NonMendelian Genetics

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NonMendelian Genetics

• Chapter 14 Mendel and the Gene Idea

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Complex patterns of inheritance

• The relationship between genotype and phenotype
  is rarely as simple as in Mendelian inheritance
  (controlled by dominant and recessive paired
  alleles)
• Principles of segregation and independent
  assortment apply to more complex patterns of
  inheritance
• Inheritance may deviate from simple Mendelian
  patterns in the following situations
• Alleles are
• A gene has
• A gene produces

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Incomplete dominance

• In complete dominance, heterozygous and
  homozygous dominant individuals have the
• With incomplete dominance, the phenotype of the
  heterozygous is
• This intermediate occurs because neither allele
  of the pair is completely dominant

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Incomplete Dominance

• If you cross a white flower with a red flower
  that exhibit incomplete dominance the first
  generation (heterozygotes) will be
• If you cross two of those heterozygotes you will
  get

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Incomplete dominance genetic problems

• We can still use the Punnett Square to solve
  problems involving incomplete dominance.
• The trick is to recognize when you are dealing
  with a question involving incomplete dominance.
• There are two steps to this ?
• 1) Notice that the offspring is showing a 3rd
  phenotype. The parents each have one, and the
  offspring are different from the parents. ?
• 2) Notice that the trait in the offspring is a
  blend (mixing) of the parental traits.

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Incomplete Dominance Questions

• 1. A cross between a black bird a white bird
  produces offspring that are grey. The color of
  birds is determined by just two alleles.
• a) What are the genotypes of the parent birds in
  the original cross? ?
• b) What is/are the genotype(s) of the grey
  offspring?
• ?
• c) What would be the phenotypic ratios of
  offspring produced by two grey birds?

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Incomplete Dominance Questions

• 2. The color of fruit for plant "X" is determined
  by two alleles. When two plants with orange
  fruits are crossed the following phenotypic
  ratios are present in the offspring 25 red
  fruit, 50 orange fruit, 25 yellow fruit.
• What are the genotypes of the parent
  orange-fruited plants?

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Codominance

• In codominance, two dominant alleles affect the
  phenotype in separate, distinguishable ways
• Codominant alleles cause the phenotypes of
  to be produced in heterozygote individuals.
• In codominance
• For example, red cows crossed with white will
  generate roan cows. Roan refers to cows that have
  

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Codominance

• The genetic gist to codominance is pretty much
  the same as incomplete dominance.
• A hybrid organism shows a --- not the usual
  "dominant" one not the "recessive" one.
• With incomplete dominance we get a blending of
  the dominant recessive traits so that the third
  phenotype is something in the middle (red x white
  pink).
• In codominance, the "recessive" "dominant"
  traits in the phenotype of hybrid
  organisms.
• red x white ---gt red white spotted

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Codominance Punnett Squares

• Some texts use letters superscripts when
  dealing with codominance.
• Others use different letters, noting the type of
  nonMendelian cross.
• Lets use the second method for our example
• R allele for red flowers ?
• W allele for white flowers
• red x white --gt red white spotted flowers
• RR x WW ----gt 100 RW
• The symbols you choose to use don't matter, in
  the end you end up with hybrid organisms, and
  rather than one trait (allele) dominating the
  other, both traits appear together in the
  phenotype.

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Codominance Questions

• 1. Predict the phenotypic ratios of offspring
  when a homozygous white cow is crossed with a
  roan bull.
• ?2. A cross between a black cat a tan cat
  produces a tabby pattern (black tan fur
  together). ?
• a) What pattern of inheritance does this
  illustrate? Why?
• b) What percent of kittens would have tan fur if
  a tabby cat is crossed with a black cat?

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Multiple Alleles

• It is common for
• Traits controlled by more than two alleles are
  said to have multiple alleles
• A diploid individual can possess of each
  gene

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Multiple Alleles
The number of alleles for any particular trait is
not limited to four, there are instances in which
more than 100 alleles are known to exist for a
single trait
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Multiple Alleles Blood Types

• Multiple Alleles govern blood type
• Human blood types are determined by the presence
  or absence of certain molecules on the surfaces
  of red blood cells called antigens
• As the determinant of blood type the gene I has
  three alleles IA, IB, and i
• IA (or A) allele produces
• IB (or B) allele produces
• i (or O) produces

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Importance of Blood Typing

• Incompatible blood types could clump together,
  causing death.
• Disputed parentage
• Example If a child has type AB blood and its
  mother has type A, a man with type O blood could
  not be the father.
• Why?

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Blood Typing Practice

• A woman with Type O blood and a man who is Type
  AB have are expecting a child. What are the
  possible blood types of the kid?
• What are the possible blood types of a child
  who's parents are both heterozygous for "B" blood
  type?
• What are the chances of a woman with Type AB and
  a man with Type A having a child with Type O?
• A test was done to determine the biological
  father of a child.The child's blood Type is A and
  the mother's is B. Man 1 has a blood type of O,
  Man 2 has blood type AB. Which man is the
  biological father?

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Pleiotropy

• Most genes have a property called
  pleiotropy
• For example, pleiotropic alleles are responsible
  for the multiple symptoms of certain hereditary
  diseases, such as cystic fibrosis and sickle-cell
  disease
• In the garden pea, gene for flower color also
  affects color of seed coat

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Epistasis

• In epistasis, a gene at one locus
• For example, in mice and many other mammals, coat
  color depends on
• One gene determines the pigment color (B for
  black and b for brown)
• The other gene (C for color and c for no color)
  determines whether the pigment will be deposited
  in the hair
• Dominance
• Epistasis

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Polygenic inheritancepoly many genic
genes

• contributes to a phenotype
• Effects of dominant alleles are additive
• More dominant genes
• Number of dominant determines phenotype
• are polygenic traits
• Many disorders may be polygenic
• Cleft palate, club foot, diabetes, schizophrenia,
  allergies, cancer

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Skin color example

• If skin color was related to 3 gene pairs
• Dominant gene A, B or C produces pigment
• Incompletely dominant to a, b or c
• So of dominant genes determines
• AABBCC
• AaBbCc
• aabbcc
• 2 heterozygotes (AaBbCc) could have a child with
  any pigment range

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Environmental Influences

• Genes are also influenced by the
• Temperature and Siamese cats
• The darker colors on the extremities are due to a
  
• Gene that codes for production of the pigment in
  the Siamese cat only functions
• Many diseases, such as heart disease and cancer,
  have both genetic and environmental components

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Pedigree

• A is a family tree that describes the
  interrelationships of parents and children across
  generations
• Inheritance patterns of particular traits can be
  traced
• Can also be used to make predictions about future
  offspring
• Many genetic disorders are inherited in a
• Recessively inherited disorders show up only in
  individuals
• are heterozygous individuals who carry the
  recessive allele but are phenotypically normal

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Pedigree Symbols
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Albinism

• Albinism is a recessive condition characterized
  by a
• If a recessive allele that causes a disease is
  rare, then the chance of two carriers meeting and
  mating is low
• (i.e., matings between close relatives)
  increase the chance of mating between two
  carriers of the same rare allele
• Most societies and cultures have laws or taboos
  against marriages between close relatives

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Cystic Fibrosis

• Recessive condition
• Cystic fibrosis is the most common lethal genetic
  disease in the US, striking one out of every
  
• The cystic fibrosis allele results in defective
  or absent
• Symptoms include mucus buildup in some internal
  organs and abnormal absorption of nutrients in
  the small intestine

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Sickle-cell disease

• Recessive condition
• Sickle-cell disease affects one out of
• The disease is caused by the substitution of a
  single amino acid in the hemoglobin protein in
  red blood cells
• Symptoms include physical weakness, pain, organ
  damage, and even paralysis

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Dominant Genetic Diseases

• Some human disorders are caused by dominant
  alleles
• Dominant alleles that cause a lethal disease are
  rare and arise by mutation
• is a form of dwarfism caused by a rare
  dominant allele
• is a degenerative disease of the nervous
  system caused by a dominant allele
• The disease has no obvious phenotypic effects
  until the individual is about 35 to 40 years of
  age

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Genetic Tests

• There are many genetic diseases that exist (way
  beyond the scope of what we will discuss)
• can provide information to prospective
  parents concerned about a family history for a
  specific disease
• Using family histories, they help couples
  determine the odds that their children will have
  genetic disorders
• For a growing number of diseases, tests are
  available that identify carriers and help define
  the odds more accurately

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Amniocentesis

• In amniocentesis, a long thin needle is used to
  remove
• The amniotic fluid contains , which can be
  tested for genetic diseases
• The DNA from fetal cells is

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Chorionic Villus Sampling

• In chorionic villus sampling (CVS), a sample of
  the chorionic villus is removed and tested
• The chorionic villus cells contain the ,
  making them fetal cells
• The DNA from fetal cells is

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Karyotypes

• Karyotypes (picture of chromosomes arrested
  during mitosis) are prepared, which determines

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Other Genetic Tests

• Other techniques, such as and ,
  allow fetal health to be assessed visually in
  utero
• Some genetic disorders can be detected at birth
  by simple tests that are now routinely performed
  in most hospitals in the US
• Phenylketonuria (PKU)
• Congential Hypothyroidism

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Review Questions

1. Name 3 examples of when inheritance patterns may
   not follow Mendelian rules.
2. Explain, identify, and solve genetics problems
   involving incomplete dominance, codominance,
   multiple alleles.
3. Complete genetics problems involving blood types.
4. Explain, differentiate between, and complete
   nontraditional genetics problems involving
   pleiotropy, epistasis, and polygenic inheritance.
5. Explain the effect of the environment on the
   expression of our genes.
6. Define and analyze a pedigree in order to answer
   inheritance questions.
7. Identify the most common pedigree symbols.
8. Identify the inheritance patterns and major
   characteristics of the following genetic
   conditions albinism, cystic fibrosis,
   sickle-cell disease, achondroplasia,
   Huntingtons disease.
9. Explain the purpose, benefits, and risks of
   genetic testing.
10. Differentiate between amniocentesis and chorionic
    villus sampling.
11. Explain the purpose and use of a karyotype.
12. List 3 pieces of information that can be obtained
    from a karyotype.