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Chromosomes and Human Inheritance CHAPTER 12 Disorders caused by individual genes - dominant Individuals with the disorder only need 1 allele to have the disorder Huntington’s Disease Caused by a repeat of the sequence CAG Mental retardation and uncontrollable movements that usually don’t appear until middle age Achondroplasia (mom and son) A form of dwarfism Polydactyly 6 fingers or toes Disorders caused by individual genes – some say recessive – really codominant To have the disorder you need 2 copies of the gene Sickle cell disease Defect in hemoglobin (oxygen carrying molecule in red blood cells) causes red blood cells to “sickle” Carried by 1/12 African Americans Carriers are healthy but are highly resistant to malaria. 12.1 Human Chromosomes In humans, two sex chromosomes are the basis of sex – human males have XY sex chromosomes, females have XX All other human chromosomes are autosomes – chromosomes that are the same in males and females Sex Determination in Humans Sex of a child is determined by the father Eggs have an X chromosome; sperm have X or Y Sex Determination in Humans The SRY gene on the Y chromosome is the master gene for male sex determination Triggers formation of testes, which produce the male sex hormone (testosterone) Without testosterone, ovaries develop and produce female sex hormones (estrogens) What makes a boy a boy and a girl a girl? It’s not that simple………. SRY gene Karyotyping Karyotype A micrograph of all metaphase chromosomes in a cell, arranged in pairs by size, shape, and length Detects abnormal chromosome numbers and some structural abnormalities Construction of a karyotype Colchicine stops dividing cells at metaphase Chromosomes are separated, stained, photographed, and digitally rearranged Fig. 12-3a, p. 187 Fig. 12-3b, p. 187 Down Syndrome Trisomy 21 Turner Syndrome 1/ 2,000 live female births Short stature, loss of ovarian function, lack or incomplete development at puberty Heart defect, infertility, webbed neck Klinefelter’s Syndrome 1/500-1/1000 males Small testes and less testosterone than normal (less facial hair, infertility, breast development) Tall Nondisjunction Chromosomes don’t separate in meiosis Chromosomal Mutations Deletion – involves the loss of part of a chromosome Duplication – involves an extra copy of part or all of the chromosome Inversion – reverses the direction of part of the chromosome Translocation – when part of one chromosome breaks off and attaches to another Changes in Chromosome Structure Cri-du-chat – “Cat’s cry” Deletion in Chromosome 5 Mental impairment, abnormally shaped larynx that causes infants to make distinct cat-like meow , low set ears Disorders caused by individual genes – sex-linked Genes in sex-linked disorders are found on the X or Y chromosome (most are on X) Color blindness (X chromosome) Humans have 3 genes for color vision – all on the X Red-green colorblindness = 1/12 males & 1/200 females Males have 1 X chromosomes – females have 2 (the allele for colorblindness can be masked in females) Colorblindness Hemophilia – Remember the Romanovs? X-linked recessive interferes with blood clotting – bruise easily, internal bleeding causes joint problems Duchenne Muscular Dystrophy (DMD) 1/3500 people – x-linked recessive Causes abnormal dystrophin (a protein that supports muscle fibers) that causes muscle cells to die Progressive disorder – usually diagnosed between 3 and 7, wheelchair by 12 and early death from heart or respiratory failure. Sex linked problems 1) A female is a carrier for hemophilia and the dad is normal What percent of sons will have hemophilia? What percent of daughters will have hemophilia? 2) Show how a daughter can be a carrier for colorblindness from either her mom or dad. Pedigree Analysis How is this trait inherited? How is this trait inherited? What is the genotype of individual III 1? I 2? How is this trait inherited? Well…..it’s not that simple…… Epigenetics – “above the genome” Factors that control gene expression that do not change the DNA or RNA Nova – Ghost in Your Genes!!!!