Robbins Gen Path - Genetics

A missense mutation is a type of point mutation where a single nucleotide change in the DNA sequence leads to the substitution of one amino acid with another during protein synthesis. This alteration in the genetic code occurs within a triplet of bases, also known as a codon, resulting in the replacement of one amino acid by another in the protein sequence. Therefore, a missense mutation best defines a point mutation that alters the code in a triplet of bases, leading to the replacement of one amino acid by another.

Frameshift mutation is the best term to define the insertion or deletion of one or two base pairs in the DNA strand, leading to alterations in the reading frame. This type of mutation causes a shift in the grouping of codons, which changes the way the genetic code is read and often results in a completely different amino acid sequence. This can have significant effects on the structure and function of the resulting protein.

Trinucleotide repeat mutations almost always affect guanine and cytosine nucleotides. These mutations involve the expansion of a sequence of three nucleotides, typically CAG, CTG, or CGG, which are repeated multiple times within a gene. As the number of repeats increases, it can lead to various genetic disorders. Since guanine and cytosine are two of the four nucleotides that make up DNA, these mutations specifically affect these nucleotides.

X-linked recessive inheritance accounts for a small number of well-defined clinical conditions. The Y chromosome, for the most part, is not homologous to the X, and so mutant genes on the X do not have corresponding alleles on the Y. Thus, the male is said to be hemizygous for X-linked mutant genes, so these disorders are expressed in the male. Although gene expression and mendelian traits are usually described as dominant or recessive, in some cases both of the alleles of a gene pair contribute to the phenotype—a condition called codominance.A single mutant gene may lead to many end effects, termed pleiotropism; conversely, mutations at several genetic loci may produce the same trait (genetic heterogeneity).

Codominance is the term that best defines the full expression of both alleles of a pair in the heterozygote. In codominance, both alleles are fully expressed and neither allele is dominant or recessive to the other. This means that both alleles contribute to the phenotype of the individual in a heterozygous state. This is different from incomplete penetrance, where the phenotype associated with a particular genotype is not always expressed, and genetic homozygosity, which refers to having two identical alleles for a particular gene. Pleiotropism refers to a single gene having multiple effects on different traits, and complete penetrance refers to the presence of a genotype always being associated with the expected phenotype.

Genetic heterogeneity refers to mutations at several genetic loci that produce the same trait. This means that different mutations in different genes can lead to the same phenotype or trait. It is the opposite of genetic homozygosity, which refers to mutations at a single genetic locus. Pleotropism refers to a single gene mutation that affects multiple traits, hemizygosity refers to the presence of only one allele of a gene, and autosomal recessive disorders refer to genetic disorders that require two copies of a mutated gene to be present.

Chromosome mutation is the best term to define the rearrangement of genetic material that results in visible structural changes in the chromosome. This term specifically refers to alterations in the structure or number of chromosomes, which can lead to significant changes in an organism's genetic makeup. Point mutation, frameshift mutation, and genome mutation do not specifically refer to changes in the chromosome structure, making them less appropriate choices for this definition. Genome rearrangement is not a commonly used term in genetics and does not specifically indicate visible structural changes in the chromosome.

Some individuals inherit the mutant gene but are phenotypically normal. This is referred to as incomplete penetrance. Penetrance is expressed in mathematical terms. Thus, 50% penetrance indicates that 50% of those who carry the gene express the trait.

point mutation = single nucleotide substitution
missense mutation = point mutation that changes the meaning of sequence of protein
frameshift mutation = small deletions or insertions that change the reading frame of the DNA strand
chromosome mutation =

Transcription may be suppressed with gene deletions and point mutations involving promoter sequences. Abnormal mRNA processing may result from mutations affecting introns or splice junctions or both. Translation is affected if a stop codon (chain termination mutation) is created within an exon. Finally, some point mutations may lead to the formation of an abnormal protein without impairing any step in protein synthesis.