Single-Locus Inheritance: Dominance, Penetrance, Lethal Alleles, and Multiple Alleles

Single-Locus Inheritance and Dominance Patterns

• Complete Dominance:

  • Occurs when the phenotype of the heterozygote is identical to the phenotype of one of the homozygotes.

  • Homozygous genotype $A^1 A^1$ encodes red flower color.

  • Homozygous genotype $A^2 A^2$ encodes white flower color.

  • If the $A^1 A^2$ heterozygote exhibits a red flower phenotype, the $A^1$ allele is completely dominant over the $A^2$ allele.

  • If the $A^1 A^2$ heterozygote exhibits a white flower phenotype, the $A^2$ allele is completely dominant over the $A^1$ allele.

• Incomplete Dominance:

  • Defined as a pattern of inheritance where the phenotype of the heterozygote falls quantitatively between the phenotypes of the two homozygotes.

  • Heterozygotes ($A^1 A^2$) express an intermediate phenotypic spectrum (such as varying shades of pink) bounded by the red ($A^1 A^1$) and white ($A^2 A^2$) homozygous phenotypes.

Co-Dominance

• Definition:

  • Co-dominance is an allele relationship in which the phenotype of the heterozygote simultaneously expresses the phenotypes of both homozygotes without blending.

  • Neither allele masks the other; both phenotypic traits are fully and distinctly observable in the heterozygous individual.

• Flower Color Example:

  • Observed in dahlia flowers where petals simultaneously manifest fully red and fully white sections, rather than blending into a uniform pink color.

Penetrance and Expressivity

• Penetrance:

  • Definition: The percentage of individual organisms carrying a specific genotype that actually express the expected phenotype.

  • Quantitative Example: In a study examining $42$ individuals known to possess an allele for polydactyly, if $38$ of these individuals display the polydactylous phenotype, the penetrance is calculated as:     $\text{Penetrance} = \frac{38}{48} = 0.90 \quad (90\%)$

• Expressivity:

  • Definition: The degree or severity to which a genotype is expressed phenotypically within an individual organism.

  • Polydactyly Example: Individuals possessing the polydactyly allele can exhibit varying expressivity—some may display a minor anatomical alteration resembling a small skin tag, while others possess one or more completely formed additional digits.

Comparison of Incomplete Dominance and Co-Dominance

• Phenotypic Mechanisms:

  • Incomplete Dominance: Produces a blended, intermediate phenotype in heterozygous individuals (e.g., a uniform pink camellia blossom).

  • Co-Dominance: Produces distinct, concurrent expression of both homozygous traits across different regions of the organism (e.g., a camellia blossom featuring distinct, separate patches of red and white).

Lethal Alleles

• Definition:

  • Lethal alleles are gene variants that cause the death of an organism, frequently during early embryonic development.

  • Because affected homozygous individuals die before birth, lethal alleles modify standard Mendelian phenotypic and genotypic ratios among surviving offspring.

• Mouse Coat Color Genetic Cross ($Y$ Allele):

  • Parental ($P$) Generation Cross: Two yellow heterozygous mice ($Yy \times Yy$).

  • Meiosis & Gametes: Each parent produces $\frac{1}{2}$ $Y$ gametes and $\frac{1}{2}$ $y$ gametes.

  • Progeny ($F_1$ Generation) Genotypic Distribution:

  • $\frac{1}{4}$ $YY$: Homozygous dominant individuals die during embryonic development (Dead).

  • $\frac{1}{2}$ $Yy$: Heterozygous individuals survive and express yellow coat color.

  • $\frac{1}{4}$ $yy$: Homozygous recessive individuals survive and express non-yellow (agouti/gray) coat color.

  • Modified Phenotypic Ratios:

  • Among surviving progeny, $\frac{2}{3}$ are yellow ($Yy$).

  • Among surviving progeny, $\frac{1}{3}$ are non-yellow ($yy$).

  • The standard Mendelian $3:1$ phenotypic ratio is modified to a $2:1$ ratio of yellow to non-yellow surviving offspring due to embryonic lethality in the $YY$ genotype.

Multiple Alleles and the ABO Blood Group System

• Multiple Alleles:

  • Defined as the presence of more than two alternative allele forms at a single gene locus within a population, although each individual carries a maximum of two alleles at that locus.

• Allelic Interactions at the ABO Locus:

  • Controlled by three primary alleles: $I^A$, $I^B$, and $i$.

  • $I^A$ and $I^B$ alleles demonstrate co-dominance with respect to each other.

  • Both $I^A$ and $I^B$ alleles show complete dominance over the recessive $i$ allele.

• Comprehensive ABO Phenotypes, Genotypes, Antigens, and Antibodies:

  • Type A:

  • Genotypes: $I^A I^A$ or $I^A i$

  • Antigen Present on Red Blood Cells: $A$ antigen

  • Antibodies Produced: Anti-$B$ antibodies

  • Type B:

  • Genotypes: $I^B I^B$ or $I^B i$

  • Antigen Present on Red Blood Cells: $B$ antigen

  • Antibodies Produced: Anti-$A$ antibodies

  • Type AB:

  • Genotype: $I^A I^B$

  • Antigens Present on Red Blood Cells: Both $A$ and $B$ antigens

  • Antibodies Produced: None (neither anti-$A$ nor anti-$B$ antibodies)

  • Type O:

  • Genotype: $ii$

  • Antigens Present on Red Blood Cells: None

  • Antibodies Produced: Both Anti-$A$ and Anti-$B$ antibodies

• Transfusion Reactions and Compatibility:

  • Incompatibility & Agglutination: When donor red blood cells carry antigens corresponding to antibodies present in the recipient's bloodstream, the recipient's antibodies attack the donor cells, causing red blood cells to clump together (agglutinate). Donor and recipient blood are incompatible.

  • Compatibility: When donor red blood cells do not react with recipient antibodies, the red blood cells remain evenly dispersed in suspension. Donor and recipient blood are compatible.

  • Universal Donors: Individuals with Type O blood ($ii$) possess red blood cells that lack $A$ and $B$ antigens. As a result, Type O red blood cells do not trigger agglutination in recipients with anti-$A$ or anti-$B$ antibodies, making Type O individuals universal donors.

  • Universal Recipients: Individuals with Type AB blood ($I^A I^B$) produce neither anti-$A$ nor anti-$B$ antibodies. Consequently, Type AB individuals can receive red blood cells of any blood type without initiating an antibody-mediated reaction, making Type AB individuals universal recipients.