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

2. 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

3. One nucleotide

4. II. DNA Based Disorders A. Dysfunctional Genes (mutation)
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

5. 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 in stressed conditions and die in 10 to 20 days (normal blood cells live for 120 days) leading to anemia
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

6. Huntington’s Disease
Produced by a single dominant allele
A gradual change of the nervous system occurs around the age of 30 to 45
Marked by a painful, progressive loss of muscle control and mental function until death occurs
Approximately 6/100,000 affected

7. III. Chromosomal Based Disorders
Causes
1. Nondisjunction
during meiosis I, homologous chromosomes do not separate properly
Results in 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

8. 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

9. D. Disorders involving Sex Chromosomes
1. Klinefelter Syndrome
 males with extra X-chromosomes
 can be XXY or XXXY
Affects 1/500 births
abnormal development of the testis, leading to infertility
Affected individuals are often tall and produce relatively small amounts of testosterone

10.  females with extra X-chromosomes (XXX, or XXXX)
2. Triple X
 females with extra X-chromosomes (XXX, or XXXX)
Affects 1/1000 female births
Affected individuals are usually taller than average and have slender builds
normal development of sexual traits and are fertile
3. Turner Syndrome
 Females with only one X-chromosome
Affects 1/2500 live female births
only 1 in 40 affected zygotes develops to term
Individuals are short in stature, generally lack prominent female secondary sexual characteristics and may be sterile
Health problems such as heart disease, kidney problems, diabetes and thyroid problems.

11. 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 – trisomy in chromosomes 13, 18 or 21.
Most frequently, the baby dies in utero.
The life expectancy of a baby born with trisomy 13 or 18 is days to weeks.
F. Disorders Involving Autosomal Chromosomes
1. Down Syndrome
 one extra chromosome 21 -written as
(47,+21)
1 in 800 live births
Life expectancy has improved but heart problems persist
lower than average mentality
decreased muscle tone, a flat face, eyes slanting up, irregular shaped ears, large tongue relative to the mouth.

12. 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

13. B. Prenatal Diagnosis
Carrier Identification Tests
CVS (chorionic villus sampling) – a needle is inserted into the mother’s abdomen to test for DNA, gene, and chromosome disorders.
Ultrasound – detects major physical deformations
Amniocentesis – fluid is removed from the sac around the embryo to detect chromosomal abnormalities
Fetal blood sampling – needle is used to removed cells from the fetus to test for blood disorders

14. Preventing Genetic Disorders
Blood Test
Simple blood test in males and females can screen for more than 400 genetic mutations
Approximately $400 and results are available in less than a week
In Vitro Fertilization
Doctors screen embryos after fertilization and only implant those that appear free of disease