Mosquitoes can have bigger or smaller body sizes. Two parents that have medium-sized bodies have offspring. One of their offspring has a small body, and one has a large body. How did the offspring end up with traits that are different from the traits of their parents? How did the gene combinations result in these traits?

Mosquitoes have different body size than their parents.
Explanation
It is seen that when parents genes combine together they form an offspring having half each of their parents genes although sometimes it is seen that the traits that parents carry are not carried by the offspring and they end up looking different from the parent genes.
The difference in genes of the offspring than the genes of the parent is due to the suppressed genes that the grandparents had which was not transferred in the next generation but was present and transferred to the third or fourth generation.
This means that the traits that offspring have might have been there in the genes of previous generations that were present earlier to their parents but just the transfer of genes happened in the generation where treats are showing.
Here the mosquito having a different body size than their parents body size is a trade that the parents previous generations were having and ended up passing the genes to the next generations skipping 1 or 2 generations in between.
to know more:
Genes that are involved in turning on or off the transcription of structural genes are- (a) redundant gene (b)operator gene. (C)regulatory gene. (d) polymorphic genes