Most of the time, meiosis goes normally. But, occasionally, things do not go quite as they are supposed to. Down syndrome, for example, is a genetic disorder that is caused by an error in meiosis. The error results when a gamete, usually an egg, has two copies of chromosome 21 instead of one copy. When a normal gamete joins with a gamete that contains two of chromosome 21, the resulting fertilized egg contains three of chromosome 21. This is the reason that Down syndrome is also called trisomy 21. At the age of 25, women have about a 1 in 1,300 chance of having a baby with Down syndrome. That risk increases to 1 in 365 by the time the woman turns 35, and then 1 in 30 by age 45 because as she gets older, the chance of meiosis going wrong increases.

Down syndrome is a result of nondisjunction during meiosis. Nondisjunction occurs when one of the chromosome pairs fails to separate correctly during one of the stages of meiosis. The result is one gamete with one too many chromosomes (two copies of the same chromosome instead of just one) and one gamete with one too few chromosomes (this gamete has zero copies of the chromosome inquestion). When the number of chromosomes is off by one or two, the condition is called aneuploidy.  e two most common forms of aneuploidy are trisomy and monosomy.

Trisomy is a condition in which a person has three copies of a particular chromosome instead of the normal two. Down syndrome is one example of a trisomy condition; it usually involves three copies of the autosome chromosome 21. Down syndrome is one of the more common chromosomal syndromes, occurring in 1 in 733 live births. The syndrome is named for John Langdon Down (1828–1896), the English physician who first described a group of people who had characteristics of the syndrome in 1866. These physical characteristics include a flattish face, small ears, an upward slant of the eyes, a small mouth that makes the tongue appear to be enlarged, and a single crease across the palm of the hand. The syndrome usually causes some level of physical developmental delay and mental retardation. Therefore, babies with Down syndrome may learn to sit up, crawl, and walk later than children who do not have Down syndrome. The severity of developmental and mental delays varies widely from person to person. Some children with Down syndrome exhibit no other medical conditions, but about half of them also have a congenital (present at the time of birth) heart defect. About half also have trouble with their vision and hearing. Children with Down syndrome have a higher-than-normal risk of developing leukemia, a cancer of the blood.

While Down described the characteristics of Down syndrome, it was not until 1959 that Jérôme Lejeune (1926–1994), a French physician, and Patricia Jacobs (b. 1934), a geneticist, determined that all people with Down syndrome have 47 chromosomes instead of the usual 46.

Two other types of trisomies are Klinefelter syndrome and XYY syndrome. Both trisomies involve the sex chromosomes. The genotype of a male with Klinefelter syndrome is 47, XXY. The Y in the genotype of a person affected by Klinefelter syndrome may alter his or her appearance by making him or her look less masculine. The syndrome is named after Henry Klinefelter who, in 1942, first published a paper describing a group of men who had enlarged breasts, infertility, and sparse facial and body hair. Although all men who have Klinefelter syndrome have a 47, XXY genotype, not every male with Klinefelter syndrome has all of these symptoms. In fact, about 1 in 500 males has an extra X chromosome, but because many do not exhibit symptoms, they may not know that they carry an extra chromosome.

XYY males also have an extra copy of a sex chromosome, but in this case, it is a Y. The genotype of an XYY male is 47, XYY. An XYY male results from the nondisjunction of the Y chromosome in a sperm that then fertilizes a normal egg. This genotype causes no significant physical characteristics or medical conditions. XYY males are sometimes taller than their peers, and may have severe acne and an increased risk of learning disabilities. About 1 in 1,000 males is born with XYY syndrome. Th is means that 5 to 10 boys are born every day in the United States with an extra Y chromosome.

Males are not the only ones that can be born with an unusual number of sex chromosomes. About 1 in 1,000 girls is born with an extra X chromosome, too. These girls have a genotype of 47, XXX. This is called XXX syndrome, triploidy X syndrome, or triple X syndrome. These girls may have an increased risk of developing learning disabilities or speech and language delays. But, like XYY males, many XXX girls have no symptoms and may never know they have an additional X chromosome. Occasionally, girls can be born with four, or even five, X chromosomes. Their genotypes are 48, XXXX and 49, XXXXX, respectively.

Unlike other chromosomes, the extra X chromosomes do not cause much harm, probably due to X-inactivation. Early in the development of a female fetus, one of her X chromosomes is inactivated (turned off). All her cells, thereafter, will have the same X inactivated in every cell. This effectively makes every female have only one copy of the hundreds of genes that lie on the X chromosome, just like males do. Whether the X chromosome that is inherited from the mother or the one inherited from the father is the one that is inactivated is random (except in kangaroos, where it is always the father’s X that is inactivated). This inactivation is essential for the female to get the correct number of genes. If inactivation did not occur, females would get a double “dose” of X genes while males would only have a single “dose.” The inactivated X is converted into a structure called a Barr body (named after Murray Llewellyn Barr, who discovered them). It seems that when a girl is born with extra X chromosomes, the extras are inactivated, as well.

When one gamete receives both copies of a chromosome during nondisjunction, the other gamete is left with no copies of that chromosome at all. A condition where a person has only one copy of, for example, chromosome 10, is called monosomy 10. However, most monosomies are incompatible with life and the fetus never develops.

The only exception to this rule is in Turner syndrome. Women with Turner syndrome have a genotype of 45, X or 45, XO. The O in a genotype stands for a missing chromosome—in this case, the other X. Worldwide, Turner syndrome affects 1 in 2,500 newborns. Like other monosomies, Turner syndrome is much more common in pregnancies that do not survive to term (stillbirths and miscarriages). The missing X can cause developmental problems and specific physical characteristics. Women with Turner syndrome tend to be shorter than average, and they are usually infertile because they lack functioning ovaries. Other physical characteristics may include puffiness or swelling of the feet and hands, kidney problems, heart and skeletal abnormalities, and extra skin around the neck. Developmental delays and learning disabilities may also occur, but the severity of these issues varies widely between individuals.

Polyploidy is a condition in which an organism has more than one complete set of chromosomes. Triploidy, for example, means the organism has three complete sets of chromosomes. So a triploid human would have 69 chromosomes (46 plus one extra set of 23). Triploidy differs from trisomy because an individual with triploidy has three copies of all chromosomes, not just one. One way a triploid fetus can occur is when two sperm fertilize the same egg. Triploid fetuses are very rarely born because they have numerous abnormalities.

Most triploid pregnancies end in miscarriage. Tetraploidy is even more rare of an occurrence than triploidy. But, as the name implies, a tetraploid human fetus would have four complete sets of chromosomes for a chromosome count of 92. Tetraploidy in humans is not compatible with life, but horticulturists purposely breed tetraploidy plants all the time. Plants with tetraploid genomes seem to have several advantages over normal, diploid plants of the same species. In 1937, two scientists, Albert Blakeslee and Amos Avery, discovered that if plants were exposed to a chemical from the autumn crocus (Colchicum autumnale), called colchicine, the plants would consistently develop with more than the normal complement of chromosomes. Daylilies, for example, normally have 22 chromosomes, but, when exposed to colchicines, tetraploid daylilies (with 44 chromosomes) can be bred. Tetraploid daylilies tend to have larger, brighter blooms than normal diploid daylilies.

Triploid daylilies, with 33 chromosomes, are rare, and the ones that do exist are basically infertile. However, research with other triploid plants has led to the development of seedless fruit, such as watermelon and grapes. Because these plants produce very few seeds, they are virtually infertile. The seedless watermelon, for example, is a cross between a tetraploid watermelon and a normal, diploid watermelon. The seedless variety gets two sets of chromosomes from the tetraploid parent and one from the diploid parent, making the seedless watermelon triploid. So fruit growers make use of the plant’s genetic abnormalities and the infertility it causes to produce seedless fruit.