Question

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?

Answer 1

If a person gets a disease then it may be a sufferer or carrier depending on the presence of recessive or dominant traits. In the case of the carrier, the infection is present on one allele out of the pair whereas in the case of the sufferer the disease is present in both the alleles.

Complete answer:
In the case of a disease, a carrier does not express disease but the sufferer does, therefore carriers are always heterozygous. In the case of recessive disease if an allele is recessive then the disease does not express itself therefore for expression of disease the allele should be dominant. A dominant allele expresses itself in the case of both homozygous and heterozygous allele and it leads to the patient sufferer of disease which may be fatal but a carrier only carries a disease from one generation to the next generation.

Additional Information:
The autosomal recessive diseases are sickle cell anaemia, phenylketonuria, cystic fibrosis, etc. The autosomal dominant disease is Huntington's disease and the sex-linked recessive disease are haemophilia and colour blindness. Only haemophilia C is an autosomal recessive disease that occurs in less than $1\%$ of patients.

Note:
In case if an allele is recessive but sex-linked then all carriers will be female. The genetic disorder caused by the dominant and recessive allele is also known as a mendelian disorder, chromosomal aberrations and anomalies. In humans, the disorder which is inherited by mendelian genes was discovered by Sir Archibald Garrod and William Bateson by scanning the human population.