What Are Sex Chromosomes?
Sex chromosomes are chromosomes, or packages of DNA, that are found inside the nucleus of all of your cells. There are two different kinds: X and Y, which determine whether you’re biologically male (XY) or female (XX). Chromosomes themselves are made up of proteins and DNA that carry your genetic information from cell to cell.
When an egg is fertilized, the egg itself always contributes an X chromosome, while the sperm can contribute either an X or a Y. Sex chromosomes are the genetic elements that define a person’s biological sex. But there are other genes and things in the environment that may play a role as to whether a person identifies as male or female, or even neither.
How many chromosomes do humans have?
Humans have 23 pairs of chromosomes, or 46 total, and 22 of these pairs are numbered chromosomes, known as autosomes. The 23rd pair is a pair of sex chromosomes (XX or XY).
All of these chromosomes vary in both size and shape, but are numbered roughly based on their size. The largest one, chromosome 1, has about 2,800 genes. It’s about three times the size of chromosome 22, which has around 750 genes.
Do you get your chromosomes from your parents?
You get half of your chromosomes from each parent. Each pair of numbered chromosomes contains one from your mother and one from your father. It helps to explain why you get (inherit) some physical and personality traits from Mom and some from Dad.
This inheritance pattern differs when it comes to the mitochondria, which are the structures in your cells that provide it with energy. When a person is conceived, only egg cells, and not sperm cells, keep their mitochondria. As a result, you only inherit mitochondrial DNA from your mom. Certain mitochondrial conditions that can cause symptoms like hearing loss or diabetes can be passed down from a mother to a child this way.
What Do Chromosomes Do?
Chromosomes keep your DNA wrapped around proteins known as histones. If you didn’t have them, your DNA molecules would be too big to fit inside your cells. (And if you unwrapped a DNA molecule from a single cell, it would stretch out to 6 feet.)
Chromosomes also help your body do its daily activities. As part of everyday life, your cells need to constantly divide and make new cells to replace your old ones. Chromosomes work to ensure that your DNA is copied in the right way, without any mistakes. These are a couple unique parts in your chromosomes that help them do that:
Centromere. Your chromosomes copy themselves to make new cells. The centromere is the attachment site where the two halves, known as “sister” or “daughter” chromatids, form “arms” so that they can eventually divide with identical DNA.
Telomeres. These are strands of DNA that cap the tips of your chromosomes and protect them when your DNA divides. They’re like the tips that cap your shoelaces and prevent them from unraveling. Telomeres gradually lose their DNA over time. When that happens, the cell dies. Blood cells often have a special substance to help prevent this, which is why they usually live longer.
Most of the time, this process happens smoothly. But sometimes, problems can occur. Some cancers, for example, are caused by broken or defective chromosomes. These cells also usually don’t lose their telomeres, which means that they can grow uncontrollably and cause cancer to spread.
Sex Determination System
In humans and other mammals, biological sex is determined by sex chromosomes. XX is for females, and XY is for males. But while both are important, there are also other genes on different chromosomes that may play a role. While the Y chromosome is usually related to males, and the X chromosome to females, they aren’t the only things that define your sex. There are other genes that may help explain why someone identifies as male, someone else as female, and yet another person as intersex.
Gender vs. sex
The words sex and gender are often used similarly. But they don’t mean the same thing. The term sex classifies people as male or female, according to their reproductive organs (vagina, uterus, and ovaries, versus penis and testes) as well as their chromosomal makeup (generally XX for female and XY for male).
The term gender, on the other hand, refers to how the person identifies themselves, rather than how their sex chromosomes identify them. Someone may have male reproductive organs, and be XY, but identify as female. Other ways to identify yourself include:
Nonbinary. Some people identify as neither male nor female.
Transgender. People who identify as a gender that’s different from the one they were assigned at birth. These terms are different from a person’s sexual orientation. You can be born male but identify as a woman and be attracted to women, for example.
XX Chromosomes
The X chromosome is one of two sex chromosomes in humans. Females have two X chromosomes. One of these X chromosomes is inactivated. This prevents exposure to a double dose of proteins from X-linked genes.
When the X chromosome is inactivated, it gets folded into a small, dense structure in the nucleus of the cell. This is called a Barr body.
Since the X chromosome is larger than the Y chromosome, it carries more genes. Therefore, more health conditions are likely to be X-linked, including hemophilia and color blindness. These diseases are controlled by genes that are only found on the X chromosome. Since males have just a single copy of the X chromosome that carries the mutation, they are more likely to be affected by them. Females usually have a second, non-mutated copy, which leads to milder symptoms or none at all.
It’s also thought the X chromosome has a link to aging. A study found that about 12% of women develop abnormal white blood cells as they age that only contain one X chromosome, which raises their risk of developing blood cancer.
XY Chromosomes
Males have two types of sperm: Half of the sperm have the X chromosome, and half have the Y chromosome. If the egg receives an X chromosome from the sperm, the fertilized egg is XX, or female. But if the egg gets a Y chromosome, the baby is XY, or male.
The Y chromosome itself has a gene, called the SRY gene, that creates testes. These are the two small male organs that’ll eventually make sperm. If the SRY gene doesn’t work properly, then the embryo won’t become male.
The Y chromosome is smaller than the X chromosome. It’s about a third as long. While the X chromosome has about 900 genes, the Y chromosome only has about 100 – the lowest out of all 46 chromosomes.
Like the X chromosome, you can lose part of the Y chromosome as you age. Smoking raises your chances of this happening. This may raise the risk of developing certain health conditions, such as bladder cancer.
Everyone who has an XY chromosome is related via a single Y-bearing ancestor who lived about 300,000 years ago.
Other Chromosome Combinations
XXY chromosomes
People born with an XXY chromosome combination have a condition called Klinefelter syndrome. This only happens in males. While it’s one of the most common sex chromosome abnormalities, it’s still very rare. Only around one to two males per 1,000 will be born with it.
Most males with Klinefelter syndrome don’t show noticeable symptoms until they reach puberty. As babies they may sit up, crawl, and walk later than average. They may have language delays, and/or learning disabilities. As they get older, their body doesn’t produce as much testosterone as a male teenager without the condition. This can lead to:
- Small testicles and penis
- Breast growth
- Narrow shoulders and wide hips
- Taller, less muscular body
- Weak bones
- Low sperm count
In adulthood, they may not be able to father children without the help of a fertility specialist.
Since symptoms are often mild, Klinefelter syndrome may go undiagnosed until another issue crops up, such as infertility. There’s no cure for Klinefelter syndrome, but males with the condition may find that their symptoms improve with testosterone therapy.
Women with XY chromosomes
Very rarely, a baby is born that appears to be female, but actually has XY chromosomes. This condition is called Swyer syndrome. These infants have female genitalia, like a vagina, and may even have a uterus and fallopian tubes. But they don’t have ovaries. It occurs in only about 1 in 80,000 births.
Swyer syndrome usually isn’t diagnosed until a girl goes through puberty. It’s usually treated with hormone replacement therapy (HRT). Once this starts, girls will begin to develop breasts, pubic hair, and get their periods. HRT can also help to prevent bone loss.
Women with Swyer syndrome can’t conceive on their own because they don’t have ovaries. But since they have a uterus, they can get pregnant through a donated egg and carry the baby to term.
Men with XX chromosomes
XX male syndrome, or de la Chapelle syndrome, is a very rare condition where a baby with two XX chromosomes is born with a male appearance. It affects about one in 20,000 male births.
Males with de la Chapelle syndrome may have:
- Small or undescended testes (when testicles don’t move from your abdomen)
- Shorter height than other boys or men their age
- Breast development during puberty
Like other sex chromosome conditions, it may not be diagnosed until puberty. Treatment usually involves testosterone therapy to help you develop facial hair, a deep voice, and prevent breast growth.
Takeaways
Sex chromosomes are found inside the nucleus of all of your cells. There are two kinds of sex chromosomes: X and Y. Sex chromosomes decide whether you’re biologically male (XY) or female (XX), but they don’t define your gender. You may be male (XY) but identify as female, or vice versa.
Rarely, there can be changes in your sex chromosomes. You may have one X and two Y chromosomes, for example, or two X chromosomes and one Y. While people with these conditions usually lead full and productive lives, they may find they need help from fertility specialists in order to have children.
XX and XY Chromosome FAQs
What is the 23rd pair of chromosomes called?
The 23rd pair of chromosomes are called sex chromosomes. All humans have one pair. The other chromosomes – chromosomes 1 through 22 – are called autosomes. Humans have 22 pairs of autosomes, and one pair of sex chromosomes, either XX or XY.
Do humans have one or two sex chromosomes?
Humans, like most mammals, have two sex chromosomes. These are known as X and Y. Females have two X chromosomes, while males have one X and one Y.
What gender is a YY chromosome?
It’s impossible to just have a YY chromosome, since all babies inherit an X chromosome from their mother. But very rarely, males can have one X and two Y chromosomes. Males with this condition tend to be very tall, and may have behavior and learning problems.
Is XX male or female?
XX is female. One X chromosome is inherited from the mother and one from the father. The X chromosome itself is about three times larger than the Y chromosome. It has around 900 genes. The Y chromosome has only about 100 genes.
Is XXY male or female?
XXY is a genetic condition known as Klinefelter syndrome. It’s found in males only. Many people who have it don’t realize they do until later in life.
Is XY a baby girl?
XY is a baby boy. Your child’s biological sex – whether they’re male or female – is determined by the chromosome that they get from their father. If their dad gives them an X chromosome, they’ll be XX, or biologically female. If their dad gives them a Y chromosome, they’ll be XY, or biologically male.
In about 1 out of 80,000 births, a child is born who’s biologically male but has a female appearance and genitals. This is a condition known as Swyer syndrome.
Can an XXY female get pregnant?
XXY, or Klinefelter syndrome, only occurs in males. As a result, they can’t get pregnant. Males with XXY have low fertility, but they may still be able to father a child if they use reproductive technology.
Can men with XYY have kids?
Men with XYY syndrome have normal fertility. They can have kids like other males without the condition. Most signs of XYY can be subtle but include being tall (6 feet, 3 inches on average), bad acne during adolescence, and behavior problems.
