Question

Select from the terms below to answer questions #8-12. Each term is used once.

H) Genetic anticipation

I) Onset of Genetic Expression

J) Penetrance

K) Expressivity

L) The Threshold Effect

1. In individuals affected with Tay-Sach’s disease, there is almost no activity of an enzyme called hexosaminidase A. Heterozygotes, with only a single copy of the mutant gene, have 50% enzyme function but are phenotypically normal. This illustrates _________.

2. If 15 percent of fruit flies with homozygous mutant alleles show a wild-type appearance, then the gene is said to have reduced _________.

3. Fruit flies homozygous for the recessive mutant phenotype eyeless exhibit phenotypes that range from the presence of normal eyes to the complete absence of both eyes. This case illustrates variation in _________.

4. The first symptoms of Lesch-Nyhan syndrome begin to affect newborns at around six to eight months, whereas Huntington’s disease strikes in middle age. This illustrates differences in _________.

5. With myotonic dystrophy, medical doctors have noticed that over several generations of a family, symptoms can appear at earlier and earlier ages with increasing severity of muscle weakness. This led to the discovery of _________.

Answer 1

1. In individuals affected with Tay-Sachs disease, there is almost no activity of an enzyme called hexosaminidase A. Heterozygotes, with only a single copy of the mutant gene, have 50% enzyme function but are phenotypically normal. This illustrates L) The Threshold Effect.
   
   The threshold effect is a phenomenon where a minimum concentration of an effetor, hexosaminidase A in this case, is sufficient to acheive a particular phenotype.

2. If 15 percent of fruit flies with homozygous mutant alleles show a wild-type appearance, then the gene is said to have reduced J) Penetrance.
   
   Penetrance is the percentage of the individuals with the same genotype for that locus that show the phenotype. Here, homozygous mutants show a wild-type phenotype and therefore have reduced penetrance.

3. Fruit flies homozygous for the recessive mutant phenotype eyeless exhibit phenotypes that range from the presence of normal eyes to the complete absence of both eyes. This case illustrates the variation in K) Expressivity.
   
   Expressivity is the variation of degree of expression within individuals. An individual may express the phenotype fully or not at all; this range is the expressivity of the gene/locus.

4. The first symptoms of Lesch-Nyhan syndrome begin to affect newborns at around six to eight months, whereas Huntington’s disease strikes in middle age. This illustrates differences in I) Onset of Genetic Expression.
   
   The Onset time, or the age at which a disease manifests itself is the Onset of of Genetic Expression.

5. With myotonic dystrophy, medical doctors have noticed that over several generations of a family, symptoms can appear at earlier and earlier ages with increasing severity of muscle weakness. This led to the discovery of H) Genetic anticipation.
   
   Genetic Anticipation is a phenomenon of increased severity of a disorder in successive generations of a family. This is typically caused by dynamic mutations, where the probabiity of mutation at a particualr locus is dependent on the genotype of that locus. Mutations like the change in number of trinucleotide repeates affect the expression of the locus, altering the phenotype.