Question

What happens when two genes are located on the same chromosome?

Answer 1

Mendel’s law of independent assortment can only occur with 100% accuracy if the two genes are located in different chromosomes. But if the two genes are located in the same chromosomes, this law may or may not be followed. This phenomenon was discovered and experimentally proved by Morgan.

Complete answer:
When the two genes or traits are located in the same chromosome, it is the phenomenon of crossing over that decides whether these traits would be independently assorted or not. If crossing over takes place between these two genes, then in the gametes they get segregated and they will be assorted independent to each other. But if there is no crossing over between the two genes, then there is no segregation, hence only the parental combination will be found in gametes.
Thus, the genes which are present on the same chromosome are actually termed to be ‘linked’ between each other.

Note:
Having said this, we should define and make a clear-cut distinction between the two terms ‘linkage’ and ‘recombination’ from Morgan’s experiments.
Morgan performed two crosses to study the genes that were X-linked.
Cross A: Between yellow-bodies (y) and white-eyed (w) female and brown-bodies (y+) red-eyed (w+) male.
Cross B: Between white-eyed miniature winged female (wwmm) with wild-eyed (w+) normal-winged (m+) male.
The results obtained were:

F2 generation	Cross A	Cross B
Parental type	98.7%	62.8%
Recombinant type (non-parental type)	1.3%	37.2%

With this result, Morgan concluded and coined out the term linkage to define the physical association of genes present on the same chromosomes.
Thus, the most important point to be noted is that those genes which are tightly linked (or the inter-distance between the two is small), they may not get segregated.
The genes which are loosely linked may get segregated due to crossing over.