A chromosome is basically a specific strand of DNA (deoxyribonucleic acid) that is tightly coiled up into into an individual package rather than the long strand form that it is usually found as. Chromosomes are found in the nucleus of a cell and are only present when the cell is undergoing mitosis (cell division). The formation of chromosomes allows the DNA to be neatly packaged for replication as the new daughter cells are being produced. Otherwise, mitosis would be a very slow process.
Chromosomes were discovered in the late 1800s while cells were being observed under microscopes but were not formally described and named until the 1880s by scientist Walther Flemming. The word chromosome originates from the Greek words for color (chroma) and body (soma). When observed under a microscope, chromosomes appear as little x-shapes with two short upper arms and two longer bottom arms that are joined in the middle. The top portion is known as the p arms and the bottom portion is known as the q arms. The central point that joins the two sections together is called the centromere. Naming the sections of the chromosome in this way allows scientists to identify where specific genes are located on a chromosome.
There are two types of chromosomes: sex chromosomes and autosomes. Sex chromosomes are the chromosomes responsible for determining the gender of an organism while autosomes are the other chromosomes present in the organism’s genome. Sex chromosomes in humans are denoted as the X chromosome and the Y chromosome. A human with two X chromosomes (XX) will always be female while a human with one X and one Y chromosome (XY) will always be male. There are some exceptions to this rule, but that will be covered at a later time. Organisms that reproduce sexually produce specific cells, known as gametes, that only have half the number of chromosomes. These cells are known as haploid and are either eggs or sperm. Eggs and sperm must be haploid in order to create the resulting embryo, which will be diploid (having a complete amount of chromosomes, one set each from the sperm and the egg).
While most organisms always possess autosomes, not all will have sex chromosomes. Different types of plants may have sex chromosomes depending on their reproductive strategy while others may not. Plants that are known as dioecious (plants that are strictly either male or female) have been found to have sex chromosomes present in their genome that determine what specific flower anatomy (anthers with pollen or pistil and ovary) they will produce at maturity. Flowers that are either male or female are known as incomplete flowers.
All organisms have a specific number of chromosomes present in their genome and each cell will contain the same amount. This is known as a karyotype. For example, humans have a total of 46 chromosomes in each cell while other organisms such as the fruit fly (Drosophila melanogastor) and thale cress (Arabidopsis thaliana, a plant belonging to the mustard family) have a total of 8 chromosomes and 10 chromosomes respectfully. Other organisms have very large numbers such as with domestic dogs that possess 78 chromosomes, or very few in the case of Myrmecia pilosula (a type of ant) that only possesses 2 chromosomes.
As mentioned previously, there are exceptions to the rules of sex determination by the sex chromosomes. In some cases, there are male and female humans that may look like males or females but do not have either the XX or the XY chromosomes or some other variation to the norm. Although rare, some females are born genetically as males (they have XY, known as Swyer syndrome) and some males are born as XXY (known as Klinefelter’s syndrome) who may also possess female characteristics. Unfortunately, these individuals are usually sterile and will require hormone therapy as the gonads (sex organs) are usually abnormally formed and do not secrete the necessary hormones (estrogens or androgens, depending on the case).
Possessing extra or fewer chromosomes than what is considered normal is known as aneuploidy. In humans, the common types of aneuploidy result in conditions known as trisomosies (having an extra chromosome) or monosomies (missing a chromosome). Aneuploidies can occur with either the autosomes or the sex chromosomes. In the vast majority of cases, embryos with aneuploidy are miscarried, but there are also many cases of babies suriviving to birth with aneuploidy. Down syndrome (trisomy 21), Patau’s syndrome (trisomy 13), and Edward’s syndrome (trisomy 18) are all examples of aneuploidy of the autosomes. Klinefelter’s syndrome (XXY) and Triple X syndrome (XXX) are examples of aneuploidy of the sex chromosomes. Turner syndrome (X) is an example of a monosomy of the X chromosome. These individuals will only possess one X chromosome and will always be female. Individuals with extra or missing chromosomes that survive to birth may have a wide range of both physical and mental abnormalities. These abnormalities are most commonly seen in individuals who have trimosomies of the autosomes. The most common complications are with cognitive development and internal organ development. Individuals with aneuploidy of the sex chromosomes have less severe complications but will most likely need hormone therapy and will have issues dealing with infertility.