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Selective Breeding


Many species of animals and plants have been "improved" or otherwise modified by the commonsense application of knowledge of the process of reproduction, notably by simply breeding from chosen individuals.

Inbreeding and outbreeding


If the parents chosen are closely related, the process may be described as inbreeding. In the human context this has bad connotations, but in the correct context it may fix characteristics (caused by recessive genes?) which would otherwise only be sporadically expressed in a breeding line, resulting in greater uniformity. On the other hand, outbreedingcombines characteristics from unrelated individuals, and may result in more vigorous offspring, but with a broader spectrum of associated characteristics. Records of breeding history are called pedigrees, and there is considerable importance attached to their administration.

Over the years, this has led to the development of different distinctive varieties (also called breeds or strains, etc) of various species. This is particularly noticeable in animals and plants used for food.
Give some examples of different breeds in the following categories - add more categories of your own if you can:
Milk cattle, e.g. > Friesians, Holsteins, Jerseys, Guernseys
Beef cattle, e.g. > Herefords, Charolais etc

It is also noticeable in animals kept as pets, and in garden flowers.
Give some examples of varieties of pets together with different varieties bred by Man.
> Dogs e.g. poodles - Alsatians (German shepherds) - Doberman Pinscher etc

However, a more rapid increase has been brought about through more systematic application of the principles of genetics and reproductive technology. This is especially useful in economically important species, e.g. cattle, sheep, pigs, etc.

It is perhaps interesting to note that different levels of technology can be applied to the different sexes!

Artificial insemination (AI)

Edexcel
It is a reasonably straightforward process to remove semen from a chosen male animal, and transfer it into a female, at the appropriate stage of the oestrous cycle (counterpart of the human menstrual cycle).
This process has been well developed in the case of dairy (milk) cattle breeding.

It is expected that you have a reasonable knowledge of the background to cattle breeding; the role of bull (male) and cow (female), and the events leading to the production of milk.

What is the difference between semen and sperms?
> semen is fluid containing sperms

Why do you think AI is especially useful for dairy cattle?
> problems dealing with (stroppy!) bulls
>economic rewards - milk production
>the process of milk production relies on regular pregnancy of cows, so lost opportunities for pregnancy result in loss of revenue

What stage/event during the oestrous cycle should AI be synchronised with?
> ovulation (egg release) - oestrus ("heat"): time when female will accept male

AI versus natural breeding


Fill in the following table, summarising the comparison between these two methods.

Factor Artificial Insemination Natural breeding
choice best bulls - of proven quality (cost
shared)
must take best (?) available on the farm
(limited cost)
flexibility may use semen from several different
bulls on different cows in the herd
most farms are not likely to support
several bulls
geographical
factors
(frozen) semen can be transported
globally
bull must be in same field as cow
health can be checked for possibilities of
disease
disease may be spread by sexual contact
process cost about 10 per straw (dose of semen) Up to 10 000 per bull
process risk cows are relatively straightforward to
deal with by AI
bulls can be dangerous
- to farm workers
- to public when loose in fields
longevity semen can be stored for several years bulls have limited life
long-term costs straws are stored in insulated flasks
containing liquid nitrogen which needs
regular replacement
bulls are expensive to rear and maintain,
and may become sterile due to
disease/accident


Semen collection

A bull ejaculates into an artificial vagina, possibly inside an artificial cow!
After collection, the quantity and quality of the semen are checked using a microscope.

The following factors are checked:
- concentration of sperms (number per ml)
- absence of contaminating material (blood, pus, etc)
- normality and motility of sperms (some sperms have more than one head or tail, or do not swim consistently in a straight line)

Semen dilution

Semen is usually diluted to about 1/50 of its normal concentration, and chemicals are added to protect sperms against the freezing process, and to "provide energy" and other requirements, for the following reasons:
Reason Additive
protection against shock due to the first stages of
cooling
albumen (white of egg) or milk
("antifreeze") protection against damage due to
ice crystals formed during freezing
glycerol
pH buffer - against lactic acid citrate buffer solution
protection against osmotic damage
and as energy source for respiration
glucose solution
as insurance/ to kill any bacteria present antibiotics


How might lactic acid be produced?
> as a result of anaerobic respiration (by sperms?)

What is meant by osmotic damage?
> water entering or leaving sperm cells due to differences in concentration on either side of cell membrane (partially permeable)

Semen storage

The diluted semen is placed into plastic "straws" (20 million sperms per straw, contained in 0.25 cm3 of fluid), sealed and immersed in liquid nitrogen (which has a temperature of -196 C).
Special refrigeration plant is required to provide this temperature at "sperm banks", but liquid nitrogen is quite readily available, and straws may be transported in small flasks.

The straws of semen may be kept for several years - even after the death of the "donor" bull - but the sperms become gradually less and less effective. Comprehensive records are kept.
Why do you think these records are necessary?
> so that a bull's semen is not used on closely related cows
> for pedigree purposes

The insemination process


This may be carried out by AI technicians, vets or licensed farmers. The farmer or herdsman will note changes in the behaviour of individual cows in the herd which indicate oestrus - the animal's readiness to mate or be artificially inseminated - and the timing of insemination is fairly crucial.

A straw containing semen is quickly thawed in warm water, then inserted into a syringe-like "insemination gun" or catheter, and the semen is deposited into the cervix at the far end of the cow's vagina. It is hoped that sperms will be in the oviduct when eggs are released.

Progeny testing

In the process of proving the worth of a bull in an AI programme, the performance of that bull's progeny must be tested. The possible scale of the operation is set out below.

1) Young candidate bull (1-2 years old) provides semen.

2) This semen used to impregnate about 500 cows

3) These 500 cows (pregnant for 9 months) give birth to (500?) calves.

4) Some of these calves are raised to maturity (2 years) - impregnated and have own offspring (after another 9 months) - then they commence lactation (1 year).

5) Lactation figures are collated to give performance result for progeny, used to determine worth of bull for use in the programme.


What is meant by the term progeny?
> offspring
What is meant by the term impregnated?
> made pregnant - either by artificial or normal means
Why are only some of the calves in step 4 raised to maturity?
> half will be male, so will not give milk
> others will be culled /used for meat
What is meant by the term lactation
> production of milk - extended to include total quantity of milk produced after each pregnancy
What figures give a performance result, and why do they need to be collated?
> total milk production - cows may be on different farms

Total up the time taken between stages 1 and 5.
How old is a bull likely to be by the time he is accepted for large scale use?
> 6-7 years

Bulls whose progeny do not produce satisfactory results are likely to be culled.

The same fate is likely to befall a bull who has contributed a reasonable amount of semen, "in order to reduce maintenance costs"!

Interestingly, artificial insemination is not approved for the development of thoroughbred horses!


Manipulation of reproductive processes - from the female side

Background information

Edexcel The oestrous cycle lasts 21 days in cows (compared to the human menstrual cycle 28 days), the gestation period for cows is about 40 weeks (compared to 38 for humans), and usually only one calf is born. When about 2 years old, a cow can mate and have a calf. The result is that each cow usually produces only 2 or 3 calves in her lifetime.
This limits the contribution of any particularly outstanding cow to the herd, as well as slowing the development of a particular characteristic.

Stimulation of ovulation (superovulation) and Embryo transfer

"Genetically superior" (!) or "desirable" (!!) "cows of fine pedigree" can be treated with hormones to increase the number of eggs they release at ovulation - making them effectively act as potential embryo donors.

The cow is then artificially inseminated using semen from a proven bull "in the usual way".

Each egg will be fertilised and start to develop into an embryo, which takes some time to implant.

At an appropriate stage (6-7 days after insemination), the embryos (still not implanted) are flushed using a catheter, i.e. pipe placed into the uterus. Normally 4-7 embryos are recovered.

Meanwhile, (presumably 4-7) other cows have had their oestrous cycle manipulated to be receptive and the embryos are placed into their uteri, so they act as surrogate mothers to the embryos.

Alternatively, embryos can be frozen and stored - in a similar but more closely controlled version of the techniques used in sperm freezing. In this frozen state, they may be shipped to any part of the world. In the past, embryos have even been implanted into completely different species, e.g. rabbits, and re-implanted at their destination into surrogate mothers.

Other simpler versions include:
- "non-surgical" removal of "ripe" eggs from a normal healthy cow as the follicles mature at the surface of the ovary, monitored by ultrasonic equipment
- removal of immature eggs from the ovaries of cows slaughtered at the abattoir (for whatever reason). These may need laboratory treatment to mature them before in vitro fertilisation, and eventually implantation into surrogate mothers.

It has been said that the techniques described above have the potential to double the rate of genetic improvement achievable with AI used on its own.

There are many ethical issues associated with biological developments such as these, and you should be able to express an informed opinion about the implications of these procedures. The relationship between science and technology needs to be underpinned by wider public knowledge of the facts involved, and a higher quality of debate.


Other issues


Cloning
- The technology behind the production of Dolly the sheep has been applied to other species, but there are indications that there are some problems still to be solved. The frequency of success is not as high as it should be, and not all offspring develop as normally as can be expected.

Choosing the sex of offspring
- Male offspring are an inconvenience in many situations! Although it is theoretically possible to separate sperms carrying Y chromosomes from those carrying X chromosomes, the best approach may be to test the sex of embryos. only implanting females.

It is to be hoped that these techniques will not be applied to the human species in the foreseeable future!




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