They are thread-like structures in which DNA is tightly packed within the nucleus.
DNA wraps around proteins called histones , which provide structural support. Chromosomes help ensure that DNA is replicated and distributed properly during cell division.
Each chromosome has a centromere , which divides the chromosome into two sections: the p (short) arm and the q (long) arm. The centromere is at the constriction point of the cell, which may or may not be the center of the chromosome.
In humans, 46 chromosomes are arranged in 23 pairs, including 22 pairs of chromosomes called autosomes. Autosomes are labeled 1-22 for reference. Each pair of chromosomes consists of one chromosome inherited from the mother and one from the father.
In addition to the 22 numbered autosomes, humans also have a pair of sex chromosomes called an allosome. Instead of labeling these chromosome pairs with numbers, allosomes are labeled with letters like XX and XY (which would be chromosome 23, sex chromosomes ).
Definition of sex chromosomes
Sex chromosomes are chromosomes that determine whether the individual is male or female. Although these two chromosomes pair with each other during meiosis, there is usually very little homology or recombination between them, mainly due to a large difference in their genetic content and size.
Often times, a chromosome is smaller and appears to retain only those genes that are necessary for sex determination. On evolutionary time scales, the appearance of distinctly different sex chromosomes, or heteromorphic sex chromosomes, is a relatively recent event.
The first instances of sexual dimorphism, in which the male and female reproductive organs are found in different individuals, are believed to have arisen through temperature-dependent sex determination, where some genes are turned on or off depending on the ambient temperature. .
These genes give rise to male or female external characteristics and some species of lizards continue to use this method. Over time, it is assumed that this developed in the system of different sex chromosomes.
Other methods of sex determination include haplodiploidy, in which males develop from unfertilized eggs and therefore have only one set of chromosomes and females are diploid.
Bees, ants, and wasps are common examples where male drones are haploid and worker bees are diploid. Komodo dragons can even preferentially produce males through parthenogenesis.
X and Y chromosomes
In the XY system, males contain one X chromosome and one Y chromosome, while females contain two X chromosomes. Therefore, males are considered heterogametic: they can produce two different types of gametes, depending on whether the sperm carries a chromosome. X or one Y.
Females are homogeneous, all their eggs carry an X chromosome. Many primates, including humans, use the XY sex determination system.
A variant of this is the method used by some grasshoppers. Here, males have only one X chromosome and no Y chromosomes. In such systems, a male or female is believed to develop based on the relationship between the X chromosomes and the number of sets of autosomes.
For example, if a diploid individual has two X chromosomes, it develops into a female, while males arise from diploids that have one X chromosome.
The impact of ratios is particularly important in sex determination in Drosophila melanogaster fruit flies and C. elegans roundworms where XXY or XXYY individuals are female and XO individuals are male.
This is in contrast to the case in humans where the mere presence of a Y chromosome confers masculinity, regardless of the number of X chromosomes or the ratio of sex chromosomes to autosomes.
Until recently, there was a widespread belief that the Y chromosome in primates undergoes rapid gene loss and that the chromosome would disappear completely in about ten million years, leading primates to an XX / XO system of determination. of sex.
This is now being questioned and research on the evolution of sex chromosomes is leading to new discoveries.
Accumulation of harmful mutations on the Y chromosome
The evolution of sex chromosomes is believed to arise through the mutation of autosomes that carry sex-determining genes.
At some point, when there is a cluster of genes for sex determination in one of the two autosomes, there is a suppression of recombination to ensure that the group of genes is inherited in a block.
Once this occurs, however, a fledgling Y chromosome begins to form, accumulating transposable elements, chromosome rearrangements, and other deleterious mutations, hitchhiking with genes for sex determination.
This is said to lead to completely heteromorphic sex chromosomes and sex determination.
The inability of the Y chromosome to self-correct mutations through recombination during meiosis makes it particularly prone to accumulation of errors.
Additionally, sperm are formed in large numbers, involving many cell division events, all of which increase the chances of error accumulation. Sperm is also stored in a highly oxidative environment in the testes, again increasing the possibility of a genetic mutation.
One hypothesis states that these factors have contributed to a situation in which the Y chromosome has lost most of its genes, except for those that are crucial for determining the sex and survival of the fetus.
This leads to homogametic women having almost twice as many genes on their sex chromosomes compared to their heterogametic partners. In some animals where the males are XY, heterochromatin.
Alternatively, some insects choose to overexpress genes on their X chromosome in heterogametic individuals.
This modification of gene expression is called dose compensation. Recently, a dosage compensation has also been observed for the first time in the dioecious plant S. latifolia.
Genetic disorders in sex chromosomes
Heterogametic individuals are more susceptible to genetic disorders that affect the sex chromosome because they receive only one copy of each gene.
For example, if the mother is a carrier of a recessive genetic disorder, she has a 50 percent chance of transmitting the disease to her male child, depending on which allele is retained in the egg.
On the other hand, it is likely that none of his daughters will be affected, since they would inherit another X chromosome from their father that would have the normal allele.
An important historical event was precipitated by an X-linked genetic disorder. Alexis Nikolaevich’s hemophilia, the son of the last Russian Tsar, was inherited from his mother.
The initial mutation originated in Queen Victoria and, through her daughters, spread to many royal families in Europe. Alexis’ illness is even believed to have contributed to the fall of the Russian monarchy.
Other X-linked disorders include color blindness that is seen much more often in men. Furthermore, Y chromosome mutations are also inherited by male progeny without the possibility of recombination or change. Y-linked inheritance is associated with reduced fertility and baldness.
On the other hand, dominant X-linked disorders affect both male and female progeny. A mother who has an X-linked dominant disorder can transmit her disease to fifty percent of her daughters and fifty percent of her sons.
A father will pass his condition on to all his daughters and none of his sons. However, these are rare events because the presence of dominant genetic abnormalities dramatically reduces reproductive opportunities.
Occasionally, a nondisjunction event during meiosis leads to aneuploidy: the fertilized egg has an irregular set of chromosomes with some present in multiple copies or totally absent.
When this happens with the sex chromosomes, it can lead to individuals who have an unusual set of X and Y chromosomes. For example, some women have three X chromosomes and this condition is usually discovered when there are other symptoms, such as poor muscle tone or difficulties. Learning.
On the other hand, women with only one X chromosome have Turner syndrome and suffer from a number of physical, reproductive and neurological deficiencies. Klinefelter males are people with two X and one Y chromosomes.
This is one of the most common chromosomal sex aneuploidies in humans with several subtle and severe symptoms.
Men are often sterile, taller than average but have poor muscle tone and coordination. On the other hand, men who have an extra Y chromosome show increased height but no other symptoms.
Most cases of sex chromosomal aneuploidy are discovered when individuals show neurological symptoms, learning difficulties, or infertility.