Understanding Genetic Relationships: How Much DNA Do Cousins Share Compared to Half Siblings?

When discussing genetic relationships, it's easy to find explanations for common scenarios, but sometimes the nuances between different types of relatives can be confusing. This article aims to clarify the genetic relationships between half-siblings and first cousins, focusing specifically on how much DNA they share.

Let's start with some basic genetic concepts. DNA is inherited in units called alleles. Each person inherits one allele for each gene from each parent, resulting in a pool of alleles from both parents. When discussing genetic relatedness, we often use the concept of 'identical by descent' (IBD), which refers to alleles that are copies of a common ancestral allele.

Half Siblings: A Closer Look

Half-siblings share one parent. This means they have a 50% chance of inheriting the same allele from that parent. On average, half-siblings will share 25% of their DNA due to sharing one parent. This can be expressed mathematically as follows:

Each parent has two alleles for each gene. The common parent gives a particular allele to the first child with a probability of 1/2. Say the same allele is passed to a half-sibling, the probability that both siblings inherit the same allele from that parent is 1/4.

This simplistic view captures the essence, but the real calculation involves multiplying the probabilities. Each parent's alleles are independent, leading to a joint probability of their sharing the same allele, which is 1/4. However, since there are two parents, the overall probability is 2 * 1/4 1/2.

Full Siblings: A More Detailed Explanation

Full siblings have two parents in common. They each inherit one allele from each parent, leading to a pool of unique combinations. The probability of sharing any particular allele is 1/2, as each parent independently contributes an allele from either of their own two alleles.

Assign a couple's alleles for a particular gene as 1, 2, 3, and 4. The mom can contribute either 1 or 2 to a child, and the dad can contribute either 3 or 4 to a child. Regardless of which allele the mom contributes to her first child, there is a 50% chance that she will contribute the same allele to her second child, leading to 50% shared maternal alleles. The same logic applies to paternal alleles, resulting in a 50% shared paternal allele probability.

Therefore, full siblings share 50% of their DNA, as both their maternal and paternal alleles are 50% shared.

First Cousins: A More Complex Relationship

First cousins have one set of grandparents in common. Their genetic relationship is more complex compared to half-siblings and full siblings. The genetic relatedness between first cousins can be calculated by considering the shared grandparents' alleles.

Let's break it down:

There is a 50% chance that the grandparents share an allele with each other. Each grandparent passes half of their alleles to each child, so the first cousins have a 50% chance of inheriting the same allele from the common grandparent. Assuming the grandparents share an allele, the probability that both first cousins inherit the same allele from that grandparent is 1/2 * 1/2 1/4. Thus, the overall probability that first cousins share an allele that is identical by descent is 1/2 * 1/2 * 1/2 1/8.

This means that first cousins share approximately 12.5% of their DNA, on average.

Conclusion: Genetic Relatedness Explained

Understanding the genetic relationships between half-siblings and first cousins can be complex, but breaking down the probabilities of allele sharing provides clarity. Half-siblings share 25% of their DNA due to sharing one parent, while first cousins share approximately 12.5% of their DNA due to sharing one set of grandparents. Both of these relationships are important in genetic counseling, familial genealogical research, and even in legal contexts related to inheritance and paternity.

In summary, the differences in genetic sharing between half-siblings and first cousins are due to the varying levels of shared parental or grandparental genetics. This knowledge helps in understanding genetic tests and results, making healthcare and family planning more informed.