Which X chromosome from mom

- A curious adult from Oklahoma

March 23, 2016

Believe it or not, a woman actually passes some of each of her X’s to her kids. And each child gets different parts of each X.

So whether you are a boy or a girl, you got some of your maternal grandma’s X and some of your maternal grandpa’s X. You have a unique combination of the two of them.

DNA is made up of units of information called genes. We have genes for hair color, eye color, and so on. If DNA is the recipe book for life, you can think of chromosomes as the pages, and genes as the individual instructions.

Although the two chromosomes in a pair both code for the same genes, they are also slightly different. After all, not everybody makes, say, brownies the same way – someone may use nuts, while another person might not.

Also, the daughter’s and son’s X’s don’t have to be mirror images of each other. I just drew them this way for simplicity. There are many different combinations of the mom’s X chromosomes they can receive!

That’s because there are almost an infinite number of ways that mom’s two X’s can swap DNA. And this is true for the other 22 chromosome pairs in both mom and dad.

A Weakened Gene Pool

In the Last Mimzy, a scientist uses time travel to get good, undamaged DNA from the past. In this film the DNA damage is caused by the environment. Something similar might actually happen in the future but ironically, it may be caused by, among other things, good healthcare.  

Risk for parents who are first cousins once removed

-A graduate student from India

March 15, 2016

The more closely related two people are, the more likely their kids will end up with some sort of genetic problem. Since the two of you are first cousins, once removed, that means your chances of having a child with a genetic problem are higher.

But as discussed here, this higher chance for cousins isn’t that bad. Yes, there is a higher chance but it is still relatively small. And for you, the chance is even a bit less since you are actually first cousins, once removed.

Different Diseases, Different Gene Combinations

So, genetic diseases often happen when someone has a version of a gene that doesn’t work. For example, people with cystic fibrosis have a CFTR gene that can’t do its job.

Remember how we have two copies of each gene?

Sometimes it takes just one nonworking gene to cause a problem. These are called dominant traits or conditions.

Autosomal Recessive Inheritance

As you may remember, recessive means both copies of a gene have to not work to have a disease. If this was the case here, then your cousin would have two nonworking copies.

But since he just contributes one of his copies, his child would also need to get a copy from the mom. If mom doesn’t have the condition, then the only way this could happen would be if she was a carrier. Carriers have one nonworking copy, but since they also have one working copy, they don’t have the disease.

Rh positive parents with multiple Rh negative children

-A curious adult from Pennsylvania

March 8, 2016

Not necessarily. You might be Rh negative (Rh-) but you could also be Rh positive (Rh+).

This is because Rh- is something called a recessive trait. This means that someone who is Rh+ might have a hidden Rh- in their DNA. If that person and his or her partner both pass an Rh- down to a child, that child will be Rh-. Even though at least one parent is Rh+!

The RHD Gene

Whether or not you are Rh- or Rh+ is decided by the RHD gene. It comes in two versions or alleles. We’ll call one version Rh+ and the second Rh-. (Click here to learn why this is a bit of a simplification.) 

Like most every other gene, we have two copies of the RHD gene. One copy came from your mom and the other from your dad.

If both copies are Rh+, it is easy to see why you would be Rh+. That is all you have.

Cats and quick reflexes

But why are there so few copies of the Rh- allele? And why does it vary so widely between populations?

There is no clear answer, but there is a very interesting hypothesis involving cats and quick reflexes.

There is a parasite called Toxoplasma gondii that normally infects cats. When this parasite infects humans, it can cause people to react more slowly. Studies have shown that Rh+ heterozygotes (people who carry the Rh+ and Rh- allele) may be resistant to this symptom.

Factors affecting gender ratio number of sons or daughters

-A curious adult from Washington

March 3, 2016

This is a great question. The easy answer is that no, you are not really more likely to have a boy.

Well, you are a bit more likely to have a boy, but so is everyone else. There are about 1.03 boys born for every girl (we will call this the gender ratio).

But the fact that your boyfriend had four sons in a row doesn’t change these odds. From a statistics perspective, what came before doesn’t affect what comes next. 

Y Marks the Spot

To start with let’s talk about how someone is born either a boy or a girl. The answer is in our DNA. Which makes sense since DNA has the instructions for building your entire body.

For example, DNA tells you what color eyes and hair you will have, how easily you can digest milk and so on.  It also tells you what gender you will be.

Things that Affect Gender Ratio

What if we really stressed the coin before we flipped it? What if we stressed it out so much that it bent? Then we might also affect whether it is coming up heads or tails.

Children of identical twin parents

Having children who look the same, like these identical triplets, would be pretty cool! But this doesn’t happen very often. (Wikimedia Commons)

-A middle school student from North Carolina

February 17, 2016

Seeing Double: Chromosomes and Twins

DNA is the information that makes you who you are. It is found in the cells of our bodies, the building blocks for our different body parts. DNA in skin cells makes sure our skin works properly, DNA in hair cells gives us our hair color, and so on.

DNA is organized in long pieces called chromosomes. Most cells have 23 pairs of chromosomes for a total of 46. But germ cells like sperm and eggs have 23 unpaired chromosomes.

All DNA Are Not Created Equal

But you certainly don’t have to be an identical twin in order to feel unique. We all inherit a unique combination of features from our parents. And this has to do with how DNA is passed down.

As I mentioned earlier, sperm and egg cells have 23 chromosomes, instead of the usual 46. This happens through a process called meiosis.

In meiosis, each pair of chromosomes gets copied to make four chromosomes total. The four chromosomes are then divvied into four new cells like this:   

The Y Chromosome Passes Virtually Unchanged from Father to Son

-A curious adult from California

February 5, 2016

The Y chromosome can be a useful way to trace your family roots if you don't have a complete family tree like this one. (Flickr)

A Journey to the Past

DNA is the information that makes us who we are. We get our DNA from our parents, and they get theirs from their parents, and so on and so forth in a process called inheritance. This is why families tend to look alike.

Could there be DNA swimming around in your body from relatives who lived thousands of years ago? Actually, yes! In fact, unless you have only African ancestors, you probably still have DNA from Neanderthals, who died out 30,000 years ago!

Studying our Forefathers

Now you may be wondering what makes a Y chromosome Italian. After all, it doesn’t speak in an Italian accent or eat pasta…

More seriously, shouldn’t the Y chromosome have stayed the same since the beginning of time, since it doesn’t recombine? 

Actually, the DNA on the Y chromosome does change a bit over the generations. These changes aren’t because of recombination though. Instead, they come from tiny changes or mutations in the DNA that happen because of damage or mistakes the cell makes when the Y gets copied.

Overpruned Brains and Schizophrenia

A self portrait by someone with schizophrenia. (Wikimedia Commons)

Minimum number needed to survive near mass extinction

If numbers get as low as on NBC's Last Man on Earth, we may need some planning/genetic engineering to save the human race. (Flickr)

-A graduate student from Austria

January 27, 2016

Colonizing Space

For colonization, you might first look at all the potential colonists’ DNA. You’d then try to pick only those who are the most different.

That way, whatever pairing happens out in space will work out from a genetic point of view. Random, spontaneous love could continue to flourish. 

It might even be easier than you think for the space colonizers running the ship. You’d probably only need for them to be healthy for 10 or 20 generations (or however long it took to get to where they were going).

Real Life Near Death Experiences

Human populations have started out with very small numbers before and survived with no planning. For example, it is thought that all humans today came from a few thousand who survived something truly terrible 50-100 thousand years ago.

We did OK because there were still thousands of us. It is different when many fewer survive.

Simple Mutation, Complex Life

A single, simple mutation may have been key for animals to make the leap from being single-celled to having multiple cells. (DeviantArt)

January 15, 2016

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