Why genotypic characteristics of siblings are different from each other though they belong to the same parents and share similar hereditary material?

Genetic variation refers to differences among the genomes of members of the same species. A genome is all the hereditary information—all the genes—of an organism. For instance, the human genome contains somewhere between twenty and twenty-five thousand genes.

Genes are units of hereditary information, and they carry instructions for building proteins. The genes that are encoded within these proteins are what enable cells to function. Most organisms that reproduce sexually have two copies of each gene, because each parent cell or organism donates a single copy of its genes to its offspring. Additionally, genes can exist in slightly different forms, called alleles, which further adds to genetic variation.

The combination of alleles of a gene that an individual receives from both parents determines what biologists call the genotype for a particular trait, such as hair texture. The genotype that an individual possesses for a trait, in turn, determines the phenotype—the observable characteristics—such as whether that individual actually ends up with straight, wavy, or curly hair.

Genetic variation within a species can result from a few different sources. Mutations, the changes in the sequences of genes in DNA, are one source of genetic variation. Another source is gene flow, or the movement of genes between different groups of organisms. Finally, genetic variation can be a result of sexual reproduction, which leads to the creation of new combinations of genes.

Genetic variation in a group of organisms enables some organisms to survive better than others in the environment in which they live. Organisms of even a small population can differ strikingly in terms of how well suited they are for life in a certain environment. An example would be moths of the same species with different color wings. Moths with wings similar to the color of tree bark are better able to camouflage themselves than moths of a different color. As a result, the tree-colored moths are more likely to survive, reproduce, and pass on their genes. This process is called natural selection, and it is the main force that drives evolution.

A colleague related the following story: while running errands with her 11- and 7-year-old daughters, a back seat battle began to rage. My colleague’s attempts to defuse the situation only led to a shouting match about who was to blame for the skirmish. Finally the 11-year-old proclaimed to her sister, “You started it the day you were born and took away Mom’s love!”

This pair of sisters fight frequently, and from their mother’s perspective, part of the reason is that the two have little in common. As it turns out, their situation is not unique.

Despite the fact that siblings are, on average, 50% genetically similar, are often raised in the same home by the same parents, attend the same schools and have many other shared experiences, siblings are often only as similar to each other as they are to children who are growing up across town or even across the country.

So, what is it that makes two siblings from the same family so different?

What makes the difference?

As researchers of sibling and family relationships, we knew that at least one answer to this question comes from theory and data showing that, at least in some families, siblings try to be different from one another, and seek to establish a unique identity and position in their family.

From a child’s perspective, if an older brother excels at school, it may be easier to attract her parents’ attention and praise by becoming a star athlete than by competing with her brother to get the best grades. In this way, even small differences between siblings can become substantial differences over time.

But parents may also play a role. For instance, when parents notice differences between their children, children may pick up on parents’ perceptions and beliefs about those differences. This, in turn, can increase sibling differences.

We wanted to test these ideas to see what makes siblings different. So, we used data from first- and second-born teenage siblings from 388 two-parent families to examine sibling differences in school performance.

We asked mothers and fathers to report on whether they thought the two siblings differed in their academic abilities, and if so, which sibling was more capable. We also collected school grades from both siblings’ report cards.

Preference for the firstborn

Our analyses showed some interesting results: parents tended to believe that the older sibling was better in school. This was even when older siblings did not actually receive better grades, on average.

This may be a product of parents having greater expectations for firstborns or that, at any given time, the older sibling is undertaking more advanced school work.

There was, however, an exception to this pattern: in families with older brothers and younger sisters, parents rated the younger sibling as being more capable. In fact, in those families, younger sisters received better grades than their older brothers.

Our findings also showed that it was not sibling differences in school grades that predicted parents’ ratings of their children’s abilities. Rather, parents’ beliefs about differences in their children’s abilities predicted later sibling differences in school grades.

In other words, when parents believed one child was more capable than the other, that child’s school grades improved more over time than their sibling’s.

Sustaining beliefs

Although we expected that children’s school grades and parents’ beliefs about their children’s relative abilities would be mutually influential, it turned out that parents’ beliefs did not change much over their children’s teenage years.

Instead, sibling differences in school grades did change, and were predicted by parents’ beliefs. In this way, parents’ beliefs about differences between their children may encourage the development of actual sibling difference.

The above comment by an 11-year-old highlights that children are sensitive to their place and value in the family – relative to those of their siblings. Parents may strive to show their love for their children, but they also should be aware that small differences in how they treat their children can have large effects – including on their children’s development and adjustment, and also on the sibling relationship.

Indeed, some research suggests that sibling conflict arises when children try to be different from their siblings.

My colleague may be correct that her daughters fight frequently because they have nothing in common. But their conflicts may also be motivated by her daughters’ perception that their differences started on the day her sister was born “and took away Mom’s love.”

Why genotypic characteristics of siblings are different?

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