Understanding the Rh Factor
Human blood types are determined by specific markers on red blood cells, inherited from our parents. Understanding how these traits are passed down provides insight into individual biological makeup.
The Rh factor is a specific protein found on the surface of red blood cells, separate from the more commonly known ABO blood group system. Individuals who possess this protein are classified as Rh-positive (Rh+), indicating its presence. Conversely, those who lack this particular protein are designated as Rh-negative (Rh-). This distinction is important in medical contexts, particularly concerning blood transfusions and pregnancy.
Basic Principles of Genetic Inheritance
Inherited traits, including blood type, are determined by genes on chromosomes. Each gene has different forms called alleles, with one inherited from each parent. These alleles combine to form an individual’s genetic makeup (genotype), which influences observable characteristics (phenotype).
When two different alleles are present, one is dominant, meaning its trait is expressed even with a single copy. The other is recessive, expressed only if two copies are inherited. This interplay forms the basis of how traits, including the Rh factor, are passed through generations.
Passing Down Rh Factor Genes
The inheritance of the Rh factor follows a clear pattern, with the Rh-positive trait being dominant and the Rh-negative trait being recessive. This means an individual only needs one copy of the dominant Rh-positive allele (often symbolized as ‘R’) to be Rh-positive. The Rh-negative allele is recessive (often symbolized as ‘r’), requiring two copies for the Rh-negative trait to manifest.
An Rh-positive individual can have two dominant alleles (RR) or one dominant and one recessive allele (Rr). Both genotypes result in the Rh-positive phenotype because the ‘R’ allele is expressed. An Rh-negative individual must have two copies of the recessive allele (‘rr’).
The ‘Positive Parents, Negative Child’ Scenario
Given these genetic principles, two Rh-positive parents can have an Rh-negative child. This occurs when both Rh-positive parents are heterozygous, meaning each carries one dominant Rh-positive allele (R) and one recessive Rh-negative allele (r), making their genotype ‘Rr’.
When two ‘Rr’ parents conceive, each parent can pass on either their ‘R’ or ‘r’ allele. If both parents pass on their recessive ‘r’ allele, the child’s genotype will be ‘rr’, resulting in an Rh-negative blood type. This outcome has a 25% chance with each pregnancy.