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Sex-linked Genes & Genetic Disorders Unit 5: Genetics Ch. 11-3 & Parts of Ch. 14 Learning Goals • 1. Describe where sex-linked traits are located and and why these traits generally affect men more than women. Give an example of a sexlinked trait. • 2. Describe dominant and recessive disorders and give an example of each. • 3. Explain nondisjunction and how it leads to trisomy and monosomy. Sex-Linked Genes X Chromosome • Some genes are carried on the sex chromosomes (X & Y) – XX = female, XY = male • Several disorders are carried on the X chromosome Y Chromosome • Because men have only 1 X, they don’t have a “backup” X like women • If the X is bad, men have the disease • EX: Colorblindness, hemophilia, muscular dystrophy Color-Blindness – Three genes for color vision are located on the X chromosome. – In males, a defective version of any one of these genes produces colorblindness. • Sex-linked Crosses – 1) Must show sex chromosomes for each parent & alleles for each X chromosome – 2) Answer must include male or female Predict Gender & Phenotypes • Colorblindness is a recessive trait • Geno: Pheno: – XCXC Normal Female – XcXc Colorblind Female – XCXc Normal / Carrier Female – XCY Normal Male – XcY Colorblind Male Pedigree Charts • A chart used to examine the appearance of traits, especially diseases, over several generations. Genetic Disorders • Some genes code for nonfunctional proteins. • These cause hereditary diseases. Dominant Disorders • If you have the gene, you have the disease – May not show up until later in life, after you’ve had kids and passed it on – Huntington disease, Dwarfism Huntington’s Disease • Results in the early death of some neurons in the brain. People loose the ability to control their movements. • Symptoms usually don’t start until the 20s to early 30s. • The disease is fatal, usually by the late 30s or early 40s. Recessive Disorders • Can remain hidden and skip generations • Heterozygous individuals (Rr) are called “carriers” – Albinism, cystic fibrosis, Tay-Sachs, phenylketonuria • Cystic Fibrosis- Protein does not fold properly. Causes a build up of mucus in the lungs Chromosomal Disorders • 1) Nondisjunction: chromosomes do not separate properly during meiosis • The results are cells with abnormal numbers of chromosomes. – Cells end up with extra chromosomes (47) or fewer chromosomes (45). During Meiosis • Trisomy: If there are three homologous chromosomes instead of two –Down syndrome is caused by trisomy of chromosome 21 Where is the mutation? Where is the mutation? • Monosomy: the cell is missing one chromosome –Turner’s syndrome is a female with one “X” chromosome not 2 “X” chromosomes. • 2) Alterations in Chromosome Structure – Deletion: portion of chromosome is deleted • EX: Cri du chat (Cry of the cat) – Duplication: piece of chromosome is repeated • Ex: Fragile X Syndrome – Translocation: piece of chromosome is deleted from one homologous pair and reattached to another homologous pair Deletion: Cri du chat Duplication: Fragile X Translocation Learning Goals • 1. Describe where sex-linked traits are located and why these traits generally affect men more than women. Give an example of a sex-linked trait. • 2. Describe dominant and recessive disorders and give an example of each. • 3. Explain nondisjunction and how it leads to trisomy and monosomy. Whiteboard Practice 1. Legend & Parents 2. Punnett Square & Answers • Queen Victoria was a carrier of hemophilia (h), a recessive, sex-linked disease. King Edward was normal (H). – – – – 1. Make a legend 2. Genotypes of parents? 3 & 4. Show the cross. 5. Give probabilities for both sons and daughters • Muscular Dystrophy (d), is a recessive, sex-linked disease. A man who has muscular dystrophy marries a woman who is homozygous normal – – – – 1. Make a legend 2. Genotypes of parents? 3 & 4. Show the cross. 5. Give probabilities for both sons and daughters