Genetics are a fascinating thing. Most of us have forty-six chromosomes in every one of the some fifty-trillion cells that are in our body at any given moment. Each of those chromosomes is paired up with another into twenty-three pairs. Twenty-two of these pairs are called autosomes and the twenty-third pair is called the sex chromosomes. All chromosomes are made of two strands of DNA. There’s a total of 20,000 to 25,000 genes located in segments on those two strands of DNA, and roughly 10,000 to 12,500 of those genes are given to us from each parent.
Sperm and egg cells, known as gametes, are the only cells in a human’s body that do not contain the DNA double helix, but instead house only one strand of DNA each. This is due to the fact that reproduction for humans and many other living things, requires the conjoining of genetic material from both parents. DNA, or when not abbreviated Deoxyribo-Nucleic Acid, is what contains the information required to instruct your stem cells on what type of cell they should become and how to do their job once they are formed. This process of determination is done through the production of proteins.
These instructions are outlined in the sequences of the DNA alphabet, molecules that pair up along the sugar and phosphate “backbone” of the two DNA strands. These molecules, known as nucleotides, are Adenine, Thymine, Cytosine, and Guanine. Through hydrogen bonds Adenine always pairs up with Thymine and Cytosine always pairs up with Guanine. The specific sequences in which these molecules pair up within a gene are called an allele and will determine everything about you, from the color of your eyes and hair, to your height, and whether or not you like chocolate. Since we inherit one allele of a certain type of gene from each parent, these pairs of alleles can be the same or different, and if they are different then one can be dominant over the other. Even identical twins cannot have identical DNA, that is how unique the sequences of paired nucleotides are in each human’s genome.
A mutation in the gene’s nucleotide sequence can also affect whether it gets expressed or not, and even the way in which it is expressed. Let’s take a look at homozygous and heterozygous pairs of alleles and how inheritance works when one is dominant and one recessive.
In the above image, known as a Punnett square, it shows the possible children for a parent with brown eyes who did not inherit the blue eyes allele and the other with brown eyes who did inherit the blue eyes allele. Since blue eyes is a recessive trait, represented by the lower case letter, it requires both parents to carry this allele in order to have a child where the blue eyes allele is expressed. The only way a child in this family will have blue eyes is if one of the two children with the recessive blue eyes allele marries someone who either has blue eyes or at least has inherited the blue eyes allele.
Now in the above image both parents inherited the recessive blue eyes allele, and so now they have a 25% chance of having a child with blue eyes, even though neither of the parents have blue eyes. Let’s take a look at one more example.
Due to one of the parents having inherited the recessive blue eyes allele from his/her parents and it is therefore expressed in them (they have blue eyes), these two individuals are able to up the odds that they will have a child with blue eyes to a 50% chance.
While determining eye or hair color can be interesting, what is the most interesting thing about genetics is the twenty-third pair of chromosomes. A man has an X and a Y chromosome for his 23rd pair and a woman has an X and another X. These pairs determine their gender and when they have a child, each will pass on one of their two chromosomes to their child. Though the X chromosomes can change – what’s called “recombination” – when they are passed on to subsequent generations, the Y chromosome does not (aside from slight random mutations).
Sometimes the X a male inherits from his mother is an exact copy of one of her X’s she inherited from her father or mother, but most of the time the X chromosome is a mixture of the two. The Y chromosome is the only one that is always passed on from male generation to male generation unchanged, it is the connection a male has to every male ancestor on his father’s side that has ever lived all the way back to the beginning of that bloodline.
For daughters, their genetic inheritance is often times more mixed as she by default inherits two X chromosomes that are typically a recombination of X chromosomes inherited from her parents and grandparents. This process can be confusing so let’s look at some charts to better understand how the X chromosome works.
For a basic first look we will remove the Y chromosome from the equation all together, this first graphic shows how a daughter inherits one X from her father and a combination of her mother’s two X’s. A disclaimer here, in reality the X chromosomes of the father and mother should already be a mix colors, signifying that they inherited normal combination X chromosomes from their own parents, but for the sake of simplicity the oldest generations in these graphics will reflect solid colors. In the next graphic we will bring the grandparents into the mix.
Now we have the paternal and maternal grandparents passing on their chromosomes to the granddaughter. You will notice that since the paternal grandfather does not pass on his X to the father, that grandfather’s mother’s X chromosomes do not factor into this at all. The paternal grandmother plays a role here because she passes on a combination of her X chromosomes to the father and that combination gets passed on to the daughter as it is the only chromosome she can inherit from her father. The maternal grandmother passes on a combination of her X chromosomes to the mother, and the paternal grandfather passes on his X chromosome that he inherited from his own mother, and again his would normally be a mix of colors already but for the sake of simplicity it is left as a solid. Next we will look at a rare event involving the X chromosome.
On occasion it’s possible to inherit an entire X from one female ancestor – this is called a non-recombination X. It’s also possible that this X can remain a non-recombination chromosome for more than one generation, in which it then becomes a dominant non-recombination – essentially acting like a Y chromosome. Notice how the daughter in the above graphic has inherited a solid colored chromosome from her mother – whom inherited it from the grandfather, indicating that this X chromosome did not undergo the normal recombination process of mixing the two X chromosomes. This is called a dominant non-recombination because the daughter did not inherit any genes from her maternal grandmother’s X chromosome.
While this daughter would likely have inherited other genetic traits from her maternal grandmother on her other twenty-two pairs of chromosomes, on her twenty-third pair she has no genetic connection to her maternal grandmother. Now let’s look at the inheritance of chromosomes for a son involving the X chromosome.
Just as with the daughter example, normally a son will inherit a recombination X chromosome from his mother, consisting of the two X chromosomes she inherited from her parents.
In this graphic we can see that a non-recombination occurred for the mother, but it was not a dominant non-recombination because the son did not inherit that particular X chromosome from his mother. As I mentioned before, non-recombinations are rare and dominant non-recombinations are even more rare. However, just as with the daugher graphic shown earlier, this son has no genetic connection with his maternal grandfather on his twenty-third chromosome.
You will notice in this final graphic that the son has no genetic connection on his twenty-third chromosome to his maternal grandmother because his maternal grandfather had a dominant non-recombination X chromosome that canceled her’s out. Just as the others above, the maternal grandmother’s genes on her X chromosome have now ended and that genetic information will now never be passed on to subsequent generations.
Through this process of recombination and non-recombination it is possible to have children who have no genetic connection on their twenty-third chromosome, or have children where one of them has more genetically in common with one sibling than with another. This varying degree of genetic connection can be seen in phenotypes – the genetic and environmental traits expressed in physical features, where two siblings may look a lot a like, but a third sibling may look very different from the others.
The fact that the Y chromosome never changes is the reason that only male DNA can be used to trace male ancestry. If a woman wants to do a DNA test to learn about her father’s ancestry she has to use the DNA of her father, uncle, brother, or nephew. While it is possible that a girl every generation could inherit the same dominant non-recombined X chromosome through every subsequent generation it would be extremely unlikely to happen.
Aside from genetic mutations caused during the copying of the nucleotide sequence, the environment also can change our genes over time, in quite profound ways actually. The genes you have now are not identical to the ones you were born with. A lifetime offers many opportunities for your genetic material to be changed by various things.
From the climate you live in, to the foods you eat, to the people you socialize with, your level of physical activity, your hobbies, interests including musical instruments, all of these things have the power to affect the generations that descend from you. Everything around you has the power to change who/what you are. Some foods can actually damage your DNA if you continue to eat them over time, much in the same way that age causes your DNA to break down over time. Some viruses can also change our DNA, the more often you contract certain types of viruses the more genes in your DNA can be changed because these types of viruses replace our human genes with their own viral genes, this sharing of DNA is part of how they survive. As much as 8% of the average human’s DNA is actually not human, but viral. Extraordinary and also terrifying.