1. 14-2: Human Chromosomes Objectives
Why are sex-linked disorders more common in males than in females?
What is nondisjunction, and what problems does it cause?

2. Human Genes and Chromosomes
Molecular biologists may gradually learn more about how the arrangements of genes on chromosomes affect gene expression and development as exploration of the larger human chromosomes continues.

3. Sex-Linked Genes
There is a special pattern of inheritance for genes located on the X chromosome and Y chromosome.
Sex-linked genes: genes located on sex chromosomes
Many are found on the X chromosome
More than 100
Few are found on the Y chromosome
Only a few

4. Colorblindness
3 human genes associated with color vision are located on the X chromosome.
Color Blindness: an inability to distinguish certain colors
Red-green
Most common
1 in 10 males in the U.S.
1 in 100 females in the U.S.

5. Colorblindness cont’d
Sex-linked traits are more common in males than in females
Males have just one X chromosome
All X-linked alleles are expressed males
Even is recessive
Females have 2 X chromosomes
Both X chromosomes must have the recessive trait to show up

6. Colorblindness cont’d
Men pass their X chromosomes to their daughters
Sex-linked genes move from fathers to daughters
May show up in the sons of daughters

7. Quick Lab
Pg 351: “How is colorblindness transmitted?”

8. Hemophilia
Hemophilia: a protein necessary for normal blood clotting is missing
Sex-linked disorder
2 important genes carried on the X chromosome help control blood clotting
Recessive allele on either 2 genes may produce this disorder
People may
Bleed to death from minor cuts
Suffer internal bleeding from bumps or bruises
1 in 10,000 males born with this disorder
Can be treated with injections of normal clotting proteins

9. Duchenne Muscular Dystrophy
Sex-linked disorder
Results in the progressive weakening and loss of skeletal muscle
1 out of 3000 males in the USA
Caused by a defective version of the gene that codes for a muscle protein

10. Quick Question
What causes Duchenne muscular dystrophy?

11. Quick Answer
A defective gene that codes for a muscle protein.

12. X-Chromosome Inactivation
In female cells
One X chromosome is randomly switched off
Forms a dense region in the nucleus
Barr body
Generally not found in males because their single X chromosome is active

13. Chromosomal Disorders
If nondisjunction occurs, abnormal numbers of chromosomes may find their way into gametes, and a disorder of chromosome numbers may result.
nondisjunction: the most common error in meiosis occurs when homologous chromosomes fail to separate
XO Chromosomal Disorder

14. Down Syndrome Trisomy: 3 copies of a chromosome Most common
3 copies of Chromosome 21
Down Syndrome
1 in 800
Mild to severe mental retardation
Increased susceptibility to many diseases
Higher frequency of some birth defects

15. Sex Chromosome Disorders
Nondisjunction
In females
Turner’s syndrome
Usually inherits only 1 X chromosome
Women are sterile
Sex organs do not develop at puberty

16. Sex Chromosome Disorders cont’d
In males
Klinefelter’s syndrome
Extra X chromosome interferes with meiosis
Usually prevents reproduction

17. Sex Chromosome Disorders cont’d
Babies without an X chromosome are not viable
Shows that the X chromosome has the genes vital for the embryo’s
Survival
Development
Essential role of the Y chromosome
Male sex determination in humans
Has a sex-determining region that is necessary to create males
If the Y chromosome is missing
Embryo is female

18. Exit Ticket
Answer the following
questions.
Use complete sentences.
You may NOT use your book. You MAY use your notes.
This is a quiz grade.
Why are sex-linked disorders more common in males than in females?
What is nondisjunction, and what problems does it cause?