Biology Chapter Twelve Notes

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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Biology Chapter Twelve Notes