Human Karyotypes

Purpose
To examine and prepare human karyotype charts

Background
The term karyotype refers to a photograph of the chromosomes of a cell at a time when each still consists of two ‘sister’ chromatids attached to each other at their centromeres.
To prepare a karyotype chart, scissors are used to cut out each chromosome from an enlarged photo of the karyotype. Chromosome pairs are identified, and then arranged in chart form (figure 1).
Numbers or letters have been assigned to specific chromosome pairs. Analysis of karyotype charts is helpful in the study of chromosome abnormalities.

Figure 1: A Human Karyotype (left) and Karyotype Chart (right)

Special staining techniques can reveal light and dark regions (bands) on the chromosomes. By matching the bands, it is now quite easy to identify specific chromosomes and prepare a karyotype chart (figure 2).

Figure 2: A Human Karyotype Chart Using Chromosomes With Simulated Banding

Materials * Sample karyotypes | * Scissors | * Unlined paper | * Transparent tape/glue |

Procedure

Part 1: The structure of a human karyotype chart 1. Carefully examine the human karyotype illustrated in figure 2 (previous page) a) How many rows of chromosomes are in a karyotype chart
4 rows

b) Record the total number of chromosomes in the individual.
46 Chromosomes

c) How many chromosome pairs are shown?
There are 23 pairs shown; 22 somatic and 1 sex chromosome pair.

d) What characteristic is used to give a chromosome pair a number?
A special staining technique used to show the dark bands on the chromosome to pair them. Then, the size is analyzed and they are numbered from biggest to smallest.

e) Which pair is not given a number? Why?
The X and Y pair is not given a number because they are the sex chromosomes and only somatic chromosomes are given numbers.

f) What is the sex of this individual? Explain.
This individual is a male because he has XY chromosomes. If this individual was a female, she would have XX chromosomes.

g) What point in the life of a cell would be best for photographing a karyotype? The best time to photograph a karyotype is when the cell is undergoing prophase. This is because the centromeres attach each chromosome and you can see them right beside each other which make it easier to analyze and compare. After prophase, these chromosomes will split and will make it harder to see.

Part 2: Chromosome Abnormalities 2. Two type of chromosome abnormalities are easily seen in a karyotype: i. Abnormal number ii. Major defects in structure h) How many chromosomes are normally found in a human cell?
-Normally, you will 46 chromosomes in a human cell.

i) How many chromosome pairs are normally found in a human cell?
-Normally, there will be 23 pairs of chromosomes; hence there will be 46 individual chromosomes in total.

The major characteristics of some well documented abnormalities in humans are outlined in Table 1.

Table 1: Some Chromosome Abnormalities in Humans

Chromosome Abnormality | Syndrome (condition) | Effects on Individual (Variable) | XO | Turner’s Syndrome | * Sterile female * Underdeveloped female characteristics * Short stature * Sometimes below normal intelligence | XXY | Klinefelter’s Syndrome | * Sterile male * Abnormal male characteristics * Abnormally long arms and legs * Below normal intelligence | Trisomy #21 | Down’s Syndrome | * Low mental ability * Short arms and legs * Internal defects * Distinct facial features | Trisomy #18 | Edward’s Syndrome | * Severe abnormalities * Life expectancy about 10 weeks | Trisomy #13 | Patan’s Sydrome | * Severe defects including small, non-disfunctioning eyes * Survive only a few weeks after birth | Defective #5(part of chromosome missing) | Cri du Chat | * Severe defects, both physical and mental * Sound, when crying, similar to cats’ meow |

3. Examine Table 1 carefully: j) What is the chromosome count in an individual with Turner’s Syndrome? Down’s Syndrome?
-The chromosome count in Turner’s Syndrome is 45 Chromosomes
The chromosome count in Down’s Syndrome = 47 Chromosomes; Trisomy of #21

k) How do these numbers compare to the normal chromosome count?
-Turner’s syndrome is caused by only 1 less chromosome, whereas Down’s syndrome has 1 extra chromosome.

Questions for Application and Research 1. Why is karyotyping a major activity in medical genetics?

-Karyotyping is a major activity in medical genetics because it makes it easier to study the structure or numbers of the chromosome in the individual and discover any abnormalities with the chromosomes.

1. In many karyotype charts, the X chromosome(s) are placed at the end of row 2 and the Y chromosome at the end of row 4. Suggest a reason why this is done.

One can tell that the size of the chromosomes decreases, so it is clear they have been arranged by size. When following this pattern, X chromosome should fall at the end of row 2 because it is much larger than the Y chromosome which falls at the end of Row 4.

2. Could two individuals have both the same karyotype? Why or why not?

-Yes, two individuals can have the same karyotype when they are identical twins because they were created from the same egg. They end up inheriting identical genes from their parents and will also end up having the same karyotype.

3. With reference to figure 2, how many chromosomes would be found in a gamete of this individual? Explain.

-In a gamete of an individual that is affected with Turner’s syndrome, only 22 chromosomes will be found. In a normal gamete, or sex cell, there is 23 chromosomes. However due to the trisomy of #21, this results in only 22 chromosomes in a gamete cell of an individual affected by Turner’s Syndrome.

4. Down’s syndrome is caused by the process of non-disjunction during meiosis. a. What is non-disjunction?
-Non-disjunction refers to the failure of chromosome pairs separating properly during the cell division process.

b. How could it result in an individual with Down’s syndrome?
-The failure of the separation of chromosomes will result in a trisomy of the chromosome #21; which is what causes Down’s Syndrome.

c. Could an individual with Down’s syndrome ever have normal children? Explain.
-Yes, an individual with Down’s syndrome can have normal children. They have 47 chromosomes; 44 normal somatic cells, 2 sex chromosomes, as well as the extra #21 chromosome. There is a chance that the gamete produced by the affected person will contain the extra copy of chromosome #21, resulting in Down’s Syndrome. However, there is also a chance that it will not contain that extra copy.

d. What are the chances that a 40 year old pregnant woman could have a child with Down’s syndrome? Explain.
-The chances of a 40 year old pregnant woman giving birth to a child with Down Syndrome is 1:112. As women age, the risk of her producing gametes that contain chromosomal abnormalities increase. Due to this, the chance of having a child with Down’s Syndrome also increases significantly with age.