The Basics of Genetic Relatedness

• Parent-Offspring Relatedness: Each child inherits exactly half of their DNA from each parent. This means the genetic relatedness between a parent and their child is 0.5, or 50%.

• Sibling-Sibling Relatedness: Full siblings also share, on average, 50% of their DNA. This is because each sibling receives a random half of each parent’s DNA, so the probability that a given gene is shared between siblings is also 0.5.

Why Is the Relatedness the Same?

Both relationships involve the transmission of half of the parents’ genes. For siblings, the specific halves inherited may differ, but statistically, siblings still end up sharing about half their genes. This is confirmed by multiple scientific sources and is a foundational concept in genetics.

Variations and Exceptions

• Identical twins are an exception, sharing 100% of their DNA.

• Half-siblings share only 25% of their DNA.

• X-chromosome inheritance and mitochondrial DNA can create slight differences, but for most purposes, the average relatedness holds at 0.5 for both parent-offspring and full siblings.

Conclusion

The degree of genetic relatedness between the offspring and their parents is the same as that between sister and brother.

Correct answer: (3) the same as that between sister and brother.