short arm, band 15.4Because 15.4 is a fairly high number, the gene sits well away from the centromere, towards the tip of the arm.
WORKED EXAMPLE 2
Three genes have the loci 4q21.3, 4q22.1 and 9p13.2. Deduce which pair is most likely to be inherited together, and explain your answer.
Step 1: check the chromosome numbersTwo genes are on chromosome 4; one is on chromosome 9Step 2: rule out the unlinked pair
The gene on chromosome 9 is on a different chromosome, so it assorts independently of both others
Step 3: compare the two on chromosome 4
Both are on the q arm, at bands 21.3 and 22.1 — very close together
4q21.3 and 4q22.1 are most likely to be linkedSay “likely”, not “certain”. Crossing over can still separate them — it is just far less likely when they are close together.
The rule in one line. Different chromosome numbers → unlinked, always. Same chromosome → linked, and the closer the band numbers, the tighter the linkage.
💡 Exam tip
Break the locus into three parts in your answer — chromosome, arm, band. One mark each is common.
p is short, q is long. Remember “p for petite” if that helps.
Compare chromosome numbers first when asked about linkage; different numbers settle it immediately.
Use “likely” for linkage on the same chromosome. Distance changes the probability, not the certainty.
Name a database (for example EMBL) if the question asks how the information would be found.
⚠ Common mix-up
Swapping p and q. p is the short arm.
Thinking a larger band number means a bigger gene. It only tells you the position.
Reading 31.2 as a distance in bases. It is a band label, not a measurement.
Assuming genes on the same chromosome are always inherited together. Crossing over separates them, especially when they are far apart.
Confusing the gene name with the protein name. CFTR is the gene; the CFTR protein is what it codes for.
Saying a locus proves linkage. It supports a prediction — the cross data confirms it.
Up next: Gene Linkage — what happens to dihybrid ratios when two genes share a chromosome and refuse to assort independently.
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