An allele for a particular trait that is only expressed in the presence of a second copy of the same allele is called

The correct answer is recessive.

Explanation:

The alternate forms of similar genes are known as alleles. A version of a gene, which to get expressed in the phenotype need to be homozygous at the time of inheritance is termed as a recessive allele. This shows that for a specific trait, a recessive allele only demonstrates its expression when a similar copy of the same allele is present.  

There are two forms of alleles known, these are dominant alleles and recessive alleles. The prime difference between the two is that the expression of a dominant allele is always witnessed, that is, whether it is in the heterozygous state or homozygous state. On the other hand, the expression of a recessive allele is only witnessed when it is in a homozygous state.  

c) 50% there are four chromosomes. after crossing over, 2 of the four have been changed. that means any gamete has a 2 in 4 chance or 50% chance of receiving the mutated chromosome.