Allele Interactions


  1. Define gain of function, loss of function, and haploinsufficient. Explain why gain of function mutations are often dominant and loss of function mutations are often recessive.
  2. Define the following terms, and give examples of each: complete dominance, incomplete dominance, codominance, penetrance, expressivity, lethal allele, pleiotropy
  3. Predict offspring ratios from a cross involving any of the terms defined above
  4. Use offspring ratios to predict the mode of inheritance and/or determine genotypes of parents
  5. Explain how a single allele can be described in multiple ways (dominant, recessive, incompletely dominant, codominant, etc) depending on the phenotype that is observed.

Genes contribute to an organism’s phenotype primarily through the function of the protein that the gene encodes. If the gene sequence – or the sequence of regulatory elements that affect gene expression – is changed, then the function of the protein might be affected. Changing protein function can sometimes affect phenotype.

When one phenotype predominates in a population, that trait is called wild-type. As an example, in wild populations of fruit flies – ones you might find circling your overripe bananas! – eye color is red, wings are longer than the body, and bristles are straight. For fruit flies, then, wild-type eye color is red, wild-type wings are long, and wild-type bristles are straight. Phenotypic differences are described as mutant in relation to those wild-type trait. Mutant fruit flies might have brown or scarlet eyes, short or curly wings, forked or bent bristles – or any combination of these traits. It’s important to note that the word “mutant” does not have negative connotations to a geneticist: mutations are just differences .

Note that the term wild-type is not always appropriate to use: it may not be used if there is enough variation in a population that there is no single predominant phenotype. The term wild-type is also not typically used to describe human phenotypes. The term mutant is sometimes used to describe human allelic variants associated with disease (eg. a mutant CFTR allele) but never people themselves.


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