In this video, we're going to briefly introduce maternal effect genes. And so, the maternal effect is actually a non-Mendelian pattern of inheritance that does not follow the simple Mendelian pattern of inheritance that we've talked about before in some of our previous lesson videos. And so this maternal effect will actually occur when the offspring's phenotype is not determined by the offspring's genotype. Instead, the offspring's phenotype is going to be determined by the mother's genotype. And so, in other words what we can say is that in this maternal effect the mother's genotype will dictate the offspring's phenotype regardless of what the offspring's genotype actually is. Now, I know this might sound a little strange but the maternal effect does take place in many organisms and in some organisms it takes place fairly often. Now, what can be really helpful to help us better understand this maternal effect is if we reflect back on how meiosis occurs. And so, if we were to take a look at a cell that is heterozygous for a particular gene, meaning that it would have a dominant allele and a recessive allele, say a capital D and a lowercase d. Well, we know that before the cell undergoes meiosis, it would undergo DNA replication to replicate each of these alleles. And so, after DNA replication, which we can write in DNA replication here, we know that each of these alleles is going to be replicated and so we would end up with a single cell that is going to have identical replicates of each of these alleles. And so now that the cell has replicated its DNA it's ready to begin meiosis and we know from our previous lesson videos that meiosis consists of 2 rounds of cell division, meiosis 1 and meiosis 2. And so what you'll notice here is that meiosis 1, which would be this first round of cell division will separate or segregate these alleles. It will separate the dominant alleles from the recessive alleles. And so over here, we would have the 2 dominant alleles and over here we would have the 2 recessive alleles. And again this arrow here represents meiosis 1. And so what's really important to note is that although the dominant products are separate from each other, gene products of the dominant allele may still be found in this cell over here and some gene products of the recessive allele may still be found in, the left cell over here.
That is something important to keep in mind, because let's say for example, some leftover gene product of this dominant allele is found in this cell which we can just represent with a red dot. Well, as meiosis continues, again meiosis 2 would happen next where each of these cells would continue to divide and we would get these haploid cells at the end as we see here. The leftover gene products would also still be in here. The leftover gene products in some cases can end up influencing the offspring's phenotype. And so this is exactly what happens with this maternal effect and that's exactly what we're trying to explain here with this second bullet of text that we have right here. And so, basically, what this is saying is that the maternal effect can occur when the mother's egg ends up containing some leftover gene products, either some leftover mRNA or some leftover proteins and again those leftover gene products in our example or represented by this little red dot over here. And again those leftover gene products can end up influencing the offspring's phenotype and so this is how the mother's genotype ends up influencing the offspring's phenotype. Now, a classic example of the maternal effect is actually the shell coiling of this snail, Limnaea.
This snail, Limnaea, can either form right-handed coils or it can form left-handed coils. Now the right-handed coils are associated with the dominant allele and so a homozygous dominant genotype or a heterozygous genotype would form right-handed coils. Whereas a homozygous recessive genotype would end up forming left-handed coils. And so you might expect with simple Mendelian inheritance that a homozygous recessive offspring would form left-handed coils. However, because the coiling is an example of the maternal effect, it turns out that a homozygous recessive offspring will actually not form left-handed coils like what you might expect. Instead, the offspring's phenotype is only going to be dictated by the mother's genotype. And so, even though the offspring has a homozygous recessive genotype that you might expect would form left-handed coils, it turns out that if the mother is homozygous dominant or heterozygous, then this offspring is going to form right-handed coils and this is exactly what we mean by the maternal effect that the offspring's phenotype is going to be dictated by the mother's genotype. And so, down below, "
notice we've got this image of this snail's coils and its shell, and this is showing you a left-handed coil here. But again, this here really concludes our brief lesson on maternal effect genes and how the maternal effect is when an offspring's phenotype is dictated by the mother's genotype and it's not dictated by the offspring's genotype. And so, I'll see you all in our next video.
