1. Things Can Go Wrong With DNA and Chromosomes
Genetic Disorders
Things Can Go Wrong With DNA and Chromosomes

2. QQ #1
What is the difference between a gene and an allele?
Why are proteins so important?
How many chromosomes are in every cell in your body?

3. I. Overview of DNA Structure
A. Review
1. A gene is a segment of DNA that codes for a particular protein
2. Proteins determine the physical traits of an organism
3. In humans, DNA is organized into 23 pairs of homologous chromosomes
B. DNA Structure
1. The basic building block of DNA is a nucleotide
2. Nucleotide chains are held together to form a double helix
3. Nucleotides are represented using the letters A,T,C,G

4. One nucleotide

5. II. DNA Based Disorders A. Dysfunctional Genes
1. A gene can be missing some of its nucleotides or some of the nucleotides present may be in the wrong order
The protein made from that gene may not work properly
B. Missing Genes
1. A person can be born missing most or all of a gene
No protein is produced and the trait cannot be expressed properly
QQ 2 – What do you think is the result of dysfuncntional/missing genes?
C. Genetic Mutations
The result of dysfunctional genes
Can be bad, causing life threatening diseases
Can be good, contributing to evolution through variation

6. C. Examples of DNA Based Disorders 1. Sickle-cell anemia
Caused by a substitution of one nucleotide in the allele for normal hemoglobin
Red blood cells become sickle-shaped
These tend to become stuck in capillaries, leading to tissue death
Most common among individuals with African decent, because it provides a resistance to Malaria
Normal Red Blood Cell
Sickle Cell

7. Huntington’s Disease
Produced by a single dominant allele
A gradual change of the nervous system occurs around the age of 30 or 40
Marked by a painful, progressive loss of muscle control and mental function until death occurs

8. III. Chromosomal Based Disorders
Causes
1. Nondisjunction
during meiosis I, homologous chromosomes do not separate properly
QQ 3 – What are homologous chromosomes?
Produces an extra copy of a chromosome in one cell, and a loss of that chromosome from another
Results
After fertilization, the resulting person will have an abnormal number of chromosomes
A monosomic cell is missing one chromosome
A trisomic cell has one extra chromosome
QQ 4 – What is the difference between DNA and chromosomal based genetic disorders?

9. C. Nondisjunction of Sex Chromosomes
Genes critical to all embryonic development are on the X chromosome
absence of an X chromosome is lethal
absence of a Y chromosome is not lethal
Individuals can survive if they have an extra sex chromosome
Affects approximately 1 birth in every 1000

10. D. Disorders involving Sex Chromosomes 1. Klinefelter Syndrome
 males with extra X-chromosomes
 can be XXY, XXXY, OR XXXXY
Affects 1/500 births
phenotypically male
low body hair count, long legs, thin, increased possibility of sterility unless receiving treatment
mental retardation occurs when there are more than 2 X-chromosomes
QQ 5 – Is XXY a monosomy or trisomy?

11. 2. Triple X and Quad X Syndrome
 females with extra X-chromosomes (XXX, or XXXX)
Affects 1/700 female births
sometimes no visible difference from XX females
limited fertility
XXXX females are always intellectually challenged
3. Turner Syndrome
 females with only one X-chromosome
Affects 1/2500 live female births
more than 90% self abort
no ovaries, short, shield-like chest low set ears, webbed neck
no mental deficiencies

12. E. Nondisjunction of Autosomal Chromosomes
Can affect chromosomes #1 - #22
There only 3 trisomies that result in a baby that can survive for a time after birth
Most frequently, the baby dies in utero.
F. Disorders Involving Autosomal Chromosomes
1. Down Syndrome
 one extra chromosome 21 -written as
(47,+21)
1/6 die within the first year
average life expectancy is 16 years
lower than average mentality
shorter than average height, broad forehead, round head, open mouth, etc.

13. IV. Diagnosing Gene Disorders
A. Karyotype - a display of all the chromosomes in the nucleus
1. The process:
Harvested cells in metaphase are treated and stained
Chromosomes are observed under the microscope
A photograph is taken and enlarged
Chromosomes are arranged in homologous pairs
Abnormalities are identified

14. 1. Two Maternal Serum (Blood) Test
B. Prenatal Diagnosis
1. Two Maternal Serum (Blood) Test
Plasma Protein Screening (PAPP-A) – protein produced by the placenta in early pregnancy. Abnormal levels are associated with increased risk of chromosome abnormalities
Human Chorionic Gonadotropin (hCG) – a hormone produced by the placenta in early pregnancy. Abnormal levels are associated with increased risk of chromosome abnormalities
What ethical considerations are parents facing with regards to prenatal diagnosis of DNA based disorders?