Plant and animal breeding

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Heredity and breeding

The diversity in plant and animal genetics and features can be manipulated by humans through artificial selection. This can focus on traits related to productivity for plant crops, resistance to environmental stress or illness, specific appearance or function.

The nutritional value of foods can be maximised in this way, alongside the yield for maximum product or growth that is environmentally sustainable.

For example, cotton has been bred for a larger boll while dairy cows have been bred for increased high quality milk production. In order to determine the traits that are present in different cultivars of plants, field trials are carried out in different environments. This is also used to assess GM (genetically modified) crops.

Designing these field trials requires a selection of fair treatment, using replicates and randomising treatment. Fair treatment refers to carrying out tests that apply equally to the test cultivars. For example, it would be fair to expose them all to a specific pest that is an issue in that location, but would be unfair to give them fertilisers that one cultivar is unable to metabolise.




Replicates involve having multiple sets of identical test crops to gather data from. This enables statistical analysis of the data by accounting for the inherent variability present between individuals.

Randomised treatment refers to administering things or doing whatever the test involves in such a way that removes bias from the process of then measuring the effects. For example, administration of a new pesticide should be randomly distributed between test GM and non-GM crops, so that no human bias in the form of assumptions about the way the crops interact with the pesticide would result in skewed assessments being made.


Outbreeding, inbreeding and cross breeding

Outbreeding refers to introducing a different genetic makeup to a breeding lineage, while inbreeding means introducing the same genetic makeup to a breeding line. Cross breeding involves introducing two purebred lines to each other. Purebred lineages are created by selectively breeding to obtain true-to-type organisms.

Animals and cross-pollinating plants inherently outbreed, while self-pollinating plants inbreed.




Inbreeding organisms involves sequential breeding over multiple generations up to the point where breeding has become true to the desired type. At this stage, the organisms are purebred. The breeding has removed heterozygotes from the gene pool. A problem that can arise from inbreeding is inbreeding depression due to accumulation of recessive, deleterious homozygous alleles.


Basics

Versions of the same gene that give rise to different phenotypes are called alleles. For example, the gene responsible for ear shape in a cat may have 2 alleles: pointy shape and oval shape.

One of these alleles may be expressed at the expense of another, where both are present together in a cat. Say that the oval shape allele is dominant while the pointy shape is recessive.

Because cats also have 2 copies of each chromosome (and therefore gene), these alleles are written as 2 letters. 

If the gene for ear shape is abbreviated as E or e for ear, then the alleles would be:

E (dominant) for oval shape and 
e (recessive) for pointy shape

These upper case - dominant, lower case - recessive notation rules are used universally. So what combinations may be present in a cat?

EE, ee or Ee

The first two examples are called homozygous because the same allele (E or e) is present twice, while the last example is heterozygous because different alleles (E and e) are present.

So what happens in crossing?

EE x ee gives rise to 4 combinations: Ee, Ee, Ee and Ee! 100% heterozygous where the cats will appear oval-ear shaped, yet also carry the recessive allele for pointy ears. Let's do a second cross.

Ee x Ee gives rise to 4 combinations: EE, Ee, eE and ee. That's 50% homozygous and 50% heterozygous. 3/4 will have oval ears while 1/4 will have pointy ears.




Now see what happened in Mendel's pea plant experiments?

It's also possible to have multiple (more than 2) alleles for a given gene. Say there is also a round ear allele. It could be that this allele is codominant with the pointy ear allele, so that both traits are simultaneously expressed.

The reason self-pollinating plants which inbreed do not suffer from inbreeding depression is as a result of natural selection limiting the prevalence of those alleles which are deleterious.

In outbreeding populations, desired characteristics are still selected for, but the rest of the genetic information and traits are still kept diverse, thus avoiding inbreeding depression.


Cross breeding

Cross breeding involves introducing new features into a separate population by breeding it with individuals from a population with the desired trait.

In animals, the cross produces F1 hybrids as the first generation, exhibiting the "best of both" features of the parents. This cannot be maintained indefinitely, because by the F2 (second) generation the offspring have a wide mixture of different genotypes. Selection of the desired genotypes (which would be only a few of the total number of produced offspring) alongside backcrossing would be required to maintain the new crossbreed. Backcrossing involves, quite literally, crossing the offspring back with its parent, or a parent-like individual in order to maintain the desired traits.




It is also possible to keep the original cross parents and produce offspring, some of which will be the desired crossbreed.

In plants, the crossbreed F1 generation takes on the traits of its parents and produces a heterozygous crop. These hybrids have better yield and vigour (health). However, the F2 generation from these better hybrids is an underwhelming mixture of variable plants that cannot be used for the same purposes.

Their variability can be potentially useful in that it provides greater variety, and these would have to be tested by test crosses in order to identify the unwanted varieties with heterozygous recessive alleles.


Gene technology

Delivering DNA into cells for various purposes can be achieved via viruses which naturally can infect certain cells, as well as gene guns (biolistics) for plants.

Delivering DNA to plants via gene guns involves firing tiny metal pellets covered with DNA into plant cells.




Once inside the cell, the DNA delivered will be transcribed and translated by cell machinery. The protein encoded by the DNA depends on the gene included. This varies by application.


Super soya beans

The first genetically modified (GM) soya beans were introduced by Monsanto in 1994. They now occupy the majority of soya land.

The applications of this technology focused on increasing the yield of soya beans at as low a cost as possible. In time, it became apparent that various other elements could also be improved, as the use of soya spans many different products. The soya beans could be made healthier and more valuable by adding several genes foreign to itself, from bacteria and other plants (Roundup Ready Soybean). These can be delivered using a gene gun.




DuPont Pioneer is a company that developed a GM soya bean that makes the resulting soya oil more valuable. Naturally, the soya oil is very susceptible to oxidation and hence making the oil rancid. By silencing or knocking out the delta 9 and delta 12 desaturase enzymes, they made a soya bean with an altered fatty acid composition high in oleic acid and stearic acid, and low in linolenic acid. This different fatty acid profile makes it less susceptible to oxidation.

Although the scientific community has concluded that GM food is equally safe to eat as non-GM food (as tested individually for each new GM food to be marketed), public opinion is still relatively against it. GM food sparked a big debate around its advantages and disadvantages to big agriculture companies, consumers, scientists, farmers and others.

On one hand, GM food improves yield and the overall value extracted from crops. This is significant to the economy as well as areas where people struggle to have enough food to eat. Companies that develop GM food sell seeds and get to make large profits which can be used to further research GM food.

On the other hand, patenting food raises issues for small farmers in developing countries, as well as other ethical concerns of big companies owning the food source of the world. Suspicious consumers also don't fully trust GM food, so may choose to avoid it in favour of non-GM food.

These concerns that affect multiple parties constantly vie for attention, and form the dynamic of attempting to accomplish balance in this debate.

Ok byeeeeeeeeeeeeeeee

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