Video Transcript
Which of the following correctly
compares dominant and recessive traits? (A) Recessive traits are always
harmful to an organism, and dominant traits are rarely harmful. (B) Recessive traits will always be
visible in the appearance of an organism, whereas dominant traits will only be
visible 50 percent of the time. (C) Dominant traits, if present in
an organism, will mask the appearance of recessive traits.
Mendel’s experiments with pea
plants greatly improved our understanding of inheritance. One theory that he proposed is the
law of dominance. This law explains that if a parent
with two dominant alleles reproduces with a parent with two recessive alleles, they
will produce offspring that only show the dominant trait. Peas can be yellow or green, and
these two traits are controlled by two different versions of a gene called
alleles. The uppercase A is the dominant
allele, and the lowercase a is the recessive allele.
Notice that all the offspring in
this example are yellow, which is the dominant trait indicated by uppercase A. Even though the offspring contain
both the dominant and recessive allele, only the dominant trait is expressed. You need two copies of the
recessive allele for that trait to be expressed, as shown here with the green color
with the two recessive alleles.
We can summarize all of this
here. Dominant traits are always
expressed if they’re present, while recessive traits are only expressed if the
dominant trait is absent. Now let’s take a look at our answer
choices.
Answer choice (A) states that
recessive traits are always harmful, while dominant traits are rarely harmful. This is incorrect because this
isn’t necessarily true. For example, the brown eye trait is
dominant, while the blue eye trait is recessive. Neither cause harm to us.
Answer choice (B) is also
incorrect. Recessive traits will not always be
visible in organisms. In fact, in any organism that has
the dominant allele, the recessive trait will not be visible. The dominant trait will be visible
100 percent of the time if it is present.
As we have seen, the correct answer
is therefore (C). Dominant traits, if present in an
organism, will mask the appearance of recessive traits.
