Understanding Genes and Chromosomes
Your body is made up of cells. In the middle of each cell is a nucleus, and inside of the nucleus are chromosomes. Chromosomes are important because they contain genes that determine your physical characteristics, your blood type, and even how susceptible you will be to certain illnesses. Each cell in the body typically contains 23 pairs of chromosomes—46 chromosomes in total—each of which contains roughly 20,000 to 25,000 genes.
What Are Chromosomal Abnormalities?
Chromosomal abnormalities are differences in the chromosomes that can happen during development. They could be “de novo” (unique to the fetus) or inherited from a parent. Abnormalities are split into two categories: numerical and structural.
Numerical Abnormalities
Numerical refers to the fact that there are a different number of chromosomes than expected; there could be more or less. This is also called aneuploidy. Each scenario has a specific term:
Monosomy: A chromosome is missing from a pair.Trisomy: There are three chromosomes instead of two.
Structural Abnormalities
When the makeup of the chromosome presents an issue, this is known as a structural abnormality. There are several ways that structural abnormalities may occur.
Translocation: One piece of a chromosome is transferred to another. (This could be a Robertsonian translocation, where one chromosome attaches itself to another, or a reciprocal translocation, where two chromosomes are traded.)Deletion: There is a deleted or missing part of a chromosome.Duplication: A chromosome is copied, resulting in extra genetic material.Ring: A ring/circle forms as a result of a portion of a chromosome tearing off.Inversion: A piece of chromosome breaks off and turns upside down, then reattaches itself to the original structure.
Why Do Chromosomal Abnormalities Occur?
Chromosomal abnormalities occur because of cell division that does not go as planned. Typical cell division happens by either mitosis or meiosis. When a cell, comprising 46 chromosomes, splits into two cells, this is called mitosis. The new cells should also have 46 chromosomes each. Human bodies are made up of cells that have formed from mitosis. In meiosis, a cell also divides into two halves, but these halves each have 23 chromosomes. Meiosis produces sperm and eggs in reproductive organs. If either mitosis or meiosis results in a different number of chromosomes than expected, this is considered a chromosomal disorder.
Risk Factors
There are some factors that could potentially increase the chances of a chromosomal disorder. One such consideration is the age of the parents. There is a higher risk of a chromosomal abnormality occurring in women 35 and older. Although there is conflicting research and more to learn about advanced paternal age (sometimes defined as age 40 and older), this, too, has evidence of possible increased risk for chromosomal disorders. Additionally, environmental factors, like fetal drug exposure, could be involved in chromosomal abnormalities as well.
Chromosomal Disorders
There are different types of disorders that could arise from abnormal chromosomes. The following list is not exhaustive, but rather it includes the disorders that a fetus has the best chance of surviving to birth.
Down Syndrome
One of the most well-known chromosomal disorders is Down syndrome (also known as trisomy 21), which is caused by an extra copy of chromosome 21. Some of the common traits of Down syndrome are developmental delay, small stature, an upward slant to the eyes, low muscle tone, and a deep crease across the middle of the palm. The causes behind Down syndrome are not entirely clear, but scientists have noted, among other things, a common link between older maternal age and trisomy 21. Risk has been shown to increase exponentially as a woman ages, from 1 in 1,500 at age 20 to 1 in 50 by age 43.
Turner Syndrome
Turner syndrome (also known as gonadal dysgenesis), which only affects females, results when one of the X chromosomes is missing or partially missing. It could make them infertile or shorter than average. Other features of the disorder may be swelling of feet/hands, extra neck skin, kidney and heart issues, as well as skeletal abnormalities. Turner syndrome can also result in miscarriage or stillbirth.
Klinefelter Syndrome
Klinefelter syndrome, also known as XXY syndrome, is the result of an extra X chromosome in males. It is associated with high rates of sterility and sexual dysfunction. It typically goes unnoticed until puberty when it is characterized by weak musculature, tall stature, little body hair, and small genitalia.
Trisomy 13
Trisomy 13 or Patau syndrome, is caused by an extra copy of chromosome 13. The syndrome can cause severe intellectual disability as well as heart defects, underdeveloped eyes, extra fingers or toes, a cleft lip, and brain or spinal cord abnormalities. Patau syndrome occurs in one of every 16,000 births, with infants usually dying within the first days or weeks of life.
Trisomy 18
Trisomy 18, sometimes called Edwards syndrome, is caused by an extra copy of chromosome 18. The syndrome occurs in one of every 5,000 live births. Edwards syndrome is characterized by low birth weight, a small, abnormally shaped head, and other life-threatening organ defects. Edwards syndrome has no treatment and is usually fatal before birth or within the first year of life.
Triple X Syndrome
Also known as trisomy X or XXX syndrome, triple X syndrome (in which there are three copies of the X chromosome) only affects females. The chromosomal abnormality can make them taller than average with weaker muscles, introduce issues with speech, or pose challenges with coordination.
XYY Syndrome
XYY syndrome affects 1 in 1,000 males and is caused by the presence of an extra Y chromosome. XYY syndrome symptoms vary but males with the disorder could be taller than average, have speech processing disorders, or have difficulties with coordination. It may also cause behavioral issues, hand tremors, and weaker muscles.
Fragile X Syndrome
Fragile X syndrome, or Martin-Bell syndrome, results from an X chromosome gene change. It can cause intellectual and learning disabilities, social/behavioral issues, and developmental delays.
Cri-Du-Chat Syndrome
When a piece of chromosome 5 is absent, it is categorized as Cri-du-chat (cat’s cry) syndrome or 5p- (5p minus) syndrome. The abnormality is named as such because infants who have it may have a cry that mimics a cat’s. Symptoms of Cri-du-chat may include small head size, weak muscle tone (for infants), delayed development, low birth weight, heart defect, or intellectual disability.
Pregnancy Complications
Chromosomal abnormalities can lead to complications during pregnancy. Two such complications are miscarriage and molar pregnancy.
Miscarriage
Chromosomal abnormalities are one of the primary causes of miscarriage during the first trimester. In early miscarriage, chromosomal errors can prevent an embryo from developing normally. When this happens, the pregnant person’s immune system may respond by spontaneously terminating the pregnancy, though some miscarriages still require medical or surgical assistance for the tissue to pass from the uterus.
Molar Pregnancy
In other cases, a chromosomal abnormality may lead to a rare condition called a molar pregnancy. During a molar pregnancy, tissues that were meant to form into a fetus instead become an abnormal growth on the uterus. There are two types of molar pregnancy:
Complete molar pregnancy: A complete molar pregnancy is caused when the egg has no genetic information and is fertilized by one or two sperm. Due to the lack of genetic information from the mother’s side, the fertilized egg develops a placenta that looks like a cluster of grapes without an accompanying fetus.Partial molar pregnancy: A partial molar pregnancy occurs when an egg with genetic material is fertilized by two sperm. It causes the development of an embryo that has multiple copies of chromosomes, forms some abnormal placental tissue, and usually does not survive.
Chromosomal Testing
Fetal chromosomal testing, though not routine, is available to expecting parents who choose it. This testing includes both screening and diagnostic tests. Toward the end of your first trimester, you can elect to have a screening test that can provide you and your doctor with information about the probability of your baby having a chromosomal abnormality. These tests cannot diagnose a chromosomal abnormality. Screening tests include ultrasound and blood tests, such as a panel of biomarkers or testing of circulating placental DNA. While screening tests cannot diagnose a chromosomal abnormality, there are tests that can. Diagnostic tests such as chorionic villus sampling and amniocentesis are more invasive but can diagnose abnormalities that previous screening tests may have flagged.
A Word From Verywell
Learning about chromosomal abnormalities can be overwhelming. While there are tests that can be performed during the first trimester to gain insight into development or complications, genetic counseling prior to conception could also offer more information that is valuable for those trying to have a baby.
