12.2 Complex patterns of inheritance

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12.2 Complex patterns of inheritance
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Complex patterns of inheritance

• Mendels experiments show simple inheritance with
  recessive and dominant alleles
• Some patterns of inheritance are more complex

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Complex patterns of inheritance

• Incomplete dominance
• Codominance
• Multiple alleles
• Sex-linked inheritance
• Polygenic inheritance
• Environmental factors

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Incomplete dominance

• Phenotype of the heterozygote is in between the
  phenotypes of the homozygotes
• RRred pigment
• R1R1white, no pigment
• RR1pink

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Codominance

• Phenotypes of both homozygotes are produced in
  heterozygote, both alleles are expressed equally
• BBblack feather
• WW white feather
• BW checkered

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Multiple alleles

• Traits controlled by more than two alleles
• Each pigeon has two alleles for feather color,
  but there are many possible alleles

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Sex-linked inheritance

• In humans have 46 chromosomes, 23 pairs
• 22 pairs autosomes
• one pair sex chromosomes, determine sex

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Sex-linked inheritance

• Sex-linked traits are controlled by genes located
  on the sex chromosomes
• Alleles are carried on the X chromosome,
• not the Y chromosome

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Polygenic inheritance

• A trait that is controlled by more than one
  gene
• Genes may be on different chromosomes
• Each gene may have more than one allele
• Trait usually expressed in a continuous range
  of variability

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Environmental factors

• External
• Gene may not be expressed due to temperature,
  nutrition, light, chemicals, infection
• For example, this bacteria
• is different color when
• grown at different temperature

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Environmental factors

• Internal
• Males and females differ due to hormones
• Age may change trait expression

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Complex patterns of inheritance

• Mendels idea that heredity is a composite of
  many traits still holds
• Later researchers have filled in details