Each year on Mothers’ Day, we take time to show appreciation to the mothers in our lives. If you’re anything like me, you may have a special meal, gift, or shopping excursion planned to celebrate your mom. I’m here to tell you that you owe your mother thanks—not only for the hours spent preparing meals, helping with homework, and character training that your mother has invested—but also for some things you may not have ever considered before.
Mitochondria and Mitochondrial DNA
Our cells contain miniature organs called organelles, each with a specific role to play to keep our cells—and us—alive. The mitochondrion (mitochondria, plural) is one of these organelles. Known as “the powerhouse of the cell”, mitochondria provide the fuel that keep us alive. Mitochondria are the site of cellular respiration: a process through which sugar molecules supplied by the food we eat are broken down into carbon dioxide and water. Energy stored within the chemical bonds of the sugar molecules is transformed into ATP, the energy currency of the cell. ATP is able to be stored, transported, and used by the cell for any process that requires energy. Because mitochondria supply the energy necessary for all cellular processes, there are many mitochondria in each cell. Cells that require more energy (such as heart or muscle cells) can have thousands of mitochondria per cell.
The vast majority of human DNA exists as chromosomes in the nucleus of the cell. It is this nuclear DNA that determines our eye color, hair color, whether or not we have dimples, and so on. These genetic traits are defined by the genes each of us possess that are located on our nuclear DNA. However, another form of DNA, mitochondrial DNA, resides in the mitochondria and replicates independently of nuclear DNA. While the latest data estimates that nuclear, chromosomal DNA carries approximately 20-25,000 genes, mitochondrial DNA only contains 37 genes, each of which direct the function and maintenance of the mitochondria.
What does this have to do with mothers? Quite a lot, actually. I’m sure at some point you’ve been told that when you were conceived, you received half of your DNA from your mom and half of your DNA from your dad. Actually, that only applies to the chromosomal DNA found in your nuclei. It turns out that during conception, only mitochondria supplied by the mother’s egg make their way into the newly formed single-celled zygote. Because the developing zygote contains mitochondria inherited only from the mother, all cells produced by the zygote as it develops into an embryo will contain exclusively maternal mitochondria. Since all of your mitochondria are derived from the ones supplied by your mother’s egg during conception, all of your mitochondrial DNA was supplied by your mother too.
So thank your mom for more than half of your total DNA and for ALL of your mitochondria.*
Let’s revisit the chromosomal DNA found in the nuclei. You probably know that, as a human, you have 46 chromosomes in each of your cells. These 46 chromosomes come in 23 pairs. 22 of these pairs are called autosomes, and contain the same genes in the same location no matter whether you are male or female. The 23rd pair of chromosomes is special: these chromosomes, known as sex chromosomes, determine whether you are genetically male or female. Females have two X chromosomes (XX) while males have one X chromosome and one Y chromosome (XY). At conception, you received one chromosome making up each of these 23 pairs from your mom and the other chromosome from the pair from your dad.
Consider the picture below, which compares the size of the X and Y chromosomes. As you can see, the Y chromosome is significantly smaller than the X chromosome. In fact, studies have shown that the X chromosome contains 1,098 genes while the Y chromosome carries only 78. Included within the 78 genes on the Y chromosome are genes important for male development, including the SRY gene—the so-called “sex-determining region”.
As females, mothers have only X chromosomes to pass on to their offspring from their sex chromosomes. As males, fathers can pass on either an X or a Y chromosome. Let’s remember back to high school biology and Punnett Squares. Here is a Punnett Square, focusing on the possible outcome that can occur each time a child is conceived. I have colored the chromosomes that the mother is able to pass on to offspring pink, while those the father is able to pass to offspring blue. Let’s consider the results of this Punnett Square. Any female this couple conceives will have received an X chromosome from the mom and another X chromosome from the dad. But what about the male offspring? In order to be genetically male, these offspring must have inherited a Y chromosome from the father and an X chromosome from their mother.
In other words, the mother’s contribution to her offspring’s 23rd chromosome is always X. It is the father whose genetic contribution to the 23rd chromosome pair which determines whether the child is a male or female. (Side note: this makes King Henry VIII an even bigger jerk for killing the wives who wouldn’t bear him a son). Additionally, it means that males only have a single X chromosome in their 23rd chromosome pair. Whereas for most genes, individuals have two copies of every gene (one copy inherited from mom and the other from dad), males only have one copy of each gene on the X chromosome. And who do these males have to thank for their lone copies of the genes on the X chromosome? That’s right. Their mother.**
Women Are Permanently Changed When They Become Mothers
If you asked your mother, she could probably tell you many ways that her body changed following pregnancy. She may have stretch marks, scars, varicose veins, or wider hips. Her shoe size may have even changed! However, she may not be aware of all of the changes that have taken place in her body, including:
- Your mom still carries some of your cells inside her body.
- Your mother’s brain changed during pregnancy in response to pregnancy hormones. These changes are thought to lead to increased maternal bonding, empathy, and protectiveness.
- Her body may truly never be the same again.
So the next time your mom starts to get on your nerves, cut her some slack. She went through a lot to bring you into this world, and you literally wouldn’t be the same without her.
*A new study which appeared in the November, 2018 issue of the journal Proceedings of the National Academy of Sciences of the United States of America found that in a small cluster of individuals, mitochondrial DNA may be both maternally and paternally inherited ( Biparental Inheritance of Mitochondrial DNA in Humans ). While this seems to be a rare phenomenon, more research is needed to understand how and why paternal mtDNA is inherited in these individuals. To learn more, click here: Not Your Mom’s Genes: Mitochondrial DNA Can Come from Dad
**Most of the time, the fact that males only have one copy of genes on the X chromosome isn’t an issue. However, there are some diseases and disorders which are passed along on genes located on the X chromosome. Oftentimes, females are not affected by these disorders since they have two copies of the X chromosome. If one of a female’s X chromosomes has the faulty gene, it is often masked by expression of the functional gene on her other X chromosome. However, if a male has one of the faulty genes on his single X chromosome, he will be adversely affected. Such disorders are said to be sex linked, since they are carried on the sex chromosomes and clear patterns of disease can be seen between the two sexes of the species. Examples of sex-linked traits include red/green colorblindness, male-pattern baldness, Duchenne Muscular Dystrophy, and hemophilia. To learn more about sex-linked traits and other forms of so-called non-Mendelian patterns of inheritance in my online course Genetics and Heredity.