Question

Select from the terms below to answer questions #8-12. Each term is used once. H) Genetic anticipation 1) Onset of Genetic Expression J) Penetrance K) Expressivity L) The Threshold Effect 8) 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 9) If 15 percent of fruit flies with homozygous mutant alleles show a wild-type appearance, then the gene is said to have reduced 10) 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 11) 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 12) 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

8)

L) The Threshold Effect

As one copy of sufficient to produce Hexosaminidase A, and give a normal phenotype, the threshold required is met. This is an example of The Threshold Effect.

9)

J) Penetrance

Since a fraction (15%) of flies that have homozygous mutant alleles show a wild type phenotype, this means that the Mutation has a reduced penetrance as all homozygous mutants are not affected.

10)

K) Expressivity

When the same genotype shows differing phenotype in a population, such an effect demonstrates variation in Expressivity.

11)

I) Onset of Genetic Expression

Many conditions vary in the age of the organism at which the phenotype is expressed. This is variation in Onset of Expression.

12)

H) Genetic Anticipation

When the severity of a disorder increases in the descendents of an individual, the phenomenon is termed Genetic Anticipation. This occurs due to dynamic mutations, where subsequent generations accumulate further mutation at the original locus, deepening the severity of the disorder.