Variation and sexual reproduction

Sexual versus asexual reproduction

Sexual reproduction has a few disadvantages compared to asexual reproduction. It requires more than just one individual to parent each offspring, rendering about half of the individuals unable to carry through the bearing of offspring, while allowing just half of each parent's contribution to be inherited by their offspring.

The advantage that makes it significant enough to outweigh the disadvantages is increased diversity. With each generation, diversity is inevitably inscribed into offspring, and the sources of genetic variation maximised with each cross. In sexual reproduction, individuals are simply forced to mix by default. As such, even very accomplished parental genomes are inevitably disrupted.





The overall effect is a better position for responding to selection pressures, as exemplified by the Red Queen hypothesis where individuals rely on varied strategies to keep up with their parasites, predators, etc. Since the standard for fitness is ever-changing, a real time diversified gene pool is maintained in populations.

Asexual reproduction is optimal in niche environments that are more stable, or when recolonising habitats that were previously disturbed. Good example of this are plants that reproduce clonally, and parthenogenic animals. It is advantageous for plants to spread via any part of their body e.g. stem, roots, rather than have to rely on fertilisation via flowers. This ability is used by growers to study cloned plants.

Parthenogenic animals reproduce asexually using unfertilised eggs. In cooler environments, the pressures from parasites are lower, as their density is lower and they cannot thrive in the cold. This allows parthenogenic animals to reproduce asexually and forego the higher diversity enabled by sexual reproduction.

Some asexually reproducing organisms such as bacteria have options for horizontal transfer of genetic information e.g. the sharing of plasmids between individuals, even between different species.


Meiosis

Meiosis is a type of cell division which results in 4 cells that are genetically non-identical from one parent cell. In order for once cell to divide to result in 4 cells, how many divisions must take place?

Two. 1 cell becomes 2, then 2 become 4:




The first division is called meiosis I, and the second is called meiosis II.

...so far so easy? (it should be!)

Cells resulting from meiosis are gametes such as egg cells and sperm cells, hence meiosis only occurs in sexually reproducing organisms. There are 2 key points about this:

1. Gametes are genetically unlike one another - while cells in other tissues such as muscle or blood must be genetically identical to one another (clones), the very basis of sexual reproduction is genetic diversity. So somewhere in the process of division, something takes place which creates genetic diversity (we'll come to that shortly).

2. If gametes are to fuse and result in a new organism, it is essential that the number of chromosomes should stay constant. Humans have 46 chromosomes in each cell (of course, apart from cells without DNA in them, and "spoiler alert!", gametes) - if each gamete had 46 chromosomes, then fusing 2 together would result in a zygote with 92 chromosomes, whose offspring would have 184 chromosomes, and before you know it something terrible would have happened.

The above picture illustrates how the number of chromosomes is halved in the final 4 cells. The terms diploid and haploid refer to the number of sets of chromosomes. In humans, somatic cells (i.e. cells other than gametes) are diploid because there are two sets of chromosomes. Gametes are haploid because they have only one set of chromosomes.

A "set" is made up of all chromosomes which are unique, i.e. are not paired with any homologous chromosomes.

X x X X x X x x                          <----------        haploid = 1 set

XX xx XX XX xx XX xx xx         <----------        diploid = 2 sets

In the first XX, X and X are homologous chromosomes because they occupy the same space and contain DNA with similar purpose/function. Essentially, they are more or less copies of each other. So when 2 gametes fuse, they form a diploid cell with the complete number of chromosomes.

Wikipedia does us the honour with this epic picture:



On to the very important bit now...

How does meiosis achieve genetic diversity without which you would actually look *just* like your siblings?

10 words: Independent Assortment of Homologous Chromosomes, &

                Genetic Recombination by Crossing Over

What an unnecessary mouthful. You still have to learn them though.

Independent assortment of homologous chromosomes means that in meiosis I, when the original diploid line-up a.k.a. XX xx XX XX xx XX xx xx becomes X x X X x X x x in 2 resulting cells, which big X's and which small x's end up with each other in each cell is random. Pretty simple concept.

Xx

If you split the homologous chromosomes, you get Xx in 2 cells. The idea is that there is no rule saying that black must go with black, and red must go with red. You can end up with Xx and Xx, or Xx and Xx with an equal probability. What can I say, genetics likes being a bit random.

Not assigning the expected chromosome or chromatid during meiosis is called chromosome non-disjunction and results in a cell with a different number of chromosomes.




Translocation is a type of chromosomal mutation where one or more nucleotide bases are moved between non-homologous chromosomes e.g. AAGCTT on human chromosome 1 is moved and becomes AAGCTT on human chromosome 3



Genetic recombination by crossing over is a lot more interesting. It's like a bowl of spaghetti. Homologous chromosomes snuggle each other and exchange parts in the process:




Did I mention how important it is to use accurate scientific terminology in the exams? The process is called synapsis, during which mutual exchange of genetic information occurs.

Fascinating stuff.

As a finishing touch, I read of this mnemonic to remember the purpose of meiosis.

akes
ggs
n
varies,
perm
n
crotum

It is so cringe-worthy, I would rather memorise meiosis off by heart.

In some plants, mitosis follows meiosis before the gametes fuse, in order to create a haploid organism. Essentially, these plants e.g. ferns have two versions of their body, one haploid and one diploid.




This reproductive mechanism is termed alternating generations, and involves the haploid plant producing gametes that then form a diploid plant which forms different gametes (spores) to form a new haploid plant, and so on.


Determination of sex

While some species are hermaphroditic (each individual has two sexes), others have sex determination in separate individuals based on environmental cues during development such as temperature and resource availability, or based on genetic factors e.g. chromosomes.

Some factors that can shift an individual's sex in some species include their size, parasitic infection or competition. For example, crustaceans such as shrimp are born without a sex, and are prone to being directed by various cues. A paramyxean parasite was found to drive shrimps to turn female, causing an imbalance in the population and disrupting marine habitats.

The Y chromosome in mammals and some insects is the male sex determinant through a specific locus called SRY that enables subsequent cascades of expression of the male sex developmental path. All the products are contained on other chromosomes, including the X chromosome, so the Y chromosome merely carries the on switch for all of this.

This means that all genes on the X chromosome (which has a common ancestry with the Y chromosome resembling homologous chromosomes) are one copy in XY individuals, and 2 copies in XX individuals. In diploid organisms where all chromosomes are represented twice, this leaves a vulnerable loophole for recessive conditions carried on the X chromosome.

Sex linkage refers to a trait that is carried on a sex chromosome like the X chromosome. Because someone might have a different number or combination of sex chromosomes such as a single X chromosome or two X chromosomes, the expression of various traits can differ.




If for example the allele on the affected X chromosome means that an essential protein isn't being made, the carrier XX child has another unaffected X chromosome to fall back on and be able to produce the essential protein. The same loci (genes) on the second X chromosome are randomly inactivated to prevent double the dose of their products in the cell. This means that as a carrier, half the cells in tissues will be expressing one X, and the other half the other X. This prevents the expression of the illness.

The carrier XY child only has the affected X chromosome and cannot make the protein. This results in an illness for example, such as haemophilia. Haemophilia is a blood clotting disorder in which excessive bleeding takes place because the platelet plug and fibrin which are supposed to stop bleeding do not work fully.




Another example of an X-linked condition is Duchenne muscular dystrophy which affects around 1 in every few thousand children with an XY genotype. It is a progressive muscle degeneration condition that affects mobility, and in the long term requires assistive ventilation devices, with an overall impact on life expectancy. 

The inheritance pattern matches that of haemophilia.

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