• Sex linked traits: Many species have what is known as a sex chromosome. Normally, every chromosome of a given set is the same size and shape, and most importantly, they carry the same genetic information. Not so with sex chromosomes. One variation of the chromosome is shorter, and does not carry the same information. When the standard and shorter chromosome (X and Y) are in the same cell, you have a Hemizygotic state. The suffix hemi come from ancient Greek, and means half. This refers to the idea that some of the genes in an XY pairing are haploid, not diploid. If the X has a gene that the Y does not possess, then it is always expressed (as would be the case in haploid organisms).

• Epistasis: Epistasis involves a two gene system. While the genotype follows Mendelian Laws, the phenotype does not. The reason is that one gene completely masks the effect of the second gene. Below is an epistatic example. Can you describe how this is different from a standard dihybrid cross?

• Pleiotrophy: This is when an allele has widespread impact in an organism, and so is not limited to one trait. For example, would a change in microtubles affect only one aspect of a cell? Would it affect only one cell type? Pleiotrophy may also cause problems at different stages of development, such as varying affects at different ages. Here is an example of pleiotrophy.
• Heterosis: Hybrid Vigor. Inbreeding can depress the adaptive strength of a species by allowing recessive traits a greater chance to express. When you begin crossing inbreed strains, you suddenly see an increase in adaptive vigor. This is useful with coordinated breeding of animals and plants, as the hybrid produced is stronger than the parental strains. It also informs our understanding of endangered species, and helps researchers work on ways to increase not only the adaptive vigor an an endangered species, but also the population size (without increasing inbreeding depression).

• Gene expression from environmental cues: Some genes are only triggered during certain environmental conditions. Remember the concept of signal pathways and signal transduction. The body picks up an environmental signal, and then tells the cells to change in some way. There are two terms you need to understand when dealing with Environmentally Cued Gene Expression:
  • Expressivity: This describes how much gene expression you get in an individual when exposed to the proper cues.
  • Penetrance: This is a population concept. When exposed to a given condition, how many individuals of the population express the gene?

Some additional reading for those interested in genetics:

• Gregor Mendel
• Mendel’s 7 genes and their locations on pea chromosomes
• Gregor Mendel and the Principles of Inheritance
• Pedigree Analysis (Howard Hughes Medical Institute)

Blood Typing 

The ABO blood type system is an example of codominance, meaning that in a heterozygous individual, both alleles are expressed, giving the individual two phenotypes (not a blending).  In other words, the alleles are considered dominant.  The ABO blood type is also multi-allelic, meaning there are more than two alleles for this one gene.  IMPORTANT:  The ABO blood type is derived from a single gene that has multiple alleles, of which two are considered dominant (A type and B type).

The gene (FUT1) for the ABO blood type system is a glycosyltransferase.  This enzyme adds carbohydrates onto proteins during post-translational modification.  The gene is found on chromosome 19. The antigen carrying proteins (the transmembranal protein that is glycosylated) currently has an unknown function.  The following shows the glycosylation patterns of the H antigen (the correct designation for the ABO antigen).

The fucose-galactose-N-acetyl-glucosamine glycosylation seen in O is common throughout the system.  It is the precursor to the other forms.  The FUT1 gene has allelic variation based on several SNPs.  The result is a difference in glycosylation patterns.  Specifically, we see the an additional N-acetyl-glalactosamine on A and an additional Galactose on B.

• If you have the allele for A, you produce the A glycosylation pattern.
• If you have the allele for B, you produce the B gylcosylation pattern.
• If you have the allele for A and B, you produce both A and B glycosylation patterns.
• If you have neither the A or B allele, then you produce the precursor O configuration only.
• If you are Heterozygous A or B, meaning you have the (Ai^o) or (Bi^o) genotype, then you will produce the O glycosalation pattern.  Remember though, A and B are dominant to O.

An important discovery for the ABO system was the discovery of Antigen H.  This discovery began in 1952 by Y.M. Bhende.  Dr. Bhende, working in what is now known as Mumbai, India, discovered a patient who reacted to all ABO blood types.  They built antibodies against all ABO blood.   This led to the realization that O blood was antigenic to this patient.  What is now known is that the specific glycosylation of O is an antigen, and is the precursor to the A and B phenotypes.  Antigen H is the antigen found on O blood.  Antigen H is the precursor to antigen A and antigen B, as such, antigen H is found in A, B, AB and O blood.

Individuals who do not produce antigen H, described genotypically as (h,h) (heterozygoun recessive for the H antigen), are intolerant to all ABO blood (they build a reaction against it).  This blood group (h,h) is known as the Bombay Blood Group.  It represents the an additional allelic variation to the ABO blood type system.

Daily Challenge

Even with multiple alleles in a population (consider the ABO, Bombay blood groups) and multiple genes (epistasis), Mendel’s laws still hold at the level of the genotype.  Explain how the laws of inheritance still hold true, and how variations such as multiple alleles, co-dominance, incomplete dominance, and epistasis all serve to increase population diversity.