Chromosomal Disorders and Karyotypes

Essential Question

What happens when chromosomes do not separate correctly?

Meiosis usually sorts chromosomes correctly, but sometimes it does not. A karyotype is a picture of a person’s chromosomes that can show a change in chromosome number. In this lesson, you’ll learn how to read a karyotype, how errors in meiosis lead to an extra or missing chromosome, and how scientists describe conditions such as Down syndrome. People with the same karyotype can have very different strengths and needs, and a karyotype tells you only about chromosomes.

Karyotypes

• A karyotype is a picture of all the chromosomes in a cell, arranged in pairs
• Pairs 1 to 22 are autosomes, numbered from largest to smallest
• The last pair is the sex chromosomes: XX or XY in most people
• Most people have 46 chromosomes: 22 pairs of autosomes plus 2 sex chromosomes

To make a karyotype, scientists photograph the chromosomes of a dividing cell, then arrange them in pairs by size and shape. The first 22 pairs are autosomes. The last pair is the sex chromosomes: most females have two X chromosomes (XX), and most males have one X and one Y (XY). A karyotype is written as the total number of chromosomes, then the sex chromosomes. A typical karyotype is 46,XX or 46,XY. The diagram is a simplified drawing, not a real photograph.

Nondisjunction

• Nondisjunction: chromosomes fail to separate during meiosis
• It can happen in the first or the second division
• A gamete ends up with an extra chromosome (n + 1) or one missing (n – 1)
• After fertilization, the zygote has 47 or 45 chromosomes

Sometimes homologous chromosomes, or the two copies of one chromosome, fail to separate in meiosis. This error is called nondisjunction. In the diagram, only chromosome 21 is shown. If nondisjunction happens in the first division, none of the four gametes has the usual number: two have an extra chromosome and two are missing one. If it happens in the second division, two gametes have the usual number, one has an extra chromosome, and one is missing one. When a gamete with 24 chromosomes joins a gamete with 23, the zygote has 47 chromosomes and three copies of one chromosome, called a trisomy. When a gamete with 22 joins a gamete with 23, the zygote has 45 chromosomes and one copy, called a monosomy. Most embryos with an extra or missing autosome do not survive to birth. Trisomy 21 is one of the few that commonly does.

Chromosome Number Conditions

• Down syndrome: usually an extra chromosome 21, written 47,XX,+21 or 47,XY,+21
• Turner syndrome: one X chromosome and no second sex chromosome, written 45,X
• Klinefelter syndrome: an extra X chromosome with a Y, written 47,XXY
• People with the same karyotype can have very different health, abilities, and lives

Condition	Karyotype	Chromosome change
Down syndrome	47,XX,+21 or 47,XY,+21	Three copies of chromosome 21 (trisomy 21)
Turner syndrome	45,X	One X chromosome and no second sex chromosome
Klinefelter syndrome	47,XXY	An extra X chromosome along with a Y chromosome

Down syndrome is most often caused by an extra copy of chromosome 21 (trisomy 21). The extra chromosome usually comes from nondisjunction in the egg and sometimes in the sperm, and a small share of cases come from other changes involving chromosome 21. People with Down syndrome have a range of abilities and health needs, and many go to school, work, and have full lives. People with Turner syndrome have one X chromosome and no second sex chromosome (45,X). People with Klinefelter syndrome have an extra X chromosome along with a Y (47,XXY). A karyotype shows how many chromosomes a person has. It does not show their abilities, their worth, or what their life will be like. Doctors use it together with other information to understand a person’s health needs.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Read a Karyotype

The drawing shows the karyotype of one person (a drawn model). (a) How many chromosomes does the cell have? (b) Which sex chromosomes are shown? (c) Which pair is different from the others, and what is the name for having three copies of a chromosome? Write the karyotype in the form total number, sex chromosomes, change.

Show the solution

1. (a) Count: There are 22 pairs of autosomes (44 chromosomes) plus one extra chromosome 21, which makes 45, plus the 2 sex chromosomes. The total is 47.
2. (b) Sex chromosomes: One X and one Y (XY).
3. (c) The different pair: Pair 21 has three copies instead of two. Having three copies of a chromosome is a trisomy, so this is trisomy 21.
4. Karyotype: 47 chromosomes, XY, extra chromosome 21 is written 47,XY,+21. This karyotype is typical of Down syndrome.
5. Answer: (a) 47. (b) XY. (c) Trisomy 21, written 47,XY,+21.

Example 2: Counting Chromosomes After Nondisjunction

A human egg-producing cell with 46 chromosomes goes through meiosis. Nondisjunction of chromosome 21 occurs. A sperm with 23 chromosomes fertilizes each egg. (a) How many chromosomes are in an egg that has an extra chromosome 21? (b) How many are in the zygote? (c) What fraction of the four cells made by meiosis have the usual 23 chromosomes if nondisjunction happens in the first division? In the second division?

Show the solution

1. (a) Egg with an extra chromosome: 23 + 1 = 24 chromosomes.
2. (b) Zygote: 24 from the egg + 23 from the sperm = 47 chromosomes.
3. (c) First division: Both cells made by the first division have the wrong number, so none of the four final cells has 23. The fraction is 0/4 = 0.
4. Second division: One of the two cells divides as usual and makes 2 cells with 23. The other makes one cell with 24 and one with 22. The fraction with 23 is 2/4 = 1/2.
5. Answer: (a) 24. (b) 47. (c) First division: 0 of 4. Second division: 2 of 4 (1/2).

Example 3: Age and Chromosome Errors

The graph shows approximate values for the chance of a baby with trisomy 21 by the age of the parent who produced the egg. The values are rounded, and sources differ. (a) How many times more likely is a baby with trisomy 21 at age 40 than at age 30? (b) Write a claim, evidence, reasoning statement about how the chance changes with age. (c) Does the graph show that most babies with Down syndrome have older parents? Explain.

Show the solution

1. (a) Ratio: About 1 in 100 at age 40 and about 1 in 900 at age 30. 900 / 100 = 9, so it is about 9 times as likely.
2. (b) Claim: The chance rises with age, and rises faster after age 35.
3. Evidence: The chance is about 1 per 1,000 at age 30, about 3 per 1,000 at age 35, and about 10 per 1,000 at age 40.
4. Reasoning: Eggs are made before birth and wait for years. Errors in sorting chromosomes in meiosis become more likely as eggs age, so a larger share of eggs has the wrong number.
5. (c) No: The graph shows the chance per birth at each age, not how many babies are born at each age. Far more babies are born to younger parents, so most babies with Down syndrome are born to people under 35.
6. Answer: (a) About 9 times. (b) The chance increases with age, from about 1 to 10 per 1,000. (c) No. The graph gives chances, not the number of babies.

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Practice

Practice counting chromosomes after nondisjunction, reading a karyotype, writing karyotype notation, and reading a graph of chance by age. Try it on your own first, then check your work with the answer key.

• Chromosomal Disorders and Karyotypes Practice Worksheet (PDF)
• Chromosomal Disorders and Karyotypes Practice Answer Key (PDF)

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Lesson Slides

• 10.3 Chromosomal Disorders and Karyotypes Slides (PDF)

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Big Ideas

• A karyotype is a picture of a person’s chromosomes
• Nondisjunction is a failure of chromosomes to separate in meiosis
• It makes gametes with an extra or a missing chromosome
• Trisomy 21 is the usual cause of Down syndrome
• A karyotype tells you about chromosomes, not about a person’s abilities or worth

Back to the essential question: When chromosomes do not separate correctly in meiosis, a gamete has an extra or a missing chromosome, and a zygote has 47 or 45 chromosomes instead of 46. A karyotype shows this change. Next, you’ll follow single genes through families with Mendel’s rules.

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