If you have already fathered or mothered normal children, or children with some type of hereditary disorder, or this is your first time, and are considering the possibility of becoming pregnant in the future, or you are already pregnant, this web will help you find answers to some of your questions.
Life begins when a spermatozoon (a mature male germ cell) and an ovum (a mature female germ cell) uniteto form one single cell called egg" or zygote", which as of that moment, will undergo a series of divisions that will ultimately result in the birth of an infant.
This initial process is known as fertilisation, and it physiologically takes place in the female's uterine tubes.
This process is initiated through sexual intercourse during the female's monthly ovulatory period. This is the period when a female is fertile and can become pregnant.
Once sexual intercourse has taken place, spermatozoa make their way through the female's reproductive tract towards the uterine tubes in search of the ovum - released by the ovary - to fertilise it.
And voila! Fertilisation! One single sperm cell (spermatozoon) succeeds in entering the ovum and the miracle happens: a new life is created.
From the Fallopian tubes, this new life reaches the uterus where it will lodge and continue to grow during nine months until birth.
Growth takes place throughout a long stage of millions of cell divisions until a new being is totally formed.
Nonetheless, for this long and complex process to become a reality, three previous stages are necessary:
1st STAGE: GAMETE PRODUCTION (OVA AND SPERMATOZOA)
In the female, ova are produced in the ovary. Normally, each month, an ovum is released by one of the two ovaries and enters the Fallopian tube, a phenomenon known as ovulation.
Next, the ovum migrates, that is, it makes its way down the uterine tube towards the uterus. During this journey, it will only be available for fertilisation during 24 hours.
In the male, spermatozoa are continuously produced in the testis starting at puberty. The time required for their formation, from the start of development to the time a mature sperm cell with fertilising ability is obtained, is approximately 64 days. Once ejaculated, sperm cells will be capable of fertilisation for 48 hours.
The adult male emits between 2 and 8 millilitres of semen in each ejaculation. And each millilitre can contain 20 million or more spermatozoa, under normal conditions.
2nd STAGE: UNION OF THE GAMETES
3rd STAGE: NESTING OR UTERINE IMPLANTATION
The new life has to be embedded in the uterus in order to continue its growth and development.
From the moment the egg or zygote is fertilised, it starts to divide, giving rise to new cells, which group together in an orderly fashion to form new organs in our body with specialised functions.
Different sets of organs make up an interrelated ecosystem" so perfect and incredibly wonderful that it makes it possible for us to live, develop, and adapt to our environments.
So it is then easy for us to figure out that such orderly fashion must be scrupulously directed and controlled
And this is exactly the case, as all the instructions necessary for the development of the egg are contained in structures known as CHROMOSOMES that we all have inside our cells.
Chromosomes are vase-like structures where all the genetic data or necessary instructions are contained for a given individual to develop.
We could say that chromosomes are like our cook books, where all our cooking recipes are stored:
In one word, chromosomes contain our building blocks, that is to say our genetic heritage.
We receive this genetic inheritance in two equal parts from our parents at the time of conception. In turn, and by the same method, it is transmitted to our offspring, and from our offspring, transmitted to their offspring, and so traits are derived from an earlier generation to another generation. Thus, chromosomes are responsible for the origin and continuity of life.
And, how do they do that?
How can our organs be so different if all body cells contain exactly the same instructions?
In general terms, a cell is very much like an egg, and consisting of three major parts: a nucleus, a cytoplasm, and a cytoplasmatic membrane that encircles it.
The answer is easy. They are able to do this through cell specialization.
For the time being, let's just focus on the nucleus, as it is in its inner portion where chromosomes are found and which, in the human, total 46.
Given that all cells in our body have developed from one single cell, the egg or ovum fertilised by one spermatozoon", all cells are identical, that is, they contain exactly the same instructions. However, depending on the organ they are to be part of, they will use a specific part of that information. This process is known as cell specialization.
In this way, nerve cells contain the necessary information to form hair and the cells that make up hair have the necessary information to form nerves. However, once specialised, nerve cells will form nerve tissue and hair cells will form hair.
And so, since different recipes" are used, cells will form different organs.
A lung is, thus, different to a stomach because in the lung, the building blocks contained in the recipes" are designed to form cells that specialise in respiration, whereas, in the stomach, the recipes" that start operating are designed to form an organ that prepares the ingested food for subsequent absorption in the intestine. And, this process is repeated for each of the different parts that make up our body.
Because of this, every human cell has 46 chromosomes containing all our cooking recipes" or building blocks. However, these recipes are not expressed at the same time in all cells. Some of these building blocks are expressed and function in certain cells, whereas in other cells, other buildings blocks come into play.
As you can see, chromosomes that play a role in the formation of the stomach are at rest in the formation of the lung.
Chromosomes within a cell are arranged in sets. Each cell contains 46 chromosomes or 23 sets.
Such sets of chromosomes are classified according to an international nomenclature: from the largest to the smallest, followed by sex-chromosomes.
- two number 1 chromosomes
- two number 2 chromosomes
- two number 3 chromosomes
The first 22 sets are called AUTOSOMES,
and are shared by both the male and the female.
Chromosomes in set 23 are called GONOSOMES, or sex-chromosomes. These chromosomes can be X or Y, and constitute different sets depending on whether it is a man or a woman, for, as their name indicates, these chromosomes are concerned with thedetermination of sex and set the differences between a male and a female.
A woman has two X sex-chromosomes and a man has one X sex-chromosome and one Y sex-chromosome.
In other words, it is just ONE SIMPLE CHROMOSOME what makes us SO DIFFERENT from each other.
Always remember that chromosomes are the same in all human races.
Given that the number of chromosomes for everything to run smoothly should always be kept constant from generation to generation, each individual should only inherit 23 chromosomes from each progenitor at the time of conception.
Thus, if a woman has 46 chromosomes and a man has 46 chromosomes, each one has to transmit to their unborn child half of that amount (23 chromosomes) at the time of fertilisation. And so the unborn child will receive 46 chromosomes, 23 from its female parent and 23 from its male parent = 46 in total.
And, how is this possible?
Let's go back a little bit. Every cell always results from the partition of one cell into two daughter cells.
And so, from the moment in which the egg or zygote is formed, it undergoes division, producing two new cells. These, in turn, divide and each of them gives rise to two more cells, and this is successively repeated until the individual is formed.
Given that nature is very wise, depending on whether it wants to keep the number of chromosomes constant or, alternatively, reduce it by one half, it has, at its convenience, two types of cell divisions, which it uses in a very specific and controlled manner. One is mitosis and the other is meiosis.
The difference between both types of cell division lies in the way in which chromosomes are shared, that is the hereditary material at the moment of division.
Thus, in the MITOSIS cell division, the two resulting daughter cells always contain the same number of chromosomes as the parent cell from which they derive.
On the contrary, in the MEIOSIS cell division, the number of chromosomes is not kept constant, to the contrary, "it is reduced by one half". This is a reductional type of cell division, and so, at the end of the entire process, each resulting daughter cell contains only half the number of chromosomes from the parent cell. That is, from cells that contained 46 chromosomes grouped in 23 sets, cells that contain 23 chromosomes are derived, only taking one component from each set.
It is on this type of meiotic cell division that we will be focusing.
Please note that this is a very, very special type of cell division, and in the entire body, it is only used by germinal cells, that is, by cells located in the ovary in the woman and in the testis in the man.
In other words, cells that are located in the organs involved in reproduction and whose function is to keep our number of chromosomes constant from generation to generation. Otherwise, the number of chromosomes would experience a two-fold increase in each generation, and this would be incompatible with life, endangering the preservation of specie.
And, how is this distribution carried out?
The original cell that contains the 46 chromosomes divides and splits into two new cells. Before they are separated, these two cells share the chromosomes in equal numbers and each of them takes one element from each set of chromosomes.
Because they share the chromosomes between them, each of these two cells only contains 23 chromosomes, or one from each set.
During this process, germinal cells start maturing and changingtheir form until they become the future gametes or cells withfertilising capacity.
Please, note that these changes are extremely important. In the case of a male, a cell without a tail gives rise to cells with a tail that are called spermatozoa.
This tail enables spermatozoa to swim from the time of ejaculation up the uterus and the uterine tubes in the female until they encounter an ovum which they fertilise.
In the end, the percentage of spermatozoa that carry the X-chromosome is the same as the spermatozoa that carry the Y- chromosome.
In the woman, after each meiotic cell division, one of the two resulting cells is sacrificed, giving to the other cell its energy reserves so that if the other cell is fertilised, it survives while it migratesdown the uterine tubes and implants in the uterus.
The end result is that each month, the woman generates an ovumthat carries the X sex-chromosome, which can be fertilised by any of the millions of spermatozoa that are struggling to enter it, of which 50% carry the X-chromosome and the remaining 50% the Y-chromosome.
Thus, the possibility of the new individual being a boy or a girl is always determined by the male, as it is the male who bears both types of spermatozoa, the X-bearing sex-chromosome and the Y-bearing sex-chromosome. In contrast, as thewoman only has two X sex chromosomes, she will always contribute with one ovum containing one X sex chromosome.
From each male germinal cell, four spermatozoa are obtained, two of which are X-bearing sex-chromosome carriers, and two are Y-bearing sex-chromosome carriers.
From the female germinal cell, one ovum carrying the X sex-chromosome is obtained. The possibilities of recombination at the time of fertilisation are as follows:
And, what does a chromosome look like?
A chromosome is a very complex chemical structure composed of proteins and DNA (desoxyribonucleic acid: a compound composed of nitrogenous bases, sugars and phosphates).
When the chromosome is examined at a molecular level, the spatial shape it acquires resembles two strands twisted into a double helix (similar to a ladder suspended into space), where the steps correspond to the bases and the sugars and phosphates to the rail.
There are four organic bases:
- ADENINE and
The steps of the ladder are always composed of sets of GUANINE and CYTOSINE, or alternatively ADENINE and THYMINE. The succession of these bases or steps forms a lineal coded message that contains all our recipes and that is decoded inside each cell through specific mechanisms.
The spatial form DNA acquires is similar to a ladder suspended in space.
The steps of the ladder are formed by pairs of Guanine and Cytosine, or by Adenine and Thymine. And the rail by sugars and phosphates.
When it is necessary to use any of the recipes at cell level, the two DNA chains separate at this specific point and the gene or recipe is copied from beginning to end. This copy of the gene is called messenger RNA (messenger ribonucleic acid), during this process the Guanine is paired with Cytosine and Adenine instead of pairing with Thymine,does it with a new base called Uracil.
Once the process is completed, the DNA closes again and this copy of messenger RNA leaves the nucleus and moves towards the cytoplasm to join the ribosomes.
The function of the ribosomes is to read the message, and based on that information, to fabricate the proteins following a strict order. To carry out their function, ribosomes need amino acid transporters known as transfer RNA.
Remember that proteins are formed by chains of amino acids, which play a very important function as they determine how all the molecules (sugars, fats, water, minerals, DNA, etc.) that conform our body must be arranged in order to function.
The RNA message is written in a trinary code, that is, a specific amino acid is determined by a sequence of three bases. A codon is thus formed by a set of three bases.
Given that we have four pairs of bases (Guanine, Cytosine, Adenine and Uracil), in total we have 64 possible combinations of codons to recall the 20 amino acids that conform our proteins.
To make things easier, let's imagine that a chromosome is like a cassette tape, recorded over with cooking recipes one after another.
In our case, the cooking recipes are the instructions for the structures in our body, such as the brain, heart, kidney, etc. to develop and function.
5 - HOW DO WE RECEIVE OUR GENETIC HERITAGE?
Let's imagine a mother with her two daughters, who divides her belongings equally between them. The mother has two closets, two chairs, two beds, and she gives each of her daughters: one closet, one chair, one bed, etc.
In our case, what we are dividing is the 23 sets of chromosomes that we have in each of our gonadal cells - that is the cells contained in the ovaries and the testis.
Remember that the parent cell, both in the ovary and in the testis, contains 46 chromosomes arranged into 23 sets before it undergoes cell division. When meiotic division takes places, it divides the chromosomes equally between both daughter cells. Thus, from the two chromosomes contained in set 1, one is given to each daughter cell, and this process is repeated for each of the remaining sets. In the end, each daughter cell will have received 23 chromosomes, or what is the same, an element from each set.
Subsequently, when the ovum and the spermatozoon unite, an individual with 46 chromosomes (23 from the mother + 23 from the father) is obtained. In this manner, the number of chromosomes in the human race is preserved.
Let's also note that the information of the recipes contained in the chromosomes is copied twice (with the exception of sex-chromosomes in the male "XY"). From chromosome or cassette number 1, we have two copies: oneinherited from the father and one inherited from the mother. Simply because they are from the same cassette number, they must contain exactly the same recipes with exactly the same type of information, but with the specific variables that make up each individual.
Suppose that recipe number 25, inherited from the maternal "cassette" number 2, contains information for blond hair, and that the one we have inherited exactly from the same place, but from the father's side, is for black hair. Both chromosomes or cassettes are giving us exactly the same information (HAIR COLOR), but with the traits or variables that each individual presents (BLOND hair or BLACK hair).
Consequently, the instructions for hair are received twice. The new individual will on some occasions express the instructions given in both recipes, and on other occasions it will only express one set of instructions, despite having both sets, because one will dominate over the other. That is, it is easier for the dominant set of instructions to express or manifest itself than it is for the other. In our example, the individual will have black hair because black hair dominates over blond hair, but remember that the individual carries both recipes or sets of instructions: black hair and blond hair. This situation is repeated for all recipes over and over again.
Nonetheless, in life anything can go wrong and this is also true for nature.
Concerning our inheritance, these failures can be originated by;
Heredity is a set of chromosomes with their recipes that we have received or inherited from our parents and which we will pass on to our own children.
As we mentioned before, chromosomes are classified in 23 sets. The first 22 sets are called AUTOSOMES and are the same in the man and in the woman. The components of set 23 are called GONOSOMES, or sex-chromosomes, and they are different in the man and in the woman. As their name indicates, they determine the sex of the human being. A woman has two sex-chromosomes "XX" while a man has one "X" chromosome and one "Y" chromosome.
In terms of heredity, and following this classification, the recipes that determine the characteristics or traits for each person are divided into AUTOSOMAL characteristics, if they are contained in the first 22 sets of chromosomes,
and SEX LINKED characteristics, if they are part of set 23. Within this group, we will only work with those recipes or traits that are "X"-chromosome dependent, as this chromosome is shared equally by the male and the female's sex-chromosomes. When discussing these recipes, we will refer to them as traits or characters linked to the X-chromosome.
Conversely, traits that are "Y"-chromosome dependent are only transmitted from male to male, and will not be discussed here.
This whole array of characteristics or traits can manifest or express in a dominant, recessive, co-dominant, or intermediate fashion.
The terms "dominant and recessive" mean that we receive the information from a recipe twice, information from the father and information from the mother, as each of them contributes with one element from each set of their chromosomes. Then, and depending on how the information contained in those recipes manifests or is expressed, we can be faced with three different scenarios:
- A DOMINANT character or trait is when only one of the two recipes is expressed, be it the mother's or the father's, with one dominating over the other. The other recipe that is not being expressed at this moment but that belongs to the same chromosomal set (although it did not manifest in the presence of the other recipe) is called RECESSIVE character or trait because it is in RECESS, overlapping or hidden. In this case, for the character to manifest it is necessary for both progenitors to contribute with the same type of information. If only one of the progenitors provides the required information, the character does not manifest and then the individual is considered to be an asymptomatic carrier of such character or trait.
- When both recipes are expressed at the same time with the same intensity, they are called CO-DOMINANT CHARACTERS, as in the AB blood type.
- And, finally, when both recipes are expressed at the same time but the resulting character or trait is anintermediate expression of both, because neither dominates over the other; these characters are said to present an INCOMPLETE OR PARTIAL DOMINANCE, for instance if we cross-breed white flowers with red flowers, the descendants will have pink flowers. The pink flowers are the result of mixing up both pigments.