Chromosomal basis of sex determination. X and Y chromosomes, X-linkage.
- In humans and other mammals, biological sex is determined by a pair of sex chromosomes: XY in males and XX in females.
- Genes on the X chromosome are said to be X-linked. X-linked genes have distinctive inheritance patterns because they are present in different numbers in females (XX) and males (XY).
- X-linked human genetic disorders are much more common in males than in females due to the X-linked inheritance pattern.
If you’re a human being (which seems like a good bet!), most of your chromosomes come in homologous pairs. The two chromosomes of a homologous pair contain the same basic information – that is, the same genes in the same order – but may carry different versions of those genes.
Are all of your chromosomes organized in homologous pairs? The answer depends on whether you’re (chromosomally) male.
- A human male has two sex chromosomes, the X and the Y. Unlike the
autosomes (non-sex chromosomes), the X and Y don’t carry the same genes and aren’t considered homologous.
- Instead of an X and a Y, a human female has two X chromosomes. These X chromosomes do form a bona fide homologous pair.
Because sex chromosomes don’t always come in homologous pairs, the genes they carry show unique, distinctive patterns of inheritance.
Sex chromosomes in humans
Human X and Y chromosomes determine the biological sex of a person, with XX specifying female and XY specifying male. Although the Y chromosome contains a small region of similarity to the X chromosome so that they can pair during meiosis, the Y chromosome is much shorter and contains many fewer genes.
To put some numbers to it, the X chromosome has about
protein-coding genes with a wide variety of functions, while the Y chromosome has just protein-coding genes, about half of which are active only in the testes (sperm-producing organs) .
The human Y chromosome plays a key role in determining the sex of a developing embryo. This is mostly due to a gene called SRY (“sex-determining region of Y”). SRY is found on the Y chromosome and encodes a protein that turns on other genes required for male development
- XX embryos don't have SRY, so they develop as female.
- XY embryos do have SRY, so they develop as male.
In rare cases, errors during meiosis may transfer SRY from the Y chromosome to the X chromosome. If an SRY-bearing X chromosome fertilizes a normal egg, it will produce a chromosomally female (XX) embryo that develops as a male
. If an SRY-deficient Y chromosome fertilizes a normal egg, it will produce a chromosomally male embryo (XY) that develops as a female .
When a gene is present on the X chromosome, but not on the Y chromosome, it is said to be X-linked. X-linked genes have different inheritance patterns than genes on non-sex chromosomes (autosomes). That's because these genes are present in different copy numbers in males and females.
Since a female has two X chromosomes, she will have two copies of each X-linked gene. For instance, in the fruit fly Drosophila (which, like humans, has XX females and XY males), there is a eye color gene called white that's found on the X chromosome, and a female fly will have two copies of this gene. If the gene comes in two different alleles, such as
(dominant, normal red eyes) and (recessive, white eyes), the female fly may have any of three genotypes: (red eyes), (red eyes), and (white eyes).
A male has different genotype possibilities than a female. Since he has only one X chromosome (paired with a Y), he will have only one copy of any X-linked genes. For instance, in the fly eye color example, the two genotypes a male can have are
(red eyes) and (white eyes). Whatever allele the male fly inherits for an X-linked gene will determine his appearance, because he has no other gene copy—even if the allele is recessive in females. Rather than homozygous or heterozygous, males are said to be hemizygous for X-linked genes.
We can see how sex linkage affects inheritance patterns by considering a cross between two flies, a white-eyed female (
) and a red-eyed male ( ). If this gene were on a non-sex chromosome, or autosome, we would expect all of the offspring to be red-eyed, because the red allele is dominant to the white allele. What we actually see is the following:
However, because the gene is X-linked, and because it was the female parent who had the recessive phenotype (white eyes), all the male offspring—who get their only X from their mother—have white eyes (
). All the female offspring have red eyes because they received two Xs, with the from the father concealing the recessive from the mother.
X-linked genetic disorders
The same principles we see at work in fruit flies can be applied to human genetics. In humans, the alleles for certain conditions (including some forms of color blindness, hemophilia, and muscular dystrophy) are X-linked. These diseases are much more common in men than they are in women due to their X-linked inheritance pattern.
Why is this the case? Let's explore this using an example in which a mother is heterozygous for a disease-causing allele. Women who are heterozygous for disease alleles are said to be carriers, and they usually don't display any symptoms themselves. Sons of these women have a
chance of getting the disorder, but daughters have little chance of getting the disorder (unless the father also has it), and will instead have a chance of being carriers.
Why is this the case? Recessive X-linked traits appear more often in males than females because, if a male receives a "bad" allele from his mother, he has no chance of getting a "good" allele from his father (who provides a Y) to hide the bad one. Females, on the other hand, will often receive a normal allele from their fathers, preventing the disease allele from being expressed.
Case study: Hemophilia
Let's look at a Punnett square example using an X-linked human disorder: hemophilia, a recessive condition in which a person's blood does not clot properly
. A person with hemophilia may have severe, even life-threatening, bleeding from just a small cut.
Hemophilia is caused by a mutation in either of two genes, both of which are located on the X chromosome. Both genes encode proteins that help blood clot
. Let's focus on just one of these genes, calling the functional allele and the disease allele .
In our example, a woman who is heterozygous for normal and hemophilia alleles (
) has children with a man who is hemizygous for the normal form ( ). Both parents have normal blood clotting, but the mother is a carrier. What is the chance of their sons and daughters having hemophilia?
Punnett square showing the potential genotypes of children produced by a father with normal clotting (
) and a heterozygous carrier mother ( ).
All daughters (
, ) have normal blood clotting because they have at least one alelle. of the daughters are carriers, while the other half are homozygous for the allele.
Since the mother is a carrier, she will pass on the hemophilia allele (
) on to half of her children, both boys and girls.
- None of the daughters will have hemophilia (zero chance of the disorder). That's because, in order to have the disorder, they must get a
allele from both their mother and their father. There is chance of the daughters getting an allele from their father, so their overall chance of having hemophilia is zero.
- The sons get a Y from their father instead of an X, so their only copy of the blood clotting gene comes from their mother. The mother is heterozygous, so half of the sons, on average, will get an
allele and have hemophilia ( chance of the disorder).
Check your understanding
Want to join the conversation?
- In the second paragraph of the section titled "Sex Chromosomes in Humans", do Chromosomally female (XX) embreyos that develope into males make them have a more girlish appearance? Do Chromosomally male (XY) embreyos that develope into females make them have a more boyish appearance? Some boys might look a lot like girls or vice versa.(11 votes)
- Hi Tanya, my understanding is that XX individuals with an SRY translocation (who develop as male-bodied) may need hormone supplementation at puberty to develop some male secondary sex characteristics (e.g., facial hair). However, they generally have what their cultures recognize as a male appearance. You can learn more in the Genetics Home Reference entry about SRY translocation: https://ghr.nlm.nih.gov/condition/46xx-testicular-disorder-of-sex-development.(19 votes)
- What is Gene-linkage? How is it different from the Sex-Linkage?(5 votes)
- Sex linkage referes to a gene being linked to (or "on") a sex chromosome. Gene linkage refers to two genes that are on the same chromosome, and are thus "linked" (inherited/transferred together). :)(8 votes)
- "SRY is found on the Y chromosome and encodes a protein that turns on other genes required for male development.
If an SRY-bearing X chromosome fertilizes a normal egg, it will produce a chromosomally female (XX) embryo that develops as a male."
So, the SRY induces other genes to produce male character.
Does that mean that the X chromosome also contains other genes that are required for male development?(genes that would be dormant in the absence of SRY)(6 votes)
- I'm not an expert on this, but my understanding is that SRY† is (usually) sufficient for embryonic testis formation and that the hormonal effects of having testis are (usually) sufficient for male primary and some secondary sexual traits§. One way of looking at this is that male and female sexual anatomy aren't as different as they appear, they just have different structures emphasized and elaborated with a few small changes in "plumbing".
†Note: SRY encodes TDF (testis determining factor) a transcription factor that enhances expression of genes needed for testis development and possibly suppresses expression of genes that promote ovary development. This is an active area of research and there is evidence that SRY is not always necessary for male development and also may not always be sufficient for male development.
If you want to learn more about this, here is a good (and freely available) review article:
§Note: Some other genes on the Y chromosome are necessary for sperm production, but they don't appear to be needed for "maleness".(5 votes)
- It says in the 2nd paragraph of 'sex chromosomes in humans' that the X chromosome has 800-900 protein-coding genes while the Y chromosome has only 60-70, half of which are responsible for roughly the same task or processes in the same area. How does the male genome make up for that lack of proteins? Are they just not needed or are they found somewhere else? Surely the second X chromosome in females carries something which would be important in males too.(4 votes)
- Any time you have dominant and recessive alleles of a gene it is only the dominant allele that gets expressed. There doesn't have to be a second allele for the trait to be present.
A male having a single X chromosome any genes that are on the X that are not present on the Y chromosome become by default dominant.(5 votes)
- so I know a little boy who has three chromosomes. He has an Xyy chromosomes. What does that mean and how does that affect him?(5 votes)
- what does the X mean(3 votes)
- In mammals, including humans, there is a pair of chromosomes that define the genotypic sex of that organism. These chromosomes are labeled X and Y and having 2 X chromosomes is a genotypic female and having X and Y is a genotypic male.
So, an X linked gene is a gene that is found on an X chromosome. Someone who has two X chromosomes is less likely to exabit recessive traits that are carried on the X chromosome since they would have to have the recessive allele on both. Whereas for someone with X and Y there is only 1 X chromosome so any gene that is on it will be expressed.(4 votes)
- If the father is colorblind, but the mother is not, then their children can't be colorblind. The males would get their dad's Y and the non-colorblind X. Thus, they would not be colorblind. The females would have a colorblind X, but it would not be expressed, because it is recessive. Is this correct? Am I understanding this correctly?(3 votes)
- How does the XY chromosomes relate the the blood type of the offspring(3 votes)
- Blood type is not a sex-linked trait. Its an autosomal trait determined by gene I located on chromosome 9. The gene I has 3 allelic forms, Ia, Ib and i. The genotype (allelic forms present in an individual for a trait) of blood type A can either be IaIa (homozygous dominant) or Iai (heterozygous dominant). Similarly, the genotype of blood group B can either be IbIb or Ibi. The blood group AB has genotype IaIb (co-dominant alleles) only while blood group O has only ii (homozygous recessive).(4 votes)
- I hope someone will answer this question: what happens if the father has the disease, how will it affect his son / daughter?(3 votes)
- What is hemizygous?(3 votes)
- Hemizygous is a term used to describe an individual who has only one member of a chromosome pair or chromosome segment rather than the usual two.(3 votes)