Mendelian Genetics

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CHAPTER 10 Mendelian Genetics
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MENDELIAN GENETICS

• Genetics - branch of biology that studies how
  genetic characteristics are inherited
• First investigated by Gregor Mendel, an
  Augustinian monk (1822-1884)
• His work was not accepted until 1900 (30 years
  later)
• The study of inheritance is often called
  Mendelian genetics.

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GENE INHERITANCE

• Diploid organisms have two forms of a gene one
  from each parent
• Several different forms (alleles) of each gene
  may exist within a population
• Alleles are found at the same location on a
  chromosome (called the locus)
• Ex In humans there are two alleles for ear lobe
  shape (attached and unattached)

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Ear Lobe Shape
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GENE INHERITANCE

• An organisms genome is the set of all its genes.
• Genotype genetic makeup of the organism
• Phenotype how alleles are expressed (physical
  characteristics)
• Alleles can be described as
• Dominant trait that is expressed
• Designated by a capital letter (EE)
• Recessive trait is masked by dominant and not
  expressed
• Designated by a lowercase letter (ee)

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GENE INHERITANCE

• Homozygous - alleles are of the same type
• both are dominant or both are recessive
• AA, aa
• Heterozygous alleles are different
• one is dominant and the other is recessive
• Aa

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GENE INHERITANCE

• In homozygous individuals, the trait expressed
  depends on what alleles are present (AA, aa)
• Heterozygous individuals are known as carriers
• Only dominant allele is expressed or
• Trait will be expressed only under certain
  conditions
• Ex traits which are sex-linked
• The environment also can determine whether or not
  a trait is expressed
• nature vs. nurture

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• Dark color in cats is expressed on the parts of
  the body that stay cool.
• The gene for freckles expresses itself more fully
  when a person is exposed to the sun.

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GENE INHERITANCECODOMINANCE

• Both alleles are expressed (dominant)
• Ex petal color in snapdragons
• Homozygous plants (RR, WW) produce either red or
  white flowers
• Heterozygous plants (RW) produce pink colored
  flowers

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GENE INHERITANCEX-LINKED GENES

• Refers to genes found on X-chromosomes
• Also called sex-linked genes
• These genes are inherited together because they
  are found on the same chromosome called a
  linkage group
• Females get two copies of X-linked genes (XX),
  males get one copy (XY)

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GENE INHERITANCEX-LINKED GENES

• The Y is much smaller than the X chromosome, thus
  it has less genes.
• In men, because there is only one X, genes on the
  X-chromosomes will be expressed.
• X-linked genes with abnormal traits are
• color blindness
• Hemophilia
• brown teeth
• muscular dystrophy (Beckers and Duchennes)
• These are much more common in men

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SEX CHROMOSOMES
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MENDELS LAW OF HEREDITY

• Gregor Mendel developed a method to predict
  inheritance
• His work focused on
• Garden peas - determined dominant and recessive
  traits
• Obvious characteristics (Height, flower color,
  seed shape, etc.)
• One trait at a time
• Lead to the development of three laws

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MENDELS LAWS

• Law of dominance an allele that is expressed
  over another allele is said to be dominant.
• Law of segregation during meiosis, alleles
  separate from one another into different gametes,
  retaining their individuality
• Law of independent assortmentmembers of one gene
  pair separate independently of other gene pairs
  
• Do not apply to linked genes

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PROBABILITY

• Genetic problems are based on probability
• The chance that an event will happen
• In monohybrid crosses, a single trait is studied.
  
• Crossing tall plants with short plants, green
  peas with yellow peas, etc.
• Punnett squares are tools that are used to
  determine probability

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MONOHYBRID CROSSES

• A tall pea plant crossed with a tall pea plant
  will produce offspring that are all tall
• T T
• T
• T

• T T

• T T

• T T

• T T

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MONOHYBRID CROSSES

• A short pea plant crossed with a short pea plant
  will produce offspring that are all short
• t t
• t
• t

• t t

• t t

• t t

• t t

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MONOHYBRID CROSSES

• A tall pea plant crossed with a short pea plant
  will produce all tall offspring
• t t
• T
• T
• Tall is dominant - The phenotype is 100 tall.
• The genotype is 100 heterozygous tall (Tt)

• T t

• T t

• T t

• T t

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MONOHYBRID CROSSES

• A heterozygous tall pea plant crossed with a
  heterozygous tall pea plant will produce varying
  offspring
• T t
• T
• t
• One will be homozygous tall, two will be
  heterozygous tall and one will be homozygous
  short.
• Genotypic ratio - 121
• Phenotypic ratio - 31 (3 tall, 1 short)

• T t

• T T

• t t

• T t

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MONOHYBRID CROSSESCODOMINANCE

• A red snapdragon (RR) crossed with a white
  snapdragon (WW) will produce offspring that are
  all pink
• W W
• R
• R
• All offspring will be heterozygous, and because
  red and white are codominant, all the flowers
  will be pink.
• Genotypic ratio all are heterozygous (RW)
• Phenotypic ratio all are pink

• RW

• RW

• RW

• RW

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MONOHYBRID CROSSES CODOMINANCE

• A heterozygous pink crossed with a heterozygous
  pink, will produce varying offspring
• W R
• W
• R
• One will be homozygous white, two will be
  heterozygous pink, and one will be homozygous
  red.
• The genotypic ratio will be 121 and the
  phenotypic ratio will also be 121.

• RW

• WW

• RR

• RW

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DIHYBRID CROSSES

• Two pairs of alleles are followed from parents to
  offspring.
• Crossing two individuals heterozygous for
  earlobes and color of hair, the gametes can
  combine in 16 different ways.
• EeDd x EeDd
• E free earlobes e attached earlobes D
  dark hair d light hair

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DIHYBRID CROSSES

• ED Ed eD ed
• ED
• Ed
• eD
• ed
• The probability for a given phenotype will be
  9331.
• 9 free earlobes, dark hair
• 3 free earlobes, light hair
• 3 attached earlobes, dark hair
• 1 attached earlobes, light hair

• EeDd

• EeDD

• EEDd

• EEDD

• Eedd

• EeDd

• EEdd

• EEDd

• eeDd

• eeDD

• EeDd

• EeDD

• eedd

• eeDd

• Eedd

• EeDd

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ALTERNATIVE INHERITANCEMULTIPLE ALLELES

• Some genes are expressed by the presence of more
  than 2 alleles.
• Each person only gets two alleles, but in the
  population, there are more possibilities.
• ABO blood type in humans
• Determined by 3 alleles (IA, IB, i)
• 2 alleles are codominant, one is recessive
• Genotype Phenotype
• IAIA type A
• IAi type A
• IBIB type B
• IBi type B
• IAIB type AB
• ii type O

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ALTERNATIVE INHERITANCEPOLYGENIC INHERITANCE

• Traits are determined by several genes at
  different loci
• Genes may be found on the same chromosome or on
  different chromosomes
• Skin color in humans is an example
• Dark skin is dominant over light skin
• 3 different loci for skin color
• Expressed as many possible variations

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POLYGENIC INHERITANCE
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ALTERNATIVE INHERITANCEPLEIOTROPY

• Describes the multiple effects that a gene may
  have on a phenotype
• Examples include PKU and Marfan Syndrome
• PKU a single gene affects many chemical
  reactions (autosomal recessive disorder)
• Determines how a cell metabolizes phenylalanine
  (amino acid)
• Causes retardation if not treated with a proper
  diet
• Very light skin color

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ALTERNATIVE INHERITANCEPLEIOTROPY

• Marfan syndrome affects the connective tissue but
  can also affect other tissues (autosomal dominant
  disorder)
• Symptoms include
• Disproportionately long arms, legs, and fingers
• Skinniness and scoliosis of the spine
• Myopia (near-sightedness) and dislocated lenses
• Weak or defective heart valves, aneurysms, and
  aortic dissections
• Collapsed lungs and sleep apnea

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MARFAN SYNDROME
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ENVIRONMENTAL INFLUENCES

• Gene expression may vary on an individual basis
• Polydactylism (6 fingers)
• Hormones on voice changes at puberty
• Male-pattern baldness
• PKU expression (proper diet prevents symptoms)
• Familial diabetes can be delayed by reducing
  sugar
• Tanning

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CHAPTER 10 Mendelian Genetics
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• Dark color in cats is expressed on the parts of
  the body that stay cool.
• The gene for freckles expresses itself more fully
  when a person is exposed to the sun.

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• T T

• T T

• T T

• T T

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• t t

• t t

• t t

• t t

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• T t

• T t

• T t

• T t

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• T t

• T T

• t t

• T t

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• RW

• RW

• RW

• RW

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• RW

• WW

• RR

• RW

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• EeDd

• EeDD

• EEDd

• EEDD

• Eedd

• EeDd

• EEdd

• EEDd

• eeDd

• eeDD

• EeDd

• EeDD

• eedd

• eeDd

• Eedd

• EeDd

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