Chromosomal Theory of Inheritance
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Direct answer
The chromosomal theory of inheritance, put forward by Walter Sutton and Theodor Boveri in 1902-1903, states that Mendel's factors are carried on chromosomes, because chromosomes behave in meiosis and fertilisation exactly as genes must — they occur in pairs, segregate cleanly and recombine at fertilisation. Thomas Hunt Morgan's work on Drosophila melanogaster confirmed the theory and refined it: genes situated on the same chromosome are linked and inherited together, while crossing over during meiosis generates recombinants, and the frequency of recombination measures how far apart two genes lie.
What you must remember
- The parallels: Sutton noted that chromosomes occur in homologous pairs, the two members separate (segregate) during gamete formation, and homologues from two parents unite at fertilisation — behaviour matching that of alleles.
- Locus: every gene occupies a definite fixed position, its locus, on a chromosome; alleles of a gene occupy corresponding loci on homologous chromosomes.
- Linkage (Morgan): the physical association of genes on the same chromosome; linked genes fail to assort independently and tend to move into the same gamete.
- Morgan's evidence: dihybrid crosses in Drosophila with genes for body colour and eye colour, carried on the same chromosome, gave progeny ratios that deviated sharply from the expected 9:3:3:1 because the genes were linked.
- Recombination: the appearance of non-parental gene combinations in the progeny, produced when crossing over between homologues exchanges segments during meiosis.
- Distance rule: recombination frequency rises with the distance between linked genes — genes lying close together cross over rarely; genes far apart recombine more often.
- Genetic maps: Alfred Sturtevant, Morgan's student, converted recombination frequencies into relative distances and mapped genes along chromosomes; such linkage maps remain in use, including during the Human Genome Project.
Common confusion
Linkage must not be read as absolute co-inheritance. Linked genes can still be separated by crossing over, so linked genes give many parental and fewer recombinant progeny rather than only parental types. Independent assortment, by contrast, applies to genes on different chromosome pairs, where 9:3:3:1 expectations hold. A recombination frequency well below 50 per cent is itself the evidence that two genes are linked.
Exam-focused takeaway
NEET-UG frames this topic as assertion-reason and statement questions on the Sutton-Boveri parallels, on why Morgan's Drosophila ratios deviated from the dihybrid expectation, and on Sturtevant's mapping logic of converting recombination frequency into map distance. Match-the-following items pair linkage, recombination and locus with their definitions, and numericals ask which progeny class (parental or recombinant) will be larger for genes lying close together on a chromosome.
Frequently asked questions
Who proposed the chromosomal theory of inheritance?
Walter Sutton and Theodor Boveri, working independently in 1902-1903, correlated the behaviour of chromosomes in meiosis with Mendelian segregation and proposed chromosomes as the vehicles of heredity.
What is linkage?
Linkage is the physical association of genes located on the same chromosome, because of which such genes tend to be inherited together and do not assort independently.
Why did Morgan's dihybrid crosses deviate from the 9:3:3:1 ratio?
Because the two genes he followed lay on the same chromosome and were linked, so most progeny showed parental combinations, with only a small fraction of recombinants from crossing over.
How did Sturtevant construct genetic maps?
He used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between them, assigning genes relative positions along the chromosome.
What exactly is a recombinant?
An offspring carrying a gene combination absent in both parents, generated by crossing over between linked genes during meiosis.