Genetic Differences

and Disorders

Things Can Go Wrong With DNA and Chromosomes

QQ #1

A)What is the difference between a

gene and an allele?

B)Why are proteins so important?

C)How many chromosomes are in

every cell in your body?

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. Genes and Evolution

A. How do genes influence evolution?

1. Genes (and their differences) allow

variation.

2. Variation allows for natural selection to

occur.

B. What is the basis for evolution?

1. Mutations: changes in DNA to increase

variation

a. Mutations are COMMON

b. Mutations can be inherited

c. Mutations can be good, bad, or

neutral

C. Popularity of Genes

1. Are dominant alleles always more

common?

a. NO, but they can be

b. It depends on which alleles are more

beneficial for that population

c. Changes based off of natural

selection

III. 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

a. 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

a. No protein is produced and the trait cannot be expressed properly

QQ 2 – What do you think is the result of dysfunctional/missing genes?

C. Genetic Mutations

1. The result of dysfunctional genes

a. Can be bad, causing life threatening diseases

b. Can be good, contributing to evolution through variation

C. Examples of DNA Based Disorders

1. Sickle-cell anemia

a. Caused by a substitution of one nucleotide in the allele for normal hemoglobin

b. Red blood cells become sickle-shaped

c. These tend to become stuck in capillaries, leading to tissue death

d. Most common among individuals with African descent, because it provides a resistance to Malaria

Normal

Red Blood

Cell

Sickle Cell

2. Huntington’s Disease

a. Produced by a single dominant allele

b. A gradual change of the nervous system occurs around the age of 30 or 40

c. Marked by a painful, progressive loss of muscle control and mental function until death occurs

A. Causes

1. Nondisjunction

a. during meiosis I, homologous chromosomes do not separate properly

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

IV. Chromosomal Based Disorders

B. Results

1. After fertilization, the resulting person will have an abnormal number of chromosomes

a. A monosomic cell is missing one chromosome

b. A trisomic cell has one extra chromosome

QQ 4 – What is the difference between DNA and

chromosomal based genetic disorders?

C. Nondisjunction of Sex Chromosomes

1. Genes critical to all embryonic development are on the X chromosome

a. absence of an X chromosome is lethal

b. absence of a Y chromosome is not lethal

2. Individuals can survive if they have an extra sex chromosome

3. 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

• low body hair count, long legs, thin,

increased possibility of sterility unless

receiving treatment

• Intellectual disabilites occurs when there

are more than 2 X-chromosomes

QQ 5 – Is XXY a monosomy or trisomy?

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

E. Nondisjunction of Autosomal Chromosomes

1. Can affect chromosomes #1 - #22

2. There only 3 trisomies that result in a baby that can survive

for a time after birth

3. Most frequently, the baby dies in utero. F. Disorders Involving Autosomal Chromosomes

1. Down Syndrome

a. one extra chromosome 21 -written as

(47,+21)

b. 1/6 die within the first year

c. average life expectancy is 16 years

d. lower than average mentality

e. 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:

a. Harvested cells in metaphase are treated and stained

b. Chromosomes are observed under the microscope

c. A photograph is taken and enlarged

d. Chromosomes are arranged in homologous pairs

e. Abnormalities are identified

V. Diagnosing Gene Disorders

Der Chr14

Chr14

Der Chr18

B. Prenatal Diagnosis

1. Two Maternal Serum (Blood) Test

a. Plasma Protein Screening (PAPP-A) – protein produced by the placenta in early pregnancy. Abnormal levels are associated with increased risk of chromosome abnormalities

b. 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?