Hi people out there, I’m Sofia and this is The Biologist Apprentice The Hox genes are a set of transcription factor genes that exhibit an unusual property They provide a glimpse of one way in which gene expression is translated into the many different forms that animals (metazoans) exhibit. In other words, they are master regulatory genes that direct the development of specific structures or segments of the body. the genome seems to be a welter of various genes scattered about randomly with no order present in their arrangement on a chromosome the order only becomes apparent in their expression through the process of development The Hox genes, in contrast, seem like an island of comprehensible structure These are genes that specify segment identity whether a segment of the embryo will form part of the head, thorax, or abdomen, for instance and they are all clustered together in one tidy spot. Within that cluster, there is even further evidence of order. To better understand the arrangement and role of Hox genes, take a look at the Drosophila melanogaster ex. in Drosophila there are eight Hox genes in a row, and the genes’ order within that row reflects their order of expression in the fly body the gene found on the left or 3′ end of the DNA strand, remember that there are two extremes the 5 ‘and the 3′ according to the directionality, or the chemical orientation of the ends of the DNA denoted lab (labial), is expressed in the head; on the other hand, the gene at the right end of the DNA strand, Abd-B (Abdominal-B), is expressed at the end of the fly’s abdomen. Knocking out individual Hox genes in Drosophila causes homeotic transformations in other words, one body part develops into another. A famous example is the Antennapedia mutant, in which legs develop on the fly’s head instead of antennae. The Hox genes are early actors in the cascade of interactions that enable the development of morphologically distinct regions in a segmented animal Indeed, the activation of a Hox gene from the 3′ end is one of the earliest triggers that lead the segment to develop into part of the head. And also there are examples with vertebrates including mice, have Hox genes that are homologous to those of the fly by homologous I meant similar sections and these genes are clustered in discrete locations with a 3′-to-5′ order reflecting an anterior to posterior order of expression. However, there are several differences between the mouse and fly Hox genes, One obvious difference is that there are more Hox genes on the 5’ side of the mouse segment these correspond to expression in the tail and flies do not have anything homologous to the chordate tail. Another difference is that, in the mouse, there are four banks of Hox genes HoxA, HoxB, HoxC, and HoxD Vertebrates have these parallel, overlapping sets of Hox genes, which suggest that morphology could be a product of a combinatorial expression of the genes in the four Hox clusters. in the fly, the situation is much simpler. Because each segment more or less expresses only one Hox gene mutating or knocking out a single Hox gene will have an effect on the corresponding body segment In vertebrates, though, each segment has at least two, and in some cases four, Hox genes that may be involved in its development. As a result, there is the possibility of redundancy. and redundant means that there are some genes that are repeated In mice, the HoxA3 gene is expressed in the anterior cervical vertebrae, near the region where the first neck vertebra articulates with the skull Deleting HoxA3 has no detectable effects on that joint; either its influence is too subtle to measure it affects some other aspect of cervical specification, or it has a partner gene that takes over its job in its absence in this case, HoxA3 has a paralog, or copy, called HoxD3, which is expressed in a very similar place. When HoxD3 is mutated all by itself, there is a serious abnormality; here, the first neck vertebra has a partial fusion with the base of the skull. but knocking out both HoxA3 and HoxD3 shows that HoxA3 is important after all without it, the first neck vertebra doesn’t form. So all this explanation and all this words, tell us that a combination of Hox genes is required for the proper development of the first cervical vertebra. This phenomenon is also one reason why homeotic mutations in vertebrates are so rarely seen in flies, one gene can be mutated, resulting in a haltere being transformed into a wing, or an antenna turning into a leg; in the mouse, two to four genes must be simultaneously removed to get a similar complete transformation. What the Hox code represents is a somewhat a mechanism for regulating axial patterning By mixing and matching combinations of the expression of a small number of Hox genes organisms generate a greater range of morphological possibilities. Genetic control of body shape is a difficult process to comprehend but the Hox system is one place in which researchers are getting closer to comprehending this process. if you want to know more about the Hox Genes, please check the description box you always going to find there sources, papers and more links so you can learn more and do your own research so thats it for this video, hope you guys liked it and learned something new don’t forget to subscribe to this channel and share these videos, also visit thebiologistapprentice.com follow me on my social media, i’ll be giving away promotion codes and that’s it, i’ll see you on the next video, byeee Subscribe!!