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8/2/2019 Biology Chapter Twelve Notes
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CHAPTER 12
Laura Walsh
Hon. Bio
Francis Collins and his lab group discovered the gene responsible for cystic
fibrosis (CF), which is an often fatal genetic disorder in which thick mucus
builds up and blocks ducts, making it difficult to breath. Due to work by
geneticists in the early 1900s, they were able to study the CF gene. Early
Work Researcher Thomas Morgan began experimenting with the small fruit
fly Drosophila melanogaster in the early 1900s. He observed that flies have
four pairs of chromosomes, and that three of the pairs were identical in
males and females. In females, the fourth pair had two identical
chromosomes, now called X chromosomes. In males, the fourth pair had one
X chromosome as well as a shorter chromosome, now known as a Y
chromosome. Today, geneticists call the X and Y chromosomes as sex
chromosomes.
Sex Chromosomes and Autosomes
The sex chromosomes contain genes that determine the gender of an
individual. The remaining chromosomes that are not involved in determining
the sex of an individual are called autosomes. autosomes Below you can see
the karyotype of a human female, where there are two X chromosomes. The
karyotype of a male has one X chromosome and one Y chromosome. In
certain organisms, such as chickens and moths, males have two identical sex
chromosomes, and females have two different sex chromosomes. Some
organisms such as most plants and some fish lack sex chromosomes
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entirely.
Sex Determination
Sex chromosomes pair during meiosis I, like other homologous
chromosomes. As meiosis proceeds, the paired chromosomes separate and
move toward different cells. As a result, a sperm cell can receive either an X
chromosome or a Y chromosome. Each egg receives an X chromosome, so
the gender of a gamete depends on the sex chromosome the sperm that
fertilizes it will have. This system of gender determination results in a one-to-
one ratio of males to females. Each sperm and egg cell also receives a single
copy of each autosome. In mammals, when an egg that carries an X
chromosome is fertilized by a sperm with a Y chromosome, the resulting
offspring has an XY pair and is male. The same thing happens with a sperm
that has an X chromosome, except the offspring will be female. In a male
mammal, the Y chromosome contains a gene called SRY for Sex Determining
Region Y, which codes for a protein that causes the gonads of an embryo to
develop as testes. Since female embryos lack this gene, the gonads develop
as ovaries.
EFFECTS OF GENE LOCATION When Morgan was doing his research with fruit
flies, one of the lab members noticed that single male fruit fly had white eyes
instead of the normal red eyes. When Morgan crossed this white-eyed male
with a normal red-eyed female, he found all the F1 offspring had red eyes, as
expected. When he crossed a female and a male from this generation, the
offspring had the expected 3 to 1 ratio of dominant to recessive, red-eyed to
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white-eyed. What was not expected was that all of the white eyed flies were
male!
Sex-Linked Genes and Traits
Based on his observation that all of the white-eyed flies were male, Morgan
hypothesized that the gene for eye color is carried on the X chromosome and
the Y chromosome lacks an allele for the eye-color gene. Thus, a Y
chromosome cannot contribute an allele only an X chromosome. This
means that if a fly has the trait for having white eyes and is a male, there is
no chance of having a dominant allele to give the fly the red eye color.
Morgan called genes located on the X chromosome X-linked genes. He called
genes located on the Y chromosome Y-linked genes. The term sex-linked
trait refers sex to a trait coded for by an allele on a sex chromosome. Since
the X chromosome is larger than the Y chromosome, there are more X-linked
than Y-linked trait.
Linked Genes
Morgan and other geneticists hypothesized that if genes are inherited
together, it must be because they are located on the same chromosome.
Morgan studied two fly genes, one for body color and one for wing length
located on the same autosome. He first crossed a homozygous (GGLL) fly
with another homozygous fly (ggll). Their offspring had the genotype GgLl
and were gray with long wings. When he crossed that generation, they did
not occur in the phenotypic ratio of 9:3:3:1, which meant they were not
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assorted independently, and therefore located on the same chromosome. He
called pairs of genes that tend to be inherited together genes, linked genes
and called a set of linked genes a linkage group. He also noticed that some
offspring were unlike either parent, with gray bodies and short wings or with
black bodies and long wings. He realized that mutations were too rare to
have been the cause of these exceptions, and inferred that this was due to
crossing-over, the exchange of pieces of DNA between homologous
chromosomes. crossing over - the exchange of pieces of dna between
homoloous chromosomes. crossing over during the first division of meiosis
does not delete or create new genes. instead it just rearranges them.
Chromosome Mapping
The closer two genes are located on a chromosome, the more likely it is that
they will cross-over together. Recombinants are offspring that do not look
like their parents. The lower the recombination frequency, the closer the
genes for those traits must lie on a chromosome, because they will cross
over together. Researchers conduct breeding experiments and use the
resulting data to prepare a chromosome map a diagram that map, shows the
linear order of genes. CHROMOSOME MAP - a diagram that shows the
linear order of genes.Alfred H. Sturtevant, one of Morgans students, made
the first chromosome map for flies. To prepare his map, he compared the
frequency of crossingover for several genes. He defined one map unit as a
frequency of crossing-over 1 percent. A map of the human X chromosome
can be made using more recent techniques to map genes.
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MUTATIONS
cystic fibrosis results in mutations/ down syndrome.
A mutation is a change in the nucleotide-base sequence of a gene or DNA
molecule. Germ-cell mutations occur in an organisms gametes. These
mutations do not affect an organism itself, but can be passed on to offspring.
Somatic-cell mutations take place in an organisms body cells and can
therefore affect the organism, but not an organism offspring. They cannot be
inherited. Lethal mutations cause death, often before birth. Some
mutations result in phenotypes that are beneficial to an organism.
Organisms with beneficial mutations will thus have an evolutionary
advantage and have a greater chance of surviving and reproducing.
Mutations provide the variations upon which natural selection acts, and can
involve an entire chromosome or a single DNA molecule.
Chromosome Mutations
Chromosome mutations involve changes in the structure of a chromosome or
the loss or gain of a chromosome. A deletion is the loss of a piece of a
chromosome due to breakage. In an inversion a piece of the chromosome
breaks off, flips around backward, and reattaches. In a translocation a
piece of one chromosome breaks off translocation, and reattaches to a
nonhomologous chromosome. In nondisjunction,( down syndrome- 3 copies
of chromosome 21) a chromosome nondisjunction Human X Chromosome
fails to separate from its homologue during meiosis. Thus, one gamete will
have an extra copy of a chromosome while another will have no copies.
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Gene Mutations
The substitution, addition, or removal of a single nucleotide is a point
mutation mutation, which is a change that occurs within a single gene or
other segment of DNA on a chromosome. In a substitution one nucleotide
replaces another. If this occurs in a substitution, codon, then the amino acid
that is supposed to be produced can be changed. In a deletion mutation,
one or more nucleotides are lost. This loss can cause incorrect grouping of
the remaining codons, called a frameshift mutation making all amino acids
mutation, after the deletion change. Insertion mutations in which one or
more nucleotides are mutations, added, can also result in a frameshift
mutation.
Pedigrees
A pedigree is a diagram that shows how a trait is inherited through several
generations. In a pedigree, squares stand for males and circles stand for
females. A filled symbol means that an individual has the trait or condition.
An empty symbol means they do not. A horizontal line joining a male and a
female indicate a mating, and a vertical line indicates offspring, arranged
from left to right in order of birth. Roman numerals label different
generations. Patterns of Inheritance By analyzing patterns of inheritance, the
expression of genes over generations, biologists can learn about genetic
diseases. If a trait is autosomal, it will appear in both sexes equally and if it is
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sex-linked, it is usually seen only in males.
If a trait is autosomal dominant, every individual with the trait will have a
parent with the trait. If it is recessive, an individual with the trait can have
one, two, or neither parent exhibit the trait. If individuals with autosomal
traits are homozygous dominant or heterozygous, then their phenotype will
show the dominant characteristic. If individuals are homozygous recessive,
their phenotype will show the recessive characteristic. Two people whoa re
heterozygous carries of a recessive mutation will not show the mutation, but
can produce offspring who are homozygous for the recessive allele.
Individuals that have one copy of a recessive allele but do not have the
disease are called carriers and can pass the allele to their offspring. carriers,
GENETIC TRAITS AND DISORDERS Genetic disorders are diseases or disabling
conditions with a genetic basis.
Polygenic Inheritance Most human characteristics are polygenic influenced
by several genes. polygenic: Polygenic characters show many degrees of
variation. Eye color, height, skin color, and hair color are all polygenic
characters. Complex Characters Many human conditions are complex
characters affected by both the environment and by genes. One example
is skin color. Even if your genes determine that you have fair skin, if you go
out in the sun for a long period of time, then your skin will become darker
due to the environment (sun). Other complex characters play a role in
diseases or conditions such as breast cancer, diabetes, stroke, heart disease,
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and schizophrenia. By identifying the environmental components that
contribute to a disease, they can educate people in ways that minimize their
risk of developing the disease. Multiple Alleles Genes with three or more
alleles are said to have multiple alleles In humans, the alleles. ABO blood
types are controlled by the alleles IA, IB, and i. The alleles IA and IB are
codominant. In codominance, both alleles are expressed in codominance the
phenotype of a heterozygote. Those two alleles are both dominant to the
recessive i allele. Combinations of the three different alleles can produce
four different blood types A, B, AB, and O.
Incomplete Dominance Sometimes, an individual displays a trait
intermediate between the two parents, a condition known as incomplete
dominance For example, the child of a dominance. straight-haired parent
and a curly-haired parent would have wavy hair.
X-Linked Traits Some complex characters are determined by X-linked genes,
and a pedigree will show many affected males and no affected females. One
form of colorblindness is a recessive X-linked disorder in which an individual
cannot distinguish certain colors. Sex-Influenced Traits Sex-influenced traits
are involved in other complex characters. Males and Sexfemales can show
different phenotypes even when they share the same genotype. Sex-
influenced traits are usually autosomal. For example, an allele that is
dominant in males is recessive in females, due to higher levels of the
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hormone testosterone in men. Single-Allele Traits A single allele of a gene
controls single-allele traits. Huntingtons disease is Huntington an
autosomal dominant condition characterized by forgetfulness and irritability.
DETECTING GENETIC DISEASE Many people with a family history of a genetic
disease seek genetic screening before having children. Genetic screening is
an examination of a persons genetic makeup. Physicians can now detect
more than 200 genetic disorders in the fetus. The technique called
amniocentesis allows a physician to remove some amniotic fluid from the sac
that surrounds the fetus. By examining chromosomes and proteins in the
fluid, geneticists can analyze fetal cells for genetic disease. In chorionic villi
sampling the physician takes a sample of the chorionic villi, sampling, cells
derived from the zygote that grows between the mothers uterus and
placenta. Both procedures allow technicians to analyze fetal cells,
chromosomes, proteins, and detect genetic disease.
Genetic Counseling
Many people with a family history of a genetic disease also undergo genetic
counseling, counseling the process of informing a person or couple about
their genetic makeup. This is a form of medical guidance that informs
individuals about problems that may affect their offspring.
TREATING GENETIC DISEASE
Physicians treat genetic diseases in several ways. For many diseases, they
can treat just the symptoms, such as PKU and cystic fibrosis. Gene Therapy
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Another level of treatment currently in development involves replacing the
defective gene, called gene therapy which places a healthy copy of a gene
into the therapy, cells of a person whose copy of the gene is defective. Gene
therapy in which only body cells are altered is called somatic cell gene
therapy. This contrasts with germ cell gene therapy, the attempt to alter
eggs or sperm. This can affect future generations in unpredictable ways,
therefore, it poses more risks and ethical issues.
Section 1: Chromosomes and Inheritance Genes reside on chromosomes. Sex
chromosomes contain genes that determine an organisms sex. The
remaining chromosomes that are not directly involved in determining the sex
of an individual are called autosomes. In mammals, an individual carrying
two X chromosomes is female. An individual carrying an X and a Y
chromosome is male. Genes found on the X chromosome are X-linked genes.
A sex-linked trait is a trait whose allele is located on a sex chromosome.
Because males have only one X chromosome, a male who carries a recessive
allele on the X or Y chromosome will exhibit the sex-linked condition. Pairs of
genes that tend to be inherited together are called linked genes. They occur
close to each other on the same chromosome. The farther apart two genes
are located on a chromosome, the more likely a cross-over will occur.
Researchers use recombinant percentages to construct chromosome maps
showing relative gene positions. Germ-cell mutations occur in gametes and
can be passed on to offspring. Somatic-cell mutations occur in body cells and
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affect only the individual organism. Chromosome mutations are changes in
the structure of a chromosome or the loss or gain of an entire chromosome.
Gene mutations are changes in one or more of the nucleotides in a gene.
Human Genetics Geneticists use pedigrees to trace diseases or traits through
families. Pedigrees reveal inheritance patterns of genes. A carrier has one
copy of a recessive allele but does not express the trait. Polygenic
characters, such as skin color, are controlled by two or more genes. Complex
characters, such as height, are influenced by both genes and environment.
Multiple-allele characters, such as ABO blood groups, are controlled by three
or more alleles of a gene. The gene for colorblindness, an X-linked recessive,
is found on the X chromosome. A sex-influenced trait, such as pattern
baldness, is expressed differently in men than in women even if it is on an
autosome and both sexes have the same genotype. Genetic screening
examines a persons genetic makeup and potential risks of passing
disorders to offspring. Amniocentesis and chorionic villi sampling help
physicians test a fetus for the presence of genetic disorders. Genetic
counseling informs screened individuals about problems that might affect
their offspring. Genetic disorders are treated in various ways. Among the
treatments are symptomrelieving treatments and symptom-prevention
measures, such as insulin injections for diabetes. Gene therapy is a type of
treatment under development. IN gene therapy, a defective gene is replaced
with a copy of a healthy gene. Somatic cell gene therapy alters only body
cells. Germ cell gene therapy attempts to alter eggs or sperm.
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