Final answer:
The recurrence risk for a multifactorially inherited birth defect is higher when the proband is of the less commonly affected sex, as this suggests a stronger genetic component. In X-linked recessive disorders, males are more often affected due to having only one X chromosome, while females can be carriers without exhibiting symptoms.
Step-by-step explanation:
The recurrence risk for a multifactorially inherited birth defect is higher in the circumstance where the proband is of the less commonly affected sex. This holds true because if a disorder affects one sex more frequently or severely than the other, and the proband (the individual from whom the pedigree is initiated) is of the less commonly affected sex, the genetic component of the disorder may be stronger. Multifactorial inheritance implies that many factors, including genetic and environmental ones, contribute to the condition.
For example, in X-linked recessive disorders, males are generally more affected because they have only one X chromosome. A single recessive gene on that chromosome can lead to the disorder. Because females have two X chromosomes, they are less likely to be affected as they typically must inherit two copies of the recessive gene to show the disorder. They can be carriers with one defective gene but still show a normal phenotype because the other X chromosome has a normal copy of the gene.
Therefore, when it comes to X-linked recessive disorders like color blindness or hemophilia, a male proband tends to suggest a higher genetic load, which can increase the risk to his offspring or relatives. Similarly, if a multifactorial condition tends to be less common in females, an affected female proband would also suggest a higher genetic component to the disease, therefore increasing the recurrence risk.