Everyone, at least once in their life, has been called out on looking similar to their parents – sometimes there is more resemblance with one parent than the other. The simple reasoning for so that is accepted is that this resemblance owes to the fact that children are a combined product of their parents. In a way, we inherit (the scientific term for ‘get’) our looks from our parents. The study of this inheritance (or heredity) is called GENETICS. Genetics is primarily concerned with understanding the biological properties that are transmitted from parent to child.
Genetics is concerned with the transmission of GENES, which are the basis of biological properties that are transmitted from a parent to an offspring. Genes code for proteins, which make up the appearance of a person. Genes are located on the DNA, the genetic material present in each cell of our body. DNA is primarily what is obtained from both the parents into the offspring.
Following fertilization (i.e., the combination of an egg and a sperm into one cell), the resultant embryo formed contains half the genetic material of the mother (via the egg) and the other half is the genetic material of the father (via the sperm). So, biologically, a person is half their mother and half their father.
So, why do people look more like one of their parents?
While it seems logical for a person to be exactly like half of each of their parent, it is usually not the case. Often, it is observed that a child resembles one of their parents more than it resembles them both equally. This has to do with the expression of the said genes – expression of a gene refers to its ability to be visible on a person. For example, if a child has a parent with brown hair and one parent with blonde hair and the child has brown hair, it is because the genes encoding for the hair pigment were derived from the parent with brown hair, over that of the one with blonde hair. This is called DOMINANCE.
Dominance is a characteristic of a gene that helps it express its gene product over another, complementary gene, which is considered RECESSIVE. A dominant gene is expressed in every generation, while a recessive gene tends to express itself by skipping generations. So to put it directly, if one of the parents has dominant genes while the other has recessive ones, the dominant genes are going to be expressed. Hence, in this case, the child will heavily resemble the parent with dominant genes.
Despite the appearances, if a person is subjected to DNA testing, using both their parents as samples, the result would be presented in a 50/50 ratio of the parental genes. This refers to the GENOTYPE or the genetic composition, but the PHENOTYPE, which is the physical manifestation, is respective to the dominance of the genes.
Why don’t siblings exactly resemble each other?
With human logic, one can concur that if two people have the same set of parents, they ought to similar because they receive the same set of genes from their parents. Notice the word ‘similar’ being used, instead of the word ‘same’ – since siblings truly do not exactly look the same.
Imagine a pair of siblings – both of whom have received 50% of their genetic material from their mother and the other 50% from their father. But they look similar to each other; there are two reasons for this –
- Due to RECOMBINATION
- Due to receiving the other 50% of the genes from each parent
Recombination refers to the phenomenon wherein the parental genes are ‘mixed’ in the embryo, to form genes that are relatively different from what they were originally in the parents. It occurs at a stage in the cell cycle, and it occurs at random, meaning that it is nearly impossible to predict the extent of ‘mixing’ of the genes and to understand which genes are being mixed.
Recombination gives rise to different phenotypes – which is why siblings can look different from one another. If recombination has occurred in one child and has not occurred in the other, it is reasonable to say they will not resemble each other. In the case of the brown-haired child with one parent with brown hair and another with blonde, their sibling may have blond hair – thereby exhibiting a clear difference in phenotype.
The other reason for this dissimilarity can be attributed to the fact that the sibling may receive the other half of the genes from the parents – different from the other child. Since the genes received themselves are different, the resultant expression and appearance of them will be different as well. In this case, there will be little to no resemblance between the siblings.
As it was calculated above that a child shares 50% of their genes with each of their parents, they share merely 12.5% to 25% of their genes with their siblings, depending on which mechanism the difference stems from. Although this number seems small, it is significant enough to label someone closely-related. As the relation between people increases, the number of genes shared between them grossly decreases.
What is the pattern of similarity between twins?
Twins are of two types – MONOZYGOTIC and DIZYGOTIC twins. Monozygotic twins, commonly referred to as IDENTICAL TWINS, are the result of a fertilized embryo dividing into two – thus forming two embryos. Monozygotic twins share more than 75% of their genes – leaving 25% as a buffer since recombination (as mentioned above, recombination can occur unpredictably) can result in minute genetic changes. Still, the changes are not high enough for monozygotic twins to look different – they will look nearly the same or identical.
In the case of dizygotic twins, there are two eggs and two sperms or one egg and two sperms (which subsequently divided into two embryos). These twins are called NON-IDENTICAL TWINS and they share genes like a normal sibling; which is why they do not look identical. The shared genes like siblings do – ranging from 12.5% to 25%.
It is interesting to note that monozygotic twins are never of two different sexes, while dizygotic twins can be of the same or different sexes depending on the type of fertilization.
To summarise, the resemblance between family members is a play on genetics and inheritance of traits. The inheritance pattern for the genes is specific for an individual and GENETIC MAPPING helps with understanding this pattern. Genetic mapping involves a variety of techniques that attempts to ‘map’ or recognize the location of a gene on the genetic material. A comparison of this data with that of another individual can help in linking the biological relationship between the individuals.