Genetically Inherited Diseases

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Lecture 26

• Genetically Inherited Diseases

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The Monk and His Peas

• An Austrian monk, Gregor Mendel, developed the
  fundamental principles that would become the
  modern science of genetics.
• Mendel demonstrated that heritable properties are
  parceled out in discrete units, independently
  inherited.
• These eventually were termed genes.

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Mendels Experiments

• Mendel's experimental organism was a common
  garden pea (Pisum sativum), which has a flower
  that lends itself to self-pollination.
• The male parts of the flower are termed the
  anthers. They produce pollen, which contains the
  male gametes (sperm).
• The female parts of the flower are the stigma,
  style, and ovary. The egg (female gamete) is
  produced in the ovary.
• The process of pollination (the transfer of
  pollen from anther to stigma) occurs prior to the
  opening of the pea flower.

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Mendels Experiments

• The pollen grain grows a pollen tube which allows
  the sperm to travel through the stigma and style,
  eventually reaching the ovary.
• The ripened ovary wall becomes the fruit (in this
  case the pea pod).
• Mendel tested all 34 varieties of peas available
  to him through seed dealers.
• The garden peas were planted and studied for
  eight years.
• Each character studied had two distinct forms,
  such as tall or short plant height, or smooth or
  wrinkled seeds.

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Inherited Characteristics
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Definitions

• Genotype the specific allele composition of a
  cell.
• Phenotype the detectable manifestation of a
  specific genotype.

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Principle of Segregation -Monohybrid Cross
Cross female phenotype Purple X White male
phenotype

Phenotypically all offspring (F1) produce purple
flowers, but each has information for both purple
and white pigment production. Therefore,
genotypically the offspring are heterozygous. In
this case the allele for production of the purple
pigment is dominant
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Smooth vs Wrinkled Peas
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F1 x F1 F2
Cross female phenotype Purple X Purple male
phenotype
In looking at these offspring (F2), one can see
that phenotypically three of four (3/4) produce
purple flowers while one of four (1/4) produce
white flowers. We say that such a cross typically
produces a 31 ratio of dominate to recessive
phenotype expression. Genotypically, however, one
notes that the ratio is 1(homozygous dominant)2
(heterozygous)1 (homozygous recessive).
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Dihybrid Cross
Cross Female Green pods with Round peas X
Yellow pods with Wrinkled peas Male
In looking at the phenotypes of the first
generation offspring (F1), we find that all
produce green pods and round peas. Genotypically,
however, all F1 individuals have both green and
yellow pod color and round and wrinkled pea
appearance information (heterozygous). Green pod
colour and round peas appearance are, therefore,
the dominant alleles
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F1 X F1 F2
In looking at the phenotypes, we find nine
exhibit the dominant allele for both traits
three that exhibit the dominant green pods but
wrinkled (recessive) peas three that exhibit
yellow (recessive) pods but the dominant round
peas one that exhibits the recessive allele for
both traits---9331. We find a more complex
assortment with regard to genotypes
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Expression of Alleles

• Dominance refers to the effects of one allele
  overriding the effects of another allele (of the
  same gene). For example, A is dominant to a.
  Dominant traits were defined by Mendel as those
  which appeared in the F1 generation in crosses
  between true-breeding strains.
• Recessives were those which "skipped" a
  generation, being expressed only when the
  dominant trait is
• Epistasis refers to the effects of one gene
  overriding the effects of another gene. For
  example, gene A (really the aa genotype) is
  epistatic to gene B.

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Polygenic Inheritance

• Many traits such as height, shape, weight, color,
  and metabolic rate are governed by the cumulative
  effects of many genes.
• Polygenic traits are not expressed as absolute or
  discrete characters, as was the case with
  Mendel's pea plant traits.

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Multiple Allelic Inheritance

• While many genes have only two possible alleles,
  and we are most accustomed to looking for
  inheritance patterns involving only two possible
  alleles, there are genes for which geneticists
  have identified more than two possible alleles.
• These traits are said to be multi-allelic.
• One of the best documented and more interesting
  multi-allelic traits is that of the ABO blood
  group antigens in humans.
• The phenotype (A, B or O) on the individual's
  erythrocytes is determined by the presence and
  identity of the terminal sugar attached to a
  molecular configuration on the surface of the
  erythrocyte membrane.