Genetic DisordersGenetic DisordersGenetic DisordersGenetic DisordersThings Can Go Wrong With DNA and ChromosomesThings Can Go Wrong With DNA and Chromosomes

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

One nucleotide

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

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

2. 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

A. Causes

1. Nondisjunction

• during meiosis I, homologous chromosomes do not separate properly

• Produces an extra copy of a chromosome in one cell, and a loss of that chromosome from another

III. Chromosomal Based Disorders

B. 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

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

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 with some

female characteristics• 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

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 mentally retarded

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

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.

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

IV. Diagnosing Gene Disorders

B. Prenatal Diagnosis

1. Amniocentesis

• Fluid is removed from the sac surrounding the embryo

• Cells are grown in the laboratory and treated with a chemical that prevents cell division

• A karyotype is prepared and analyzed for abnormalities

2. Chorionic villus biopsy

• Cells are removed directly from the membrane surrounding the embryo and analyzed for abnormalities

• Recent studies have linked limb defects in babies to CVB tests done before the 10th week of pregnancy.

What ethical considerations are parents facing with regards to prenatal diagnosis of DNA based disorders?