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3.3 predict the difference in inheritance patterns if two genes are linked

predict the difference in inheritance patterns if two genes are linked

EF ef
EF EEFF EeFf
ef EeFf eeff

producing a ratio of 3 EF : 1ef

All of Mendel’s results were for genes that were inherited independently of each other. This led to his law of independent assortment. Sutton and Boveri later explained this when they observed that chromosomes separate independently during meiosis. Mendel’s results would have been different if any of the traits that he studied had been linked.

Some results from crossing linked genes do not show the exact predicted results as some genes undergo recombination. This recombining of certain genes occurs during meiosis when homologous chromosomes have undergone DNA replication and pair up. At this time, crossing over may occur where sections of chromosomes break off at a point and then recombine with the “wrong” piece of chromosome or swap.

A pair of homologous chromosomes contains the following linked genes A, B, C and D:

____AB_____CD_____
_________X_________
AB           CD              A homologous pair of chromosomes after DNA replication. Crossing over occurs at X

____ab______cd________
________X____________
ab         cd

If crossing over did not occur, the four potential chromosomes would be: ABCD, ABCD, abcd and abcd.

If crossing over of the two inner chromatids occurs at X, then the resulting chromosomes would be: ABCD, ABcd, abCD, and abcd. Crossing over only occurs between homologous chromosomes and is only an important factor in linked genes.

____AB_____CD________
______________________
AB           cd

____ab_____CD_______
________X___________
ab          cd