Autosome
An autosome is any of the numbered chromosomes, as opposed to the sex
chromosomes. Humans have 22 pairs of autosomes and one pair of sex chromosomes
(the X and Y). Autosomes are numbered roughly in relation to their sizes. That
is, Chromosome 1 has approximately 2,800 genes, while chromosome 22 has
approximately 750 genes.
An autosome is
any chromosome that is not a sex chromosome (an allosome).
The
members of an autosome pair in a diploid cell have the same
morphology, unlike those in allosome pairs which may have different structures.
The DNA in autosomes is collectively known as atDNA or auDNA.
For example, humans have
a diploid genome that usually contains 22 pairs of autosomes and
one allosome pair (46 chromosomes total). The autosome pairs are
labeled with numbers (1–22 in humans) roughly in order of their sizes in base
pairs, while allosomes are labelled with their letters. By contrast, the
allosome pair consists of two X chromosomes in females or one X and
one Y chromosome in males. Unusual combinations of XYY, XXY, XXX, XXXX, XXXXX or XXYY,
among other allosome combinations, are known to occur and usually cause
developmental abnormalities.
Autosomes still contain sexual
determination genes even though they are not sex chromosomes. For example,
the SRY gene on the Y chromosome encodes the transcription
factor TDF and is vital for male sex determination during
development. TDF functions by activating the SOX9 gene on chromosome
17, so mutations of the SOX9 gene can cause humans with an ordinary Y
chromosome to develop as females.
All
human autosomes have been identified and mapped by extracting the chromosomes
from a cell arrested in metaphase or prometaphase and then
staining them with a type of dye (most commonly, Giemsa). These
chromosomes are typically viewed as karyograms for easy comparison.
Clinical geneticists can compare the karyogram of an individual to a reference
karyogram to discover the cytogenetic basis of certain phenotypes. For
example, the karyogram of someone with Patau Syndrome would show that
they possess three copies of chromosome 13. Karyograms and staining
techniques can only detect large-scale disruptions to chromosomes—chromosomal
aberrations
Autosomal genetic disorders
Autosomal
genetic disorders can arise due to a number of causes, some of the most common
being nondisjunction in parental germ cells
or Mendelian inheritance of deleterious
alleles from parents. Autosomal genetic disorders which exhibit Mendelian
inheritance can be inherited either in an autosomal dominant or recessive fashion
These disorders manifest in and are passed on
by either sex with equal frequency
Autosomal
dominant disorders are often present in both parent and child, as the child
needs to inherit only one copy of the deleterious allele to
manifest the disease. Autosomal recessive diseases, however, require two copies
of the deleterious allele for the disease to manifest. Because it is possible
to possess one copy of a deleterious allele without presenting a disease
phenotype, two phenotypically normal parents can have a child with the disease
if both parents are carriers (also known as heterozygotes)
for the condition.
Autosomal aneuploidy can
also result in disease conditions. Aneuploidy of autosomes is not well
tolerated and usually results in miscarriage of the developing fetus. Fetuses
with aneuploidy of gene-rich chromosomes—such as chromosome 1—never survive to term and
fetuses with aneuploidy of gene-poor chromosomes—such as chromosome 21— are still miscarried over
23% of the time Possessing a single copy of an autosome (known as a
monosomy) is nearly always incompatible with life, though very rarely some
monosomies can survive past birth. Having three copies of an autosome (known as
a trisomy) is far more compatible with life, however. A common example is Down syndrome,
which is caused by possessing three copies of chromosome 21 instead of the usual
two
Partial aneuploidy can also occur as a result of unbalanced translocations during
meiosis Deletions of part of a chromosome cause partial monosomies, while
duplications can cause partial trisomies. If the duplication or deletion is
large enough, it can be discovered by analyzing a karyogram of the individual.
Autosomal translocations can be responsible for a number of diseases, ranging
from cancer to schizophrenia
Unlike single gene
disorders, diseases caused by aneuploidy are the result of improper gene dosage,
not nonfunctional gene product smaller than a few million base
pairs generally cannot be seen on a karyogram
An autosome is one of the
22 numbered pairs of chromosomes that most of us carry in almost all of the
cells of our body. We actually have a total of 23 pairs of chromosomes in these
cells, for a total of 46 chromosomes, but two of those are referred to by letter
rather than by number and are called sex chromosomes rather than autosomes,
since they--that is the X and Y chromosome--help determine what sex, or gender,
we are. The 22 pairs of autosomes are referred to by number basically in
inverse correlation with their size. That is, Chromosome 1, with the smallest
number, is actually the largest chromosome. It has almost 3,000 genes on it.
And we go down to the smallest chromosomes, the ones with the largest numbers.
You think that would be Chromosome 22, since we have Chromosomes 1 through 22,
which only has about 750 genes, but in fact Chromosome number 22 is not the
smallest of the autosomes. We thought it was when it was first described, so
that's how it got named 22. It turns out that Chromosome 21 is actually a
little bit smaller than Chromosome 22.
