Human Genetics

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Human Genetics

• Chapter 12 Inheritance Patterns and Human
  Genetics

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Pedigrees

• Geneticists can study human genetic traits and
  trace genetic diseases from one generation to the
  next by studying the phenotypes of family members
  in a pedigree.
• A pedigree is a diagram that shows how a trait is
  inherited over several generations.

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Pedigrees
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Patterns of Inheritance

• Biologists learn about genetic diseases by
  analyzing patterns of inheritance.
• For Example
• If a trait is autosomal, it will appear in both
  sexes equally. If a trait is sex-linked it is
  usually only seen in males. Most sex-linked
  traits are recessive.
• If a trait is autosomal dominant, every
  individual with the trait will have a parent with
  the trait. If the trait is recessive, an
  individual with the trait can have one, two or
  neither parent exhibit the trait.

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Pedigrees

• Pedigrees can be analyzed to determine whether a
  trait is autosomal or sex-linked.
• Pedigrees can be analyzed to determine whether a
  trait is dominant or recessive.
• Individuals on a pedigree that have the allele
  but do not express the trait are known as
  carriers.

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Genetic Traits and Disorders

• Genes controlling human traits show many patterns
  of inheritance.
• Some of these genes cause genetic disorders.
• GENETIC DISORDERS are diseases or disabling
  conditions that have a genetic basis

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Polygenic Inheritance

• Most human characteristics (traits) are
  polygenic.
• Polygenic characters (traits) are influenced by
  several genes.
• Polygenic traits include height, nose length,
  size of foot, hair color, skin color, and eye
  color.

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Complex Characters

• Many human conditions are complex characters.
• Complex characters (traits) are influenced
  strongly by both the environment and by genes.
• Skin color and height are both polygenic and
  complex. Explain why.
• Heart disease, diabetes, cancer are also thought
  to be complex.

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Complex Characters

• Biologists hope that by identifying environmental
  components that contribute to a disease, they can
  educate people in ways that minimize their risk
  of developing the disease.

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Multiple Alleles

• Many genes have more than two alleles. Genes with
  three or more alleles are said to have multiple
  alleles.
• The ABO blood groups are governed by multiple
  alleles.

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Codominance

• Alleles that are codominant are expressed
  equally.
• The IA and IB alleles in the ABO blood group gene
  are both codominant since they both produce a
  corresponding antigen (protein) on the surface of
  erythrocytes (red blood cells).

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Incomplete Dominance

• Sometimes an individual displays a trait that is
  intermediate between two parents, a condition
  known as incomplete dominance.
• Example In Caucasians, the child of a
  straight-haired parent and a curly-haired parent
  would have wavy hair.

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X-linked Traits

• X-linked traits are carried on the X chromosome
  of the 23rd pair. This means that X-linked traits
  are carried on a sex chromosome.
• Colorblindness, Hemophilia are both X-linked
  traits.

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Sex-Influenced Traits

• Sex-influenced traits are involved in other
  complex characters.
• Males and females 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
  but recessive in females controls pattern
  baldness, the type of baldness usually found in
  men. The difference is due to higher levels of
  the hormone testosterone in men, which interacts
  with the genotype to produce pattern baldness.

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Detecting Genetic Disease

• Many people with a family history of genetic
  disease seek genetic screening before having
  children.
• Genetic Screening is an examination of a persons
  genetic makeup. It may involve karyotypes, blood
  tests for certain proteins, or direct test of
  DNA.
• Doctors can now also detect more than 200 genetic
  disorders in the fetus.

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Amniocentesis

• Performed between the 14th and 16th week of
  pregnancy.
• Allows a physician to remove some amniotic fluid
  from the amnion, the sac that surrounds the
  fetus.
• Geneticists can analyze fetal cells for genetic
  disease by examining chromosomes and proteins in
  the fluid.

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Amniocentesis
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Chorionic Villi Sampling (CVS)

• Performed between the 8th and 10th week of
  pregnancy.
• The physician takes a sample of the chorionic
  villi derived from the zygote that grow between
  the mothers uterus and the placenta.
• Technicians analyze the cells, chromosomes and
  proteins to detect genetic disease.

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Chorionic Villi Sampling (CVS)
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Genetic Counseling

• Many people with a family history of a genetic
  disease also undergo genetic counseling.
• Genetic counseling is the process of informing a
  person or couple about their genetic makeup.
• Genetic counseling is a form of medical guidance
  that informs individuals about problems that
  might affect their offspringl

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Treating Genetic Disease

• Physicians treat genetic diseases in several
  ways.
• For some diseases, physicians can implement
  symptom-prevention measures. Example insulin
  injections for diabetes.
• Physicians can even do some types of surgery to
  correct genetic defects in a fetus before birth.

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Gene Therapy

• Gene Therapy is a technique that replaces a
  defective gene with a healthy copy of the gene.
• A virus is used to inject the gene into the
  cells. The gene functions until the cells die.
  Gene Therapy needs to be repeated.
• Gene therapy, in which only body cells are
  altered, is called somatic cell gene therapy.
• Germ-cell gene therapy is the attempt to alter
  sperm or eggs. This form of gene therapy is
  highly controversial.

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Critical Thinking Review

• A woman with cystic fibrosis marries a man who is
  heterozygous for cystic fibrosis. What is the
  likelihood (probability) that their children will
  have cystic fibrosis?
• (Cystic fibrosis is autosomal recessive)
• (Give yourself a key, determine the genotypes of
  parents, perform the cross)

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Critical Thinking Review

• Why is colorblindness less common among females?

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ABO Blood Group

• A man with blood type B marries a woman with
  blood type A. Their first child is blood type O.
  What is the probability their next child will be
  blood type AB? Blood Type B?

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Analyzing a Pedigree
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Analyzing a Pedigree
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