Final answer:
In pedigree analysis, an obligate carrier is typically represented by a half-shaded symbol, indicating they carry the gene for a trait but do not show symptoms, due to being heterozygous (Aa), with one normal dominant allele.
Step-by-step explanation:
In pedigree analysis, an individual that is an obligate carrier is usually represented with a half-shaded symbol. This half-shading indicates that the individual is a carrier of the gene for a trait but does not display the phenotype associated with that gene.
This is because carriers are typically heterozygous, meaning they have one normal allele and one allele for the disorder (genotype Aa), where the normal allele is dominant. Hence, while they don't show symptoms, they can pass the recessive allele to their offspring.
For an autosomal recessive disorder, when neither parent exhibits the disorder but their child does, it can be inferred that both parents are heterozygous carriers (Aa).
Thus, knowing the genotype of the offspring can often help in determining the genotypes of the parents. In a pedigree, such carriers are fundamental in understanding the inheritance pattern of genetic disorders.