Premed · Premed · General Biology 1
Lecture 18: Extensions of Mendelian Inheritance
General Biology I — Molecular & Cellular
Learning Objectives
By the end of this lecture, students will be able to:
- Explain incomplete dominance, codominance, and multiple alleles using examples
- Describe pleiotropy, epistasis, and polygenic inheritance
- Explain how environmental factors can influence phenotype
- Distinguish between Mendelian inheritance patterns and more complex patterns of inheritance
Lecture Content
I. Beyond Simple Dominance
Mendel's foundational principles remain valid, but many traits display inheritance patterns more complex than the simple dominant-recessive relationship he described. These are best understood as extensions of Mendel's work, not exceptions to it.
A. Incomplete Dominance
In incomplete dominance, the heterozygous phenotype falls intermediate between the two homozygous phenotypes because neither allele is fully dominant. Snapdragon flower color provides the textbook example: crossing a red-flowered plant (RR) with a white-flowered plant (R'R') produces pink F1 heterozygotes (RR'). When F1 plants are crossed, the F2 generation segregates in a 1 red : 2 pink : 1 white ratio--here the phenotype ratio matches the genotype ratio (1:2:1) because the heterozygote is distinguishable from both homozygotes. A medically relevant example is familial hypercholesterolemia: homozygotes for the normal allele (HH) have normal cholesterol levels, heterozygotes (Hh) have moderately elevated cholesterol, and homozygotes for the disease allele (hh) have severely elevated cholesterol and often suffer heart attacks in childhood.
B. Codominance
In codominance, both alleles are fully and simultaneously expressed in the heterozygote--neither is dominant, recessive, or intermediate. The ABO blood group system (discussed further below) illustrates this: individuals with the IAIB genotype express both A and B antigens on the surface of their red blood cells, producing type AB blood. The MN blood group provides another clean example: LMLN heterozygotes display both M and N antigens.
C. Multiple Alleles
While any individual carries at most two alleles for a given gene, the population as a whole may harbor many more. The ABO blood group system involves three alleles--IA, IB, and i--that combine to produce six possible genotypes and four phenotypes. IA and IB are codominant with each other (both expressed in IAIB individuals) but both are dominant over i (the "recessive" allele that produces no antigen). The resulting phenotypes--Type A (IAIA or IAi), Type B (IBIB or IBi), Type AB (IAIB), and Type O (ii)--have critical implications for transfusion compatibility and organ transplantation.
II. Pleiotropy
Pleiotropy occurs when a single gene affects multiple, seemingly unrelated phenotypic traits. Sickle cell disease is a paradigmatic example: a single nucleotide change in the HBB gene (replacing glutamic acid with valine at position 6 of beta-globin) produces hemoglobin that polymerizes under low-oxygen conditions, distorting red blood cells into a sickle shape. This one molecular defect cascades into anemia, excruciating pain crises, organ damage, spleen enlargement, and increased susceptibility to infection. Intriguingly, heterozygous carriers (sickle cell trait, HbAS) enjoy resistance to malaria--a heterozygous advantage that explains why the sickle cell allele persists at high frequencies in malaria-endemic regions. Marfan syndrome (mutation in FBN1, encoding fibrillin-1) affects connective tissue throughout the body, producing skeletal abnormalities, lens dislocation, and life-threatening aortic aneurysms. Phenylketonuria (PKU) results from deficiency of phenylalanine hydroxylase; the accumulation of phenylalanine affects brain development, skin pigmentation, and multiple other tissues.
III. Epistasis
In epistasis, one gene influences the expression of another gene at a different locus. The gene that exerts the overriding effect is called the epistatic gene, and the gene whose expression is masked or modified is the hypostatic gene. Epistasis modifies the expected 9:3:3:1 dihybrid ratio.
In recessive epistasis, the homozygous recessive genotype at one locus masks the expression of the other locus entirely. Labrador retriever coat color provides an elegant example: one gene (B) determines pigment color (BB or Bb = black; bb = brown/chocolate), while a second gene (E) controls whether pigment is deposited at all (EE or Ee = pigment deposited; ee = no pigment deposited = yellow coat). Dogs homozygous recessive at the E locus (ee) are yellow regardless of their B genotype, yielding a modified ratio of 9 black : 3 chocolate : 4 yellow. In dominant epistasis, a single dominant allele at the epistatic locus masks the other gene, producing ratios such as 12:3:1. Complementary gene interaction requires dominant alleles at both loci to produce a particular phenotype, yielding a 9:7 ratio.
<image>A diagram illustrating epistasis in Labrador retriever coat color. Top: A dihybrid cross (BbEe x BbEe) shown in a 4x4 Punnett square. The 16 offspring are color-coded: 9 B_E_ = black dogs, 3 bbE_ = chocolate/brown dogs, 3 B_ee = yellow dogs, 1 bbee = yellow dog. The standard 9:3:3:1 ratio is modified to 9:3:4 because the ee genotype (recessive epistasis) masks the B gene, making both B_ee and bbee dogs yellow. Photos or drawings of black, chocolate, and yellow Labrador retrievers illustrate each phenotype.</image>
IV. Polygenic Inheritance
While Mendel's traits were each controlled by a single gene with discrete phenotypic classes, many traits are influenced by multiple genes, each contributing a small additive effect. Such polygenic traits produce a continuous range of phenotypes that follow a bell-shaped (normal) distribution in the population. Human skin color involves at least three to four major genes (including SLC24A5, SLC45A2, TYR, and MC1R), with each gene contributing alleles that add varying amounts of melanin pigment. The more "dark" alleles an individual carries, the darker the skin tone. Human height is influenced by hundreds of genes, plus environmental factors like nutrition and overall health. Eye color, once thought to be a simple trait, is now known to involve at least fifteen genes, though OCA2 and HERC2 have the largest effects. Genomic regions associated with quantitative traits are called quantitative trait loci (QTL).
V. Nature vs. Nurture: Environmental Influences on Phenotype
An organism's phenotype is not determined by genotype alone--it emerges from the interaction of genotype and environment. The norm of reaction describes the range of phenotypes a single genotype can produce across different environmental conditions. Some traits have a narrow norm of reaction: blood type is genetically determined and unaffected by environment. Others have a broad norm of reaction: human height is strongly influenced by nutrition, disease exposure, and other environmental factors during development.
Striking examples of environmental influence abound. Hydrangea flowers from the same genotype turn blue in acidic soil and pink in alkaline soil. Siamese cats have a temperature-sensitive tyrosinase enzyme that produces dark pigment only in cooler body regions (ears, paws, tail, and nose), creating their distinctive point coloration. PKU is genetically determined, but the intellectual disability it causes can be prevented by restricting dietary phenylalanine--a powerful demonstration that even strongly genetic conditions can be environmentally modified.
Penetrance refers to the proportion of individuals with a particular genotype who actually display the expected phenotype. Complete penetrance means all carriers show the trait; incomplete penetrance means some do not (BRCA1 mutations, for example, confer approximately a 70% lifetime risk of breast cancer, not 100%). Expressivity describes the degree to which a trait manifests in individuals who do show it. Variable expressivity means the same genotype can produce a range of severity--polydactyly (extra digits), for instance, can vary from a small skin tag to a fully formed extra finger.
<image>A two-panel figure. Panel A: Polygenic inheritance of skin color — a histogram showing the distribution of skin pigmentation in a hypothetical population where three genes (AaBbCc) each contribute additively. The x-axis shows number of "dark" alleles (0 to 6), the y-axis shows frequency. The distribution is bell-shaped. Individuals with 0 dark alleles are lightest; those with 6 are darkest. Example genotypes are labeled at key points on the curve. Panel B: Environmental influence on phenotype — a Siamese cat diagram showing dark coloration at the cooler extremities (ears, nose, paws, tail) and light coloration on the warmer body core, with an annotation explaining that the tyrosinase enzyme is only active at lower temperatures (below ~33 degrees C).</image>
VI. Sex-Linked Traits (Introduction)
Genes located on the X chromosome display a distinctive inheritance pattern because males (XY) are hemizygous--they carry only one copy of each X-linked gene. X-linked recessive traits are therefore more commonly expressed in males, since a single recessive allele on their sole X chromosome is sufficient to produce the phenotype. Females require two copies of the recessive allele to be affected; heterozygous females are phenotypically normal carriers who can pass the allele to their sons. A hallmark of X-linked recessive inheritance is the absence of male-to-male transmission, since fathers contribute their Y chromosome (not their X) to sons. Red-green color blindness, hemophilia A, and Duchenne muscular dystrophy are well-known X-linked recessive conditions. A more detailed treatment of sex-linked inheritance follows in Lecture 19.
VII. Linked Genes (Preview)
Genes located close together on the same chromosome tend to be inherited together, violating Mendel's law of independent assortment. The frequency of recombination between linked genes depends on the physical distance separating them on the chromosome--a principle that forms the basis of genetic mapping. This topic will be explored in full detail in Lecture 19.
