Premed · Premed · Genetics
Lecture 3: Extensions of Mendelian Inheritance
Genetics
Learning Objectives
By the end of this lecture, students will be able to:
- Distinguish between incomplete dominance, codominance, and complete dominance
- Explain multiple alleles using the ABO blood group system as an example
- Define and provide examples of pleiotropy, epistasis, and gene interaction
- Describe how penetrance and expressivity modify phenotypic outcomes
- Explain the genetic basis of lethal alleles
- Discuss the role of environment in shaping phenotype
Lecture Content
I. Incomplete Dominance
In incomplete dominance, the heterozygote has a phenotype that falls intermediate between the two homozygotes. Although the F1 phenotype appears to be a "blend," the alleles themselves do not blend; they segregate normally in the F2 generation. The classic example is flower color in snapdragons (Antirrhinum), where RR produces red flowers, Rr produces pink flowers (an intermediate phenotype), and rr produces white flowers. The F2 ratio is 1 red : 2 pink : 1 white, a 1:2:1 phenotypic ratio that is identical to the genotypic ratio.
The molecular explanation for incomplete dominance is that the heterozygote produces half the amount of functional protein compared to the homozygous dominant organism, resulting in an intermediate level of the gene product, such as pigment. Another medically important example is familial hypercholesterolemia, where heterozygotes have elevated cholesterol levels while homozygotes have severely elevated cholesterol.
II. Codominance
In codominance, both alleles are fully expressed in the heterozygote, and neither allele is dominant over the other. Importantly, the heterozygote displays both parental phenotypes simultaneously rather than an intermediate blend. The classic example is the MN blood group system, where individuals with the genotype L^M L^M have type M blood (expressing the M antigen on red blood cells), individuals with L^M L^N have type MN blood (expressing both M and N antigens), and individuals with L^N L^N have type N blood (expressing only the N antigen).
Sickle cell disease provides an instructive example of how dominance relationships depend on the level of observation. At the molecular level, individuals with the genotype HbA/HbS produce both hemoglobin A and hemoglobin S, which represents codominance. However, at the clinical phenotypic level, heterozygotes show an intermediate severity, which looks more like incomplete dominance.
III. Multiple Alleles: The ABO Blood Group System
Although any individual carries at most two alleles at a given locus, a gene can have more than two alleles present in the population. The ABO blood group system, encoded by the ABO gene on chromosome 9, illustrates this beautifully. Three alleles exist: I^A, I^B, and i. The alleles I^A and I^B are codominant with each other, while both are dominant over i. Six possible genotypes give rise to four phenotypes: I^A I^A or I^A i produce Type A blood, I^B I^B or I^B i produce Type B, I^A I^B produces Type AB (an example of codominance), and ii produces Type O.
The clinical significance of the ABO system lies in blood transfusion compatibility. Anti-A and anti-B antibodies occur naturally, making Type O individuals universal red cell donors and Type AB individuals universal recipients. At the molecular level, the I^A and I^B alleles encode glycosyltransferases that add different sugar residues to the H antigen on red blood cell surfaces, while the i allele encodes a non-functional enzyme that leaves the H antigen unmodified.
<image>Panel A: Diagram showing the ABO blood group system with all six genotypes mapped to four phenotypes, including the molecular structure of A, B, AB, and O antigens on red blood cell surfaces. Panel B: Blood transfusion compatibility chart showing donor-recipient relationships. Panel C: Comparison of complete dominance (3:1 ratio), incomplete dominance (1:2:1 ratio), and codominance (1:2:1 ratio with distinct heterozygote phenotype) side by side with cross diagrams and phenotype illustrations.</image>
IV. Pleiotropy
Pleiotropy occurs when a single gene affects multiple, seemingly unrelated phenotypic traits. Sickle cell disease offers a vivid example: a single amino acid change in the beta-globin gene (HBB gene, Glu6Val mutation) leads to sickle-shaped red blood cells, anemia from red blood cell destruction, vaso-occlusive crises causing pain and organ damage, splenomegaly, and, in heterozygotes, increased resistance to malaria.
Marfan syndrome, caused by mutations in the FBN1 gene encoding fibrillin-1, affects connective tissue throughout the body, producing skeletal features (tall stature, long limbs, arachnodactyly), cardiovascular problems (aortic root dilation, mitral valve prolapse), and ocular manifestations (lens subluxation). Phenylketonuria (PKU), resulting from deficiency of phenylalanine hydroxylase, leads to intellectual disability, light pigmentation, musty body odor, and eczema. In reality, pleiotropy is the rule rather than the exception for most genes.
V. Epistasis
Epistasis occurs when the alleles of one gene mask or modify the expression of alleles at another gene. The gene doing the masking is called the epistatic gene, and the gene being masked is the hypostatic gene. Several types of epistasis produce characteristic modifications of the classic 9:3:3:1 dihybrid ratio.
In recessive epistasis (producing a 9:3:4 ratio), a homozygous recessive genotype at one locus blocks the expression of a second locus entirely. The classic example is coat color in Labrador retrievers. The E/e gene controls pigment deposition, while the B/b gene controls pigment color. The ee genotype is epistatic: it blocks all pigment deposition, producing a yellow coat regardless of the B genotype. Thus B_E_ dogs are black, bbE_ dogs are chocolate, and any dog with the ee genotype is yellow. Dominant epistasis (12:3:1) occurs when a dominant allele at one locus masks expression at another, as seen in squash fruit color. Duplicate recessive epistasis (9:7) requires at least one dominant allele at both loci for the trait to be expressed, as in flower color in sweet peas. Duplicate dominant epistasis (15:1) produces the phenotype when a dominant allele is present at either locus. Dominant-and-recessive epistasis produces a 13:3 ratio.
The key distinction to remember is that epistasis involves interaction between different genes, whereas dominance involves interaction between alleles of the same gene.
<image>Panel A: Diagram of recessive epistasis in Labrador retriever coat color, showing the two-gene pathway from pigment precursor to deposition, with genotypes mapped to black, chocolate, and yellow phenotypes. The 9:3:4 ratio is shown in a modified Punnett square. Panel B: Biochemical pathway diagram illustrating complementary gene interaction (9:7 ratio), where two enzymes encoded by different genes are both required to produce a pigmented product from a colorless precursor. Panel C: Summary table of all epistasis types with their modified ratios, gene interaction mechanisms, and classic examples.</image>
VI. Penetrance and Expressivity
Penetrance refers to the proportion of individuals with a particular genotype who actually express the associated phenotype. When penetrance is complete, 100% of individuals with the genotype show the phenotype. When penetrance is incomplete (reduced), fewer than 100% express it. BRCA1 mutations, for example, confer a roughly 70-80% lifetime risk of breast cancer, not 100%, and polydactyly does not appear in all carriers of the dominant allele.
Expressivity describes the degree or range of phenotypic expression among individuals who do express the trait. Variable expressivity means that affected individuals show different severity or manifestations of the condition. Neurofibromatosis type 1, for instance, ranges from mild cafe-au-lait spots to severe neurofibromas and learning disabilities. Marfan syndrome may present with severe cardiac involvement in some individuals while others primarily show skeletal features.
Several factors influence both penetrance and expressivity, including modifier genes at other loci, environmental factors such as diet, temperature, or exposure, epigenetic modifications, stochastic developmental variation, and age-dependent penetrance, as seen in Huntington disease.
VII. Lethal Alleles
Some alleles cause death when present in certain genotypic combinations. Recessive lethals are lethal only in the homozygous state and are the most common type. The yellow coat color allele in mice provides a classic example: A^Y/A heterozygotes have a yellow coat, A/A homozygotes display the wild-type agouti coat, but A^Y/A^Y homozygotes die in utero. Consequently, a cross between two yellow mice (A^Y/A x A^Y/A) yields a modified 2:1 ratio of 2 yellow : 1 agouti, rather than the expected 3:1. Tay-Sachs disease is another example of a recessive lethal, fatal in early childhood in the homozygous state.
Dominant lethals are lethal even in heterozygotes and are usually eliminated from the population unless they act after reproductive age. Huntington disease, a late-onset dominant lethal, persists in populations precisely because affected individuals typically reproduce before symptoms appear. Lethal alleles explain unexpected phenotypic ratios that deviate from standard Mendelian predictions.
VIII. Environmental Effects on Phenotype
The phenotype of any organism is the product of its genotype and its environment. Temperature-sensitive alleles provide elegant demonstrations of this principle. In Siamese cats, the melanin-producing enzyme tyrosinase is active only at lower temperatures, leading to dark coloring at the cooler extremities (ears, paws, tail, and face) and lighter coloring on the warmer core body. The Himalayan rabbit displays the same phenomenon.
Phenocopies are environmentally induced phenotypes that mimic genetic conditions. The drug thalidomide, for example, causes limb malformations that resemble genetic phocomelia. The norm of reaction describes the range of phenotypes that a single genotype can produce across a range of environmental conditions, emphasizing that genes do not determine phenotype in isolation. Twin studies help to separate genetic and environmental contributions to traits by comparing concordance rates in monozygotic versus dizygotic twins.
<image>Panel A: Illustration of penetrance vs. expressivity using a family pedigree where some individuals carrying an autosomal dominant allele show no phenotype (incomplete penetrance) and others show varying severity (variable expressivity), with a spectrum bar showing mild to severe expression. Panel B: Diagram of the yellow mouse lethal allele cross (A^Y/A x A^Y/A) showing the expected and observed ratios, with the embryonic lethal A^Y/A^Y class crossed out. Panel C: Photo-style diagram of a Siamese cat with temperature gradient overlay showing how cooler body extremities produce darker fur coloration due to temperature-sensitive tyrosinase enzyme activity.</image>
