Featured image: Venus, a cat that is either a clear example of the outcome of X-chromosome inactivation, or a chimera.
Here in Aotearoa New Zealand, senior high school students are currently sitting their external exams. This year’s Level 2 Biology papers have attracted quite a bit of attention from teachers, who’ve expressed concerns about the focus of particular questions.
One of those exam papers is for the Achievement Standard titled Demonstrate understanding of genetic variation and change, Question 1 begins with a section where the context is the inheritance of fur colour and tail length in cats ie the question is about dihybrid inheritance. The contextual information for the question notes that the genes involved (those coding for fur colour & tail length) are not linked, which relates back to what is known as the assessment specification. This is a document that sets out the detail or guidelines of what the exam might look like or could contain. The Achievement Standard sets out the conceptual knowledge and understanding that might be assessed, and the part relevant to Question 1 says this:
[students could be assessed on their knowledge of] …
* monohybrid inheritance to show the effect of co-dominance, incomplete dominance, lethal alleles, and multiple alleles
* dihybrid inheritance with complete dominance
* the effect of crossing over and linked genes on dihybrid inheritance
Now, that first part of the question is straightforward, as students need to consider only dihybrid interitance, but the second part has seen teachers bothered. Now that I’ve seen it, I’m bothered too.
The second section of Q1 is about sex-linkage and (nominally – we’ll come back to this) codominance, and a key concern for the teachers is that sex-linkage is not included in that list of concepts that students should master. Note that sex linkage is not the same as gene linkage (aka genetic linkage). The concept of linked genes/gene linkage describes how two genes that are closely associated on the same chromosome are frequently inherited together. Students need to be aware of gene linkage as a potential complicating factor when examining patterns of dihybrid inheritance.
However, sex linkage applies to genes that are located on the sex chromosomes … [and] are considered sex-linked because their expression and inheritance patterns differ between males and femalesA. (Sex-linked genes can also be genetically-linked, but in that instance you’d be discussing two separate genes inherited together because they are close together on – usually – the X chromosome.)
So, inheritance patterns of orange/black fur in cats is an example of sex-linkage, but not of linked genes – as the information in the Achievement Standard makes clear, linked genes are studied in the context of dihybrid inheritance. Given the Standard, teachers would have focused on the latter. Yes, students should be able to apply their knowledge in unfamiliar contexts, but examining them on unfamiliar concepts seems rather unfair. (Those setting it may have assumed that students would have covered sex-linkage in year 11, but some students might enter year 12 biology without taking relevant standards at year 11, & so this question could have thrown them completely. And students would expect to be taught and tested on year 12 content.)
Now, the codominance part. I said “nominally” because I don’t think that the paper is using a good example of codominance, which is a phenomenon in which in any given cell both alleles of a particular gene are expressed equally. If we look at this at the cellular level, codominance is seen in diploid cells, because only they will have two alleles for each gene locus on autosomal chromososmes. Perhaps the most familar example of this is the ABO blood group system in humans. Here, someone with the blood group AB has one copy of the A allele & one of the B, and is expressing both A and B proteins on the surface of every red blood cell. The human MN blood group is another exampleB.
The exam paper uses the example of orange, black/brown, & tortoiseshell coat colour in male & female cats. The gene locus for this trait is found on the X chromosome. Male cats, with a single X chromosome, could be solid orange, or black/brown. While females have two X chromosomes, one of these is randomly inactivated in each cell early in embryonic development, with the result that for coat colour, females with a copy of both the orange and the black/brown alleles end up with coats that have patches of both colours ie tortoiseshell, with orange patches where the ‘orange’ allele is on the active X chromosome & black/brown patches where the other X is active. (Two orange alleles will see them being solid orange; 2 black/brown alleles again see them one solid colour) A tortoiseshell female displays both coat colours – but because of that random X chromosome inactivation, not because of codominanceC.
There’s another question in that paper that I want to look at, but I’ll do that in a separate post.
A This means that the statement in the 2025 specs that “understanding of linked genes is considered to include sex linkage” isn’t appropriate. It’s worth noting that Campbell Biology, a commonly-used biology textbook at university level, clearly treats the two as distinct, separate concepts. I see no value in suggesting they be conflated at secondary school level because misconceptions, once acquired, are actually quite hard to change, and that won’t be helpful in uni-level genetics classes.
B I’ve seen websites that say that variegated flowers are an example of codominance, but how can this be, given their cells are diploid and so all contain both alleles at the ‘colour’ locus? However, mutations, viral infections, transposons and a trait known as pattern variegation are all known to cause variegated colour patterns in flowers.
C You can have tortoiseshell cats that are predominantly black/brownD (& calico cats that are largely white, with a few colour patches), so it can’t even be said that the two coat colours are always expressed equally.
D We had a tortoiseshell cat like that when I was a little kid. Her name was Smut. Mum said it was for her very dark fur, but dad always reckoned it was because when she meowed she sounded like she was swearing.