Chromosomal disorders and karyotyping

What makes us human? …. Our DNA… genetic code “blueprint”
What’s inside the nucleus that contains the DNA? Chromosomes

To analyze chromosomes, cell biologists photograph cells in mitosis
(metaphase).
Why is this a good time? Chromatin coils/condenses into visible chromosomes

Biologists then cut out chromosomes and group them in pairs.
What are the pairs called? Homologous chromosomes (homo=same)

The resulting picture is called a Karyotype.
A body cell contains 46 chromosomes.
Each of us begins life when a haploid sperm cell, carrying just 23 chromosomes,
fertilizes a haploid egg cell, also with 23 chromosomes.
The diploid zygote produced by fertilization, contains the full complement of 46
chromosomes.
Why 46? We receive 23 chromosomes from our mother and 23 from our father.
Total of 46 b/c 46 chromosomes are needed for each body (somatic) cell. A human
has 23 homologous chromosomes.
Complete the following chart:
Chromosomes
Autosomes
Body chromosomes
Sex Chromosomes
Gender Chromosomes
22
pairs
1
pair
XX female
XY male
How is gender determined?
Females have two copies of a large X chromosome and males have one X
chromosome and one small y
Written as: 46XX (female) and 46 XY (male)
Look at a karyotype….
How are the chromosomes arranged/identified?
1. According to size (largest to smallest), 2. Banding pattern (segment
of DNA/code for the genes), and 3. Centromere location.
What do you think that the last pair are? Why?
Sex chromosomes b/c if it is a male, the two chromosomes at 23 aren’t the same
size.
Chromosomes = like a library containing hundreds of thousands of books….Book = an
allele and the letters represent the DNA Code.
Geneticists have nearly “read everything”
Interesting info: 21 and 22 first to be discovered.
Chromosome 21 = 545 genes 43 mil DNA bases
Chromosome 22= 225 genes 32 mil
bases
Linked genes: genes on the same chromosome that are usually inherited together
Chromosomal disorders
 Nondisjunction: “not coming apart”
What do you think happens? And When? During meiosis (gamete formation) one
cell ends up with an extra chromosome and one cell ends up minus one…
This is referred to as…..
 Trisomy “3 bodies” …most common…3 chromosomes instead of the normal 2
(i.e. 24 from the mom and 23 from the dad give you one extra…and 23 from the
mom and 24 from the dad give you one extra… total 47)
 What do you think Monosomy means?
(22 from mom and 23 from dad leaves one missing…and 23 from mom and 22 from
dad leaves one missing… total 45)
Specific Chromosomal disorders
Down syndrome: Trisomy 47XX+21 (the extra chromosome is at location 21)
Turners: Monosomy 45 X0 (missing a sex chromosome, develops as female)
*cannot ever just have a Y
Klinefelter’s: Trisomy 47XXY (an extra sex chromosome, develops as a male)