KaryotypesA karyotype is a map of an individual’s chromosomes. Karyotypes are normally used if a chromosomal genetic disorder is suspected in an individual.

A karyotype shows the homologous pairs of chromosomes of an individual cell. Since every cell in the human body has the same DNA and therefore the same chromosomes, the analysis of one cell is enough.

Making a Karyotype

1. The homologous pairs are matched up.

2. They are arranged in pairs according to size, banding pattern and centromere position.

3. The autosomal chromosomes are arranged first, number 1 through 22.

4. The 23rd pair, the sex chromosomes, are placed at the end.

Labeling a Karyotype1. First write down the number of chromosomes.

2. Next to the number write down the sex chromosomes.

3. If there are any chromosomal abnormalities list them last.

46 XY Normal Male 47 XXT21 Down syndrome

4. Write down the phenotype.

Chromosomal AbnormalitiesChromosomal abnormalities are caused by mistakes in meiosis or alterations of chromosomes. There are several types of chromosomal abnormalities:

1. Trisomy-too many chromosomes caused by a non-disjunction during meiosis.

2. Monosomy-too few chromosomes caused by a non-disjunction during meiosis.3. Alterations of chromosome structure (mutations):

A. Deletion: Removes a chromosome segment.

B. Duplication: A repeat of a chromosomes segment.

C. Inversion: A reverse of a segment within a chromosome.

D. Translocation: Movement of a segment from one chromosome to another, non-homologous one.

Monosomy and TrisomySometimes during the process of meiosis (making sex cells), chromosomes are not correctly distributed into the two daughter cells. This is called a nondisjunction.

A nondisjunction can occur in anaphase I or II when the DNA is being pulled apart.

When this happens, one sex cell has both copies of a particular chromosome while the other has none.

Monosomy: one that is missing a chromosome. If fertilized the zygote will have 45 not 46 chromosomes.

Trisomy: one that has an extra chromosome. If fertilized the zygote will have 47 not 46 chromosomes.

In general, humans can tolerate excess genetic material better than it can tolerate missing genetic information. Either way there is normally severe side effects.

Disorders Caused by NondisjunctionsTurner’s Syndrome: Not enough chromosomes (A Monosomy). Karyotype label, 45 XO. Often called monosomy X.

Frequency: 1 in every 5000 female births.

Symptoms (problems): Individuals are missing a second sex chromosome. This is the only known viable human monosomy. The X chromosome is necessary for life to occur. The monosomy YO (male) results in death of the fetus .

Approximately 98% of all fetuses with Turner syndrome spontaneously abort. Children with Turner syndrome make up about 10% of the total number of spontaneous abortions in the United States.

Individuals with Turner’s Syndrome are phenotypically female, however their sex organs do not mature and secondary sexual characteristics fail to develop. Individuals are sterile have a webbed neck, and short in stature. They are usually mentally healthy but can be mentally challenged.

Highly expressed Turner’s syndrome

Klinefelter Syndrome

Klinefelter Syndrome: Too many chromosomes. Karyotype label, 47XXY.

Frequency: 1 in every 2000 male births.Symptoms (problems): Individuals with Klinefelter Syndrome have too many sex chromosomes. It only takes one Y chromosome to be male, regardless of how many X chromosomes he has. The extra X chromosome has some negative effects on the male phenotype:

1. They are very tall.

2. They have male sex organs, but they are abnormally small.

3. They are usually sterile.

4. Some have feminine characteristics, like enlarged breasts and body curves.

Some individuals are XXXY, XXXXY or XXXXXY. The symptoms above are more severe.

What does the extra X do?Molecular explanation: The extra X produces extra estrogen (a female hormone) which changes the phenotype of the male.

Trisomy XTrisomy X: Extra X chromosome. Sometimes called Meta-females or super females). Karyotype label, 47, XXX.

Frequency: 1 in every 1500 female births.

Symptoms (problems): They have limited fertility and may be mentally retarded.

If an individual with Trisomy X is fertile and decides to have a child, what are the possible genotypes and phenotypes her and a normal male can have?

How does this get passed down?

Answers

X Y

XX

X

XXX XXY

XX XY

Meta-female Klinefelter syndrome

Normal female Normal male

Jacob’s syndromeJacob’s syndrome: An extra Y chromosome. Karyotype label, 47, XYY.

Frequency: 1 in every 1000 male births.

Symptoms (problems): The extra Y chromosome may have some affects on the male phenotype:

1. They tend to be somewhat taller than the average male.

2. They tend to have slightly lower mental ability.

4. Studies of the prison system show that 85% of inmates in prison for violent crimes have this syndrome.

3. They tend to have excess acne on their body.

What does the extra Y do?Molecular explanation: The extra Y chromosome produces extra testosterone (a male hormone). This extra testosterone may cause excess aggression and acne.

The other side of the argument.

Down syndromeDown Syndrome: An extra 21st chromosome. Sometimes called trisomy 21. Karyotype label, 47XYT21 or 47XXT21.

Frequency: 1 in every 700 births.

Symptoms (problems): Down syndrome is the most common serious birth defect in the United States. Having an extra 21st chromosome severely alters the individual’s phenotype:

Phenotypic expressivity varies in Down Syndrome

1. They have characteristic facial features. A wide round face and many have trouble with their eye sight.

2. Individuals have a short stature, stubby fingers and toes, heart defects, susceptibility to respiratory infections and mental retardation. Vision problems and large tongues.

3. Prone to develop leukemia and Alzheimer’s disease.

4. There life span is below average to average, in some cases living to middle age or above.

Age and incidence of Down syndrome

Among normal parents, the incidence of Down Syndrome in offspring correlates with the age of the mother.

A mother age

20 has a 1 in 1925

30 has a 1 in 885

40 has a 1 in 110

50 has a 1 in 12

Edward’s Syndrome

Edward’s Syndrome: An extra 18th chromosome. Sometimes called trisomy 18. Karyotype label, 47XYT18 or 47XXT18.

Frequency: 1 in every 10,000 births.

Symptoms (problems): 1. Edward’s Syndrome affects all the organs of the body. It produces severe mental retardation.

2. Feeding and breathing difficulties

3. Specific body deformities: Underdeveloped or absent thumbs, underdeveloped nails, webbing of the second and third toes, small pelvis with limited movements of the hips and short breastbone

4. Specific facial deformities: Small head, low-set malformed ears, abnormally small jaw, small mouth, cleft lip/palate, widely-spaced eyes and upturned nose.

Most infants survive less than a year.

Patau SyndromePatau Syndrome: An extra 13th chromosome. Sometimes called trisomy 13. Karyotype label, 47XYT13 or 47XXT13.

Frequency: 1 in every 5,000 births.

Symptoms (problems): 1. Patau syndrome is characterized by serious eye, brain, and circulatory defects, as well as harelip and cleft palate.

2. Polydactyly, particularly of all extremities, strongly suggests trisomy 13. Extra toes are seen here on each foot.

3. baby with trisomy 13 has cyclopia (single eye) with a proboscis (the projecting tissue just above the eye).

4. Most infants survive less than a year.