6 H

6.1 Talking Glossary: Haploid (1.25 min)

https://www.genome.gov/genetics-glossary/haploid

Abstract: “Haploid is the quality of a cell or organism having a single set of chromosomes. Organisms that reproduce asexually are haploid. Sexually reproducing organisms are diploid (having two sets of chromosomes, one from each parent). In humans, only their egg and sperm cells are haploid.”

Audio: https://www.genome.gov/sites/default/files/tg/en/narration/haploid.mp3

Transcript: “Haploid refers to a cell or an organism that has only a single set of chromosomes. This is to be contrasted with diploid.”Di” means two, of course. So most animal cells and plant cells are diploid. Then they’re diploid in part because they got one chromosome from their mother and one chromosome from their father, therefore making them diploid. A haploid cell only has one set of chromosomes, and most of the time that refers to the so-called sex cells, either eggs or sperm. And these are a critical transition from a diploid cell to a haploid cell to allow normal reproduction to occur, so that when these two haploid cells come together with a single set of genetic information–single chromosomes–they can come together into a so-called zygote made of when the egg cell and the sperm cell come together that then reconstitutes a diploid cell, which can then become a new individual.”

Christopher P. Austin, M.D.

Image: https://www.genome.gov/sites/default/files/tg/en/illustration/haploid.jpg

6.2 Talking Glossary: Halplotype

A haplotype is a set of DNA variations, or polymorphisms, that tend to be inherited together. A haplotype can refer to a combination of alleles or to a set of single nucleotide polymorphisms (SNPs) found on the same chromosome. Information about haplotypes is being collected by the International HapMap Project and is used to investigate the influence of genes on disease.

Audio: https://www.genome.gov/sites/default/files/tg/en/narration/haplotype.mp3

A haplotype is in its most general sense referring to a set of DNA variations along a chromosome that tend to be inherited together because they’re very close together. They get inherited together because they’re not generally crossovers or recombinations between these markers or between these different polymorphisms because they are very, very close. So a haplotype can refer to a combination of alleles in a single gene, or it could be alleles across multiple genes. It could be single nucleotide polymorphisms that are not in a gene but are in-between genes. Basically, it just means that these are variations in the DNA that are so close together that they tend not to recombine, and therefore tend to be passed down through the generations together. And the International HapMap Project has given us a very excellent tool to detect these regions of haplotypes that are passed together and to use those in genetic studies.

Joan E. Bailey-Wilson, Ph.D.

6.3 Talking Glossary: Heterozygous

https://www.genome.gov/genetics-glossary/heterozygous (Links to an external site.)

Abstract: “Heterozygous refers to having inherited different forms of a particular gene from each parent. A heterozygous genotype stands in contrast to a homozygous genotype, where an individual inherits identical forms of a particular gene from each parent.”

Audio: https://www.genome.gov/sites/default/files/tg/en/narration/heterozygous.mp3

Image: https://www.genome.gov/sites/default/files/tg/en/illustration/heterozygous.jpg

Transcript: “Heterozygous is a state of having inherited different forms of a particular gene from each one of your biological parents. Now, by different forms we generally mean that there are different portions of the gene where the sequence is different. They may be inconsequential portions of the gene, or they may in fact be pretty important portions of the gene. That doesn’t really matter for our discussion today. The word”heterozygous” simply means that your biological mother and your biological father, when they contributed their copies of a particular gene to you, they did so in a way so that the DNA sequence is slightly different. It can be different at one point in the gene, or it can be different at dozens and dozens of different points in the gene. Now, a heterozygous genotype stands in contrast to a homozygous genotype. And in the case of a homozygous genotype, we’re talking about a case where we’ve gotten identical forms of a particular gene from each biological parent. That is, if we were to read along the DNA sequence that mom gave you and the DNA sequence that dad gave you, we would find absolutely, positively no differences in that gene or in the region of the gene that we’re concerned about. “Heterozygous” meaning different, “homozygous” meaning the same.”

Amalia S. Dutra, Ph.D.

Amalia Dutra, Ph.D, is a Uruguayan genetic biologist known for being part of the team that mapped the human genome

6.4 Homology:L When things are homolgous AND analogous!

By: Dr. Brouwer

The topics of homologous and analogous structures are often discussed in relation to convergent evolution and homoplasy. It can be difficult to keep track of the exact definitions and when each term applies; in this short reading, I’ll reiterate the difference between homologous and analogous features and discuss an important facet that is often not brought up when these things are discussed.

In this Mometrix video – like many videos on the topic – they discuss homologous structures by comparing the arm of a human, the wing of a bat, the flipper of a whale, and the front leg of a cat (1:31). All of these anatomical structures contain the exact same bones, though they are used for different forms of movement.

To be more precise and clearer, instead of calling these arms, fins etc, they should call them forelimbs. At the level of bone structure, these forelimbs are homologous. Their homology is defined based on similar development and morphology and has nothing to do with function. To me, to refer to these different organisms’ forelimbs as arms, flippers etc. brings in aspects of function which aren’t relevant to consideration of homology and can cause confusion when you start thinking about analogous structures and convergent evolution.

When they discuss analogous structures, they show the wings of bats, birds, and butterflies. Bat wings and bird wings are analogous structures. (Comparing vertebrate and insect wings is a trivial example but a useful starting point). Vetebrate wings have the same function - flight - but have key differences. For example, bats create the wing using membranes derived from skin tissue (epidermis), while birds use feathers (which are derived from scales). Moreover, bats spread out the membranes of their wings with their fingers, while birds spread out their feathers by making the feathers themselves stiff. These structures are therefore analogous when we consider them from the perspective of anatomical function. Therefore, while discussion of homology doesn’t require considering function, discussion of analogy does. In contrast, the flippers of whales and dolphins are not analogous because they share a common ancestor with flippers, and they function the same.

From the perspective of natural selection, bat wings and bird wings are also an example of convergent evolution. Starting from different initial types of organisms - terrestrial rodents and terrestrial dinosaurs, respectively - both bats and birds converged on the ecological strategy of flight. That is, they converged on the strategy of using their forelimbs as wings. (You could say birds and butterflies converged on the strategy of flight but this is trivial since they do very different things while flying; bats and birds, however, often compete for the same resources, are eaten by similar predators, etc).

But wait - in our above example of forelimbs we had humans, whales, cats and birds, and we said these were homologous. These are all tetrapods, and bats are tetrapods too, so shouldn’t bat forelimbs also be homologous to these others? Yes, bat forelimbs are homologous to all these others. But we just said that bat wings and bird wings are analogous! Yes, that’s true too. At the level of the forelimb bone structure, bird and bat forelimbs are homologous. But at the level of the functional wings, they are analogous because they achieve their function - key to consideration of analogy and convergent evolution - very differently.

Sadava et al (11th edition) puts it this way

“Any features shared by two or more species that have been inherited from a common ancestor are said to be homologous. Homologous features may be any heritable trait, including DNA sequences, protein structures, anatomical structures, and even some behavior patterns. For example, all living vertebrates have a vertebral column, as did the ancestral vertebrate. Therefore the vertebral column is judged to be homologous in all vertebrates.”…similar traits may evolve independently in different lineages, a phenomenon called convergent evolution. For example, although the wing bones [I say forelimbs] of bats and birds are homologous, having been inherited from a common tetrapod ancestor, the wings of bats and birds are not homologous because they evolved independently from forelimbs of different nonflying ancestors. Functionally similar structures that have independent evolutionary origins are called analogous characters.” Sadava et al (11th edition, pg 451).

6.5 Talking Glossary: Homozygous

https://www.genome.gov/genetics-glossary/homozygous (Links to an external site.)

Abstract: “Homozygous is a genetic condition where an individual inherits the same alleles for a particular gene from both parents.”

Audio: https://www.genome.gov/sites/default/files/media/audio/2019-04/homozygous_narration.mp3

Image: https://www.genome.gov/sites/default/files/tg/en/illustration/homozygous.jpg

Transcript: “Homozygous describes the genetic condition or the genetic state where an individual has inherited the same DNA sequence for a particular gene from both their biological mother and their biological father. It’s often used in the context of disease. We talk about a situation where an individual has inherited a mutant allele or an error in DNA sequence from their mother and they have inherited the identical mutant allele from their father. We would then say that individual is homozygous for that mutation. They have two identical copies of the deleterious version of that gene and, as a result, they are then going to be predisposed to the genetic condition the gene codes for.”

Amalia S. Dutra, Ph.D.