3.3: Karyotype Analysis
Background Information
After the sperm fertilizes the egg the zygote begins dividing to generate more cells. Errors during replication of the chromosomes can result in breakages, deletions or duplications in the number of chromosomes in each cell. Sometimes the chromosomes in the egg or sperm are already altered even before fertilization occurs. Typically, an embryo missing large portions of chromosomes or essential genes will not result in a live birth. However, there are many chromosomal differences, such as extra chromosomes, deletions of chromosomes or deletions of parts of chromosomes, that do not have lethal effects. The phenotypic effects of the chromosomal differences may be significant or subtle, depending on the genes that are added, altered or missing. Many of these chromosomal alterations can be identified by examining chromosomes in the form of a karyotype. A human karyotype is an arrangement of the autosome pairs from longest to shortest, followed by the sex chromosomes as the last arranged pair.
Below are descriptions of some human chromosomal syndromes.
Down syndrome , occurring in 1 in 900 live births, is the most common chromosomal syndrome in humans. This condition is also called Trisomy 21 because it results in three copies (instead of the normal 2 copies) of chromosome 21. Individuals with Down syndrome often have physical growth delays, intellectual disability and congenital heart defects.
Edwards syndrome , also known as Trisomy 18, results from a third copy of all or part of chromosome 18. This affects about 1 in 2,500 pregnancies, contributes to stillbirths, resulting in the syndrome prevalence of 1 in 6,000 live births. Babies born with Edwards syndrome are often born smaller in size, have heart defects, clenched fists with overlapping fingers, smaller heads and jaws, deformed feet, and severe intellectual disabilities.
Turner syndrome is a condition occurring in 1 in every 2,000-4,000 female live births, in which a female is missing some or all of an X chromosome (X0). Individuals with Turner syndrome often have short stature, extra folds of skin on the neck, and many are born with heart defects. Affected girls with Turner syndrome do not undergo expected sexual changes during puberty, including development of breasts and ovaries, without the assistance of hormonal treatment. Individuals with Turner syndrome have normal intelligence. Depending on severity of symptoms, diagnosis may occur before birth, during infancy or childhood, or during the teen and adult years.
Fragile X syndrome is characterized by changes in a gene FMR1, on the X chromosome. Most people have few (5-40) repeats of a CGG sequence in this gene. But, people with Fragile X syndrome have more than 200 repeats within the gene. This causes mild to moderate intellectual disability and behavioral challenges.
Klinefelter syndrome occurs in males who have an extra X chromosome, resulting in XXY genetic makeup. Klinefelter syndrome occurs in 1 in 500-1,000 male births. Affected individuals have male sex organs, but poor sexual development during puberty. Affected males often produce lower testosterone, have reduced body muscle mass and body hair, have enlarged breast tissue and are often sterile.
Prader-Willi syndrome results when genes on chromosome 15 are not functional. In most cases this is due to the deletion of part of chromosome 15 received from the father. Most common symptoms of Prader-Willi syndrome are behavioral problems, intellectual disability and short stature. Due to hormonal imbalances, people with Prader-Willi syndrome are often constantly hungry, leading to obesity and diabetes. Prader-Willi syndrome affects about 1 in every 15,000 births.
Materials needed
Human karyotypes provided by your instructor.
Exercise 2: Karyotype Analysis
Method: Your instructor will provide your lab group with several karyotypes. Examine the karyotypes with your lab group. Answer the questions for each karyotype.
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Karyotype 1
- Is the individual biologically male or female? _______________
- What chromosomal abnormality is shown? _______________
- The likely syndrome is _______________
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Karyotype 2
- Is the individual biologically male or female? _______________
- What chromosomal abnormality is shown? _______________
- The likely syndrome is _______________
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Karyotype 3
- Is the individual biologically male or female? _______________
- What chromosomal abnormality is shown? _______________
- The likely syndrome is _______________
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Karyotype 4
- Is the individual biologically male or female? _______________
- What chromosomal abnormality is shown? _______________
- The likely syndrome is _______________
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Karyotype 5
- Is the individual biologically male or female? _______________
- What chromosomal abnormality is shown? _______________
- The likely syndrome is _______________