Lesson 7.1 Chromosomal Disorders

Can you have too many chromosomes?

Yes, it's not a good thing to have extra chromosomes. An extra chromosome can be fatal to an embryo, in fact. In the case of a few chromosomes, however, a baby may be born with an extra chromosome. This child will have a chromosomal disorder.

Chromosomal Disorders

Some children are born with genetic defects that are not carried by a single gene. Instead, an error in a larger part of the chromosome or even in an entire chromosome causes the disorder. Usually the error happens when the egg or sperm is forming. Having extra chromosomes or damaged chromosomes can cause disorders.

Extra Chromosomes

One common example of an extra-chromosome disorder is Down syndrome (Figure below). Children with Down syndrome are mentally disabled and also have physical deformities. Down syndrome occurs when a baby receives an extra chromosome 21 from one of his or her parents. Usually, a child will receive one chromosome 21 from the mother and one chromosome 21 from the father. In an individual with Down syndrome, however, there are three copies of chromosome 21 (Figure below). Therefore, Down syndrome is also known as Trisomy 21. These people have 47 total chromosomes.

Another example of a chromosomal disorder is Klinefelter syndrome, in which a male inherits an extra “X” chromosome. These individuals have an XXY genotype. They have underdeveloped sex organs and elongated limbs. They also have difficulty learning new things.

Outside of chromosome 21 and the sex chromosomes, most embryos with extra chromosomes do not usually survive. Because chromosomes carry many, many genes, a disruption of a chromosome can cause severe problems with the development of a fetus. Individuals with one (or more) fewer chromosome usually don't survive either. Can you explain why?

Damaged Chromosomes

Chromosomal disorders also occur when part of a chromosome becomes damaged. For example, if a tiny portion of chromosome 5 is missing, the individual will have cri du chat (cat’s cry) syndrome. These individuals have misshapen facial features, and the infant’s cry resembles a cat’s cry.

Summary

• Changes in chromosome number can lead to disorders like Down syndrome.
• Chromosomal disorders also occur when part of a chromosome becomes damaged.

Review

1. What is a chromosomal disorder?
2. Explain what causes Down Syndrome.
3. When do chromosomal defects occur?
4. What happens to most embryos with extra chromosomes? Explain your answer.

Explore More

Use the resource below to answer the questions that follow.

1. What do all people diagnosed with a chromosome disorder share?
2. What is a clinical geneticist? What are they trained to do that is different from a regular doctor?
3. What is a karyotype?
4. Do chromosomal disorders always involve extra genetic material? Explain your answer.