A single gene mutation results from

DNA changes in one particular gene

Explanation:

Single gene disorders are caused by DNA changes in one particular gene, and often have predictable inheritance patterns. Over 10,000 human disorders are caused by a change, known as a mutation, in a single gene. These are known as single gene disorders.

because of

the answer would be 2, if crossing over doesn’t occur 2 out of 4 of the daughter cells will have that allele.

hope this : )