Final answer:
Carriers have one dominant and one recessive allele and do not express the recessive trait but can pass on the recessive allele to their offspring. The true statements about carriers are A) They have one dominant allele, C) They are heterozygous for a recessive disorder, and D) They can pass a gene for a recessive disorder.
Step-by-step explanation:
In the context of genetics and inheritance, a carrier refers to an individual who has one dominant allele and one recessive allele for a particular trait (heterozygous), especially one that relates to a recessive disorder.
The correct statements about carriers are:
A) They have one dominant allele. Carriers have one dominant and one recessive allele. The presence of the dominant allele means they do not express the recessive trait, but they can pass the recessive allele to their offspring.
C) They are heterozygous for a recessive disorder. Being heterozygous means they have two different alleles for a gene. If one of these is a recessive allele that can cause a disorder, they are carriers without expressing the symptoms.
D) They can pass a gene for a recessive disorder. Carriers inherit and can pass on the recessive allele to their offspring. The recessive disorder could manifest if the offspring receives another recessive allele from the other parent.
The statement B) They express the recessive trait is not true for carriers, as they have a dominant allele that masks the recessive one, and therefore, they do not express the related phenotype of the recessive allele.