Beyond Mendel

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Beyond Mendel
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Beyond Mendel

• Despite the importance of Mendels work, it would
  be a mistake to characterize the principles he
  discovered as laws because there are important
  exceptions to most of them.
• Not all genes show simple patterns of dominant
  and recessive alleles.
• In most organisms, genetics is more complicated,
  because the majority of genes have more than two
  alleles.
• In addition, many important traits are controlled
  by more than one gene.

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Incomplete Dominance

• A form of intermediate inheritance in which
  heterozygous alleles are both expressed,
  resulting in a combined phenotype.

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Incomplete Dominance

• For example, in cross-pollination experiments
  between red and white snapdragon plants the
  resulting offspring are pink.

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Codominance

• A number of human traits are the result of 2
  types of alleles that are equally dominant.
• Such traits are said to be codominant for that
  trait.

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Codominance

• When an individual is heterozygous for such
  traits, the resulting phenotype or expression of
  these two traits is a blending, because both
  traits are expressed equally.

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Codominance

• The alleles for curly hair and straight hair are
  examples of alleles for a trait that are
  codominant.
• Individuals with curly hair are homozygous for
  curly hair alleles.
• Individuals with straight hair are homozygous for
  straight hair alleles.
• Individuals who are heterozygous, with one of
  each allele have wavy hair, which is a blend of
  the expressions of the curly and straight hair
  alleles.

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Multiple Alleles

• Diploid organisms naturally have a maximum of 2
  alleles for each gene expressing a particular
  characteristic, one deriving from each parent.
• In some cases, however, more than two types of
  allele can code for a particular characteristic,
  as is the case of genetic coding for blood type
  in humans or coat color in rabbits.
• Multiple alleles result from different mutations
  of the same gene.

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Multiple Alleles

• Coat color in rabbits is determined by four
  alleles.
• Human ABO blood types are determined by alleles
  A, B, and O.
• A and B are codominants which are both dominant
  over O.

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Multiple Alleles

• The only possible genotype for a type O person is
  ii.
• Type A people have either IAIA or IAi genotypes.
• Type B people have either IBIB or IBi genotypes.
• Type AB have only the IAIB (heterozygous)
  genotype.

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Multiple Alleles

• The A and B alleles of gene I produce slightly
  different glycoproteins (antigens) that are on
  the surface of each cell.
• Homozygous A individuals have only the A antigen,
  homozygous B individuals have only the B antigen,
  homozygous O individuals produce neither antigen,
  while a fourth phenotype (AB) produces both A and
  B antigens.

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Rh Blood type system

• When we are told our blood type, it is usually
  expressed as a letter followed by either a
  positive () or negative (-). 
• This or - refers to the Rh system of blood
  testing and is usually quoted in combination with
  the ABO system described earlier in this lecture
• e.g. AB or O-

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Rh Blood type system

• The Rh system is actually much more complicated
  than the ABO system because there are more than
  30 combinations possible when inherited, however
  for general usage, the Rh proteins are grouped
  into two families
• either positive () or negative (-).

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Rh Blood type system

• As with the ABO system, there is a dominant
  allele which happens to be the positive family. 
• This means that the genetic pairs that can exist
  in humans are as follows
• Genotype Blood type
• Rh
• - Rh
• -- Rh -

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Who can donate to who?
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Polygenic Traits

• Traits controlled by two or more genes are called
  polygenic traits.

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Polygenic traits

• Usually polygenic traits are distinguished by
• Two or more gene pairs contribute to the
  phenotype.
• Traits are usually quantified by measurement
  rather than counting.
• Phenotypic expression of polygenic traits varies
  over a wide range.

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Polygenic Traits

• Human polygenic traits include
• Height
• Weight
• Eye Color
• Intelligence
• Skin Color
• Many forms of behavior