Because T is dominant all the plants are tall. According to Mendel's First Law, when these plants produce pollen grain may fuse with any ovule when the pollen is released, and there are therefore four possible combination in the F2 generation-TT,Tt, tT and tt. Three of these combinations contain T and these plants are therefore tall while the remaining quarter are short. Two-thirds of the tall plants contain the shortness factor and; when self-pollinated, produce the monohybrid 3:1, tall: short ratio. The other tall F2 plants, however, are pure breeding as are all the short ones they have only one type of factor and are called homozygotes. The impure plants (i.e. those with two different factors) arc heterozygotes.

Similar results are obtained with any pair of opposed characters. Mendel then went on experimenting with peas which differed in two characters. He chose two types of plant-one with round, yellow seeds (RY), the another with wrinkled, green seeds (wg), and cross pollinated them. The resulting FI generation plants all had round, yellow seeds, so we can see that the factor for round seeds is dominant to that for wrinkled and that yellow dominates green. When the F2 generation plants grew up and produced seed they showed all four characters in every possible combination. The proportions were very close to 9 round yellow: 3 round, green: 3 wrinkled, yellow: I wrinkled, green. Mendel then stated his second law known as the law of independent assortment: the factor for each one of a pair of opposed characters many combine with any one of another pair when the sex-cells are formed. He reasoned that the impure F1 plants would be RY wg and that they would produce the gametes RY. Rg. wY and wg. Rw and Yg cannot he produced because according to the first law a gamete can carry only one of a pair of opposed factors. Any pollen grain can again combine with any ovule and from the table (Plate 58) we can see how Mendel explained the 9:3:3:1 dihybrid ratio on this basis. Whenever R and Y occur together there will be round yellow seeds, for these two factors are dominant.

When R is absent they will be wrinkled, and when Y is absent they will be green.

When Mendel published his results in 1865 scientists paid little attention to his work, and it was not until after 1900 that the truth of his statements was realised. By then the chromosomes had been discovered and it was realised that these were the 'vehicles' on which Mendel's factors' could be, carried. The factors are the genes that we now know to be carried on the chromosomes.

The behaviour of the chromosomes when sex cells are formed is exactly as Mendel had suggested in his laws governing the behaviour of his 'factors'. When sex cells are formed the chromosome pairs separate, one of each pair going to each sex cell. Therefore, even if the parent cells contain two opposed factors, the sex cell can have only one or the other, as Mendel stated in his first law. Again; when sex cells are formed, anyone of a pair of chromosomes can go to a sex cell with either one of any other pair and so Mendel's second law is obeyed as far as the chromosomes are concerned. But it does not always hold true for the genes.

Because of the enormous number of genes necessary to controal all the features of an animal it is obvious that each chromosome must carry a large number of genes. Because they are linked in this way the genes on each chromosome do not normally separate when sex cells are formed.

Wing length and body colour are linked in Drosophila. When a long winged, grey bodied insect is crossed with a short winged, black bodied one, the first generation will all be long winged and grey, because these factors are dominant. In the second generation, without linkage, according to Mendel's Second Law, one would expect long winged black t1ies and short winged grey ones to appear. However, because the genes are linked, only long winged grey and short winged black appear in the ratio 3:1. The linked genes therefore act as one gene and produce the monohybrid ratio associated with a single pair of characters.

Mendel was very lucky in his choice of characters for his experiments with peas. Had he selected any that were linked he may not have arrived at his second law.

Mendel's laws can be used to predict the characteristics of the offspring of particular animals as long as the genetic make up of the parents is known. This has enormous practical importance in the breeding of new and improved strains of plants and farm animals.

The examples so far have been concerned with complete dominance of one gene over another. There are however, instances of incomplete dominance. When the opposed genes occur together in an. impure individual they act together to produce an intermediate form. Some genes in the Andalusian fowl act in this way. This chicken has a fine blue sheen and is in demand by breeders. It occurs when black and splashed white forms are crossed. The genes controlling cotoration act together to produce the blue. When the blue ones are crossed among themselves the results .are one quarter black, one quarter white and one half blue. This is the monohybrid 3:1 ratio modified because there is incomplete dominance.

Many animals and the majority of plants are hermaphrodite i.e they have both male and female features. In those where the sexes are separate a genetic mechanism normally decides whether an individual may be male' or female. Although it is usually stated that man has 23 pairs of chromosomes, there are, in the male, only 22 pairs, plus two odd ones called the sex chromosomes or the X and Y chromosomes, making 23 true pairs. When the sex cells are formed the pairs separate so that each female cell (egg) has an X chromosome. Half the male cells will contain a Y chromosome and the other half an X. If a male cell containing an X chromosome fuses with an egg-cell a female will develop, whereas a male cell containing Y will produce a male embryo.  Because the X and Y are present in equal numbers boys and girls will be produced in nearly equal numbers too. The female is not the XX sex in all animals. In birds the hen is XY while the male is XX.

Hermaphrodite - Having both testes and ovaries in one individual. Many animal groups, notably earthworms and snails, ar hermaphrodite and special features have been evolved which prevent self fertilisation.

Heterocercal Tall - Upturned or asymmetrical tail typical of sharks.

Heterodont - Having teeth of various types (i.e. mammals) as opposed to the homodont condition of reptiles whose teeth differ on y in size.

Heterotrophic - Needing complex organic food. All animals are heterotrophic and obtain their food from plants or other animals. Plants, however, can manufacture their food requirements from simple inorganic substances and are said to be autotrophic.

Heterozygous - (See Gene).

Hexapoda - (=Insecta).

Hibernation - In cold and temperate regions, many animals disappear at the beginning of winter. They may disappear because they cannot withstand the cold, or because they are unable to obtain food during the cold season. Many birds migrate to warmer lands but among other animals, hibernation is common. This is a state of inactivity or deep sleep during which the body processes slow down almost to a stop. The body temperature, even in mammals, falls to within a degree or two of that of their immediate surroundings.

Aquatic animals do not normally hibernate although many become lethargic during cold weather. Most soil dwelling animals merely burrow deeper to avoid the cold, but many free living invertebrates hide away for the winter. Probably the majority of insects pass the winter as eggs which are very resistant to drought and cold, but many overwinter as larvae, pupae, or adults.

Among vertebrates, amphibians and reptiles are well known hibernators. Frogs, totoises, snakes and lizards all bury themselves away from the effect of forst. They often huddle together and this habit undouubtedly helps to keep their temperature a degree or two above that of the surroundings. Some of these animals seem able to expel water from their tissues. This makes the remaining fluids more concentrated and lowers their freezing point. The animals can then endure temperatures below 0°C without becoming frozen solid.

True hibernation is not known among birds although the poor will, an American night jar, has recently been found to pass the winter in a state of semi hibernation. The temperature of these sleeping birds is about 65°F instead of the normal 100° of the active bird. Many mammals, too, hide away and remain drowsy during the winter months. Bears, badgers, tree squirrels and others go to sleep for varying periods of time, but they wake periodically and their temperature does not drop more than afew degrees below normal. True hibernation, where the body temperature falls almost to that of the surroundings, is found in only a few groups of mammals. The egg laying monotremes and some of the opossums are known to hibernate in cold winters. Bats of temperate and cold climates hibernate because they cannot catch insects in winter. Bats are peculiar, however, in that their temperature drops considerably every time they sleep, even in the summertime. In this torpid condition they use less energy and they can be more active when they are awake.

Some insect eating mammals notably the hedgehog and many rodents (e.g. dormice, ground squirrels and hamsters) also go into a deep winter sleep. Even so, these hibernators often wake up and may feed on stored food. It seems that periodic waking is essential for getting rid of accumulated waste.

Before they go into hiberna animals often put on weight the form of fatty deposits. The extra material is drawn upon during the winter sleep. Others food on which to draw when they wake at intervals. It is not known what stimulus causes these preparations, nor are the physiological processes of hibernation fully understood. temperature and lack of food probably contribute to the onset of hibenation and the length of the day may possibly be concerned. There must also be some internal control because close relatives of hibernating animals often remain active in the cold season.

At the start of hibernation the temperature regulating mechanism isto prolonged cold. A short cold spell will not necessarily lead to hibernation. As the body temperature falls, the other activities slow down. Less oxygen is used, less food material is used, breathing slows down and the heartbeat rate also slows down. Its metabolic rate is less than a thirtieth (even as low as one hundredth) of that of the active animal. These changes are probably the result of hormone action. All the while, however, the nervous system is in control. If the outside temperature drops too much, the heart beat quickens and the body temperature increases to maintain life.

The end of hibernation is brought about by some stimulus such as the temperature rise in the surroundings, and is probably controlled by the nervous system. The body processes increase their rate and shivering often occurs producing more heat. In the hamster, an two is all that is necessary for the animal to wake and regain its temperature. Bats possibly an even shorter time. During the waking process a great deal of energy is used and so, although a hiber animal does wake periodically, too frequent disturbances can beAn animal will quickly use up its applies of fat and, unless it has a store of food probably contribute to the onset of hibernation fully understood. Temperature and lack of food probably contribute to the onset of hibernation and the length of the day may possibly be concerned. There must also be some internal control because close relatives of hibernating animals often remain active in the cold season on which to draw, it will soon perish.

Hinge Joint - A joint so arranged that movement can occur in one plane only. The knee and elbow joints are of this type,

Hirudines -Class of Annelida, with marine, fresh water, and terrestrial leeches. The body is relatively short and composed of a fixed number of segments, but the visible rings do not correspond with the segments as in earthworms, and except for one genus, there are no chaetae. Both ends of the leech bear a sucker the front one surrounding the mouth. The coelom is reduced to narrow tubes, the rest being filled in with tissue. The mouth is filled in with tissue. The mouth is equipped with teeth which tear off the leech's food or, in blood sucking forms, make the initial wounds. Bloodsucking leeches have a number of pouches in the gut in which blood is stored, sometimes for a long time. Like the earthworms, leeches are hermaphrodite.