Genetics - Exam 2 - Patterns of Gene Expression

Gene expression

How genetic information is used to produce proteins or functional RNAs.

Mutation

A change in the DNA sequence.

Allele

Different versions of a gene.

Wildtype

The most common allele in a population.

Mutant

An allele that differs from the wildtype.

Polymorphism

Presence of multiple alleles in a population; can be phenotypic or genetic.

Copy number variation (CNV)

Differences in the number of copies of a gene.

Norm of reaction

The range of phenotypes produced by a genotype in different environments.

Penetrance

Proportion of individuals with a genotype who express the phenotype.

Variable expressivity

Degree to which a phenotype is expressed in individuals with the same genotype.

Pleiotropy

A single gene influences multiple traits.

Epistasis

When expression of one gene masks/modifies the effect of another.

X-inactivation

Process in female mammals where one X chromosome is silenced → Barr body.

Mosaic

Female mammals express different alleles in different cells due to X-inactivation.

Describe the relationship between mutations and alleles

• Mutations create new alleles of a gene.

• These alleles can change protein function, expression level, or not affect the gene at all.

Distinguish between wildtype and mutant

• Wildtype - Common, “normal” version of a gene in a population.

• Mutant - Variant allele caused by mutation (not necessarily harmful).

List and describe the different effects that alleles can have in terms of protein function

• Normal protein (functional).

• Protein not functional (loss of function).

• Different protein function (altered activity).

• No protein produced (null mutation).

Distinguish between copy number variation and allelic variation

• Allelic variation - Different sequence versions of a gene (A vs. a).

• Copy number variation - Differences in how many copies of a gene are present.

Distinguish between simple traits and complex traits

• Simple (Mendelian) traits - Controlled by a single gene; follow predictable inheritance (e.g., Huntington’s disease).

• Complex (multifactorial) traits - Influenced by multiple genes and environment (e.g., height, diabetes).

Describe the patterns of gene expression of complex traits

• Polygenic - Many genes contribute.

• Environmental influences interact with genes.

• Norm of reaction - Same genotype can produce different phenotypes under different conditions.

Describe allelic interaction : how two different alleles of a single gene can interact in a heterozygote to produce the three basic patterns of gene expression

• Complete dominance/recessiveness - One allele completely masks the other (e.g., MC1R and hair color).

• Incomplete dominance - Heterozygote has intermediate phenotype (e.g., LDLR gene in familial hypercholesterolemia).

• Codominance - Both alleles are expressed equally (e.g., ABO blood types).

Explain why the terms dominance/recessiveness, codominance, and incomplete dominance make sense only when discussed relative to the specific environment and phenotype being examined

• An allele’s effect depends on which trait is being measured and under what environmental conditions.

• Example - The sickle-cell allele is recessive for normal RBC shape, but codominant for hemoglobin type, and advantageous in malaria environments.

Explain the patterns of gene expression of multiple genes interacting in different biochemical pathways versus in the same biochemical pathway

• Different pathways - Effects are independent (e.g., snake pigmentation patterns).

• Same pathway - One gene can mask or alter the effect of another (epistasis, e.g., Labrador retriever coat color).

Explain why the patterns of expression of X- and Y-linked genes is unique relative to autosomal genes

• Males have only one X and one Y (hemizygous).

• Females undergo X-inactivation, creating mosaics.

• Y-linked genes (e.g., SRY) are passed only father → son.

Explain why the patterns of gene expression are unique when there is only one copy of a gene (as in mitochondria) vs. two (as typical for nuclear genes)

• Mitochondrial genes are usually inherited maternally.

• With one copy, no “backup” allele exists → mutations have a direct effect.

• Expression is not influenced by dominance/recessiveness.

CFTR gene

Mutations cause cystic fibrosis; loss of chloride channel function.

LDLR gene

Incomplete dominance → heterozygotes partially affected (familial hypercholesterolemia).

MC1R gene

Complete dominance/recessiveness; certain alleles → red hair.

Huntingtin gene

Dominant mutation causes Huntington’s disease.

ABO gene

Codominance determines blood type.

β-globin gene

Sickle-cell allele shows multiple patterns depending on phenotype considered.

Corn snake pigmentation

Independent gene pathways determine pattern and color.

Labrador retrievers

Epistasis in coat color pathway.

Opsin genes (X-linked)

Mutations cause red-green color blindness.

SRY gene (Y-linked)

Determines male sex.

Mitochondrial genes

Mutations affect energy production; allelic interaction not relevant.