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This lecture delves into karyotypes, examining the number and structure of chromosomes in eukaryotic cells. It explains how karyotypes are prepared, analyzed, and interpreted, focusing on chromosome pairs, sex chromosomes, and unique banding patterns. The lecture covers normal human karyotypes, variations leading to developmental abnormalities, and the implications of trisomy and monosomy. By identifying abnormalities, such as Turner’s and Down’s Syndromes, we can better understand genetic disorders and their impact on individuals.
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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
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.
