Michigan Test for Teacher Certification (MTTC) Secondary Integrated Science Practice test

What is a defining characteristic of a recessive gene?

1. It is always expressed

2. It requires two copies to be expressed

3. It masks the dominant gene

4. It only appears in homozygous individuals

The correct answer is: It requires two copies to be expressed

A recessive gene is defined by the characteristic that it requires two copies of itself to be expressed in an organism's phenotype. This means that an individual must inherit recessive alleles from both parents for the recessive trait to manifest. If an individual has just one dominant allele, the presence of the recessive allele will not affect the phenotype, as the dominant trait will be expressed instead. In contrast to this, a recessive allele will not be expressed if a dominant allele is present, which is why it cannot be observed in heterozygous individuals (those with one dominant and one recessive allele). Therefore, the defining feature of a recessive gene fundamentally lies in this need for two copies (homozygosity) to be visible in the phenotype, which contrasts with the attributes of dominant genes, which can express their trait with only one copy present. This is why the understanding of alleles being dominant or recessive is crucial in the study of genetics.