Mutant Alleles: Dominance, Recessiveness, and Phenotypic Effects

The video tutorial explores whether mutant alleles are dominant or recessive, depending on their effects in heterozygotes. Recessive mutations often do not affect phenotype unless homozygous, as seen in enzyme-related genes. Dominant mutations, like those causing Alzheimer's, affect phenotype even in heterozygotes due to abnormal protein accumulation.

What determines whether a mutant allele is dominant or recessive?

• The number of alleles present

• The location of the gene on the chromosome

• The effects of the mutation in a heterozygote

• The size of the gene

Why are many mutations in genes encoding enzymes classified as recessive?

• Because enzymes are not essential for survival

• Because heterozygotes produce enough enzyme to maintain normal function

• Because enzymes are only active in homozygous individuals

• Because mutations in enzymes are always lethal

How can a heterozygote display a normal phenotype despite having a mutant allele?

• By having a backup copy of the gene

• By producing twice the normal amount of gene product

• By producing half the normal amount of gene product, which is still sufficient

• By not expressing the mutant allele at all

What role do enzymes play in the classification of mutations as recessive?

• Enzymes are always produced in excess

• Enzymes are only produced by dominant alleles

• Enzymes can catalyze many reactions, so a small amount is sufficient

• Enzymes are not involved in mutation classification

What is an example of a dominant mutation affecting phenotype?

• A mutation that results in the production of neurotoxic proteins

• A mutation that is only expressed in homozygous individuals

• A mutation that causes a loss of enzyme function

• A mutation that does not affect phenotype at all

How do abnormal proteins contribute to Alzheimer's disease in heterozygotes?

• They are degraded quickly and do not accumulate

• They accumulate and are neurotoxic, affecting phenotype

• They have no effect on brain function

• They enhance normal brain function

Why is the mutation causing Alzheimer's disease considered dominant?

• Because it does not affect phenotype at all

• Because it only affects individuals with two mutant alleles

• Because it affects phenotype even in the presence of a normal allele

• Because it is located on a dominant chromosome

What happens to the abnormal peptides in Alzheimer's disease?

• They are converted into normal peptides

• They are degraded normally

• They are beneficial to brain function

• They accumulate and are not degraded