1. IV. Human Heredity & Sex-linked Disorders A. Human Chromosomes -Humans have 46 chromosomes in their cells. -Cell biologists analyze chromosomes using karyotypes. Karyotype – a picture of chromosomes arranged/grouped in order in pairs.

2. Sex-chromosomes vs. Autosomes *Two of our 46 chromosomes are known as sex chromosomes, because they determine a person’s sex. -Females have two copies of a large X chromosome (XX) & males have one X & one Y chromosome (XY). -Females can only pass on an X to a child & males pass on either X or Y, determining the sex of a baby. -All human egg cells contain an X chromosome, while ½ of the sperm cells carry X’s & ½ carry Y’s. -The remaining 44 chromosomes (non-sex chromosomes) are known as autosomal chromosomes or autosomes. -The total # of chromosomes in human cells (autosomes & sex chromosomes) is represented as : 46,XX for females & 46,XY for males.

3. Chromosomes 21 & 22 -The smallest of human autosomes, chromosomes 21 & 22, were the first to be studied. -Genetic disorders on chromosome 22 are responsible for one form of leukemia (cancer of the blood) & neurofibromatosis, a tumor causing disease of the nervous system. -Genetic disorders on chromosome 21 include amyotrophic lateral sclerosis (ALS) or Lou Gehrig’s disease.

4. B. Sex-linked Genes & Disorders Sex-linked gene – gene located on either the X or Y chromosome. -Special patterns occur on each sex chromosome. -Because the X chromosome is larger, it carries more sex-linked genes & disorders.

5. Types of Sex-linked Disorders 1. Colorblindness – sex-linked disorder in which an individual can’t perceive certain colors. It is passed to offspring on the X chromosome. -If an X carries the recessive allele for colorblindness it may or may not be expressed in a female but will be expressed in a male. -Males receive one X chromosome, so all X-linked alleles are expressed even if they are recessive. -Sex-linked genes move from fathers to daughters and from those daughters to their sons.

6. Types of Sex-linked Disorders 2. Hemophilia – sex- linked disorder in which the blood is unable to clot because it lacks a certain protein. -It is caused by a point mutation & is carried on the X chromosome, so it usually affects males.

7. Types of Sex-linked Disorders 3. Duchenne Muscular Dystrophy – sex-linked disorder that results in the progressive weakening & loss of skeletal muscle. -Affected muscle tissue starts to break down during childhood. -Is carried on the X chromosome & is caused by a defective version of a gene that codes for a muscle protein.

8. C. Dominant, Recessive, & Chromosomal Disorders *All genetic info, including human disorders, is in the Human Genome – our complete set of genetic info. Recessive Alleles and Disorders (pg. 345) Recall : Recessive alleles aren’t expressed in heterozygous individuals. heterozygous individuals. *Disorders caused by recessive alleles don’t affect a carrier’s health but may be passed on to offspring.

9. Types of Recessive disorders : 1. Phenylketonuria (PKU) – genetic disorder in which the body lacks the enzymes to break down the amino acid phenylalanine (in milk & other foods). Can be tested & treated. genetic disorder in which the body lacks the enzymes to break down the amino acid phenylalanine (in milk & other foods). Can be tested & treated. Result : Phenylalanine builds up in the tissues causing severe nerve damage & possible mental retardation.

10. Types of Recessive disorders : 2. Tay-Sachs disease – a fatal genetic disorder that causes lipids to accumulate in the brain when the body fails to break them down. *Appears frequently in Jewish families of central/eastern European ancestry. Result : Nervous system breakdown including : blindness, brain damage, and eventually death in the first few years of life. Has no cure.

11. Types of Recessive disorders : 3. Cystic fibrosis (CF) – common genetic disorder in which genetic disorder in which the body produces excessive secretions of thick mucus. the body produces excessive secretions of thick mucus. *Appears frequently in people whose ancestors came from Northern Europe. Result : Malfunction of tissues in the body & mucus accumulation in the digestive tract (digestive difficulty) & in the lungs (difficulty breathing).

12. Types of Recessive disorders : 4. Albinism – genetic disorder in which individuals have no skin, hair, or eye pigment. *Occurs in ALL races.

13. Types of Recessive disorders : 5. Galactosemia – genetic disorder in which galactose (a sugar) accumulates in tissues. Result : Mental retardation, eye and liver damage.

14. Dominant Alleles and Disorders Recall : Dominant alleles are always expressed in a person’s phenotype, ex : freckles, widow’s peak, farsightedness, broad lips,& polydactyly (extra/long fingers/toes). -If passed on, dominant disorders usually kill the offspring before he/she is capable of reproduction. *For this reason, dominant disorders are very rare.

15. Types of Dominant Disorders : 1. Huntington’s Disease – a fatal genetic disorder in which the nervous system gradually deteriorates, especially the brain. fatal genetic disorder in which the nervous system gradually deteriorates, especially the brain. Results : Progressive loss of muscle control and mental function until death occurs. *Signs of the disease show up later in life (late 30’s or 40’s).

16. Types of Dominant Disorders : 2. Achondroplasia – a form of dwarfism. 3. Hypercholesterolemia – genetic disease in which a person has excess cholesterol in the blood. Result : Heart disease

17. Codominant Alleles and Disorders *Codominant alleles are both expressed at the same time. Types of Codominant Disorders : 1. Sickle Cell Disease – genetic disease in which one allele causes a change in the shape of normal red blood cells. -Affected blood cells are moon-shaped or “sickle-shaped” instead of round. Result : Poor blood-flow during which the cells clump together blocking & damaging parts of the circulatory system. Can cause weakness, anemia, brain damage, spleen damage, heart damage, & even death.

18. What causes Sickle-cell disease? *Sickle-cell disease results from Pleiotropy – when a single gene affects more than one trait. *Is commonly found in African Americans whose ancestors are from west-central Africa. *People who are heterozygous for sickle-cell disease are generally healthy & are resistant to malaria.

19. Chromosomal Disorders *Most chromosomal disorders are caused by mutations. *The most common form of mutation that results in chromosomal disorders is Nondisjunction – the addition or loss of a whole chromosome when the chromosomes are supposed to separate in meiosis.

20. Types of Chromosomal Disorders : 1. Down Syndrome chromosomal disorder that results from an extra copy of chromosome number 21. chromosomal disorder that results from an extra copy of chromosome number 21. An affected person has three number 21 chromosomes in each cell. Result : Mental retardation & Physical abnormalities. *Down Syndrome is also called Trisomy 21, Trisomy meaning “three bodies” or three copies.

21. Types of Chromosomal Disorders : 2. Turner’s syndrome – sex chromosome disorder in which females have underdeveloped sexual characteristics. *It is caused by the presence of only 1 sex chromosome (Monosomy), an X, in the cells. Allele pair = XO due to a mutation in sex cells (germ mutation). Result : Affected females are sterile and their sex organs DON’T develop at puberty.

22. Types of Chromosomal Disorders : 3. Klinefelter’s syndrome – sex chromosome disorder in which males have underdeveloped sex organs. *It is caused by having two X chromosomes and one Y chromosome in each cell. Allele pair = XXY. Result : The extra X chromosome interferes with Meiosis & usually keeps them from reproducing.