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 , 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 Abnormalities Chromosome abnormality Syndrome nameEffects on individual XTurner’s●sterile female ●underdeveloped female characteristics ●short stature ●sometimes below normal intelligence XXYKlinefelter’s●sterile male ●abnormal male characteristics ●abnormally long arms and legs ●below normal intelligence Trisomy #21Down’s●low mental ability ●short arms and legs ●internal defects ●distinct facial features Trisomy #18Edwards●severe abnormalities ●life expectancy about 10 weeks Trisomy #13Patans●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

Patan’s Syndrome

Practice an X-linked Pedigree

Practice an Autosomal Pedigree