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Transcript
Human Genetics Chromosomes • Genes are found on chromosomes – Chromosomes are rod-shaped structures located in the nucleus of every cell in an organism. • Control all the traits of an organism. – Sex cells are produced during a process called meiosis “Me”. • Only half the information of an organism can be handed down to an offspring. Nucleus Chromosome – Parent = 46 chromosomes – Sex cell = 23 chromosomes Chromatid Centromere Cell Chromatid Mitosis vs. Meiosis 4 Body Cells 4 Sex Cells “Me” “My toes” 8 8 4 4 4 4 2 2 2 2 Karyotype • A picture of the 23 chromosome pairs a person has. – Can be used to identify the sex of an individual or see if an individual has any abnormalities. Normal Karyotype 23 Sex Chromosomes • The 23rd set of chromosomes are called the sex chromosomes. – Determine the sex of the offspring: • Male = XY • Female = XX 23 Determining Gender Mom = XX Dad = XY X Y X X XX XX XY XY Genotype: 50% - XX 50% - XY Phenotype: 50% = Female 50% = Male Sex-Linked Traits • All the genes on the X or Y chromosome are inherited together. • This means that certain traits are always inherited with certain sex chromosomes. • Examples: – X-linked traits • Color-blindness • Hemophilia – Y-linked traits • Hairy ears X-Linked Traits • Color Blindness – Inability to distinguish different colors, usually reds and greens. – Recessive trait (Xb), Normal vision (XB) – Inherited on the X chromosome – More common in males. Why? • You only need one affected X chromosome to inherit the trait. (XY) • Females can inherit one affected X chromosome but not show the trait (XX)= carriers Males Females XBY - normal XbY – color blind XBXB – normal XBXb – carrier but normal XbXb – color blind Color Blindness Xb Xb Mom = XbXb = Colorblind Dad = XBY = Normal Genotypes: 50% - XBXb 50% - XbY XB Y XBXb XBXb XbY Xb Y Questions: What percentage of children are carriers, who? _50%_- females What percentage of children are color blind, who? 50%_- males Can males ever be carriers, why? NO – only receive 1 X What percentage of the children are normal? 50% X-Linked Traits • Hemophilia – blood clotting disorder. – Recessive trait (Xh), Normal clotting (XH) – Inherited on the X chromosome – More common in males. Why? • You only need one affected X chromosome to inherit the trait. Males XHY - normal XhY – hemophiliac Females XHXH – normal XHXh – carrier but normal XhXh – hemophiliac Hemophilia Mom = XHXh XH Xh XHXH XHXh = carrier Dad = XHY XH = Normal Genotype: 25% - XHXH 25% - XHXh Y XHY XhY 25% - XHY 25% - XhY Questions: What percentage of the children are carriers, who? _25% - females What percentage of the children are hemophiliacs, who? 25% - males What percentage of the children are normal? 75% Y-Linked Traits • Hairy Ears – a trait handed down on the Y chromosome that produces men with hairy ears. – Why not hairy eared women? • Women don’t inherit Y chromosomes! Males XYH - normal XYh – hairy ears Females XX – normal Hairy Ears X X X XX XX Yh XYh XYh Mom = XX = normal Dad = XYh = hairy eared Genotype: 50% - XX 50% - XYh Questions: What percentage of the children are carriers, who? 0% What percentage of the children are hairy eared, who? 50% - males Can females have hairy ears, why? No – no Y chromosome Pedigree Charts • Another model for understanding inheritance is a pedigree chart. – Each level represents a new generation. • Lines connect offspring to parents – Males are squares and females are circles. – Blank squares or circles usually represent individuals who do not carry a recessive trait – Shaded in squares or circles represent individuals who carry recessive traits. – Circles that are partially shaded in represent carriers, females who carry a recessive trait but do not show the trait and would therefore be considered normal. 1 2 3 4 Questions: • What is the Mom considered in the Parent generation? Carrier • How many offspring did the parents have? 4 • How many of the offspring, in the second generation, are: • Normal, who? 3 • Carriers, who? 1 – female • Colorblind, who? 1 - male Nondisjunction – “Not Coming Apart” • Nondisjunction – the failure of chromosome pairs to separate properly during cell division. – Result is a cell with an imbalance of chromosomes. Kleinfelter’s Syndrome • 47, XXY Syndrome – males have an extra X chromosome. • Most common sex chromosome disorder. • Characteristics: – Smaller testicles – Reduced fertility Karyotype Down Syndrome – “trisomy 21” • 3 copies of chromosome 21 • Characteristics: – – – – Mental & developmental disabilities Almond-shaped eyes Deep crease on palms of hands Short limbs Mutations • Mutation – A change in a gene or chromosome. – If in a body cell, it affects only the organism carrying it. • Example: Cancer – If in a sex cell (egg, sperm), mutation can be passed on to an offspring. • Found in 3 ways: – Positive – Negative – Neutral Positive Mutation • Positive – Benefits the organism – Example: Mutation causes disease resistance • Plants that can resist disease or drought • Humans that can resist HIV infection • Mutations that cause pleasing new feather colors in birds Neutral Mutations • Neutral – Does not benefit or harm the organism – Example: Extra toes and fingers – Example: White skin Negative Mutations • Negative – Harms the organism or reduces its chances of having offspring – Example: Sickle-cell anemia • Often in people of African descent • Blocks blood vessels, causes pain, strokes, death • Can also be a positive mutation – Africa – lots of malaria – One copy of the mutant gene protects people from malaria