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Basic Genetics for a Layman- Short and Simple!

Why this post?

In today’s world, it highly important today that each one of us at least has basic understanding of genetics, even if one is not a doctor. You must know at least the basic laws of the town in which you live, even if you are not a lawyer!

Nothing in the world of biology can be understood without basic knowledge of genetics. Every entity in this living world is an outcome of the genes that the organism harbors. Every disease we know today is influenced by genetics.

Unfortunately the science of genetics is viewed as cryptic. I know too many well read doctors who bravely accept that they don’t understand genetics and feel its a daring job to make any such attempt. Complicated terminologies, hopelessly long chemical formulae and long reaction process maps can easily dissuade any enthusiastic learner.

But basic genetics need not be so complicated. Its actually a beautiful story that epitomizes the miracle called life. And in this post, I will try to tell this story in a simple fashion using analogies such that even a layman with no training in biology will end up learning the basics of genetics.

Do let me know in the comments section if I have been successful in this aim!

Chromosomes as SOPs

Amazon is a giant organization. It’s a huge multinational business with enormous complexities. But still Amazon has been a successful business establishment. And this has been possible because of its thousands of employees working together as a system. This system divides Amazon business into several functions such as Finance, Marketing, Sales, HR etc. And in each function, there are several employees who have pre- defined roles. And to ensure seamless execution of these roles and their synchrony with the overall organization, there are well defined detailed processes in place. There is seldom any confusion, because of these detailed processes perfectly outlined in the form of manuals known as Standard Operating Procedures (SOPs).

Genetics- Chromosmes SOPs

A human body is just like a big organization like Amazon. We are made of millions of living units called cells. Each cell is functional entity capable of living on its own. These cells are like the employees at Amazon. Several cells come together to form organs. Each of these organs has a specific role in our body, and their co-ordination allow us to live the way we do. Cells that form liver specialize in detoxifying the toxins entering our body. Cells forming our kidneys are specialists in filtering out unwanted substances from our blood. Cells that make our heart are the only ones who know how to work as a rhythmically beating bellow.

Genetics- Organ Functions

In Amazon most of the employees are extremely intelligent. But any employee can really do most of his functions without thinking much. This is because everything is so well defined in the instruction manuals- the SOPs. However, unlike the employees at Amazon, cells inside any organ cannot think. A liver cell cannot think and determine how to detoxify the highly toxic ammonia into urea. But we all agree that our body is more efficient than any organization existing on this planet. This is because a liver cell is not supposed to think at all to do its job. It does exactly what is written in the instruction manuals. And these instruction manuals are known as chromosomes. These chromosomes are located inside every cell. Inside a human cell there are 23 pairs of such instruction manuals (chromosomes). There are 23 chromosomes and for each of these 23 chromosomes there is a corresponding back- up copy, thus making 23 pairs. So, in every cell, we have 46 chromosomes. Within these chromosomes are located pages of instructions that define each and every activity performed by a cell. Each page of instructions is known as a “gene”. Each chromosome bears thousands of genes. The cell just does exactly what’s written in those genes. Of these 23 pairs of chromosomes, the chromosomes belonging to 23rd pair are called the “sex chromosomes”- they decide the gender of the person. Rest of the 22 pairs are known as autosomes.

The 23rd pair in any female consists of has two X chromosomes. In case of males, the same 23rd pair consists of one X chromosome and one Y chromosome. Hence, in the world of human genetics, males are denoted as XY and females are denoted as XX. In a female, one X comes from her mother and other X comes from her father. In a male, the X always comes from his mother (because she has no Y) and Y always comes his father (because only father has Y and not the mother).

Genetics- Chromosomes pairs

Let me again repeat that when I say there are 23 pairs of chromosomes what I am trying to emphasize is that each chromosome has a back-up. Both chromosomes in each pair are similar to each other. They define specific functions and processes to be performed by the cell, and they both harbor “similar” set of instructions. “Similar” is a key word here and we will now elaborate on this concept of “similar”.

The Kitchen Analogy in Genetics

Genetics- Chromosmes recipe book

Imagine a kitchen where the chef has strict instructions not to follow his brains but to cook exactly the way the recipe book says. This recipe book (let’s call it Book A1) is there on the kitchen shelf, and has a back- up copy alongside (let’s call it Book A2). Thus, each page of this recipe book (Book A1) has a similar corresponding copy in its back- up book (Book A2). Let’s say the customer has placed order for chicken soup. The recipe for making chicken soup is on page no. 106 of both Book A1 and Book A2.

Page 106 of Book A1 says:

  1. Ingredients
    1. 1 whole chicken (about 4 pounds), cut into pieces (including back)
    2. 8 cups water
    3. Coarse salt
    4. 3 medium onions, thinly sliced (4 cups)
    5. 2 celery stalks, sliced crosswise 1/4 inch thick
    6. 4 garlic cloves, crushed
    7. 6 medium carrots, sliced 1/2 inch thick
  2. Steps
    1. Bring chicken, water, and 1 tablespoon salt to a boil in a large stockpot. Skim foam. Add onions, celery, and garlic. Reduce heat. Simmer, partially covered, for 30 minutes.
    2. Remove breast, and set aside. Add carrots. Simmer, partially covered, for 40 minutes.
    3. Remove remaining chicken; discard back and wings. Let cool slightly. Remove meat from bones, and cut into bite-size pieces.
    4. Stir in desired amount of chicken; reserve the rest for another use. Skim fat. Season with salt.

The chef uses the entire machinery of the kitchen to ensure that these instructions to prepare chicken soup are exactly followed. Because the instructions are clear and chef was competent and the kitchen was well equipped, a tasty chicken soup got delivered to the customer!

Note that here the chef used Book A1.

At this point, there are 3 important questions that need to be discussed and resolved. Believe me, these are really important questions whose answers will help you understand genetics.

  1. Can the chef use Book A2 primarily instead of Book A1?
    • Answer: Of course, he can. Both books are back -up copies of each other and any can be used.
  2. How does the chef decide whether to use Book A2 or Book A1?
    • Answer: It’s quite a random pick. Theoretically all pages of both books are similar and equally competent. So, book selection is a “random event”. We will soon see that this random event of book selection is called “lyonization”.

We earlier saw the details of recipe on Page 106 of A1 book (for the sake of brevity we will call it Recipe A1).

Now let’s look at Page 106 of book A2 book (again, for the sake of brevity we will call it Recipe A2).

  1. Ingredients
    1. 1 whole chicken (about 4 pounds), cut into pieces (including back)
    2. 4 cups water
    3. Coarse salt
    4. 5 medium onions, thinly sliced (6 cups)
    5. 2 celery stalks, sliced crosswise 1/4 inch thick
    6. 6 garlic cloves, crushed
    7. 6 medium carrots, sliced 1/2 inch thick
  2. Steps
    1. Bring chicken, water, and 2 tablespoon salt to a boil in a large stockpot. Skim foam. Add onions, celery, and garlic. Reduce heat. Simmer, partially covered, for 30 minutes.
    2. Remove breast, and set aside. Add carrots. Simmer, partially covered, for 40 minutes.
    3. Remove remaining chicken; discard back and wings. Let cool slightly. Remove meat from bones, and cut into bite-size pieces.
    4. Stir in desired amount of chicken; reserve the rest for another use. Skim fat. Season with salt.

If you compare this with Recipe A1, it’s not same. It’s similar, but not same. If the chef follows this Recipe A2, it will still lead to chicken soup. It will be a bit different though. It will be a bit more spicy, and thick. But still, it will be a tasty chicken soup- not same as Recipe A1, but “similar”.

Now imagine a scenario where Recipe A2 looks something like this:

  1. Ingredients
    1. 1 whole chicken (about 4 pounds), cut into pieces (including back)
    2. 8 cups water
    3. Coarse salt
    4. 3 medium onions, thinly sliced (4 cups)
    5. 2 celery stalks, sliced crosswise 1/4 inch thick
    6. 4 garlic cloves, crushed
    7. 6 medium carrots, sliced 1/2 inch thick
  2. Steps
    1. Bring chicken, water, and 100 tablespoon salt to a boil in a large stockpot. Skim foam. Add onions, celery, and garlic. Reduce heat. Simmer, partially covered, for 30 minutes.
    2. Remove breast, and set aside. Add carrots. Simmer, partially covered, for 40 minutes.
    3. Remove remaining chicken; discard back and wings. Let cool slightly. Remove meat from bones, and cut into bite-size pieces.
    4. Stir in desired amount of chicken; reserve the rest for another use. Skim fat. Season with salt.

This new Recipe A2 is actually more similar to Recipe A1 than the earlier Recipe A2. However, this new recipe will have 100 tablespoon salt making the soup 100 times saltier than Recipe A1. Customers will hate it. It will be inedible. But still the Chef will go ahead and cook as per this new Recipe A2. This is because Chef is not allowed to use his brains and will cook exactly as entailed in the recipe. Because the selection of recipe book is random, whether this kitchen will make a perfect chicken soup using Recipe A1 or an incredibly salty and unpalatable one using this “defective” Recipe 2, is purely a matter of chance.

Let’s take an extreme example to drive home the point I am trying to make.

Imagine Recipe 2 (the recipe on Page 106 of A2 book) looking like this:

  • Ingredients
    1. 8 cups water
    2. Coarse salt
    3. 3 medium onions, thinly sliced (4 cups)
    4. 2 celery stalks, sliced crosswise 1/4 inch thick
    5. 4 garlic cloves, crushed
    6. 6 medium carrots, sliced 1/2 inch thick
  • Steps
    1. Remove breast, and set aside. Add carrots.
    2. Remove remaining chicken; discard back and wings. Let cool slightly. Remove meat from bones, and cut into bite-size pieces.

This recipe is just “non- sense”. There is no mention of chicken in the ingredients. Then, the steps also are illogical. Just by following this recipe, no chef will be able to prepare any chicken soup. So, if the chef of this kitchen randomly selects A2 book to read such “non- sense” instructions for making chicken soup, there won’t be any chicken soup!

Now, let’s expand our imagination. Imagine that there are 10 such kitchens belonging a company. Each kitchen has 2 recipe books- A1 and its corresponding copy A2. 106th page in every A1 has instructions that would lead to a normal tasty chicken soup. But the same 106th page in every A2 is “non- sense” as described above. As we discussed, which recipe book gets chosen by which kitchen is completely random and is a matter of chance.

Now, let’s say the company, which manages these 10 kitchens, get an order for 10 bowls of chicken soup. This order is to serve a group of let’s say 7 people who have gathered for a party. There can now be 5 scenarios:

  • Scenario 1: All of the kitchens randomly select A1

Here you will get 10 bowls of “normal” soup

  • Scenario 2: Most of the kitchens randomly select A1 (lets say 8)

Here you will get 8 bowls of “normal” soup and 2 bowls “empty”

  • Scenario 3: Half of the kitchens randomly select A1, other half select A2

Here you will get 5 bowls of “normal” soup and 5 bowls “empty”

  • Scenario 4: Majority of the kitchens randomly select A2 (lets say 8)

Here you will get just 2 bowls of “normal” soup. Rest 8 bowls will be “empty”

  • Scenario 5: All of the kitchens randomly select A2

Here all bowls will be empty

Scenario 1 is the ideal scenario. Customers get exactly what they ordered and they will be happy. In scenario 5 it’s clear that the customer will end up hungry and very very angry. For scenarios 2,3,4- of course the customers will not be happy; but their anger and dissatisfaction will be higher for scenario 4 compared to scenario 2.

Genetics- Chromosomes chicken soup ordering

A similar range of outcomes will show up if the 106th page of A2 details a recipe which is 100 times saltier- just that the company can still argue that soup was delivered in the exact quantity ordered!

Let’s now start connecting whatever we imagined, with what really happens in our body. What I am going to explain now is a little bit repetition of an earlier discussion- but an important one to understand genetics on a deeper level.

Our body is made of millions and millions of microscopically small independent living units called cells. Each cell has its own little “kitchen” called “nucleus”. Inside each such kitchen there are 23 pairs of “recipe books”. Each such recipe book is called a chromosome. Thus, there are 23 pairs of chromosomes (total 46) inside the nucleus of each cell. In each pair, one is a back- up copy of the other. Each recipe book (chromosome) is made of thousands and thousands of “pages”. Each page contains detailed recipe for one particular “dish”. Inside any chromosome, each such page is known as a “gene”.

Genetics in Action- Chromosome 12

To give an example, let’s consider chromosome No. 12. There are 2 copies of chromosome 12, each serving as a back-up for the other. Within first copy of chromosome 12 there are around 1600 pages or genes. Each page (gene) is a detailed recipe of how to prepare one specific “dish”. Thus, on this copy of chromosome 12 there are recipes for around 1600 unique dishes. Now the other copy of chromosome 12 is also has “similar” 1600 genes. We have already discussed what this “similar” means in detail earlier when we looked at recipe books A1 and A2.

Now here comes the interesting part. In each chromosome pair, one copy comes from father and the other copy comes from mother! So, every gene has two versions- one coming from father and other coming from mother. Hence, the recipe on page 106 of A1 recipe book is SIMILAR to the recipe on page 106 of A2 recipe book and NOT SAME- this is because one page has been contributed by the father and the other copy has come from mother.

Genetics- Chromosome 12 mom dad

Genetics in Action- vWF gene

Let’s go a bit deeper into chromosome 12. Each segment of this chromosome carries around 1600 pages of instructions for similar number of chemicals to be prepared; and these chemicals in turn orchestrate thousands of processes that contribute to the survival and the functioning of the cell. Amongst these 1600 genes on each copy chromosome 12, there is a gene called the vWF gene. vWF gene carries the “recipe” to prepare a chemical called von Willebrand Factor (vWF).  

vWF is a very important chemical necessary for clotting of blood. If vWF is defective or deficient in a person, that person is aid to be suffering from a bleeding condition called von Willebrand’s Disease (vWD). Such a patient of vWD can easily bleed to death if not treated appropriately.

We have seen that there are two chromosome 12s. Both have similar genes at similar locations carrying similar instructions. So, there is a gene which carries instructions for making vWF on chromosome 12 and a similar corresponding gene on the other chromosome 12.

Let’s call the first copy of chromosome 12 as 12.1 and other copy as 12.2. There are two copies of vWF gene, one on each copy of chromosome 12. Let’s call the first copy of vWF gene as V-1 and the other corresponding copy as V-2. Let’s say V-1 is on 12.1 and V-2 is on 12.2. In a healthy individual, the recipe on V-1 will encode for a normal structure of vWF and the recipe on V-2 will also encode for a similar normal structure of vWF. Since the recipes are “similar” and not same, the structure of vWF from V-1 will be slightly different from the one out of V-2. Nevertheless, in a healthy individual both recipes of vWF are equally competent in developing functionally healthy vWF.

Genetics- Chromosome-12-genes

Concept of “Lyonization” in Genetics

At this moment, you may want to refresh our discussion on normal and defective recipes of chicken soups. It will now start connecting.

There are millions of cells that prepare vWF. Each of these million cells has two copies of chromosome 12 (we named them as 12.1 and 12.2). While preparing vWF, one cell can use either 12.1 or 12.2- never both. If a cell uses 12.1, that particular cell automatically inactivates 12.2 and vice- versa. In one cell whether 12.1 will be used or 12.2, is a matter of chance and the selection is random. So, in any such cell, either 12.1 will be inactivated or 12.2. This random inactivation of one copy of chromosome is known as “lyonization”. This phenomenon of random inactivation of one copy of chromosome in a pair was first proposed by the English geneticist Mary Lyon in 1961.

Since either 12.1 or 12.2 will be used and never both, a cell producing vWF will be able to use either V-1 or V-2 and this usage is purely random.

(Please note that in majority of literature lyonization is generally referred to random inactivation of one X chromosome in females)

The “Defect” and the “Disease” in Genetics

Now, just for the sake of our understanding, let’s say imagine a small living body in which there are 100 cells which are assigned to produce vWF. This hypothetical body’s requirement is, let’s say, 100 mg of normal functioning vWF so that clotting mechanism of its blood works with perfection. However, this body can manage with 50 mg of vWF as well. So, optimal level is 100 mg and minimum required is 50 mg. (Remember this is example is hypothetical and 50 mg, 100 mg etc are imaginary levels used here just to make understanding easier). In this example, lets imagine that one cell can produce 1 mg of vWF. It means if all the 100 cells are healthy, irrespective of which cell uses V-1 and which uses V-2, this body will be easily able to produce 100 mg of vWF.

Now let’s consider a diseased scenario for this body. In this disease, V-1 bears a defective recipe for vWF. The defect is so bad the cell won’t be able to produce any vWF if it reads V-1. Now, we know that which cell will select V-1 and which cell will select V-2 is a matter of random choice.

So, in this example, there can now be 5 possible scenarios:

  • Scenario 1: All the 100 cells randomly select V-2 (the normal gene)

Here this body will be able to prepare 100 mg of functional vWF and there will be no issue.

  • Scenario 2: Most of the cells randomly select V-2 (let’s say 80)

Here this body will be able to prepare 80 mg of functional vWF. This is less than optimal but is higher than the minimum requirement. So, most often, this body will not suffer. But in extreme scenarios (let’s say an accident leading to intense bleeding), this body won’t be able to quickly control the bleeding.

  • Scenario 3: Half of the cells randomly select V-2, other half select V-1

Here this body will be able to prepare just 50 mg of functional vWF. This is way less than optimal but just meets the minimum requirement. So, in normal situations this body will not suffer much. But in little extreme scenarios similar to Scenario 2, this body can easily bleed to death.

  • Scenario 4: Majority of the cells randomly select V-1 (let’s say 80)

Here this body will be able to prepare just 20 mg of functional vWF. This is too less and is below the minimum requirement. This body will frequently suffer from bleeding episodes even without any accident

  • Scenario 5: Majority of the cells randomly select V-1

Here this body will not have any functional vWF. Most probably this body won’t survive for long (unless treated) and will soon bleed to death

Genetics- vWF gene defect scenarios

What if we tone down the defect in V-1 and say that cell can read and prepare vWF, but the recipe lacks some important instructions because of which the resultant vWF is structurally defective? In such a case also the same 5 earlier scenarios will be applicable, just that the severity of disease will be a lot lesser.

(If you have understood everything whatever have been discussed in this section, pat your back! It’s a very complicated topic and when I was learning genetics during my initial days these concepts remained an enigma to me for a very long time. It took a long time before I realized that the concept of lyonization can be extrapolated and used to understand manifestations of autosomal genetic diseases as well!)

Challenging Genetics of Sex Chromosomes

Now, let’s complicate things for you here; not because I want to, but it’s essential. Remember, we said that amongst the 23 pairs of chromosomes within each nucleus in any human cell, the chromosomes belonging to 23rd pair are called the “sex chromosomes” and that they decide the gender of the person. The 23rd pair in any female consists of has two X chromosomes. In case of males, the same 23rd pair consists of one X chromosome and one Y chromosome. Hence, in the world of human genetics, males are denoted as XY and females are denoted as XX. In a female, one X comes from her mother and other X comes from her father. In a male, the X always comes from his mother (because she has no Y) and Y always comes his father (because only father has Y and not the mother).

As you can easily see, X is more similar to another X than Y. An X has 4 arms. A Y is an X with one arm less. In a female cell, there are 2 Xs in every cell. Each X chromosome is similar to the other X in terms of the genes it harbors. So, when each female cell randomly inactivates one X (known as lyonization) it’s perfectly all right. The other copy of X ,which is not inactivated, is good enough and contains all the information that is loaded in the inactivated X. But a male cell has one X and one Y. They are not similar. They are essentially 2 “different” recipe books each with “different number” of “dissimilar” pages. Here, random inactivation of any sex chromosome here in a male will lead to disaster. Hence, lyonization is not applicable in case of 23rd pair of chromosomes in males. In short, if a cell has XY pair lyonization is not applicable for that XY pair. Remember- the concept of lyonization can be applied for all other chromosome pairs in every male cell- however, it is not applicable for the XY pair.

This understanding is very critical to grasp manifestations of several diseases. Let’s take example of a bleeding condition known as Hemophilia-A. It is similar to von Willebrand’s Disease (vWD) we discussed above. Like vWD, it is a genetic disease. And like vWD, the patient suffers from uncontrolled bleeding. In vWD the defect lies in a gene located in chromosome 12. But in Hemophilia-A the defect lies in a gene located in X chromosome. Because of this defect in X chromosome, production of a chemical called Factor 8 (also referred as FVIII) is impacted. FVIII is a critical component required for clotting of blood. Now let’s take example of a female whose one X chromosome (X-1) has come from her father and other X (X-2) from her mother. Let’s say X-2 carries the defective gene that causes hemophilia. Due to lyonization few cells in this females will inactivate X-1 and few cells will inactivate X-2. If majority cells inactivate X-1, they will read from the unhealthy X-2, and the body may not then produce enough FVIII and the female might end up showing signs and symptoms of Hemophilia- A. But the quantity of FVIII actually required is quite small. All what is needed is enough number of cells with active X-1. In most females bearing defective gene on X chromosome that causes Hemophilia A, there are enough cells inactivating the X chromosome with defective gene and reading the healthy gene from the other X. Hence females are generally “carriers” in case of X- linked genetic diseases such as Hemophilia A. They CARRY the disease, but usually don’t suffer.

Ofcourse, there can be an unlucky female who has defective genes on both the X chromosomes. In such scenario, the female will 100% suffer (in such cases father is patient and mother is a carrier).

In males, situation changes dramatically. Lyonization doesn’t exist for XY pair and there is only one X and one Y. X has no back up. Neither does Y. So, if there is a defect in any gene in X, there is no option. So, if there is a defective gene in X chromosome impacting FVIII production in a male, that male will definitely suffer from Hemophilia A. Same is true if the defect is on Y chromosome- the only logical difference is that a female can never ever suffer from Y linked genetic diseases (since it’s Y what make one male!)

Genetics- Sex Chromosome Inheritance

Of the 23 pairs of chromosomes in any, we saw that the first 22 pairs are known as “autosomes” and that the chromosomes of the 23rd pair (XX in females and XY in males) are called the “sex chromosomes”. So genetic diseases from defects in genes located in any of the autosomes are known as “autosomal diseases”, those located on X chromosome are called “X- linked diseases” and those located on Y chromosome are called “Y- linked diseases”.

Dominant And Recessive genes

For each of millions of chemicals prepared by any cell based on the recipes in chromosomes, there is bare minimum quantity that needs to be produced so that things run normally. For some chemicals, this minimum required quantity is quite low. Let’s call one such chemical as “Chem- A”. So, in case of a gene defect impacting production of Chem- A (assuming that the corresponding copy of this gene is normal), all what is required is few cells with active chromosomes bearing normal copy of gene for that Chem- A so that that minimum amount of Chem A gets produced. If such minimum production happens, no disease will be seen. Such individual bearing gene defect for Chem A but no symptoms will be called as a “carrier” for this disease. An individual will suffer only when even the minimum required amount of Chem A is not produced. And this most probably will only happen when both the copies of gene for Chem A is defective. Such genetic diseases, which do not generally show up with defect in single copy of gene, are known as “recessive”.

However, for some chemicals the minimum required quantity is not low enough unlike Chem- A. Let’s call one such chemical as “Chem- B”. So, in case of a gene defect impacting production of Chem- B (assuming that the corresponding copy of this gene is normal), you will need a large number of cells (maybe majority) with active chromosome bearing normal copy of gene for that Chem- B, so that that minimum amount of Chem B gets produced. In most of such cases, even if single copy of gene is impacted and there are cells which have active copy of healthy gene, the minimum quantity required is not reached. Such genetic diseases, which generally show up even with defect in single copy of gene, are known as “dominant”.

It’s obvious that if both the copies of a gene are defective, the disease will definitely show up irrespective of “recessive” or “dominant” situation. But if only one copy of gene has the defect, it depends on individual whether the disease will show up or not. If its “dominant”, even if majority of the cells inactivate the defective gene, the disease will show up if the minimum amount of corresponding chemical doesn’t get produced.  If this minimum quantity gets produced, “dominant” genetic disease won’t show up in such individual (although the chances are rare in dominant genetic diseases). On a similar note, if it’s recessive and only single copy of gene is defective, in certain individuals the disease might show up if majority of the cells inactivate the normal gene and the minimum amount of corresponding chemical doesn’t get produced. 

Genetics- Dominant Recessive Inheritance

Penetrance and Expressivity in Genetics

So, in a large population a “dominant” genetic disease will show up in majority (there can be few who will escape from the suffering), and a “recessive” genetic disease will be seen in minority. What proportion of the population will be impacted by a genetic disease (assuming all have atleast one copy of the defective gene causing the disease) is called “penetrance”. “Complete penetrance” means everyone who has the defective gene will suffer from the disease. “Incomplete” or ‘reduced’ penetrance the disease will be seen in only a part of the population. The extent of penetrance may also change in different age groups of a population and depends on a combination of genetic, environmental, and lifestyle factors.

The logical conclusion of this discussion is that dominant genes have higher penetrance than recessive genes.

Genetics- Penetrance Genes

This entire discussion of “dominant”, “recessive” and “penetrance” is irrelevant when we talk about XY chromosome pair in males. And you already must have guessed the reason. There is only one X in males with no back- up. There is only one Y and it has also doesn’t have any back- up. Any defect in any gene in X will show up in that male- its 100% penetrance for males. Similarly there is 100% penetrance in males for any defect in Y.

If you have understood the concept of penetrance, expressivity is quite simple to follow. Expressivity just refers to the degree of expression of a gene that has “penetrated”. Let’s relook at the example of von Willebrand’s disease. If you DON”T have enough number of cells reading the normal vWF gene, you will suffer. To what extent your suffering will be, will then depend on how many cells are using the abnormal vWF gene. Lower the number of reading abnormal gene, more will the amount of normal vWF generated and lesser will be your suffering. The signs and symptoms of vWD will depend of “how much” of vWF is prepared and of “what quality” So, the same disease may be different signs and symptoms in different patients based on the random choice made by cells regarding which gene will be used. This variable expression of gene is known as “expressivity”

Genetics- Expressivity

Let me give example of a disease called Marfan Syndrome. People with this genetic disease have a defect in a gene called FBN1 located on Chromosome 15. However, symptoms vary from one patient to the other. Some patients have only mild features making them tall and thin with long, slender fingers. While few other patients experience life-threatening complications involving cardiovascular system. This variable expression of same genetic defect is known as expressivity.

Genetics and Mutations

The discussion on genetics cannot be complete without mention of an important terminology- “mutation”. When the recipe in any gene is slightly different from its corresponding copy, and both recipes lead to production normal and healthy versions of the same chemical, we call both the genes as “variants” of each other. The recipes in such variants can be called as “variations”. But sometimes one gene codes a recipe which is significantly different from that in the other gene. Such difference may lead to production of a chemical which is not healthy or functional. We call such a “different gene”, which has recipe significantly different from what is expected, as a “mutant”; and the new recipe is known as “mutation”. However, it’s a common practice to use the term “variant” and “mutant” interchangeably. All one needs to remember is that both terms, whenever used, will mean that the referred gene has “difference” compared to its “normal” copy.

Genetics- Mutation

On Science Genetics- Final Notes

Most of the religions across the world believe that every individual is a puppet of his/ her destiny which is already scripted by god during his/her birth. Followers of Hinduism believe that a fate of a person is written on his forehead by god during birth. Similarly, followers of Islam assert that the destiny of every living person on this earth is written is a holy tablet by God and it cannot be changed.

Our initial understanding of genetics too may make us feel that we are hopelessly enslaved to what is written in our genes. But this is not completely true. The “window of opportunity” provided by our genes is too large!

Imagine yourself as a car, and the genes as your engine. If the engine’s capacity is to achieve a a speed of 500 kmph and you are running at 100 kmph, you cannot blame the engine for your sluggish pace. Even an engine with double capacity can’t help you. Hard work and perseverance are underestimated virtues. But success is never a function of talent attributed by genes. Its generally a direct function of hard work and determination.

There are enough evidences to show that even genes adapt and change to support you, if you are willing to put the required hard work to achieve what you are passionate about. So, never blame your “genes” or “fate” for your failures!

Lastly, there are nothing called perfect genes. No one is perfect. No one should be. A set of genes that makes a person weak and timid might be the same set that make him a great painter. Both painters and warriors are equally important in their own ways. Let me stop here by quoting Angie Karan:

“Our imperfections make us unique and also BEAUTIFUL. Some beautiful things are more impressive when left imperfect than when too highly finished, Our flaws and weaknesses can make us more beautiful! People who make mistakes are more like-able than those who appear perfect. We can’t connect with perfect…. but we like and LOVE people who are real. That’s beauty from the inside out. So lets accept our self for who we are and meant to be. We are all perfectly and authentically beautiful in our own special way, and nothing more or even less”

Hope you like this article. Please share your thoughts/views/feedbacks in the comments section below.

Dr Subir Roy

7 thoughts on “Basic Genetics for a Layman- Short and Simple!

  1. I have read so many posts about the blogger lovers however this post is really a good piece of writing, keep it up

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