What does it mean when a person is said to be a carrier of a trait or disease? Being a carrier, does it automatically mean that the person is heterozygous if the trait is an autosomal dominant trait?
1 Answer
YES. a carrier is heterozygous but in a carrier the trait/disease is not expressed. Thus the allele responsible for the trait is RECESSIVE.
Explanation:
A carrier is not affected by the disease it carries, thus the unfavorable allele is a recessive condition. This means an undesirable allele is present in the genotype of the carrier; but as the allele is recessive, there is no expression of disease symptoms, i.e. there is no development of the disease in the carrier. The carrier expresses the trait(s) controlled by corresponding dominant allele.
Following illustration is true for a case of autosomal recessive trait:
So carriers 'silently' carry a genetic load, an undesirable allele, from one generation to next. When two carrier organisms reproduce, recessive alleles from both may come together and affected progeny (about 25%) will appear. In humans, several congenital diseases are reported due to such pattern of inheritance: Tay sach disease, Albinism, Cystic fibrosis and Sickle cell anaemia are only few examples.
Please note, if an allele is dominant, it would be expressed even in heterozygous form. If the undesirable dominant allele is present, even on a single allele, the disease will appear. A disease in humans caused by autosomal dominant allele is Huntington's chorea, which develops after middle age.
If the allele is recessive but sex-linked, all carriers will be female.
