A1258690777miUTs

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Patterns of Inheritance

• Chapter 9

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Heredity

• Genetics- the science of heredity
• True-breeding individuals (purebred lines) are
  essential in genetic research
• In purebred lines , it is easier to predict
  genetic probabilities because the family line all
  has a very similar genetic makeup
• If lines are not purebred, it is more difficult
  to predict genetic outcome because so many
  variations may occur

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Heredity

• Until the 20th century, many biologists
  erroneously believed that
• Particles called pangenes traveled from each part
  of an organisms body to the eggs and sperm and
  are then passed on to the offspring
• Characteristics acquired during lifetime could be
  passed on to offspring
• Characteristics of both parents blended
  irreversibly in their offspring

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Mendelian Genetics

• Modern genetics began with Gregor Mendels
  quantitative genetic experiments
• Austrian monk
• Used peas because
• Easy to grow and available
• Many distinguishable characteristics
• Self-fertilization

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Mendel

• Mendel crossed pea plants that differed in
  certain characteristics and traced the traits
  from generation to generation
• Controlled breeding through cross-fertilization

HYBRID
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Mendel

• Seven pea characteristics
• Made true-breeding lines
• Looked at what would happen when breeding certain
  lines to each other
• Examined type and number of progeny

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Mendels Principle of Segregation

• Four Hypotheses
• There are alternative forms of genes, the units
  that determine heritable traits.
• These alternate forms of genes are called
  alleles.
• Examples
• Blue eyes or brown eyes.
• Just like the variations of genes on homologs.

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Mendels Principle of Segregation

• For each inherited characteristic, an organism
  has two genes, one from each parent. These
  genes may both be the same allele, or they may be
  different alleles
• Blonde hair from mother, brown hair from father

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Mendels Principle of Segregation

1. A sperm or egg carries only one allele for each
   inherited trait because allele pairs separate
   from each sperm and egg during the production of
   gametes. During fertilization when the egg and
   the sperm unite the paired condition is restored
   in the offspring

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Mendels Principle of Segregation

1. When the two genes of a pair are different
   alleles and one is fully expressed while the
   other has no noticeable effect on the organisms
   appearance, the alleles are called the dominant
   allele and recessive allele, respectively

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Mendels Principle of Segregation

• Phenotype- an organisms expressed trait
• Purple or white flowers
• Genotype- an organisms genetic makeup
• Which alleles it contains
• An organism that has a pair of identical alleles
  for a characteristic is called homozygous for
  that trait
• PP or pp
• An organism with two different alleles for a
  trait is called heterozygous for that trait
• Pp

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Homologous Chromosomes and Alleles

• Homologous chromosomes carry different versions
  (different alleles) for the same trait
• These alleles reside at the same locus on
  homologous chromosomes

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Principle of Independent Assortment

• Mendel wondered if traits were inherited
  separately or if certain traits were inherited as
  a package
• By looking at two characteristics at once, Mendel
  found that the alleles of a pair segregate
  independently of other allele pairs during gamete
  formation
• This is known as the principle of independent
  assortment

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Principle of Independent Assortment
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Test Cross

• If the genotype of an organism is not known,
  scientists can use controlled breeding to reveal
  the genotype
• The offspring of a testcross often reveal the
  genotype of an individual when it is unknown

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Probability and Inheritance

• Inheritance follows the rules of probability
• The rule of multiplication and the rule of
  addition can be used to determine the probability
  of certain events occurring
• Independent event- if the outcome of an event is
  not dependant on the outcome of another it is
  independent
• Use the rule of addition and add the
  probabilities
• Puppy is black OR fuzzy
• Dependent event you multiply
• Puppy is black AND fuzzy

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Probability and Inheritance

• The probability that an event can occur in two or
  more alternative ways is the sum of the separate
  probabilities of the different ways this is
  known as the rule of addition
• Example What is the probability that a puppy
  from a litter is brown or has long hair?
• X probability that the dog is brown
• Y probability that the dog has long hair
• XY probability that the dog is brown or has
  long hair

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Using Pedigrees to Determine Inheritance

• Family pedigrees are used to determine patterns
  of inheritance and individual genotypes
• This is like a test cross

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Inherited Disorders

• Most human genetic disorders are recessive
• Offspring born to normal parents that are
  heterozygous for the gene
• Can use pedigree analysis to determine the
  probability
• Can use this with any gene that is controlled by
  one gene
• Examples Cystic fibrosis, albinism
• Increase in the risk of birth defects is shown
  with inbreeding

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Inherited Disorders

• Some disorders are due to dominant alleles
• Examples extra toes and fingers
• Achondroplasia- dwarfism
• Homozygous dominant causes death, homozygous
  recessive does not have dwarfism, only
  heterozygous individuals become dwarves

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Fetal Testing for Disorders

• Karyotyping and biochemical tests of fetal cells
  and molecules can help people make reproductive
  decisions
• Fetal cells can be obtained through amniocentesis

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Genotype and Phenotype

• Mendels principles of inheritance are valid for
  all sexually reproducing species, but inheritance
  is not always this predictable
• Cases which include types of gene dominance,
  alternate alleles and other special cases can
  alter whether the genotype will dictate the
  phenotype seen in the speices.

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Incomplete Dominance

• When the offspring shows a phenotype that is in
  between the phenotypes of the parents this is
  called incomplete dominance

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Genes With More Than Two Alleles

• In a population, multiple alleles often exist for
  a characteristic
• The three alleles for ABO blood type in humans is
  an example
• A, B and O are representative of a carbohydrate
  present on the red blood cells of the person
• This is why you have to receive the right blood
  type
• Antibodies attack the wrong carbohydrate

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Blood Types
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A Single Gene May Have Many Phenotypic Affects

• In many cases, one gene influences several
  characteristics. This is called pleiotropy.
• An example is the gene for sickle-cell disease.
  This one gene when it is present in both
  homologous chromosomes causes many symptoms
• Directly causes red blood cells to produce
  abnormal hemoglobin molecules

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A Single Gene May Have Many Phenotypic Affects
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Genetic Testing for Disease-Causing Alleles

• Genetic testing can be of value to those at risk
  of developing a genetic disorder or of passing it
  on to offspring
• Parental testing before conception, prenatal
  testing during pregnancy and testing right after
  birth
• For families that have a history of a certain
  disease
• Problems with how these tests are used

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Characteristics Influenced by Many Genes

• Some characteristics are influenced by many genes
  such as skin color and height
• Many of these features come from the additive
  effects of two or more genes, this is called
  polygenic inheritance
• This can lead to many variations such as there
  are in skin color and height

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Polygenic Inheritance
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Environmental Effects

• Phenotypic variations are influenced by the
  environment
• Skin color is affected by exposure to sunlight
• Susceptibility to diseases, such as cancer, has
  hereditary and environmental components

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Chromosome Behavior and Mendels Principles

• Mendel published his work in the 1800s and soon
  after scientists worked out the mechanisms for
  mitosis and meiosis
• They observed the correlation between chromosome
  behavior and gene frequency
• Chromosome Theory of Inheritance- genes are
  located on chromosomes and that the behavior of
  chromosomes during meiosis and fertilization
  accounts for inheritance patterns

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Chromosome Behavior and Mendels Principles
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Linked Genes

• Genes that are located close together on the
  chromosome and tend to be inherited together are
  called linked genes
• Do not follow Mendels principles

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New Alleles Produced by Crossing Over

• Crossing over recombines linked genes into
  assortments of alleles that are not found in the
  parent
• Produces new recombinant gametes

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Mapping Genomes Using Crossover Data

• Crossing over is more likely to occur between
  genes that are farther apart
• Recombination frequencies can be used to map the
  relative positions of genes on chromosomes

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Mapping Genomes Using Crossover Data
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Sex Determination

• A human male has one X chromosome and one Y
  chromosome
• A human female has two X chromosomes
• Whether a sperm cell has an X or Y chromosome
  determines the sex of the offspring

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Sex Determination in Other Species

• The X-O system
• The Z-W system
• Chromosome Number

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Sex Determination in Other Species

• Not all organisms have separate sexes
• Most plant species and some animal species
  produce both sperm and eggs
• Plants of this type are said to be monoecious
• Animals that produce both sperm and eggs are said
  to be hermaphroditic
• Earthworms
• Garden Snails

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Sex-linked Genes

• Sex chromosomes also carry genes, any gene
  located on the sex chromosome is called a
  sex-linked gene
• Most are found on X chromosomes

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Sex-linked Disorders

• Most sex-linked human disorders are due to
  recessive alleles
• Examples hemophilia, red-green color blindness
• These are mostly seen in males
• A male receives a single X-linked allele from his
  mother, and will have the disorder, while a
  female has to receive the allele from both
  parents to be affected

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Sex-linked Disorders

• A high incidence of hemophilia has plagued the
  royal families of Europe