Reading Passage 1

Gender selection-the choosing of a baby's gender prior to birth-occurs in many parts of the world. In China and India, for example, a baby's gender is considered to be of vital importance to the family, and male babies are often preferred over females for cultural reasons. In Western countries as well, there are many reasons why a family might want to choose a baby's sex. Often, parents wish to have a mix of both boys and girls in the family.

There are also health reasons for gender selection: many diseases affect children of only one sex, and a family that is susceptible to these diseases may wish to choose a baby's gender to avoid having an affected child.

This demand for gender choice by parents has led scientists worldwide to investigate gender selection prior to conception. Conventional wisdom states that the father's sperm is the main determinant of a child's gender, but recent research has begun to reveal a number of other possible determining factors.

Elissa Cameron's research, conducted in 2007 at the University of Pretoria, South Africa, investigated the effects of diet on sex ratios at birth. In one experiment, she changed the blood sugar level of female mice prior to conception by putting a chemical in the animals' water. Mice that received the additive saw their blood sugar levels fall from 6.47 to 5.24 millimols/litre. A separate control group of mice received pure water, without the additive.

After a few days, the two groups of mice were allowed to mate. In the control group, 41 % of the mice were born female, as compared to 47% in the group that received the additive-a disparity that Dr Cameron ascribed to the differences in the mothers' blood sugar levels.

Interestingly, the idea that blood sugar levels affect a baby's sex follows traditional wisdom. It has long been believed that mothers should eat more red meat and salty foods-which raise blood sugar for a long period-if they want to have a boy; they are advised to eat chocolates and sweets-which raise blood sugar levels for a short time-if they want a girl.

Another researcher in this field, Fiona Matthews of the University of Exeter, England, has come up with further evidence in support of the effect of diet on the sex of the unborn child. Her study followed 7 40 pregnant women who kept detailed records of their diets before conception. Her study found that mothers who consumed high-energy foods prior to conception were slightly more likely to have boys. The food with the greatest effect seemed to be breakfast cereals, which tend to be high in energy and often high in sodium content as well. Among women eating cereals on a daily basis, 59% had boys, compared with 43% of women who ate less than one bowl of breakfast cereal per week. These results are said to echo those seen in other animals, for example horses and cows, which statistically bear more males when well-fed.

The eating habits of women in rich Western countries could explain the slight fall in male births that has been reported over the past several years. In the UK, male births are falling by 1 per 1,000 births per year. This decrease could be ascribed to the decline in the number of adults and adolescent girls eating breakfast on a regular basis. In addition, the popularity of low-calorie diets for females of child-bearing age could also be a factor contributing to the reduction in male births.

The recent decline in male births in Western countries appears to make sense if one looks at it from an evolutionary standpoint. Historically, more boys tend to be born in times of food plenty, while females tend to be born in times of scarcity. One explanation is that when food is scarce, it is better for the survival of the species for female children to be born-as one male can father offspring by many females. Lower-calorie diets among Western women could be biologically echoing the effects of scarcity-hence, the decline in male births.

So what can we conclude from this complicated picture? If you would like to have a son, it might be a good idea to eat a breakfast that includes cereal. On the other hand, if you would prefer to give birth to a daughter, then cut out breakfast and continue a weight reduction diet, at least until after conception.

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Reading Passage 2

The Disease Multiple Sclerosis

A- Multiple sclerosis (MS) is a disease in which the patient's immune system attacks the central nervous system. This can lead to numerous physical and mental symptoms, as the disease affects the transmission of electrical signals between the body and the brain. However, the human body, being a flexible, adaptable system, can compensate for some level of damage, so a person with MS can look and feel fine even though the disease is present.

B- MS patients can have one of two main varieties of the disease: the relapsing form and the primary progressive form. In the relapsing form, the disease progresses in a series of jumps; at times it is in remission, which means that a person's normal functions return for a period of time before the system goes into relapse and the disease again becomes more active. This is the most common form of MS; 80- 90% of people have this form of the disease when they are first diagnosed. The relapse-remission cycle can continue for many years. Eventually, however, loss of physical and cognitive function starts to take place, and the remissions become less frequent.

C- In the primary progressive form of MS, there are no remissions, and a continual but steady loss of physical and cognitive functions takes place. This condition affects about 10-15% of sufferers at diagnosis.

D- The expected course of the disease, or prognosis, depends on many variables: the subtype of the disease, the patient's individual characteristics and the initial symptoms. Life expectancy of patients, however, is often nearly the same as that of an unaffected person-provided that a reasonable standard of care is received. In some cases, a near-normal life span is possible.

E- The cause of the disease is unclear; it seems that some people have a genetic susceptibility, which is triggered by some unknown environmental factor. Onset of the disease usually occurs in young adults between the ages of 20 and 40. It is more common in women than men; however, it has also been diagnosed in young children and elderly people.

F- Hereditary factors have been seen to have some relevance. Studies of identical twins have shown that if one twin has the disease, then it is likely that the other twin will develop it. In addition, it is important to realise that close relatives of patients have a higher chance of developing the disease than people without a relative who has MS.

G- Where people live can be seen to have a clear effect, as MS does not occur as frequently in every country. It commonly affects Caucasian people, particularly in North America, Europe and Australia. It has been recognised that MS is more common the further the country is away from the equator, and the incidence of MS is generally much higher in northern countries with temperate climates than in warmer southern countries.

H- Three things, which do not normally occur in healthy people, happen to people who have MS. First, tiny patches of inflammation occur in the brain or spinal cord. Second, the protective coating around the axons, or nerve fibres, in the body start to deteriorate. Third, the axons themselves become damaged or destroyed. This can lead to a wide range of symptoms in the patient, depending on where the affected axons are located.

I- A common symptom of MS is blurred vision caused by inflammation of the optic nerve. Another sign is loss of muscle tone in arms and legs; this is when control of muscle movement, or strength in the arms or legs, can be lost. Sense of touch can be lost so that the body is unable to feel heat or cold or the sufferer experiences temperature inappropriately; that is, feeling heat when it is cold and vice versa. Balance can also be affected; some people may eventually have to resort to a wheelchair, either on a permanent or temporary basis. The course of the disease varies from person to person.

J- A diagnosis of MS is often confirmed by the use of a magnetic resonance imaging (MRI) scan, which can show defects in the brain and spinal cord. Once diagnosed, MS is a lifelong disease; no cure exists, although a number of medical treatments have been shown to reduce relapses and slow the progression of the disease. It is important that patients with the disease are diagnosed early so that treatment, which can slow the disease, can be started early.

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Reading Passage 3

Surge Protection

With more devices connecting to the world's electrical networks, protecting electrical systems and devices from power surges-also known as distribution overcurrent-has become more important than ever. Without adequate overcurrent protection, interruptions to electrical service can have catastrophic effects on individuals, cities and entire nations.

In a normal electrical system, customers are supplied with a steady electrical current-a predetermined voltage necessary to operate safely all electrical equipment connected to that system. This steady electrical supply is subject to minimal variations-variations that are imperceptible to the consumer and do not normally harm electrical devices. An overload current is any surge that exceeds the variances of this normal operating current.

The higher the overcurrent, the more potential it has to damage electrical devices. One of the most important principles of overcurrent protection, therefore, is that the higher the magnitude of the overload current, the faster the overcurrent must be disrupted.

How do over-currents occur? Most over-currents are temporary and harmless, caused when motors start up or transformers are energised. Such things as defective motors, overloaded equipment or too many loads on one circuit, however, can cause harmful, sustained over-currents, which must be shut off quickly to avoid damaging the entire distribution system. An inadequately protected system can cause damage ranging from electrical shocks to people coming in contact with electrical equipment, to fires caused by the thermal ignition of electrical materials on the overloaded circuit.

Electrical storms and lightning are among the biggest causes of major distribution overcurrent worldwide. In the United States alone, 67 people are killed every year by these types of storms (including those killed by falling trees and power lines-not only surges).

The intense current of a lightning discharge creates a fleeting, but very strong, magnetic field. A single lightning strike can produce up to a billion volts of electricity. If lightning strikes a house, it can easily destroy all the electrical equipment inside and damage the distribution system to which that house is connected.

To protect people and devices adequately, overcurrent protection needs to be sensitive, selective, fast and reliable. IN the interest of conservation, most power systems generate different loads at different times of day; overcurrent protection must therefore be sensitive enough to operate under conditions of both minimum and maximum power generation.

It also needs to be selective so that it can differentiate between conditions that require immediate action and those where limited action is required; in other words, it should shut down the minimum number of devices to avoid disrupting the rest of the electrical system. Overcurrent protection also needs to be fast; it should be able to disconnect undamaged equipment quickly from the area of overcurrent and thus prevent the spread of the fault. Of course, the most basic requirement of protective equipment is that it is reliable, performing correctly wherever and whenever it is needed.

When an overcurrent occurs at a major electricity supply point such as a power station, the resulting surge, if it is not checked, can damage the entire distribution system.

Like a flooding river-which breaks its banks and floods smaller rivers, which in turn flood streets and houses-the extra voltage courses through the network of wires and devices that comprise the distribution system until it discharges its excessive energy into the earth. This is why each piece of equipment within the electricity manufacturing and distribution system must be protected by a grounding or earthing mechanism-the grounding mechanism allows the excess electricity to be discharged into the earth directly, instead of passing it further down the distribution system.

Within the distribution system, surge protection is provided by overcurrent relays. Relays are simply switches that open and close under the control of another electrical circuit; an overcurrent relay is a specific type of relay that operates only when the voltage on a power line exceeds a predetermined level. If the source of an overcurrent is nearby, the overcurrent relay shuts off instantaneously. One danger, however, is that when one electrical circuit shuts down, the electricity may be rerouted through adjacent circuits, causing them to become overloaded. At its most extreme, this can lead to the blackout of an entire electrical network. To protect against this, overcurrent relays have a time-delay response; when the source of an overcurrent is far away, the overcurrent relays delay slightly before shutting down-thereby allowing some of the current through to the next circuit so that no single circuit becomes overloaded. An additional benefit of this system is that when power surges do occur, engineers are able to use these time delay sequences to calculate the source of the fault.

Fuses and circuit breakers are the normal overcurrent protection devices found in private homes. Both devices operate similarly: they allow the passage of normal currents but quickly trip, or interrupt, when too much current flows through. Fuses and circuit breakers are normally located in the home's electrical switch box, which takes the main power coming into the house and distributes it to various parts of the home. Beyond this level of home protection, it is also advisable to purchase additional tripping devices for sensitive electrical devices such as computers and televisions. While many electrical devices are equipped with internal surge protection, the value of these devices usually warrants the additional protection gained from purchasing an additional protective device.

The modern world could not exist without reliable electricity generation and distribution. While over-currents cannot be entirely avoided, it is possible to mitigate their effects by providing adequate protection at every level of the electrical system, from the main power generation stations to the individual home devices we all rely upon in our daily lives.

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