Remarks such as, “It runs in the family.” “It’s in our blood.” are very common in conversations related to family history.
What dictates the differences and similarities between individuals of the same lineage or family?
How can understanding lineage and inheritance prove beneficial?I like the color of your eyes, blue as the ocean.
Your ears—they look just like your mother’s!
Sounds familiar?
These remarks refer to particular characteristics (aka attributes or traits) of an individual. A trait is defined as any distinguishing characteristic or quality like skin color, hair color and type, eye color, and blood type—to name a few. For the layman, it suffices to associate these remarks with individual-specific traits inherited from parents and ancestors. However, geneticists dig deeper into the purpose of these traits. Just like tools have a utilitarian purpose, traits are utilized by organisms to overcome hurdles and survive. The traits that prove useful in facilitating survival are passed on to the next generation via reproduction.
What makes you what you are?
Traits vary considerably among different populations, with the most desirable traits occurring most frequently and commonly in any particular population. However, the presence of a trait does not imply that the trait will be expressed in the organism.
A trait may be expressed (dominant trait) or suppressed (recessive trait) in an organism depending on multiple factors. There are two copies of a gene (aka alleles) that dictate any particular trait; one of the copies (aka dominant allele) overrides the function of the other copy (aka recessive allele).
Traits that are expressed or suppressed depending on the allelic expression of the genes are known as Mendelian traits, named after Gregor Johann Mendel, the father of genetics. The expression of Mendelian traits does not depend on other factors; i.e., if one dominant allele is present, the trait will be expressed and if both the alleles are recessive, the trait will be suppressed.
Example of a common trait—The ability to roll your tongue
The allele that enables tongue rolling is dominant, while the allele that suppresses tongue rolling is recessive. This means that you can roll your tongue only if you express at least one copy of the dominant allele that facilitates this function. If both the alleles are recessive, you may not be able to roll your tongue!
Two halves of one
The interactions of genes are not always straightforward; in fact, they are very complex and the perfect balance of their interactions is crucial for optimal functioning of any organism. Dominant traits will be expressed, irrespective of their pairing; be it the pairing of two dominant alleles or the pairing of one dominant and one recessive allele. Contradictorily, the expression of recessive traits requires the pairing of both recessive alleles. As if this isn’t complicated enough, factoring in heredity further complicates the implications of allele pairing.
The understanding of the processes involved in gamete formation demystifies this interplay between genes and heredity to a large extent. So, what are gametes? A gamete is half the genetic material (DNA) passed on from one parent to their offspring, which combines with another gamete (from the other parent) to produce the complete genome of the organism.
Example of a dominant trait—Melanin production
Melanin is the pigment that imparts color to your skin, hair, and eyes. The melanin producing trait is considered to be dominant because the larger part of the world’s population demonstrate color in their skin, hair, and eyes. Therefore, the trait suppressing melanin production is considered to be recessive.
In the scenario that an individual with a pair of dominant alleles for melanin production (MM) mates with an individual with a pair of recessive alleles (mm), the gametes formed will be ‘M, M’ from the dominant gene and ‘m, m’ from the recessive gene. The genes containing pairs of the same alleles are known to be homozygous for the trait, and the resulting offspring will express the dominant melanin production trait with colored skin, hair, and eyes. However, the gene will comprise of one dominant and one recessive allele i.e., Mm. The demonstrable attributes resulting from the expression of a trait is known as the phenotype, while the allelic pairing of the gene is known as the genotype.
The determination of the genotype and phenotype of the offspring resulting from the mating of two individuals with the ‘Mm’ genotype is slightly more complicated. The genes containing pairs of different alleles are known to be heterozygous for the trait. In this case, each gene splits into ‘M, m’ gametes.
A table format (aka Punnett square) was developed for easier understanding of the multiple possibilities of genotypes and phenotypes in the offspring resulting from the fusion of these gametes.
| 'M' | 'm' |
'M' | MM (Dominant; melanin producing) | Mm (Dominant; melanin producing; aka carrier) |
'm' | Mm (Dominant; melanin producing; aka carrier) | mm (Recessive albino) |
According to this, mating of two individuals expressing the melanin producing phenotype but having a heterozygous genotype may result in a 25% chance of the offspring having albinism, a condition resulting from the lack of melanin production. There is also a 25% chance that the individual will express the dominant trait both genotypically and phenotypically. However, there are higher (50%) chances that the offspring may carry the recessive trait but not express it. Such individuals are known as a carrier of the recessive trait.
So, the genotype also needs to be accounted for!
Based on the principle of gamete formation and allele pairing, mating of an individual homozygous for the dominant trait with an individual who is a carrier will result in equal chances of the offspring being homozygous for the dominant trait and being carriers.
(See the Punnett square below.)
| 'M' | 'M' |
'M' | MM (Dominant; melanin producing) | MM (Dominant; melanin producing) |
'm' | Mm (Dominant; melanin producing carrier) | Mm (Dominant; melanin producing carrier) |
Similarly, mating of an individual homozygous for the recessive trait with an individual who is a carrier will result in equal chances of the offspring being homozygous for the recessive trait and being carriers.
(See the Punnett square below.)
| 'M' | 'm' |
'm' | Mm (Dominant melanin producing, carrier) | mm (Recessive albino) |
'm' | Mm (Dominant melanin producing, carrier) | mm (Recessive albino) |
23 pairs and us
Every characteristic of a human being is attributable to the expression of one of the innumerable traits comprised in a human genome. The expression of some of these traits can also cause lifelong debilitating medical conditions known as 'genetic disorders'.
Human chromosomes are divided into 22 pairs of autosomes and one pair of sex chromosomes (X/X or X/Y). Based on the chromosome and the inheritance pattern (dominant or recessive) involved, the inheritance of genetic disorders may be autosomal dominant, autosomal recessive, and autosomal dominant, autosomal recessive, Y-linked, X-linked, X-linked dominant, X-linked recessive, codominant, or even mitochondrial.
Credit: Jerome Walker/ Own work/ https://creativecommons.org/licenses/by/2.5/
Source: https://commons.wikimedia.org/wiki/File:Autosomal_Dominant_Pedigree_Chart.svg
Tree of knowledge
Analysis of family trees and histories can facilitate the identification of specific traits that have been passed down through the generations — the most common ones being diabetes, male pattern baldness, and blood group type. Collection of accurate data is crucial to the correct understanding and interpretation of the inheritance patterns of these traits. In addition, the ability of traits to skip generations increase the challenge involved in interpretation of the data, especially if the data is inaccurate or unreliable.
Credit: Innovative Genomics /https://creativecommons.org/licenses/by-nc-sa/4.0/
Source: https://innovativegenomics.org/glossary/allele/
The curious case of kinship
Genetic counsellors are very much like detectives who collect the required data by simply talking to relevant family members about the various conditions and traits that they may have observed in the family through the generations. Of course, it is very important to ensure that the data is collected only under circumstances of confidentiality and reliability. The collected information can facilitate the creation of a family tree with the oldest relative on the top and gradually going down through the generations. Any disorder or unique genetic trait that an individual may have is noted alongside the name and relationship. That’s it! This family tree, also known as a pedigree chart, acts as the perfect tool to identify genetic inheritance patterns within families, and can prove to be a valuable asset in detecting and diagnosing genetic disorders given that the collected data is accurate. A pedigree chart is also useful in predicting the potential risk of an individual inheriting or developing certain conditions.
The implications of this kind of information are profound; it can save lives!
Genetic disorders are often chronic, debilitating, immensely challenging, and compromises the quality of life—not only for the affected individual but also for the near and dear ones. Although some genetic disorders require minor lifestyle alterations, most of the disorders take a toll on the physical, mental, and emotional health of those involved. The troubles and challenges associated with genetic disorders that are life-altering or life-threatening conditions are passed through the generations.
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