CH. 4 Gene Interaction

What are the main types of gene interactions?

Two or more genes may affect one trait (gene-gene and gene-environment interactions).

What determines dominance of one allele over another?

The activity of the protein product produced by the allele.

What does haplosufficient mean?

One copy of the allele is enough for normal function.

What does haploinsufficient mean?

One copy of the allele is not enough for normal function.

What are the two main categories of mutant alleles?

Loss-of-function and Gain-of-function mutations.

What are null mutations?

Mutations that result in complete loss of function (amorphic).

What are leaky (hypomorphic) mutations?

Mutations that cause partial loss of function.

What are dominant negative mutations?

Mutations that spoil the function of multimeric proteins, preventing normal interaction.

What are hypermorphic mutations?

Mutations that produce more gene activity than normal.

What are neomorphic mutations?

Mutations that acquire a new function not present in the wild type.

What is incomplete dominance?

When heterozygotes show an intermediate phenotype between homozygotes.

What is codominance?

When both alleles are expressed in heterozygotes (e.g., ABO blood type).

Which alleles are codominant in ABO blood groups?

IA and IB.

What genotypes give blood type O?

ii

What are ABO blood group antigens based on?

A five-sugar H antigen that is modified with an extra sugar (A, B, or none for O).

What does it mean if a gene has multiple alleles?

A single gene locus has more than two alleles within a population.

What is an allelic series?

A sequential dominance order among multiple alleles.

What enzyme does the C gene produce for coat color?

Tyrosinase, which is needed for melanin production.

Which C-gene allele produces full coat color?

The wild-type allele C.

Which C-gene allele is temperature sensitive?

ch (Himalayan allele).

Which C-gene allele produces an albino phenotype?

c, a fully recessive null allele.

What are lethal alleles?

Mutations that cause death, usually when homozygous.

What ratio of living to dead seeds suggests embryonic lethals?

3:1 (living : dead).

Why are AY alleles in mice lethal when homozygous?

Because the deletion removes the Raly gene, which is essential for embryonic development.

What is a key example of a dominant lethal with delayed onset?

Huntington disease.

What does complete penetrance mean?

The phenotype always matches the genotype.

What is variable expressivity?

Individuals with the same genotype show the trait with different degrees of severity or forms.

What disease shows gene–environment interaction and can be prevented by diet?

PKU (phenylketonuria).

How can PKU symptoms be prevented?

Restricting phenylalanine in the diet.

What is pleiotropy?

One gene affects multiple traits.

What causes sickle cell disease?

Mutation in the β-globin gene.

What is gene interaction?

Genes working together for a trait.

What did Beadle and Tatum propose?

The one gene–one enzyme hypothesis.

What does the one gene–one enzyme hypothesis state?

Each gene makes one enzyme.

What is genetic dissection used for?

To determine steps in a pathway.

What does epistasis refer to?

Gene interaction altering phenotypic ratios.

What F2 ratio shows no gene interaction?

9:3:3:1.

What F2 ratio indicates complementary gene interaction?

9:7.

What F2 ratio indicates duplicate gene action?

15:1.

What is dominant gene interaction characterized by?

9:6:1.

What is the auxotroph in Beadle and Tatum’s experiments?

A mutant strain requiring supplements.

What is a prototroph?

A wild-type strain that grows without supplements.

What is recessive epistasis and its ratio?

Recessive epistasis occurs when homozygosity for the recessive allele at one locus masks the phenotypic expression of alleles at a second locus, producing a 9:3:4 ratio.

Example of recessive epistasis in animals?

In Labrador retrievers, one gene controls eumelanin production, while another controls deposition of pigment in hair. If pigment cannot be deposited, the coat is yellow regardless of eumelanin genotype.

What is dominant epistasis and its ratio?

Dominant epistasis occurs when a dominant allele at one locus masks the phenotypic expression of alleles at another locus, producing a 12:3:1 ratio.

Example of dominant epistasis in plants?

In summer squash, a dominant allele prevents conversion of a colorless precursor to a green intermediate, masking the effect of a second locus that converts green to yellow.

What is dominant suppression and its ratio?

Dominant suppression occurs when a dominant allele at one locus completely suppresses expression of alleles at another locus, producing a 13:3 ratio.

Example of dominant suppression in plants?

In the blue pimpernel, the L allele produces blue pigment. The dominant D allele suppresses pigment production, resulting in white flowers in both L– and ll individuals with the D– genotype.

What questions do geneticists ask in complementation analysis?

(1) Do the organisms have mutations in the same or different genes? (2) How many genes are responsible for the mutations observed?

What is genetic heterogeneity?

Mutations in different genes can produce the same or very similar mutant phenotypes.

How does genetic complementation analysis work?

Two pure-breeding mutants with similar phenotypes are crossed. If complementation occurs, F1 offspring are wild-type, showing mutations affect different genes. If no complementation, F1 offspring show mutant phenotype, meaning mutations affect the same gene.

What is a complementation group?

A group of mutations that fail to complement one another, representing a single gene.