14-2 Human Chromosomes

I. Human Genes & Chromosomes

1. Genes that are located close together on the same chromosome are linked

2. Those genes may be inherited together

3. The total human genome (all of the different genes in humans) is …………

Discuss w/ Partner

•How many genes make up the human genome?

Answer:

•There are 20,000-25,000 genes in the human genome

II. Sex-Linked Genes

1. Sex- Linked Genes- Any gene located on a sex chromosome (X or Y)

2. Most sex-linked genes are located on the X chromosome

3. Sex-Linked genetic disorders mostly affect males

a) b/c they only have one X c’some

b) Many genes located on the X c’some do not exist on the Y c’some

4. Common Sex-Linked Genetic Disorders

•Colorblindness; a defective gene for vision color which is located on the X c’some

5. Hemophilia; results from two defective genes to control blood clotting which are located on the X c’some

III. X-Chromosome Inactivation

1. A scientist Mary Lyon discovered that in human females one X c’some is randomly switched off.

•Once the X c’some is turned off a dense area in the nucleus is formed.• Barr Body- the dense area of an inactivated X chromosome

Barr Bodies

2. The same process happens in other mammals

3. Female cats have multiple coat patterns as a result of X chromosome inactivation

IV. Chromosomal Disorders

1. Nondisjunction- happens when homologous c’somes do not separate during meiosis

2. Tisomy- An individual with three copies of a chromosome

3. Down Syndrome - a disorder that results from an extra copy of chromosome 21