Download ch 13 and genetic disorders

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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
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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
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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)
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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
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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
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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
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-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
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