Tag Archives: Chromosome

Sex vs Gender


America was settled by puritans. It may or may not be a coincidence that, hundreds of years later, we are still pretty weird about sex. Most people don’t even like saying the word “sex.” Enter “gender.” With the word “gender,” people can communicate their thoughts without the ickiness of having to say “sex” out loud. Or can they?

As a teacher, I often had to correct students who tried to tell me the gender of the cockroaches they were using in their experiments. Even biology professors, who really should have known better, would sometimes use the word “gender” when they meant “sex.” A friend of mine is having a baby soon. She and her husband recently made a big announcement about the gender of the baby. Official documents are notorious for items like, “Please indicate your gender: __male __female.” But what is the problem here? In short, sex and gender are not the same thing.

Sex is the biological component. Male mammals* have an X chromosome and a Y chromosome, have testicles, a penis, and produce sperm. Female mammals have two X chromosomes, have ovaries, a uterus, a vagina, and produce eggs. Sex is basically a categorical variable — you are either male or female. About 1% of people are “intersex.” For these people, there may be a disconnect between their arrangement of chromosomes and their anatomy — sometimes people will develop outwardly as male, but have two X chromosomes and no Y. The opposite can happen, too, with people developing outwardly as female, but having XY chromosomes. (People with XXY or YYX chromosomes are not considered intersex.) Sometimes people are born with ambiguous genitalia, or a mismatch between their external and internal genitalia. This throws a bit of a wrench into our concept of biological sex, but around 99% of people can be categorized comfortably as male or female. (For more information, go here.)

Gender is a psychological component. It deals with how you feel and how you present yourself. Unlike sex, which is mostly binary, gender is a smooth, continuous variable. Most human males are clustered towards the masculine end of the gender continuum, and most human females are clustered towards the feminine end of the gender continuum. But you can have males who are more or less masculine than other males, and females who are more or less feminine than other females. You can have females who are more masculine than some or most males, and males who are more or less feminine than most females. Most males identify as men, and most females identify as women, but this is not always the case. Some males identify as women, and some females identify as men — these people usually identify as “trans*” or “transgender.” (For more information, go here.)

So do cockroaches have gender? Not if the answer you are looking for is “male” or “female.” Male and female are choices for sex. Do cockroaches have a gender identity? Not as far as we know. Do cockroaches have any sense of self at all? Again, not as far as we know. There might be some scientific questions about cockroach gender, but most people don’t think about these things.

Likewise, official documents shouldn’t ask for your gender when what they really want to know is your sex otherwise the question would look like this: “Please indicate your gender on the following scale: masculine_ _ _ _ _ _ _ _  _ _ _ _ _ _feminine.”

My friend’s baby? Has a sex, but the gender will be up in the air until he or she develops a self-identity.

Both sex and gender are real things, but they are different. Make sure you’re using word you mean.

*In other types of animals, sex is determined in different ways.

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Mutations in Movies

With the remake of Teenage Mutant Ninja Turtles coming up, I thought I’d talk about the way mutations are depicted in movies vs how they are in real life. Again, this is not to diminish anyone’s appreciation for these movies, but to understand where the science stops and the fiction begins.
Many movies, especially superhero movies, describe some sort of mutagenic event as the cause for the hero’s superhuman abilities. In the original Teenage Mutant Ninja Turtles, the four turtles and one rat were given human size and intelligence through exposure to “radioactive ooze.” It also seems to have removed two of their fingers — turtles, like humans, have five fingers rather than the three they are depicted with in the movies.

Real turtles have five fingers and five toes. Teenage mutant ninja turtles only have three. Image from collecltions.countway.harvard.edu

In the X-Men franchise, “mutants” are caused by inheriting mutant alleles. In The Incredible Hulk, one of Dr. Bruce Banner’s experiments goes wrong and he is accidentally exposed to gamma radiation. In The Fantastic Four, the astronauts are exposed to cosmic rays. In Daredevil, Matt Murdock is exposed to radioactive waste as a child.

The Incredible Hulk gained his abilities through unspecified mutations. Image from screenrant.com

In Spiderman, Peter Parker is bitten by an irradiated spider, not exposed to radiation himself. Presumably this resulted in the transfer of some of the spider’s genes into Peter Parker’s genome, making this an event of “horizontal gene transfer,” rather than a typical mutation. I plan to discuss this at some point in the future.

Now, everyone knows that exposure to radiation or toxic waste will not give anyone super powers. But what do mutations look like in real life? In short, a mutation is any time there is a change to the sequence of the DNA of an individual or cell.

An organism’s genome can be compared to a book. The book is broken down into chapters, called chromosomes, and made up of letters called “nucleotides” (A, G, T, and C). Genes can be compared to sentences, and are made up of three-letter words called “codons.” Codons are always three letters.

In English, the three-letter word “cat” refers to a group of predatory mammals with long claws. In a sentence, “I keep two cats as pets.” In the language of DNA, the three-letter word “CAT” means the amino acid “valine.” A sentence of three letter words makes up the sequence of amino acids that build a protein. The function of a protein is created by the exact sequence of amino acids that make it up, just as the meaning of a sentence in English is created by the exact sequence of words that make it up.
The DNA sentence “TACCATAAACGGGTGACT” means “Methionine, valine, phenylalanine, histidine, alanine, [stop].” The codon “ACT” is one of three “stop codons,” which act like periods in English. It means the sequence of amino acids has come to an end. Proteins are usually thousands of amino acids long.
In most written languages, minor changes in the words or punctuation can drastically change the meaning of the sentence. A change of one letter can change “I keep two cats as pets” to “I keep two bats as pets.”
Similarly, changing one letter of a genetic code can change the meaning of a sentence. Changing “TTC” to “TTA” will change phenylalanine into leucine. These two amino acids have different properties, and will result in slightly different proteins being made. This protein might work basically the same, despite the minor difference. It also might work very poorly. There is also a small chance that the different protein will work better. The first two options are overwhelmingly the most likely of the three possibilities.
A change in punctuation can change “let’s eat, grandma” to “let’s eat grandma.” The same is true in genetics. If a stop codon gets put into a gene too early, part of the protein will not be built. This may result in a complete failure of the protein to do its job. It is very easy for a mutation to cause a stop codon very early in the sequence. CAG codes for glutamine; change the C to a T and it becomes the stop codon TAG. AGA codes for arginine; change the first A to a T and it becomes the stop codon TGA.
Having one protein that is missing or incorrectly built may not seem like a big deal — after all, the human body contains tens of thousands of different proteins. But the human body is a marvelously complex machine that requires the coordination of many different pieces. Taking one piece out of the human body is like taking one piece out of a Swiss watch. There are many genetic conditions that are caused by single mutations, such as Marfan syndrome, achondroplasia and paroxysmal nocturnal hemoglobinuria.
But mutations are not always bad. Blue eyes, for example, are a relatively new trait that first originated in eastern Europe only a few thousand years ago. There is no major advantage or disadvantage to survival in people with blue eyes. Beneficial mutations are what natural selection operates on to produces complex traits over millions of years. 

Mutations can be caused by exposure to radiation, certain toxic substances or just random chance. This is something that the movies get right.

Mutations that are inherited will be present in every cell of a person’s body. Everyone began as a single cell. If that first cell contained a mutation, then every cell that comes from that cell will also contain that mutation. Mutations that are acquired later in life will only be present in a few cells. If a person is exposed to radiation, the radiation will cause random mutations in whatever cells it interacts with. It may only cause mutations in a couple of cells, or it may cause mutation in many cells. Each mutation is random, so the likelihood of two different cells having the same mutation is vanishingly small. It is even vanishingly smaller for the same mutation to occur in more than two cells. For every additional cell and for every additional mutation, the likelihood of the same thing set of mutations happening gets astronomically smaller. For a person to acquire superhuman abilities through exposure to radiation, two things would need to happen. First, a complex mutation would have to occur. A single mutation is very unlikely to produce the type of complex, new trait that superheroes have. Second, the same suite of mutations would either have to occur in the entire body, or at the very least in the organ system that the trait operates on.

Taken together, this things only become less likely. A collection of random mutations that builds a complex, beneficial trait AND occurs simultaneously in billions of cells? Consider the following scenario for comparison: you are transcribing notes you took by hand into a computer file. Through a series of accidental typographical errors, your economics notes turn into The Rime of the Ancient Mariner. This is analogous to a series of random mutations producing a complex, beneficial trait in a single cell. Now imagine that everyone in your economics class accidentally and independently wrote The Rime of the Ancient Mariner when trying to copy their notes. This is analogous to Bruce Banner’s exposure to gamma radiation turning him into the Incredible Hulk. Now imagine that not only did everyone in your economics class accidentally write The Rime of the Ancient Mariner, but everyone in your history class accidentally writing The Song of Hiawatha, everyone in your math class accidentally writing Where the Sidewalk Ends, and everyone in your biology class accidentally writing Beowolf. This is approximately the likelihood of Reed Richards, Susan Storm, Johnny Storm, and Ben Grimm (The Fantastic Four) all being transformed into superheroes through exposure to cosmic rays.

The Fantastic Four all got their powers through exposure to cosmic rays. Image from static.comicvine.com

The original Teenage Mutant Ninja Turtles, which came out in 1990, was one of my favorite movies growing up, and I am cautiously optimistic about the remake, which comes out on August 8. I encourage everyone to enjoy some superhero movies this summer, but don’t be fooled by bad science.



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Skipping Generations, Part 2

A few weeks ago, I wrote about traits skipping generations. Be sure to read that entry to get caught up. Genes that are located on the X-chromosome sometimes, but not always, lead to traits skipping generations. But this is not the only way traits can skip generations. Skipping generations can also occur with genes that are not located on the sex chromosomes. The rules of inheritance are such that, given the right combination of alleles in parents, traits can sometimes skip a generation.

When sexual reproduction occurs, the offspring gets a random combination of alleles from their parents. Each parent has two copies of each gene, and one of those copies from each parent is given to each offspring. There are four possible alleles to choose from — two from each parent — and each offspring gets one at random from each parent. This is exactly the same way it works for the sex chromosomes.

Here is one way a recessive trait can skip a generation:

In genetics, it is the convention for dominant alleles to be expressed with a capital letter, and recessive alleles with a lower-case letter.

The parents in our first generation are Sam and Jess. Sam has two copies of the dominant allele (“AA”) — which causes him to express the dominant trait — and Jess has two copies of the recessive allele (“aa”) — which causes her to express the recessive trait. It doesn’t matter what the trait is. It could be attached vs free earlobes, widow’s peak, knuckle hair, or any other trait that follows the basic rules of inheritance. (There are traits that follow slightly different rules, but I’m not going to talk about them right now.)

sam and jess

When they have children, every child will get an “A” from Sam and an “a” from Jess, because Sam has only “A’s” to give, and Jess has only “a’s” to give. All of the children will express the dominant trait, although they all carry (but do not express) the recessive allele. Unlike with x-linked traits, the expression of these traits is not related to sex.

Ann grows up and marries Luke, who is also “Aa.”

ann and luke

There are four possibilities for the children:

(1) They can inherit Ann’s “A “and Luke’s “A” (“AA”).

(2) They can inherit Ann’s “A” and Luke’s “a” (“Aa”).

(3) They can Inherit Ann’s “a” and Luke’s “A” (“aA”).

(4) They can inherit Ann’s “a” and Luke’s “a” (“aa”).

Options (2) and (3) result in the same combination of alleles — “Aa” — even though the source of the alleles are from different parents. That is, it doesn’t matter whether the “A” comes from the father or the mother. It will always be dominant over the “a.”

In the three generations of this family, the recessive trait is only expressed in one individual in generation 1 (Jess), and in one person in generation 3 (Tim). Nobody in generation 2 expressed the recessive trait, even though people in generation 2 carried the recessive allele.

As I said before, these types of traits only sometimes skip generations, but do not always skip generations. By changing the genetics of the example above only slightly, the scenario will play out very differently. If Sam is “Aa” instead of “AA,” half of his children with Jess will be “aa” and the other half will be “Aa.” The recessive trait will be expressed in the second generation and there will be no skipping of generations.

Sometimes generation skipping can happen with dominant traits, too, although by a slightly different mechanism. Recessive alleles can be carried by individuals who do not express the trait, making it easy for the recessive traits to be silent. The only way for a dominant trait to skip generations is if the dominant allele is not passed on in a family, and it is then added in a subsequent generation.

For example, Jim (Aa) and Jill (aa) have children. There is a 50% chance that any child they have will inherit an “a” from both parents. If they have two children, there is a 25% chance that both of them will be “aa.”

jim and jill

Their daughter, Liz, grows up and marries Bill, who is also “aa.” Since neither of them has any “A” alleles, all of their children will be “aa.”

liz and bill

Their son, Rob, grows up and marries Kim, who is “AA.” All of their children will be “Aa.”

rob and kim

Between the first and second generations, the “A” allele was removed from the family lineage. It was not until Kim that the allele was introduced back into the gene pool. The dominant trait skipped generation 2 entirely. Again, this is not because of something to do with the trait itself — the various allele combinations of everyone interacted in certain ways to make the trait skip a generation.

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