Research paper (Down syndrome)

Down syndrome is a chromosomal disorder caused by an error in cell division that results in an extra 21st chromosome. The condition leads to impairments in both cognitive ability and physical growth that range from mild to moderate developmental disabilities. Through a series of screenings and tests, Down syndrome can be detected before and after a baby is born. The only factor known to affect the probability of having a baby with Down syndrome is maternal age. (http://www.medicalnewstoday.com/articles/)

Down syndrome is caused by extra genetic material from chromosome 21. Chromosomes are the structures in cells that contain the genes. Each person normally has 23 pairs of chromosomes or 46 in all. An individual inherits one chromosome per pair from the mother’s egg and one from the father’s sperm. When an egg and sperm join together, they normally form a fertilized egg with 46 chromosomes. Sometimes something goes wrong before fertilization. A developing egg or sperm cell may divide incorrectly, sometimes causing an egg or sperm cell to have an extra chromosome number 21. When this cell joins with a normal egg or sperm cell, the resulting embryo has 47 chromosomes instead of 46. Down syndrome is called trisomy 21 because affected individuals have three number 21 chromosomes, instead of two. This type of error in cell division causes about 95 percent of the cases of Down syndrome. Occasionally, before fertilization, a part of chromosome 21 breaks off during cell division and becomes attached to another chromosome in the egg or sperm cell. The resulting embryo may have what is called translocation Down syndrome. Affected individuals have two normal copies of chromosome 21, plus extra chromosome 21 materials attached to another chromosome. This type of error in cell division causes about 3 to 4 percent of the cases of Down syndrome. In some cases, the parent has a rearrangement of chromosome 21, called a balanced translocation, which does not affect his or her health. (http://www.marchofdimes.com/baby/birthdefects_downsyndrome.html).

Your topic is interesting.
On the fifth line of second paragraph you mentioned that

“ sometimes something goes wrong before fertilization.”

What is this something, you are writing a medical research work so you should have to explain to the reader what is this “something .”

Remember if the audience of your article will not understand you, or if you are missing a very important information then they will lose the interest, and eventually they will stop reading it. In this case the missing information is very important to the subject of you essay. Without explaining it all the relevant information losses its interest.