Biology – Lecture 56

Karyotypes

Karyotype

• The number and appearance of chromosomes in the nucleus of a eukaryotic cell.

• The complete set of chromosomes in a species

Human Karyotype

What to Look For

• Karyotypes describe the number of chromosomes, and what they look like under a light microscope.

• Attention is paid to their length, the position of the centromeres, banding pattern, any differences between the sex chromosomes, and any other physical characteristics.

How do they Look?

• The chromosomes are arranged in pairs, ordered by size and position of centromere for chromosomes of the same size.

• Sex Chromosomes are always last.

Difference between Sex Chromosomes

• X chromosomes are significantly larger than Y chromosomes.

Human karyotype

• The normal human karyotypes contain 22 pairs of autosomal chromosomes and one pair of sex chromosomes.

Chromosome Pairs

• There are 2 chromosomes of each number – one from the mom and one from the dad.

How a karyotype is made

• 1. Cells are blocked during mitosis.• 2. Chromosomes are taken from those cells

and stained (making dark bands). • 3. Pictures of the chromosomes are taken, cut

out and matched up according to size, banding pattern and centromere position.

Interpreting a karyotype

• Look at:• 1. the total number of chromosomes• 2. the sex chromosomes• 3. any extra or missing autosomal

chromosomes

Naming a Karyotype

• 1. Count the total number of Chromosomes• 2. Name the sex chromosomes• 3. If there are extra chromosomes, name the

placement of them after a + sign.

Examples

• 47, XY, +18 indicates that the patient has 47 chromosomes, is a male, and has an extra autosomal chromosome 18.

• 46, XX is a female with a normal number of chromosomes

• 47, XXY is a patient with an extra sex chromosome.

Normal Human Karyotype

• Females contain two X chromosomes and are denoted 46,XX;

• Males have both an X and a Y chromosome denoted 46,XY.

• Any variation from the standard karyotype may lead to developmental abnormalities.

Developmental Abnormalities

• In trisomy there is an extra chromosome resulting in three copies of the same chromosome.

• In monosomy, there is a missing chromosome and the zygote will have one copy of that chromosome.

How can cells end up with too many or too few chromosomes?

• Sometimes chromosomes are incorrectly distributed into the egg or sperm cells during meiosis. When this happens, one cell may get two copies of a particular chromosome, while another cell gets none.

How Does it Cause Abnormalities?

• People who are born with an abnormal number of chromosomes often have genetic disorders because their cells contain too much or too little genetic information.

• Scientists can predict genetic disorders by looking for extra or missing chromosomes in a karyotype.

Common AbnormalitiesChromosome abnormality

Syndrome name Effects on individual

X Turner’s ●sterile female●underdeveloped female characteristics●short stature●sometimes below normal intelligence

XXY Klinefelter’s ●sterile male●abnormal male characteristics●abnormally long arms and legs●below normal intelligence

Trisomy #21 Down’s ●low mental ability●short arms and legs●internal defects●distinct facial features

Trisomy #18 Edwards ●severe abnormalities●life expectancy about 10 weeks

Trisomy #13 Patans ●severe defects including small, non-functioning eyes●survive only a few weeks after birth

Turner’s Syndrome

Kleinfelter’s Syndrome

Down’s Syndrome

Edward’s Syndrome

Practice an X-linked Pedigree

Practice an Autosomal Pedigree