What Is Linkage and Linkage Groups? (NEET Biology)
Biology · Principles of Inheritance and Variation · NEET
Linkage means two or more genes that sit on the SAME chromosome tend to be inherited together, because they cannot separate easily during meiosis. All the genes on one chromosome form a "linkage group", so a species has as many linkage groups as it has chromosome types. Memory hook: "Same bus, same stop" — genes on the same chromosome travel together like passengers on one bus.
Left: linked genes A and B sit on one chromosome (one linkage group) and are passed on together. Right: unlinked genes on separate chromosomes assort independently, giving Mendel's 9:3:3:1 ratio. A gene changes linkage group only by translocation.
Your doubts, answered
What exactly does 'linkage' mean in simple words?
Linkage is the physical joining of two or more genes on the SAME chromosome, so they are usually passed to the offspring together. T.H. Morgan coined this term after working on fruit flies (Drosophila). Because linked genes stay together, a dihybrid cross does NOT give the normal 9:3:3:1 ratio — more offspring look like the parents (parental type). Think of two genes chained onto one chromosome: where the chromosome goes, both genes go.
What is a linkage group? Is it the same as a chromosome?
A linkage group is the complete set of genes located on ONE chromosome. So yes, one chromosome = one linkage group. The number of linkage groups in a species equals its number of chromosome TYPES (haploid number). Example: humans have 23 linkage groups (22 autosomes + 1 sex chromosome type). NEET loves this exact wording, so remember: genes travelling together on one chromosome belong to the same linkage group.
Why do linked genes NOT follow Mendel's law of independent assortment?
Mendel's Law of Independent Assortment works only when the two genes lie on DIFFERENT chromosomes, so they can sort independently in meiosis. Linked genes lie on the SAME chromosome, so they cannot separate freely — they stay together. That is why Morgan saw far more parental-type offspring than non-parental (recombinant) ones. Linkage is basically the exception to independent assortment.
What is the difference between tightly linked and loosely linked genes?
Genes that lie very CLOSE together on a chromosome are TIGHTLY linked — they rarely separate, so they show LOW recombination (Morgan found white and yellow genes had only 1.3% recombination). Genes that lie FAR apart are LOOSELY linked — crossing over separates them more often, giving HIGH recombination (white and miniature wing = 37.2%). So: close genes = tight linkage = low recombination; distant genes = loose linkage = high recombination.
How can a gene move from one linkage group to another?
By translocation. A translocation moves a piece of a chromosome (with its genes) onto a NON-homologous chromosome. Since a linkage group = a chromosome, moving a gene to a different chromosome shifts it into a different linkage group. This is a favourite NEET question (2016). Do NOT confuse it with crossing over, which only swaps segments between homologous chromosomes — that changes gene combinations, not linkage groups.
Who discovered linkage and in which organism?
Thomas Hunt Morgan discovered and named linkage while doing dihybrid crosses in the fruit fly Drosophila melanogaster. He noticed the F2 ratio was far from 9:3:3:1 and explained it by the physical association of genes on the same chromosome. NEET remembers this as the fixed pair 'T.H. Morgan : Linkage'.
⚠️ The NEET trap ✗ Thinking a linkage group is a group of similar traits, or that 'linked genes' means genes on homologous chromosomes. ✓ A linkage group is ALL the genes on ONE chromosome. Number of linkage groups = number of chromosome types (haploid number), e.g. 23 in humans. Linked genes lie on the SAME chromosome, not on homologous partners. A gene shifts to another linkage group only by TRANSLOCATION, not by crossing over. 🧠 One chromosome = one linkage group. If NEET asks 'move a gene to another linkage group', the answer is translocation, never crossing over.
Real NEET questions
NEET 2016
In a testcross involving F1 dihybrid flies, more parental-type offspring were produced than the recombinant-type offspring. This indicates:
A · The two genes are located on two different chromosomes
B · Chromosomes failed to separate during meiosis
C · The two genes are linked and present on the same chromosome ✓
D · Both of the characters are controlled by more than one gene
Solution: When two genes lie on the SAME chromosome they are linked and tend to be inherited together, so a testcross of an F1 dihybrid gives more parental-type than recombinant-type offspring. If the genes were on different chromosomes they would assort independently and give roughly equal parental and recombinant types. This higher proportion of parental combinations is the signature of linkage that Morgan described.
NEET 2016
The mechanism that causes a gene to move from one linkage group to another is called:
A · Inversion
B · Duplication
C · Translocation ✓
D · Crossing-over
Solution: A linkage group is the set of genes on a single chromosome. Translocation moves a segment of DNA (with its genes) onto a NON-homologous chromosome, so the gene now belongs to a different linkage group. Inversion and duplication rearrange genes within the same chromosome, and crossing over only exchanges segments between homologous chromosomes, so none of these move a gene to a new linkage group.
NEET 2018
Which of the following pairs is wrongly matched?
A · XO type sex determination : Grasshopper
B · ABO blood grouping : Co-dominance
C · Starch synthesis in pea : Multiple alleles ✓
D · T.H. Morgan : Linkage
Solution: Starch synthesis in pea is controlled by ONE gene with alleles B and b (incomplete dominance for grain size), not by multiple alleles, so this pair is wrongly matched. The other pairs are correct: grasshopper shows XO sex determination, ABO blood grouping shows co-dominance, and T.H. Morgan is correctly linked with the discovery of linkage.
Solved Principles of Inheritance and Variation NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. Linkage means any genes on the same chromosome are inherited together. Sex linkage is a special case where the genes lie on the sex chromosome (X or Y). All sex-linked genes are linked, but not all linked genes are sex-linked.
How many linkage groups are there in humans?
23 — equal to the haploid number of chromosome types (22 autosomes + 1 sex chromosome type). Remember: number of linkage groups = haploid chromosome number of the species.
Does crossing over break linkage?
Crossing over WEAKENS linkage by producing recombinant (non-parental) offspring, but it does not destroy the linkage group. The genes still belong to the same chromosome; only their combinations change.
What is the opposite of linkage?
Independent assortment. Genes on different chromosomes assort independently and give the normal 9:3:3:1 dihybrid ratio, while linked genes stay together and deviate from that ratio.
