Two genomes per cell

-A curious adult from Nevada

December 13, 2017

Your DNA has the instructions for making you. One complete set of instructions is called a genome.

These instructions are written in “letters” called base pairs. A human genome has around 3 billion base pairs.

The reason you sometimes see the 6 billion number has to do with how your DNA is stored in your body.

Your body is made up of trillions of cells. Each of these cells has two copies of your genome.

Sperm and Eggs Have One Genome

Are you still wondering why sperm and eggs only have one genome copy? Well, maybe it’s helpful to talk about the types of cells in the human body.

In general, there are two categories of cells. Most of the cells in our body are called somatic cells. Somatic cells have two genome copies.

Egg and sperm cells make a different category called germ cells. Germ cells start off with two genome copies. Eventually they divide into two cells with one genome copy each. This process is called meiosis.

Not All Cells Have Genomes!

So we know that the cells in our body have two genomes and germ cells have one genome. Every genome is housed in an area of the cell called the nucleus. But there’s more to it.

Not all of the cells in our body has copies of the genome. Red blood cells, and some cells in our skin, hair, and nails don’t have any genomes In their nuclei. These cells start off with genomes in order to develop properly, but then destroy their nuclei later on.

transgender intersex sex chromosomes

-A graduate student from Wisconsin

December 5, 2017

What a great question! Yes, there are some genetic conditions that can make someone not clearly fit into the category of “male” or “female”.

There are people who have differences in their sex chromosomes, or the X’s and Y’s you are talking about. There are also people who are intersex, with bodies that don’t fit typical definitions of male and female.

Sex Chromosomes and Genetics

Chromosomes are the instruction manuals of our body, and help determine things like our hair color, eye color, and body parts. Some chromosomes are called sex chromosomes which help to determine whether someone has the reproductive body parts of a boy or a girl.

Usually, someone who has female body parts has two X chromosomes, and someone who has male body parts has an X and a Y chromosome. Sometimes, this is not the case.

Being Intersex Is…

Now, there are other genetic conditions that have to do with people’s gender that don’t involve extra or missing X’s and Y’s. There are genetic conditions that cause people to be intersex.

Instead of having whole extra or missing chromosomes like with the sex chromosome conditions, many people who are intersex have specific changes in one of their genes.

Attached ear lobes genetically complicated

-An elementary school student from California

December 1, 2017

I can see why you’re asking.

We are taught in school that parents with attached ear lobes can’t have a child with unattached or hanging ones. We are taught wrong.

Parents with attached ear lobes can have kids with unattached ones. And vice versa.

Not the Bit of DNA You are Looking For

A really important point to make is that this study did not find the bits of DNA that actually affect your ear lobes. Instead they found the area of your DNA where the important bit might be hiding.

It is like in those spy movies where the CIA can narrow down where a phone call is coming from to a single city block. They know the bad guy is somewhere in that block but they don’t know on which floor of which building.

Why green peas are recessive and common

-A high school teacher from Pakistan

November 16, 2017

Because that is what we have chosen. Basically we decided we liked our peas green and so we only breed pea plants that have green peas.

There is no “yellow” gene in the peas found on farms. And unless we introduce one or one arises through mutation, there won’t be any. Green peas will be all there is forever.

Keeping Green Peas Common

The instructions for making a pea plant are found in its genes. Each gene has the instructions for one small part of the pea plant. There is a gene that determines the color of the peas the plant will produce.

Genes can come in different versions. The pea “color” gene comes in a yellow version (Y) and a green version (y).

Just like us, pea plants have two copies of most of their genes. This means that there are three possible combinations for this color gene:

Changing Ratio of Yellow and Green

Let’s start out with our early population:

Remember, we have 6 plants making yellow peas and 2 making green.

One way we can change things is what happened on farms all over the world. We can keep the plants with green peas and kill off the ones that make yellow. Now we have all green peas:

Best avuncular genetic test aunt uncle nephew niece

-A curious adult from Florida

November 9, 2017

The internet is certainly clogged with lots of genetic tests claiming they can help you tell if two people are aunt/nephew. Unfortunately, most can’t really give a definitive result every time. Or even most of the time.

Luckily there are much, much better tests out there. These tests don’t go by the official “avuncular” name because they can tell lots of different relationships in a single test. They are usually called ancestry or relatedness tests.

Few vs. Many

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

A standard test looks at a spot here and a spot there amongst these 23. A good avuncular test will look at 30, 40, or even 50 different spots.

The test sees how many of these markers (as these spots are called) two people have in common and figures out relatedness that way.

This is perfectly fine for paternity cases. But for more distant relationships, even 50 isn’t enough.

Why Turner syndrome has symptoms

-A curious adult from Saudi Arabia

November 1, 2017

It is true that one X chromosome is inactivated in women. But the key is that it is not completely shut off. Some of that second X is still turned on.

This is why women with Turner syndrome, who have a single X chromosome, have symptoms. They are missing that small part of the inactivated X that escapes the inactivation.

15% of Genes Still Working

Your DNA has the instructions for making and running you. These instructions are stored in 23 long stretches of DNA called chromosomes. The X is one of these chromosomes.

Most chromosomes come in pairs. So you have two copies of chromosome 1, two copies of chromosome 2, and so on.

If you have two copies of the X then you are usually biologically female and if you have one X and one Y, you are usually biologically male.

When One Isn’t Enough

One way to think about yourself is a chocolate chip cookie. (Bear with me here.)

Each gene is like one small part of the recipe. Add two cups of flour. Add two eggs. And so on.

In the end, if you have right number of ingredients and add them at the right time, you end up with chocolate chip cookies. And if you have the right number of genes that get turned on and off in the right way, you end up as a person without symptoms.

heterozygous homozygous genetic test

-A high school student from Massachusetts

October 27, 2017

There is genetic testing for this sort of thing. A good company is 23andMe. Another is AncestryDNA when it is combined with an online program called Promethease.

Unfortunately, genetics isn’t as cut and dried as it is often taught in school.

This means there are three possible combinations for people:

  • RR
  • Rr
  • rr

It is pretty obvious what color hair you would predict for the homozygous combinations, the ones with two of the same type of MC1R gene. RR people should not have red hair while rr people should have red hair.

But what about the heterozygote? The people who are Rr?

Because red hair is recessive, Rr people should not have red hair. The R dominates the r keeping red hair at bay.

Here is a table of data from a few years ago from 23andMe:

 

As you can see, it isn’t like in the textbook! Genetics is marvelously messy.

So there are tests that can tell you about your genes and which versions (or alleles) you have but they may not be as informative as you’d like. You may be heterozygous for an important gene but that will not tell you the whole story.

You might predict brown eyes but have blue instead. Or green or a multitude of other colors.

Odds for a color blind child

-A curious adult from Maryland

October 18, 2017

Each of their sons has around a 50% chance for being color blind. Most likely none of their daughters would be (although there is a chance the sons of these daughters, the grandsons, could be).

This is because both daughters are carriers. They have the bit of DNA that can lead to color blindness but they are not color blind themselves.

The reason they aren’t color blind but their sons are at risk has to do with where that bit of DNA is and how it works.

To make things easier to follow, I have color coded the X’s. So mom has a blue one that can lead to color blindness and a black one that can’t. And dad has an orange one that can’t lead to color blindness.

Remember that when they have kids, each parent will only pass down one chromosome from their pair.

Let’s say that mom happened to pass down her blue X and dad passed down his Y. Here is the combination their son would have:

Every Genetic Combination

Here is a table with all the possible outcomes for the most common form of color blindness. It is a big table: 

(Click here for a larger version of the table.)

Scientists Repair a Gene in a Human Embryo. Or Did They?

(Max Pixel)

October 6, 2017

Scientists from Oregon recently reported that they had fixed a gene in human embryos. Click here to get the details on how they did it and why it is such a big deal.

Dangers of converting ancestry data to health data

-A curious adult from Australia

September 28, 2017

This does sound scary but if you use sunscreen and try to stay out of the sun you should be fine. This particular DNA difference (or SNP) is linked to lighter eye, hair, and skin color. So what this test basically told you was that you have a DNA change that leads to lighter skin color.

And having lighter skin color, of course, increases your risk for skin cancer which includes melanoma. So that is where your melanoma risk comes from.

Worldwide around 1.2% of people has these two A’s and, like the one you brought up, it is much more common in Europeans and much less common in Africans. Around 0.3% of Africans and 4.6% of Europeans has these two A’s that can lead to red hair.

Another thing to keep in mind is that these risks are not always a for sure thing. Plenty of people with the same DNA won’t get the disease and plenty of people without it will. Your DNA just makes it more or less likely.

This becomes clear in your case. Imagine someone who has a G, G at rs1393350. They are at a genetically lower risk for getting melanoma compared to your A, A at the same SNP.

But if you wear sunscreen and he doesn’t, you may be at a lower risk. What we do can mean more than what our DNA says.

Pages