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

Daughter vs niece paternity test

-A curious adult from Australia

November 12, 2016

The testing company is right. For reasons we talk about here, a standard paternity test will have trouble telling two potential fathers apart if they are brothers.

Test Results

This is what the result might look like from 23andMe if she were his daughter: 

And here is what they would like if she is his niece: 

A pretty big difference. With the daughter, everything is shaded in light blue but with the niece, only some of it is.

Where All That Blue Comes From

If you know a bit about DNA, these results might be a bit confusing. We get half our DNA from our moms and half from our dads. So why does the daughter seemingly share 100% of her DNA with her dad?

She doesn’t. It kind of looks like that because of how the data is reported here.

Remember, we have two copies of each of our chromosomes. We get one from each pair from mom and one from dad.

Best half sibling DNA test

-A curious adult from Mexico

January 7, 2016

Many of the DNA tests you might come across on the internet for showing this kind of relationship wouldn’t be very useful. Most of them are too weak to see such a distant relationship.

In fact, these tests might not even see it with one of the parents included. They have a better chance but it is still an odds game. They can only say how likely it is that two people share a single parent.

Few vs. Many

Most people’s DNA is split into 23 pairs of chromosomes. Each chromosome is one long piece of DNA.

In total, there are over 6 billion individual parts of the DNA that can be looked at. The individual parts are the famous A’s, G’s, C’s and T’s you may have heard about.

A standard test looks at a spot here and a spot there. It sees how many of these markers (as these spots are called) two people have in common and figures out relatedness that way.

Your Results

Each company will report back on how much DNA the two of you share and give some possible relationships.

Half siblings share 25% of their DNA but so do an uncle and a nephew or a grandparent and grandchild. The companies will make a reasonable guess based on the data but they can get it wrong.

For example, when my half-sister and I did a 23andMe test, the result came back that we were grandfather and granddaughter. A bit mortifying but understandable given the 30 or so years between us.

Color blind less severe inheritance

-A curious adult from Minnesota

December 16, 2015

Given your parents, you were most likely either going to get your dad’s colorblindness or not be colorblind at all. And it is for the reasons you talk about.

How bad a case of colorblindness you get is often inherited. And in women, you end up with the less severe form of colorblindness that each parent happens to pass to you.

Your dad will pass his colorblindness to you each and every time. But your mom will either pass her severe form or none at all.

This matters here because colorblindness is associated with the X chromosome. And if you have two X’s, the best one for color vision wins out.

This is why men are colorblind more often than women. They have just one X and if it says to be colorblind, they will be colorblind.

The story is very different for women. Because of their two X’s, they are much less likely to be colorblind.

If they have an X that doesn’t lead to colorblindness, it doesn’t matter what the other X says. She won’t be colorblind.

Mild or No Colorblindness

OK so let’s break down your case.

Your mom is not colorblind but one of the more severe forms does run in her family. This means she either has two perfectly fine X’s (in terms of colorblindness) or one of her X’s could lead to colorblindness.

In the second case she would be called a carrier. She can pass on colorblindness but does not have it herself.

Your dad has a mild colorblindness which means his X has it too. And this is his only X.

Advantages of psoriasis

-A curious adult from Germany

December 9, 2015

We don’t know a lot about how psoriasis started or why it is still as common as it is. In fact, we don’t even have a good handle on what causes it!

What we do know is that it is a condition where people have patches of skin that are discolored or a different texture from the rest of their skin. We also know that it happens because their immune system attacks their skin by mistake. And importantly for the discussion here, we know that it is genetic.

One idea is that a lot of the genes that we find in people with psoriasis are helpful in fighting HIV. Genes that protect people from getting AIDS are obviously an advantage. But it’s important that we keep two things in mind.

Sickle cell anemia and malaria

Sickle cell anemia is a genetic disease that makes people very sick. This means that they may not be able to have children, and we would expect sickle cell disease to disappear.

But sickle cell anemia is much more common than we expect. Why is that?

It turns out that having mild sickle cell disease (called sickle cell trait) protects people from malaria. This disease has been infecting humans for almost 5,000 years and for most of that time has been incredibly deadly.

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