IV Human Heredity Sexlinked Disorders A Human Chromosomes

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.

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)

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.

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.

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.

Types of Sex-linked Disorders 2. Hemophilia – sexlinked 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.

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.

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.

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. Result : Phenylalanine builds up in the tissues causing severe nerve damage & possible mental retardation.

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.

Types of Recessive disorders : 3. Cystic fibrosis (CF) – common genetic disorder in which 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).

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

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

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.

Types of Dominant Disorders : 1. Huntington’s Disease – a 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).

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

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.

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.

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.

Types of Chromosomal Disorders : 1. Down Syndrome 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.

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.

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.