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. Parent = 46 chromosomes Sex cell = 23 chromosomes Chromosome Nucleus Chromatid Centromere Chromatid Cell

Mitosis vs. Meiosis Sex Cells Body Cells “Me” “My toes” 4 4 8 8 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 Genotype: 50% - XX 50% - XY Phenotype: 50% = Female 50% = Male X X X XX XX XY XY Y

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 XB XBXb XBXb XbY XbY Y Mom = XbXb = Colorblind Dad = XBY = Normal Genotypes: 50% - XBXb 50% - XbY XB XBXb XBXb XbY XbY Y Questions: What percentage of children are carriers, who? What percentage of children are color blind, who? Can males ever be carriers, why? What percentage of the children are normal? _50%_- females 50%_- males NO – only receive 1 X 50%

X-Linked Traits Hemophilia – blood clotting disorder. Males Females 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 Females XHY - normal XhY – hemophiliac XHXH – normal XHXh – carrier but normal XhXh – hemophiliac

Hemophilia XH Xh XH XHXH XHXh Y XHY XhY Mom = XHXh = carrier Dad = XHY = Normal XH XHXH XHXh Genotype: 25% - XHXH 25% - XHXh 25% - XHY 25% - XhY Questions: What percentage of the children are carriers, who? What percentage of the children are hemophiliacs, who? What percentage of the children are normal? Y XHY XhY _25% - females 25% - males 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 Females XYH - normal XYh – hairy ears XX – normal

Hairy Ears X X X XX XX Yh XYh XYh Mom = XX = normal Dad = XYh = hairy eared X XX XX Genotype: 50% - XX 50% - XYh Questions: What percentage of the children are carriers, who? What percentage of the children are hairy eared, who? Can females have hairy ears, why? Yh XYh XYh 0% 50% - males 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? How many offspring did the parents have? How many of the offspring, in the second generation, are: Normal, who? Carriers, who? Colorblind, who? Carrier 4 3 1 – female 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