basic prin disease chapter 3

. Genes and Inheritance Patterns

A. Genes: Segments of DNA chains that determine cell properties (structure

and functions); basic units of inheritance

1. Occupy specific site on chromosome (locus)

2. Exist in pairs (alleles), one on each chromosome

a. Homozygous: Alleles are the same

b. Heterozygous: Alleles are different

3. Expression of genes varies

a. Dominant gene expressed in either heterozygous or

homozygous state

b. Recessive gene expressed only in the homozygous state

c. Codominant: When both alleles expressed

4. Sex-linked genes

a. Carried on X chromosome

b. Female carrier of recessive X-linked trait is normal, effect

of defective allele offset by normal allele on other X

chromosome

c. Male carrier of recessive X-linked trait: Defective X

chromosome functions like a dominant gene

5. Polymorphism

a. Common variant forms of alleles found in populations

B. The Lyon hypothesis

1. X-chromosome inactivation: Only one of the two X

chromosomes in females is genetically active; one is inactivated

around the 16th day of embryonic development; theorized by

Mary Frances Lyon

2. Barr body or sex chromatin body: Inactive X chromosome

3. X inactivation occurs so female, with two X chromosomes, does

not have twice as many X-chromosome gene products as the

male

4. Choice of which X chromosome will be inactivated is random;

when inactivated, it remains inactive throughout the lifetime of

the cell

II. Chromosome Analysis

A. Chromosome composition and abnormalities in number or structure

studied

B. Method of analysis

1. Blood cells cultures and lymphocytes induced to divide

2. Cells arrested in metaphase and caused to swell

3. Stained smears prepared

4. Chromosomes arranged to form karyotype

C. Sex chromosomes

1. Determine genetic sex by composition of X and Y chromosomes

2. Normal female: Two X chromosomes; one X inactivated and

appears attached to nuclear membrane

3. Normal male: One X and one Y chromosome; Y chromosome

appears as bright fluorescent spot in intact cell

4. Identification of sex chromosomes in intact cells

a. A combination of staining and ultraviolet light makes it

possible to determine the X and Y chromosome

composition of intact cells