or, more specifically, a question of lethal allelesA.
Lethal alleles were first noticed back in 1905 by Lucien Cuénot, and identified for what they were by Castle & Little in 1910. These two studies, looking at inheritance of yellow coat colour in mice, found that only mice heterozygous for this gene had yellow coats, despite yellow coat colour being the dominant trait, and that homozygous yellow mice died at the embryonic stage. That is, the lethal nature of the yellow coat allele is recessive (needs two copies to be expressed), while the actual pigmentation it confers is dominant to eg white coats, and so is expressed in heterozygous mice.
Similarly, in 1907 Edwin Baur was studying snapdragons, & noticed that while most plants had green leaves, some were golden. He characterised this trait as aurea, and found when he crossed 2 aurea plants, the resulting mature plants showed a colour ratio of 2 aurea: 1 green. The remaining 25% of these plants were ivory-white (lacked any functional chlorophyll) and died 2-3 days after germinating. Thus aurea is another example of a recessive lethal, because while the colour this allele codes for is dominant over green leaf colour, there need to be 2 copies for it to be lethal. Plants with one copy of each allele produce enough chlorophyll to get by. This is the example of lethal alleles used in the 2024 L2 Biology exam.
There are human examples of recessive-lethal disorders: sickle-cell anaemia is a recessive trait, and lethal in the homozygous form, as is cystic fibrosis. Achondroplasia is an autosomal dominant disorder, with heterozygotes exhibiting dwarfism, but again two copies of the allele are needed for lethality.
There are also dominant lethal alleles, the best-known of which in humans is probably the allele underlying Huntington’s disease (HD). It’s described as a dominant lethal allele because it’s expressed in both homozygous & heterozygous individuals; that is, both the trait and its lethality are dominant. However, it takes decades to cause the neurological symptoms that eventually kill those with HD, by which time they may have already passed it on. (Apparently there’s no difference in age of onset of HD between those with 2 copies of the allele and those with one, but the disease may progress faster in homozygous individuals.) Another factor that may maintain a dominantly-lethal allele in a population is what’s known as its penetrance – the proportion of individuals who actually develop symptoms. If penetrance is less than 100% then the allele may remain in the population even while it kills of most of those who have it.
Back to the exam question: it seems to me that students were being asked to recognise (& distinguish between) the fact that a trait could be lethal, and the dominance/recessiveness of the lethality itself. It’s quite possible that a Schol Bio candidate do that, but it seems a big ask for someone in year 12, who’ll have learned that lethal alleles exist & how to recognise their presence by looking at phenotypic ratios. Students aspiring to excellence may indeed be challenged with unfamiliar contexts (otherwise questions would become very predictable & boring). Novel concepts? Not so much.
A.An allele is one of two or more versions of a given gene ie different alleles of the same gene will have somewhat different DNA sequences. An individual will have 2 alleles for each gene, inheriting one from their mother & the other from their father. If both alleles of a gene are the same, we’d say the person was homozygous. If they’re different, we’d use the term heterozygous.