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Download ch 13 and genetic disorders
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1 Human Chromosomes -there is a chance that half of the zygotes will be 46XX and half will be 46XY -all egg cells carry a single X chromosome -however, half of all sperm cells carry an X chromosome and half carry a Y -human chromosomes contain both protein and a single, double-stranded DNA molecule -many human genes have become known through the study of genetic disorders -an allele being dominant, recessive, or codominant all depends on the nature of a gene’s protein product and its role in the cell -chromosome 22 contains long stretches of repetitive DNA that do not code for proteins -chromosomes 21 and 22 are the smallest human chromosomes -human genes located on the same chromosome (linked genes) tend to be inherited together Sex-Linked or X-linked Traits -sex linked traits: traits that are carried only on the X chromosome -males have them more often than females *they only have one X chromosome -the Y chromosome does contain genes, but not sex-linked genes because they’re only carried on the X -all X-linked alleles are expressed in males, even if they are recessive -examples: -red-green colorblindness (the inability to distinguish colors) -cause: 3 human genes associated with color vision are located on X chromosomes in males— defective version of any one of these genes cause colorblindness -hemophilia (a disease where the blood doesn’t clot normally) -sex influenced traits: traits whose presence is influenced by male hormones -examples: -bird feather coloration- male birds are brighter than females -male pattern baldness -hemophilia gene for blood clotting protein female H H X X XHXh XhXh male normal XHY XhY hemophilia -it’s very rare for a girl to have hemophilia -must have a mom that’s a carrier or has the disease and a father with the disease -calico cats = always female! female male ©SarahStudyGuides 2 calico cats XBXB XbXb XBXb XBY XbY book information -thomas hunt morgan and his students Alfred Sturtevant and calvin bridges found an unusual whiteeyed male fruit fly -they crossed it with a normal red-eyed female = all red-eyed offspring -F2 generation = 3:1 ratio of red-eyed to white-eyed that confirmed the white-eyed trait is recessive *however, only the males had white eyes! -they eventually found white-eyed females -they crossed it with red-eyed males = F1 = only red-eyed females and white-eyed males -morgan’s explanation was that the gene for eye color is carried on the X chromosome and there is no eye color gene on the Y chromosome -males have only 1 X chromosome whichever eye color allele they receive is expressed -females have 2 X chromosomes the red-eye trait is dominant in females -white eye color was the first example of a sex-linked trait ©SarahStudyGuides 3 Genetic Disorders gene mutations -occurs when the correct proteins is not expressed due to a mutation -alters the proper functions of proteins -has a broader range of effects because they mess up protein functions and there are so many proteins with so many different functions -may be autosomal- mistakes in the sequence of DNA of autosomal chromosomes -ex: sickle cell anemia -may be sex-linked-mistakes in the sequence of DNA of sex chromosomes -ex: hemophilia chromosome mutations -occurs when too many or too few chromosomes are present in the zygote -most likely to be lethal -can cause there to be too much or too little information nondisjunction -improper separation of homologous chromosomes during meiosis -results in too many or too few chromosomes in daughter cells -the Y chromosome contains a sex-determining region that is necessary for male sexual development -no babies have been discovered that have been born without an X chromosome -a female with the genotype XO has inherited only one chromosome and is sterile causes of nondisjunction -aneuploidy: cells that have too many or too few chromosomes are aneuploid -monosomy: only one chromosome of a pair is present -trisomy: 3 chromosomes instead of 2 -translocation: the relocation of a segment of DNA from one chromosome to another (basically when chromosomes are “stuck” to one another) 12 13 karyotypes -visible display or picture of all the chromosomes -how biologists make it: -they photograph cells during mitosis, when chromosomes are fully condensed and easy to see -they cut out the chromosomes from the photographs and group them in homologous pairs -allows diagnosis of chromosome disorders -there may be too many or too few aneuploid -they may be “stuck” to one another translocation ©SarahStudyGuides 4 genetic testing -amniocentesis -taking a sample of amniotic fluid during pregnancy to identify chromosomes and look for any genetic defects of disorders in the baby -chronic villi sampling -taking samples of tissue from the placenta to identify chromosomes and look for any genetic defects of disorders in the baby -karyotyping -taking a picture of the chromosomes in a cell -PGD (preimplantational genetic diagnosis) -the process: -scientists take eggs from a female and fertilize them in a petri plate and wait until the zygote develops into an embryo of 8 cells -they then extract one of the cells (which doesn’t harm the embryo) -they perform a DNA test to look for any genetic disorders -they then implant the embryos that don’t carry disorders or mutations back into the female inheritance patterns -heterozygote advantage -heterozygous carriers can sometimes be immune to or resistant to another disease -example: heterozygote carriers for sickle cell anemia are resistant to malaria -genomic imprinting -the activation or inactivation of certain genes that depends on the gene’s location on a chromosome and its parental origin -example: prader-willi syndrome -genetic anticipation -age of onset: when the severity of symptoms of a genetic disorder increases and symptoms show up earlier with each generation -example: Huntington’s disease ©SarahStudyGuides 5 Book Information Sex Determination -chromosomes come in matching pairs except for the sex chromosomes, which may be different -this pair of chromosomes determines the sex of the individual -the sex chromosomes are labeled X and Y -females: XX -males: XY -all the eggs produced during meiosis have an X chromosome, and half the sperm produced have an X while the other half have a Y chromosome *so the sperm determines the sex of the offspring (if the sperm has an X =female; if the sperm has a Y=male) -insects: females have 2 X chromosomes and males only have one X but no Y -birds, some fish, and some insects have a Z-W system of sex determination -males have 2 matching ZZ chromosomes and females have ZW -some plants have separate female and male plants and have sex chromosomes that follow the X-Y system of sex determination -most plants and some animals have no sex chromosomes Multiple Alleles and Alleles without Dominance -mendel worked with traits that were either dominant or recessive; however, some genes don’t follow this pattern incomplete dominance -occurs when the dominant trait isn’t fully expressed -the heterozygote appears as a “dilute” form of the dominant trait -ex: when red snapdragons are crossed with white-flowered snapdragons, all the F1 plants are pink codominance -occurs when both dominant and recessive traits are equally expressed in the heterozygote -when both phenotypes appear in heterozygous individuals -ex: roan- when there’s both red and white in an individual although red is dominant and white is recessive multiple alleles -blood types also involve multiple alleles -there are 3 alleles for blood type, but no individual has more than 2 of the alleles -blood types are important in transfusion -if someone with type A blood receives a transfusion of type B blood the donated red cells clump together, clogging the blood vessels -this clumping is caused by antibodies, which are defensive proteins found in the blood ©SarahStudyGuides 6 -antibodies bind to foreign substances and are an important defense against infection so the person with type A blood produces anti-B antibodies Linked Genes -each chromosome carries many genes -linked genes are genes on the same chromosome -linked genes are often inherited together -alleles of linked genes don’t always stay together: because of crossing over -during crossing over, homologous chromosomes often exchange pieces when their pair in meiosis which increases variety -the farther apart two genes are on a chromosome, the more likely a break will occur between them -if two genes are far enough apart, breaks between them may be so frequent that they assort independently -researchers have identified many disease-related genes by mapping out observed traits, including Huntington’s disease ©SarahStudyGuides