Genetics Chapter: Key Terms and Concepts

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Questions and Answers

What does the phenomenon of incomplete dominance demonstrate in regards to the b + and b SC alleles?

The heterozygote exhibits a phenotype that is an average of both homozygotes. (correct)

The b + allele is always expressed as the dominant trait.

Both alleles create a new phenotype together without blending.

The b SC allele is completely recessive under all conditions.

Which statement best describes the concept of heterozygote advantage?

Heterozygotes have equivalent fitness to homozygotes.

Heterozygotes are typically selected against in natural selection.

Heterozygotes are less fit than either homozygote.

Heterozygotes exhibit greater fitness than both homozygous forms. (correct)

What factor contributes to the maintenance of the b SC allele in human populations despite early mortality associated with sickle-cell anemia?

Complete dominance of the b + allele over b SC.

Increased mutation rates of other alleles.

The presence of external environmental pressures unrelated to disease.

Protection against malaria provided by the sickle-cell trait. (correct)

How does balancing selection contribute to allele frequency in a population?

It ensures multiple alleles are maintained at higher frequencies than mutation rates alone would predict. Signup and view all the answers

How is incomplete dominance different from codominance as exemplified by the ABO blood group system?

In codominance, both alleles contribute equally to the phenotype of the heterozygote. Signup and view all the answers

What is the function of the enzyme encoded by allele IA in the ABO blood group system?

Adds N-acetylgalactosamine to the surface of red blood cells Signup and view all the answers

In achondroplasia, what is the role of the FGFR3 gene mutation?

It negatively regulates cartilage formation and bone growth Signup and view all the answers

Which ABO blood group has no antigens present on the red blood cells?

Type O Signup and view all the answers

What percentage chance does a child have of inheriting achondroplasia if one parent is affected?

50% Signup and view all the answers

Which of the following best describes the inheritance pattern of achondroplasia?

Autosomal dominant Signup and view all the answers

A mutation in which gene is responsible for causing achondroplasia?

Fibroblast Growth Factor Receptor 3 (FGFR3) Signup and view all the answers

What type of mutation does allele iO exhibit in the ABO blood group system?

Frameshift mutation Signup and view all the answers

Which of the following is a common symptom of achondroplasia?

Prominent forehead Signup and view all the answers

What defines a homozygote genotype?

Two copies of the same allele for a gene Signup and view all the answers

What is the primary difference between homologous and orthologous genes?

Homologous genes relate by descent, ortho-genes are due to duplications. Signup and view all the answers

Which statement about alleles is correct?

Different alleles can result in varying phenotypes. Signup and view all the answers

What is indicated by the term 'haploid'?

One copy of each chromosome in a gamete. Signup and view all the answers

What distinguishes sickle-cell trait from sickle-cell anemia?

Individuals with sickle-cell trait are typically asymptomatic, while those with anemia show symptoms. Signup and view all the answers

Which term describes the unique genetic makeup for a particular individual?

Genotype Signup and view all the answers

Which statement regarding β-globin gene variations is true?

β-globin gene variations can be caused by single amino acid substitutions. Signup and view all the answers

What is the significance of the term 'phenotype' in genetics?

It reflects the observable traits or characteristics of an organism. Signup and view all the answers

What is the relationship between the number of CAG repeats in the Huntington's disease gene and the age of onset of the disease?

More repeats correlate with an earlier age of onset Signup and view all the answers

Which molecular explanation describes the constitutive expression of a normally regulated protein leading to a dominant mutation?

Gain of function due to constant expression in mutants Signup and view all the answers

What effect does the phenomenon of incomplete penetrance have on individuals with a specific genotype?

Some individuals express the phenotype while others do not Signup and view all the answers

What is the definition of epistasis in genetics?

When one gene's phenotype is modified by another gene Signup and view all the answers

Which of the following is a characteristic of gain of function mutations?

The mutant protein exhibits new or enhanced functions Signup and view all the answers

How does Genetic background influence the expression of identical genes?

It can modify the expression patterns even for identical genotypes Signup and view all the answers

What happens when a mutant polypeptide 'poisons' the assembly of a multimeric protein?

It induces a loss of function in the multimeric protein Signup and view all the answers

In the context of Huntington's disease, what are the implications of a gene having 36-121 CAG repeats?

It often leads to significant symptoms at a younger age Signup and view all the answers

Study Notes

Basic Concepts and Terminology

Eukaryote: A cell with a true nucleus
Diploid: Having two copies of each gene (one maternal, one paternal)
Haploid: Having one copy of each gene (e.g., gamete)
Gene: A segment of DNA that encodes RNA
Alleles: Different forms of a gene (e.g., β-globin and sickle-cell β-globin)
Homologues (Homologous): Genes related by descent, same gene in different species (e.g., chimpanzee and human β-globin genes)
Orthologous: Genes related by descent from a common ancestor, located in different species.
Paralogous: Genes related by duplication within a species.

High Similarity

Similarity can arise from:
- Chance (short sequences)
- Convergent evolution (analogous sequences). Similar structure/function, but not homologous
Analogous sequences are similar but not homologous.

Genetic Nomenclature

β: The gene for β-globin
β⁺: Wild-type allele
β⁺/β^sc: Heterozygote- diploid, different alleles
β^sc/β^sc: Homozygote, same sickle cell alleles
β^sc: Sickle-cell allele
Genotype: Specific allele composition
Phenotype: Observable characteristics, "form that is seen"
Haplotype: DNA variations (polymorphisms) that tend to be inherited together.

β^sc Description

Valine replaces Glutamate at position 7 in the β-chain.
This results in sickle-cell disease.

Phenotype of β⁺/β^sc Heterozygote

Normal O₂: β⁺/β⁺ and β⁺/β^sc exhibit normal phenotypic expression.
**Low O₂:**β⁺/β⁺ shows normal expression, whilst β⁺/β^sc shows reduced expression.

Phenotype of β Heterozygote

Normal conditions: β⁺ is dominant over β^sc; β^sc is recessive to β⁺
Limited oxygen: β^sc is incompletely recessive to β⁺; incomplete/partial dominance of β⁺ over β^sc

Incomplete Dominance

The heterozygous phenotype is in between the phenotypes of both homozygous types

Sickle-cell Anemia

Historically, this caused early mortality
Natural selection should remove the allele from the population

Maintenance of the β^sc Allele

The β^sc allele is maintained in human populations despite its negative effect
Possibly because of its protective effect against malaria

Malaria and Sickle-cell Trait

Areas with high malaria prevalence demonstrate high frequency of sickle cell trait.
Presence of the sickle cell trait protects against malaria

Mechanism of Protection from Malaria

Selective sickling of parasitized AS red blood cells promotes selective phagocytosis of these cells.

Heterozygote Advantage

The heterozygous has a greater fitness than either of the two homozygous types
Balancing selection maintains multiple alleles of a gene in the gene pool. Can be predicted with mutations alone. Associated with clinal variation, frequency-dependent selection, or developmental-related fitness.

Codominance

Both alleles of the heterozygote contribute equally to the phenotype
Contrast incomplete dominance

ABO Blood Group System

4 phenotypes: A, B, AB, O
2 antigens: A and B
One gene (I):
- Three alleles: I^A, I^B, and i⁰ (recessive)

ABO Blood Group System (Table)

Shows phenotype, rbc antigen, serum antibody, and genotype correspondences.

Gene I description

I encodes a glycosyltransferase enzyme
The I^A allele adds N-acetylgalactosamine
The I^B allele adds galactose
The i⁰ allele produces an inactive enzyme

Dominant Mutations

Achondroplasia: A disorder of bone growth causing dwarfism. Common type of dwarfism.
- Symptoms:
  - Bowed legs
  - Large head-to-body size difference
  - Prominent forehead
  - Shortened arms and legs
  - Short stature
  - Spinal stenosis
  - Spine curvatures (kyphosis/lordosis)
- Inheritance: Autosomal dominant trait (50% chance if one parent has it, 75% chance if both)
- Usually spontaneous mutation. Prevalence approximately 1 in 25,000 births.
- FGFR3 gene mutation.
Huntington's disease (chorea): Late-onset, neurodegenerative disease.
- Symptoms: Late onset (35-50), neurodegenerative
- Inheritance: Autosomal dominant trait
- CAG repeat expansion (polyglutamine). 6-35 repeats in normal gene; 36-121 repeats in disease gene.
- Number of repeats inversely correlated with onset age.

Possible Molecular Explanations of Dominant Mutations

Gain-of-function:
- Regulated protein expressed constitutively (all the time).
- Novel function of the mutant protein (produces different product)
Loss-of-function:
- Structural protein insufficient in one copy
- Multimeric protein—mutant polypeptide poisons the assembly

Epistasis

Gene interactions that affect phenotype
Two or more loci interact to create new phenotypes
One allele masks the effects of one or more other genes
Purple is epistatic to Pattern; Purple is epistatic to Transporter; Transporter is epistatic to Pattern

Penetrance and Expressivity

Penetrance: The percentage of individuals with a particular genotype who express the corresponding phenotype
Expressivity: The variation in the degree of expression of a phenotype in individuals with the same genotype

Incomplete Penetrance

The presence of a particular allele does not alwaysresult in the phenotype in all individuals. Example: 85% penetrance mean that 85% of individuals (that carry same genotype) will show the phenotype.

Expressivity

Individuals express the same genotype with varying degrees. Example: Polydactyly (extra toes) in cats vary in degree (and number) of toes.

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Description

Test your understanding of essential genetics terminology and concepts. This quiz covers definitions and distinctions related to eukaryotes, diploids, haploids, alleles, and more. Perfect for students looking to solidify their knowledge in genetics.