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Human Genetics Chapter 14 Human Chromosomes How many kinds of chromosomes do humans have? 23 How many chromosomes do humans have? 46 (23 from each parent) What are the sex chromosomes? The two chromosomes that determine your sex (X & Y; females = XX; males = XY) The X is much larger (X has 1200 genes; Y has about 140). Human Chromosomes What are autosomes (autosomal chromosomes)? The other 22 chromosomes that are not sex chromosomes. (they are named chromosomes 1, 2, 3 . . . 22) The sex chromosomes are the 23rd pair. Human Chromosomes What is a karyotype? 1. An arrangement of pictures of chromosomes taken during metaphase. This picture is called a metaphase spread. 2. Chromosomes are arranged based on size, centromere location, and banding patterns. (The banding was the inspiration for bar codes.) 3. Karyotypes are used to diagnose genetic disorders based on chromosomal number and for sex determination. Human Chromosomes Metaphase Spread Karyotype of a normal human male. 46,XY Human Pedigree What is a pedigree? It is a chart that traces a Mendelian trait through a family. Examples of Autosomal Dominant Traits: 1. Dwarfism 2. Progeria 3. Polydactyly and Syndactyly Examples of Autosomal Recessive Traits: 1. Albinism 2. Cystic Fibrosis 3. Phenylketoneuria (PKU) Human Blood Types ABO blood types are traits inherited by multiple alleles that show both codominance & complete dominance. Rh blood group determines the (+) or (–) part of the blood type. (+) is completely dominant over the (–). Human Autosomal Disorders Cystic Fibrosis 1. 2. 3. Caused by a recessive allele on chromosome 7 that affects a chloride ion channel in the plasma/cell membrane. The CFTR protein folds improperly. Causes the production of a thick mucus that interferes with digestion & breathing. Allele is found in populations originating from Northern Europe. The heterozygous people during the Middle Ages had an evolutionary advantage against typhoid fever. Sickle Cell Disease 1. 2. 3. 4. 5. Found in people of Sub-Saharan African descent. The allele is the result of a point mutation that changes just one amino acid in hemoglobin. The affected hemoglobin causes the red blood cells to be misshapen when O2 levels drop. The allele shows codominance (sickle betaglobin gene along with the normal beta-globin gene). Homozygous recessive people are most affected, heterozygous are partially affected, and homozygous dominant are not affected. The allele is relatively common because the heterozygous people have resistance to malaria. Sickle Cell Disease Human Chromosomes 1. 2. 3. Each diploid cell has 6 billion base pairs. Only 2% actually codes for proteins. The average gene is 3000 base pairs; the longest is 2 million base pairs. PS-1, PS-2, APOE, & APP are all genes involved in Alzheimer’s Disease Sex-Linked Traits These are traits whose genes are found on one of the sex chromosomes. 1. Most of these genes are found on the X chromosome because it is larger. 2. X-linked traits have a tendency to show up more in males because they only have one X chromosome. Sex-Linked Traits 1. 2. 3. Red/Green Colorblindness: X-linked recessive allele Can’t see the colors red and green very well. Most common in males. Sex-Linked Traits Hemophilia: 1. X-linked recessive 2. One of the proteins that is involved in the clotting process is defective. 3. Once known as the “royal disease” because it was common in the royal families of Europe. XH Xh XH XHXH XHXh Y XHY XhY Sex-Linked Traits Duchenne Muscular Dystrophy: 1. X-linked recessive 2. Causes progressive weakening & loss of skeletal muscle. 3. Affects 1 out of 3000 American males X-Chromosome Inactivation 1. 2. 3. 4. In females, one of the X chromosomes is randomly inactivated. This process is called Lyonization after Mary Lyon, the scientist who discovered it. The inactive X chromosome forms a dense region in the nucleus called a Barr body. This phenomenon can cause interesting traits like the calico color pattern in cats. The genes for the black and orange color are on the X chromosome. Nondisjunction 1. 2. 3. 4. Sometimes during meiosis, the homologous chromosomes fail to separate. This causes the gametes to have more or less than 23 chromosomes. Down Syndrome is caused by nondisjunction of chromosome 21. One gamete had two copies of chromosome 21. The resulting child has 3 copies of chromosome 21 (trisomy 21) Nondisjunction Nondisjunction can also occur with the sex chromosomes. A. Turner’s Syndrome (monosomy X) - 45,X (1 in 2500 females) - Mentally normal, typically short, webbed neck, sterile, underdeveloped sex organs & 2o sex characteristics, heart problems. B. Klinefelter’s Syndrome (47, XXY) - 1 in 1000 males - Some cognitive problems, tall & slim, some female 2o sex characteristics, sterile.