Most of the time, the correct number of chromosomes develops in a fetus. However, just because a fetus has the correct number of chromosomes does not necessarily mean that all of those chromosomes are intact.

When a deletion occurs, there is a loss of some of the genetic information on a chromosome. This results in what is called a partial monosomy because the genes on the missing part of the chromosome only have one copy instead of two inside the cell. Cri du chat syndrome, for example, is the result of a partial monosomy. Cri du chat is caused by a deletion of a part of chromosome 5. The syndrome is sometimes called 5p- syndrome. The “5p-” describes the part of chromosome 5 that is missing.

The name of the syndrome means “cry of the cat” in French. The name comes from the distinctive high-pitched cries of infants with cri du chat, which sound like that of a cat. This cry is caused by abnormal development of the larynx (the voice box) in the newborn. After a few weeks, the larynx of babies born with cri du chat becomes normal. But children with cri du chat tend to have smallerthan-average heads, low birth weight, developmental delays, and mental retardation.

One in 50,000 babies is born with cri du chat. Most cases of the syndrome are not inherited and there is no family history. Instead, cri du chat syndrome is caused by the random deletion of part of the short arm of chromosome 5 early in embryonic development or during gamete production.

In about 10% of the cases of cri du chat, one parent carries a balanced translocation. A balanced translocation does not involve any gain or loss of genetic material. The DNA is just shuffled between two different chromosomes. Because there is no gain or loss of genetic material, balanced translocations usually do not cause medical problems (unless the places where the chromosomes break and reattach are in the middle of an important gene) and many people never know they have them. In fact, about 1 in 500 people carries a balanced translocation.

However, a balanced translocation can become unbalanced when it is passed on to a child. Recall that chromosomes separate randomly into gametes during meiosis. Th e child may inherit the chromosome that is missing genetic information (which is the case with cri du chat) or the one that has too much genetic information. Because a parent that carries a balanced translocation is likely to produce gametes with genetic abnormalities, a couple may have difficulties conceiving a child or experience repeated miscarriages. If the couple does conceive a baby and the child is carried to full term, there is a chance that the child will have genetic abnormalities. Not all of the gametes made by a carrier of a balanced translocation will be abnormal, however.

A procedure called an amniocentesis is the best way to determine if a fetus has an unbalanced translocation. During amniocentesis, some of the amniotic fluid the baby is surrounded by in the womb is removed from the mother’s body (through a needle in the mother’s abdomen). The fluid contains cells shed by the fetus. This amniotic fluid is sent to a laboratory where the baby’s chromosomes are analyzed under a light microscope.

Like translocations, inversions do not involve any loss of genetic material. Unless an important gene is disrupted by the inversion, the person’s phenotype will be normal. They may never even know that they have a chromosomal inversion. An inversion is created when a chromosome breaks in two places; the broken part of the chromosome then flips 180 degrees, and is reincorporated into the chromosome. The flipped section is inverted. Inversions can also be thought of as an intrachromosomal (inside a chromosome) translocation. Like other translocations, inversions may produce abnormal gametes that occur due to crossing over during meiosis. If crossing over involves the area of the chromosome that contains the inversion, some gametes may contain only one copy of certain genes, while other gametes may have more than one copy of those genes.

Chromosomes can also have duplications, which occur when a section of DNA is accidentally copied more than once during DNA replication, resulting in extra genetic material. Pallister Killian syndrome, for example, is caused by duplication of part of chromosome 12. Babies born with Pallister Killian syndrome have four copies of the p arm of chromosome 12 instead of the usual two. Because Pallister Killian syndrome is caused by four copies of the p arm of chromosome 12, it is also called tetrasomy 12p. Mental retardation, streaks of skin that have no color, extra nipples, seizures at birth, joints that do not move, and developmental delays are some of the features of the syndrome. Most people affected with Pallister Killian syndrome do not have the abnormality in every cell; instead, they are mosaic for the syndrome.

A mosaic is an individual who has more than one genotype. The different genotypes are usually the result of an error in DNA replication in one of the cells of a zygote early in the development that creates two different cell lines in the same individual. For example, most people affected with Pallister Killian syndrome have some cells that have tetrasomy 12p in them and another line of cells that are normal. A large majority of women with Turner syndrome are also mosaics with the genotypes 45, X and 46, XX. Whether or not a woman will have symptoms of Turner syndrome depends, to a great degree, on how many 45, X cells she has in relation to normal 46, XX cells. The more normal cells she has, the milder the symptoms tend to be.

Chimeras also have two distinct lines of cells. The difference between a mosaic and a chimera is that a mosaic arises from the same fertilized egg. At some point in the development of the embryo, a cell mutates and becomes abnormal. That abnormality is then carried through all of that cell’s daughter cells, resulting in an abnormal cell line and a normal cell line within the same individual. A chimera, however, forms from two separate zygotes that fuse in an early stage of development. This type of chimera is also called a tetragametic chimera because the fetus is formed from four gametes—two eggs and two sperm. If these zygotes did not fuse and had developed normally, fraternal twins would have been the result. If the two zygotes that are fused are of different sexes, a chimera with one 46, XX cell line and one 46, XY cell line is formed. Human chimeras are very rare.

Scientists have, however, successfully made chimeras in the laboratory. In 1984, the first “geep” was made by fusing a goat embryo with a sheep embryo. Chimeras of two diff erent varieties of mice have been around even longer. Producing chimeras from two differently colored or otherwise genetically distinct mice, for example, helped scientists study how embryos developed. Th e purpose of the geep was also to study developmental biology. Chimeras and mosaics are diff erent from hybrids. Hybrids occur when the gametes from two diff erent species combine to make an embryo. A mule, for example, is created from the egg of a horse and the sperm of a donkey. Mules are usually sterile because a horse has 64 chromosomes while a donkey has 62. Mules have 63 chromosomes, an uneven number that does not segregate properly during meiosis, leading to sterility. Th e off spring of a female donkey and a male horse is called a hinny, but the genetic results are the same.