A regular human cell has 46 chromosomes: 44 autosomes, which come in pairs,
and 2 sex chromosomes, which specify gender (XX for female and XY for male).
The pairs of autosomes are called "homologous chromosomes." One of each pair
came from mom and the other came from dad. Homologous chromosomes have all of
the same genes arranged in the same order, but with slight differences in the
DNA sequences of the genes.
What happens when a person has something different, such as too many or too
few chromosomes, missing pieces of chromosomes, or mixed up pieces of chromosomes?
Before we look at how the process can go wrong, let's take a look at how cells normally get 46 chromosomes.
Egg and sperm cells have just 23 chromosomes each. That's half as many chromosomes as regular cells. Through
the process of fertilization, egg and sperm join to make a cell with 46 chromosomes, called a zygote.
Before the zygote divides to make two cells, it copies each chromosome. These copies, called sister
chromatids, are identical. When the zygote divides, each cell gets one identical copy of each
chromosome. The cells continue dividing this way to make a person. This type of cell division is called mitosis.
When egg and sperm form, they go through a special type of cell division called meiosis. One purpose
of meiosis is to reduce the number of chromosomes by half. The other is to create genetic diversity.
Meiosis begins like mitosis: the cell copies each chromosome. But unlike in mitosis, homologous
chromosome pairs line up and exchange pieces—a process called recombination. Remember, homologous
chromosomes have the same genes but with slight differences. Recombination increases genetic diversity
by putting pieces of chromosomes that came from mom together with pieces of chromosomes that came from dad.
Next, the newly recombined homologous chromosomes are divided into two daughter cells. Then
the sister chromatids are pulled apart into a total of four cells. Each of these cells has one
copy each of 23 chromosomes, all with a unique combination of mom's and dad's genes.
Too Many or Too Few Chromosomes
Sometimes chromosomes are incorrectly distributed into the egg or sperm cells during
meiosis. When this happens, one cell may get two copies of a chromosome, while another
cell gets none. Incorrect distribution of chromosomes is called nondisjunction.
If a sperm or egg cell with too many or too few chromosomes participates
in fertilization, it will produce a zygote with too many or too few chromosomes. A zygote with 3 copies
of a chromosome is said to have trisomy (pronounced TRY-so-mi). A zygote that is missing
a chromosome is said to have monosomy (MOH-no-so-mi).
Most of the time, autosomal (non-sex chromosome) trisomy and monosomy are lethal because
the zygote ends up with too much or too little genetic information. But sometimes,
babies are born with extra or missing autosomes—most commonly one of the smaller
chromosomes that have fewer genes. Usually these babies have a genetic disorder,
which scientists can diagnose by looking for extra or missing chromosomes in a karyotype.
When monosomy or trisomy involves sex chromosomes, individuals usually survive and many
are quite healthy.
Genetic disorders resulting from too many or too few chromosomes include:
Sometimes pieces of chromosomes are lost or rearranged during meiosis. This happens
during the recombination step, when maternal and paternal chromosomes swap pieces.
When genetic material is missing, a chromosome is said to have a deletion. Deletions
of the tips of chromosomes are called terminal deletions. Internal deletions, where
a chromosome has broken, lost material, and rejoined, are called interstitial
Chromosomes with deletions large enough to be visible on a karyotype are missing
many genes. In humans, such large deletions are less common than smaller deletions,
which often cannot be seen on a karyotype. As with too many or too few
chromosomes, deletions can cause genetic disorders. The symptoms of deletion
disorders vary, depending on which genes are missing.
Genetic disorders resulting from chromosome deletions include:
A translocation is a chromosome rearrangement in which part of a chromosome
breaks off and then reattaches to a non-homologous chromosome. Types of
translocations include reciprocal and Robertsonian.
A reciprocal translocation is a swap between two chromosomes. In a balanced
translocation, the individual has all of their genes, and they are typically
healthy. A translocation where genes are also duplicated or deleted is called
an unbalanced translocation. Individuals with extra or missing genetic material
usually have a genetic disorder.
A Robertsonian translocation occurs when the long arms of two acrocentric
chromosomes fuse at a centromere. The two short arms are lost, leaving a
total of 45 chromosomes. But because the short arms carry very little
genetic information, individuals with Robertsonian translocations are usually healthy.
Funding provided by grant 51006109 from the Howard Hughes Medical Institute, Precollege Science Education Initiative for Biomedical Research.
Genetic Science Learning Center. (2016, March 1) Using Karyotypes to Diagnose Genetic Disorders.
Retrieved June 22, 2017, from http://learn.genetics.utah.edu/content/basics/diagnose/
Using Karyotypes to Diagnose Genetic Disorders [Internet]. Salt Lake City (UT): Genetic Science Learning Center; 2016
[cited 2017 Jun 22] Available from http://learn.genetics.utah.edu/content/basics/diagnose/
Genetic Science Learning Center. "Using Karyotypes to Diagnose Genetic Disorders." Learn.Genetics.March 1, 2016. Accessed June 22, 2017. http://learn.genetics.utah.edu/content/basics/diagnose/.