The Study of

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GENETICS

• The Study of
• Inheritance

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Gregor Mendel The Father of Genetics
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Garden Peas
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Traits studied by Mendel
round
wrinkled
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Crosses by Mendel
round
wrinkled
All round
¾ round
¼ wrinkled
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Mendels Laws

• Gregor Mendel was an Austrian monk who in 1860
  developed certain laws of heredity after doing
  crosses between garden pea plants.
• Gregor Mendel investigated genetics at the
  organism level.
• Examples of traits that can be observed at the
  organism level include facial features that cause
  generations to resemble each other.

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Gregor Mendel

• Gregor Mendel combined his farmers skills with
  his training in mathematics.
• Mendels law of segregation states that each
  individual has two factors (called genes today)
  for each trait.
• Alternative forms of a gene having the same
  position on a pair of homologous chromosomes and
  affecting the same trait are now referred to as
  alleles.

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• Today we know that alleles occur at the same loci
  (position) on a chromosome.
• The factors segregate during the formation of the
  gametes and each gamete has only one factor from
  each pair.
• Fertilization gives each new individual two
  factors again.

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Gene locus
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The Inheritance of a Single Trait

• A capital letter indicates a dominant allele
• An example is W for widows peak.
• A lowercase letter indicates a recessive allele
• An example is w for continuous hairline.

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Widows peak
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Genotype and Phenotype

• Genotype refers to the genes of an individual
  which can be represented by two letters or by a
  short descriptive phrase.
• Homozygous means that both alleles are the same
• Heterozygous means that the members of the
  allelic pair are different

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• Phenotype refers to the physical or observable
  characteristics of the individual.
• Both WW and Ww result in widows peak, two
  genotypes with the same phenotype.

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Gamete Formation

• Because homologous pairs separate during meiosis,
  a gamete has only one allele from each pair of
  alleles.
• If the allelic pair is Ww, a gamete would contain
  either a W or a w, but not both.
• Ww represents the genotype of an individual.
• Gametes are represented by W or w.

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One-Trait Crosses

• In one-trait crosses, only one trait such as type
  of hairline is being considered.
• When performing crosses, the original parents are
  called the parental generation, or the P
  generation.
• All of their children are the filial generation,
  or F generation.
• Children are monohybrids when they are
  heterozygous for one pair of alleles.

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• If you know the genotype of the parents, it is
  possible to determine the gametes and use a
  Punnett square to determine the phenotypic ratio
  among the offspring.
• When a monohybrid reproduces with a monohybrid,
  the results are 3 1.
• This ratio is used to state the chances of a
  particular phenotype.
• A 3 1 ratio means that there is a 75 chance of
  the dominant phenotype and a 25 chance of the
  recessive phenotype.

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Monohybrid cross
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One-Trait Crosses and Probability

• Laws of probability alone can be used to
  determine results of a cross.
• The laws are
• (1) the probability that two or more independent
  events will occur together is the product of
  their chances occurring separately, and
• (2) the chance that an event that can occur in
  two or more independent ways is the sum of the
  individual chances.

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• In the cross of Ww x Ww, what is the chance of
  obtaining either a W or a w from a parent?
• Chance of W ½, or chance of w ½
• The probability of these genotypes is
• The chance of WW ½ x ½ ¼
• The chance of Ww ½ x ½ ¼
• The chance of wW ½ x ½ ¼
• The chance of ww ½ x ½ ¼
• The chance of widows peak (WW, Ww, wW) is ¼ ¼
  ¼ ¾ or 75.

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The One-Trait Testcross

• It is not always possible to discern a homozygous
  dominant from a heterozygous individual by
  inspection of phenotype.
• A testcross crosses the dominant phenotype with
  the recessive phenotype.
• If a homozygous recessive phenotype is among the
  offspring, the parent must be heterozygous.

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One-trait testcross
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The Inheritance of Many Traits

• Independent Assortment
• The law of independent assortment states that
  each pair of alleles segregates independently of
  the other pairs and all possible combinations of
  alleles can occur in the gametes.
• This law is dependent on the random arrangement
  of homologous pairs at metaphase.

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Segregation and independent assortment
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Two-Trait Crosses

• In two-trait crosses, genotypes of the parents
  require four letters because there is an allelic
  pair for each trait.
• Gametes will contain one letter of each kind in
  every possible combination.
• When a dihybrid reproduces with a dihybrid the
  results are 9 3 3 1.

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Dihybrid cross
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Two-Trait Crosses and Probability

• It is possible to use the two laws of probability
  to arrive at a phenotypic ratio for a two-trait
  cross without using a Punnett square.
• The results for two separate monohybrid crosses
  are as follows
• Probability of widows peak ¾
• Probability of short fingers ¾
• Probability of straight hairline ¼
• Probability of long fingers ¼

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• The probabilities for the dihybrid cross
• Probability of widows peak and short fingers ¾
  x ¾ 9/16
• Probability of widows peak and long fingers ¾
  x ¼ 3/16
• Probability of straight hairline and short
  fingers ¼ x ¾ 3/16
• Probability of straight hairline and long fingers
  ¼ x ¼ 1/16

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The Two-Trait Testcross

• A testcross is done when it is not known whether
  a dihybrid individual is homozygous dominant or
  heterozygous for both or one of the traits under
  consideration.
• A cross of a person heterozygous for both traits
  with a homozygous recessive person produces a 1
  1 1 1 ratio.

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Two-trait testcross
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Genetic Disorders

• Patterns of Inheritance
• When studying human disorders, biologists often
  construct pedigree charts to show the pattern of
  inheritance of a characteristic within a family.
• The particular pattern indicates the manner in
  which a characteristic is inherited.

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• Pedigree charts represent males as squares and
  females as circles.
• Recessive and dominant alleles have different
  patterns of inheritance.
• Genetic counselors construct pedigree charts to
  determine the mode of inheritance of a condition.

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Autosomal recessive pedigree chart
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Autosomal dominant pedigree chart
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Autosomal Recessive Disorders

• Tay-Sachs Disease
• Tay-Sachs disease is common among United States
  Jews of central and eastern European descent.
• An affected infant develops neurological
  impairments and dies by the age of three or four.
  
• Tay-Sachs results from a lack of hexosaminidase A
  and the storage of its substrate in lysosomes.

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Cystic Fibrosis

• Cystic fibrosis is the most common lethal genetic
  disorder among Caucasians.
• A chloride ion transport protein is defective in
  affected individuals.
• Normally when chloride ion passes through a
  membrane, water follows.
• In cystic fibrosis patients, a reduction in water
  results in a thick mucus which accumulates in
  bronchial passageways and pancreatic ducts.

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Cystic fibrosis therapy
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Phenylketonuria (PKU)

• Individuals with phenylketonuria lack an enzyme
  needed for the normal metabolism of
  phenylalanine, coded by an allele on chromosome
  12.
• Newborns are regularly tested for elevated
  phenylalanine in the urine.
• If the infant is not put on a phenylalanine-restri
  ctive diet in infancy until age seven when the
  brain is fully developed, brain damage and severe
  mental retardation result.

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Autosomal Dominant Disorders

• Neurofibromatosis
• Small benign tumors, made up largely of nerve
  cells, occur under skin or on various organs.
• The effects can range from mild to severe, and
  some neurological impairment is possible this
  disorder is variably expressive.
• The gene for this trait is on chromosome 17.

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Huntington Disease

• Individuals with Huntington disease experience
  progressive degeneration of the nervous system
  and no treatment is presently known.
• Most patients appear normal until middle age.
• The gene coding for the protein huntingtin
  contains many more repeats of glutamines than
  normal.

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Huntington disease
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Beyond Simple Inheritance Patterns

• Polygenic Inheritance
• Polygenic traits are governed by more than one
  gene pair.
• Several pairs of genes may be involved in
  determining the phenotype.
• Such traits produce a continuous variation
  representing a bell-shaped curve.

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Polygenic inheritance
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Skin Color

• The inheritance of skin color, determined by an
  unknown number of gene pairs, is a classic
  example of polygenic inheritance.
• A range of phenotypes exist and several possible
  phenotypes fall between the two extremes of very
  dark and very light.
• The distribution of these phenotypes follows a
  bell-shaped curve.

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Polygenic Disorders

• Many human traits, like allergies, schizophrenia,
  hypertension, diabetes, cancers, and cleft lip,
  appear to be due to the combined action of many
  genes plus environmental influences.
• Many behaviors, such as phobias, are also likely
  due to the combination of genes and the effects
  of the environment.

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Multiple Allelic Traits

• Inheritance by multiple alleles occurs when more
  than two alternative alleles exist for a
  particular gene locus.
• A persons blood type is an example of a trait
  determined by multiple alleles.
• Each individual inherits only two alleles for
  these genes.

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Multiple Alleles

• Some traits are controlled by more than two
  alleles that can be inherited. Each person only
  inherits two of those alleles one from each
  parent.
• This is the case with human blood types
• There are 3 alleles in the population
• IA allele
• IB allele
• i allele recessive to the
• other alleles

Incompletely dominant to each other
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ABO Blood type crosses

• The key is complicated
• IAIA or IAi type A
• IBIB or IBi type B
• IAIB type AB
• ii type O

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ABO Blood Types

• A person can have an allele for an A antigen
  (blood type A) or a B antigen (blood type B),
  both A and B antigens (blood type AB), or no
  antigen (blood type O) on the red blood cells.
• Human blood types can be type A (IAIA or IA i),
  type B (IBIB or IBi), type AB (IAIB), or type 0
  (ii).

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A Blood test will show which type you are
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Inheritance of blood type
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Incompletely Dominant Traits

• Codominance means that both alleles are equally
  expressed in a heterozygote.
• Incomplete dominance is exhibited when the
  heterozygote shows not the dominant trait but an
  intermediate phenotype, representing a blending
  of traits.
• Such a cross would produce a phenotypic ratio of
  1 2 1.

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Incomplete dominance
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Sickle-Cell Disease

• Sickle-cell disease is an example of a human
  disorder controlled by incompletely dominant
  alleles.
• Sickle cell disease involves irregular, sickle
  shaped red blood cells caused by abnormal
  hemoglobin.
• HbA represents normal hemoglobin and HbS
  represents the sickled condition.

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Many Genetic Diseases are Autosomal Recessive
Traits
Whats an autosome?
Sickle cell anemia is a recessive autosomal
disease common in areas where malaria is endemic.
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What Works in Peas Works (genetically speaking)
Works in People
¼ of offspring of two carriers of a recessive
allele are expected to show the recessive trait
½ of offspring are expected to be carriers.
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• HbAHbA individuals are normal HbSHbS individuals
  have sickle-cell disease and HbAHbS individuals
  have the intermediate condition called
  sickle-cell trait.
• Heterozygotes have an advantage in
  malaria-infested Africa because the pathogen for
  malaria cannot exist in their blood cells.
• This evolutionary selection accounts for the
  prevalence of the allele among African Americans.

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Sex-Linked traits gene is found on
X-chromosome, not the Y

• Genes for colorblindness
• and hemophilia are
• common in humans
• Females have two genes
• for this trait
• Males have only one gene
• so they have no chance
• to be a carrier

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Sex-Linked Traits

• Traits controlled by genes on the X or Y
  chromosomes are sex-linked although most are
  unrelated to gender.
• An allele on the X chromosome that is in the
  region where the Y chromosome has no alleles will
  express even if recessive it is termed X-linked.
  
• A female would have to have two recessive genes
  to express the trait a male would only need one.

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X-Linked Alleles

• The key for an X-linked problem shows the allele
  attached to the X as in
• XB normal vision
• Xb color blindness.
• Females with the genotype XBXb are carriers
  because they appear to be normal but each son has
  a 50 chance of being color blind depending on
  which allele the son receives.
• XbXb and XbY are both colorblind.

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Cross involving an X-linked allele
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X-Linked Disorders

• In pedigree charts that show the inheritance
  pattern for X-linked recessive disorders, more
  males than females have the trait because
  recessive alleles on the X chromosome are
  expressed in males.
• A grandfather passes an X-linked recessive
  disorder to a grandson through a carrier
  daughter.
• X-linked recessive disorders include red-green
  color blindness, muscular dystrophy, and
  hemophilia.

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X-linked recessive pedigree chart
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Hemophilia

• Hemophilia refers to the lack of one of several
  clotting factors that leads to excessive bleeding
  in affected individuals.
• Hemophiliacs bleed externally after injury, but
  also bleed internally around joints.
• Hemorrhages can be stopped with blood
  transfusions or a biotechnology clotting factor.

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Sex-linked Traits hemophilia pedigree in royal
families of Europe
Current royalty in England
Russian family before revolution
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Are you colorblind?Can you see the shapes?
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X-linked Inheritance When Men and Woman Play by
Different Rules
Behind the 8-ball? Colorblindness is an X-linked
recessive trait.
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Color Blindness

• Three types of cones are in the retina detecting
  red, green, or blue.
• Genes for blue cones are autosomal those for red
  and green cones are on the X chromosome.
• Males are much more likely to have red-green
  color blindness than females.
• About 8 of Caucasian men have red-green color
  blindness.

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X-linked Inheritance
There are many X-linked recessive traits.
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• Would you expect any of their children to be
  colorblind?
• What happens when the carrier daughters grow up
  and marry men who have normal vision?
• Do that cross at your seats.Can they have
  colorblind children? Colorblind daughters?
• Colorblind sons?

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Punnett Square for colorblindness
Cross XBXB x XbY
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Sex-Linked Traits the gene for that trait is on
the X-chromosome, not on the Y-chromosome

• Ex in humans, both colorblindness and hemophilia
  are sex-linked
• Key for colorblindness
• XBXB normal visioned female
• XBXb normal visioned female
• XbXb colorblind female
• XBY normal visioned male
• XbY colorblind male

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Muscular Dystrophy

• Muscular dystrophy is characterized by the
  wasting of muscles.
• The most common form is Duchenne muscular
  dystrophy this is an X-linked disorder,
  occurring in 1 of 3,600 males.
• Muscles weaken, frequent falls and difficulty in
  rising occur early death occurs by age 20.

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• Duchenne muscular dystrophy involves the absence
  of a protein called dystrophin that is involved
  in the release of calcium from the sarcoplasmic
  reticulum of muscle cells.
• The lack of dystrophin causes calcium to leak
  into the cell, which promotes the action of an
  enzyme that dissolves muscle fibers.
• A test is now available to determine the carriers
  of Duchenne muscular dystrophy.

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Chapter Summary

• Alleles are alternative forms of a gene located
  at one site on a chromosome alleles determine
  the traits of individuals.
• Chromosomes and their alleles separate and assort
  independently when gametes form this increases
  variety among offspring.

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• Many genetic disorders and other traits are
  inherited according to laws first established by
  Gregor Mendel.
• Inheritance is often more complex, providing
  exceptions to Mendels laws but helping to
  explain an even wider variety in patterns of gene
  inheritance.

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Chapter Summary

• Humans normally inherit 22 pairs of autosomes and
  one pair of sex chromosomes for a total of 46
  chromosomes.
• Abnormalities arise when humans inherit an extra
  or missing autosome or abnormal autosomes.
• In humans, males are normally XY and females are
  XX.

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• Abnormalities arise when humans inherit an
  incorrect number of sex chromosomes.
• Traits unrelated to the gender of an individual
  are controlled by genes located on the sex
  chromosomes.
• Males express X-linked recessive disorders
  because they inherit only one X chromosome.
• Genes that occur on the same chromosome form a
  linkage group and tend to be inherited together.