INTRODUCTION TO GENETICS. Aim Explain genetic influence on the characteristics exhibited by animals.

Transcription

1 Lesson 1 INTRODUCTION TO GENETICS Aim Explain genetic influence on the characteristics exhibited by animals. GENETICS The study of cells is called cytology. All living plants and animals are made up of a number of very small cells which are visible only under a microscope. The structure, function and reproduction of these cells vary greatly. Those students who have completed the Animal Anatomy and Physiology course have learnt about the different parts of a typical animal cell, and also about some of the specialised cells that occur. It is important to remember that all young cells are more or less the same, while specialised cells become more complicated and specialised as they grow. In order to refresh your memory on basic cell structure, we will briefly look at some different cells. Plant Cells Figure 1 (below) shows a typical plant cell. The outer layer of the cell is made up of a carbohydrate (complex sugar) called cellulose. Inside the cell wall is a layer of cytoplasm, which is a semi-permeable membrane. This membrane allows water to flow in and out of the cell. This happens in the process called osmosis. Inside the layer of cytoplasm is a dense mass of protein called the nucleus. The nucleus is the most important organ in the cell as it controls the growth and reproduction of the cell. 1

2 The middle of the cell is called the vacuole and this is filled with a sugary liquid called cell sap. This cavity is filled with water by a process of osmosis. Plants do not have a rigid skeleton and stand upright only because the cells are filled with liquid (a condition known as turgid). If, for any reason, water flows out of the cell, the plant will lose rigidity, become limp (wilt) and may collapse altogether. In this condition, the plant is described as being flaccid. The cellulose in many plant cells is replaced by a much harder carbohydrate known as lignin which is, in fact, wood! Ruminants can digest cellulose using an enzyme called cellulase, but they cannot digest lignin. Ruminant animals (e.g. cows, sheep and goats) obtain all their protein and carbohydrate needs from plant cells. In winter, many native grasses have very high lignin levels. In addition, they are very low in protein. These two factors make such grasses poor feed for animals during the winter months. Animal Cells Animal cells differ from plant cells in that the outer membrane of the cell is thin and difficult to see under the microscope. The cell is usually filled with cytoplasm and there may be vacuoles present if the cell is excreting waste products or producing hormones and enzymes. The nucleus is large and easily seen (see below). 2

3 Some organisms consist of only a single cell. Examples of these are bacteria and amoeba. Other simple plant or animal organisms consist of several non specialised cells and take in food, excrete waste, respond to stimuli and reproduce all through these simple cells. In most plants and animals which contain many complex cells, some cell loses the ability to perform basic tasks like excretion, sensitivity and reproduction. As a result, specialised cells gather together to perform a single function for the body. For example, excretory cells are massed into a unit called the kidney while the sensory cells are organised into eyes and ears. The ability to reproduce is taken over by specialised cells in the gonads (testes and ovaries). Growth, maintenance and defence against disease are performed in other centres of the body. For example, red blood cells deteriorate after about three months and are removed by the spleen and replaced by the bone marrow. White blood cells work to protect the body from infection and to help repair damaged tissue. Figures 3 and 4 (below) show two more specialised cells. The first are mammary cells which are responsible for the production of milk. The second are the motor nerve cells which respond to stimuli and allow a reaction to occur (e.g. moving away from light or heat). 3

4 CELL DIVISION It is most important at this stage to understand the difference between mitosis and meiosis. Both mitosis and meiosis take place in the nucleus of the cell and are concerned with the division of the nucleus. Mitosis (Asexual reproduction) In the case of MITOSIS, the nucleus and its cell divide into two halves which then grow and form two new cells. The cell distributes its genetic material equally and exactly to the offspring. This process is one of simple growth or asexual cell reproduction. Plants grow from small seedlings into mature plants by mitotic division of the cells, usually at the root tips and the ends of the shoots (called the apical meristem). Animals also grow from fertilised cells through the stages from foetus to young animals to mature animals by mitosis. The process of the division of a cell nucleus by mitosis is shown in Figure 5 (below). The orderly distribution of duplicated chromosomes from a mother cell to the daughter cells is the essence of mitosis. The nucleus of the cell not only controls the growth and activities of that cell but also contains the "blueprint" or design for that cell and its functions. The "blueprint" is carried on the chromosomes of the nucleus. Chromosomes consist of long, thin threads of protein called chromatin and are woven into a dense mass inside the nucleus. If the chromosomes are examined under a very powerful microscope, it is possible to see that they are made up of sections called genes. Each gene controls one or a number of physical characteristic of the plant or animal. In cattle, things like coat colour and whether the animal has horns are controlled by genes. In humans, physical height and eye and hair colour are controlled by genes. Each nucleus contains hundreds or thousands of genes, but the number of chromosomes is the same for all members of the same species. However, the number of chromosomes varies from species to species. Cattle have 60 chromosomes, chickens has 78, sheep 54, pigs 38, goats 60, horses 64 and man 46. 4

5 Meiosis (Sexual reproduction) The process of meiosis involves a mechanism that reduces the number of chromosomes by half. In cell division by MEIOSIS, the first thing to happen is that the threads of chromatin become thicker and divide into individual chromosomes. These then split into separate parts and move to opposite ends of the nucleus. The nucleus and cell divides into two (see Figure 6, below). As mentioned, the important thing to realise is that the two cells resulting from this cell division contain only half the number of chromosomes of the original cell. The number in the original cell has been halved when the pairs of chromosomes separated and moved to opposite ends of the nucleus. These cells, containing only half the correct number of chromosomes, are called gametes (sexual cellular reproduction). In animals, gametes are produced in the gonads (the testes and ovaries). Another important point to remember is that, while each of the two cells at the end of meiosis contains half the chromosomes of the original cell, the chromosomes in each cell are an exact replica of those in the other cell (If you remember, during meiosis, the chromosomes joined up into matching pairs and then separated - you can see this quite clearly from Figure 6). So we end up with two new cells made from the original cell, each containing half the chromosomes of the original cell but each matching the other half. The reason for this is quite simple really and can be easily explained when fertilisation takes place. Take the example of cattle. The sperm from the bull joins with the egg (or ovum) of the cow. The sperm and ovum are gametes as they each contain half the number of chromosomes of the parent cell. When the sperm and ovum join together, the number of chromosomes in the fertilised egg cell is brought back up to the normal number (see Figure 7, below). 5

6 Figure 8 (below) shows how gametes are formed in cattle by meiosis. The number of gametes then "grows" by mitosis. During fertilisation, the gametes combine and bring the number of chromosomes back up to normal. The resultant embryo will grow by mitosis (simple growth). Two more terms with which you should be familiar: Diploid: Haploid: a nucleus or cell which contains the full number of chromosomes for the species is called a Diploid nucleus or cell. a nucleus or cell which contains half the number of chromosomes for the species is called a haploid nucleus or cell (easy to remember by thinking "H" for Haploid and Half the number of chromosomes). To sum up the two types of cell division revised in this module: Mitosis: Meiosis: a method of asexual cellular division which produces exact copies of the original cell and is the way in which organisms grow by means of cell division. a method of sexual cellular division which produces special cells (called gametes) for reproduction. Gametes have only half the number of chromosomes of the original cell. 6

7 GENES We have learnt that the cell nucleus in both plants and animals contain threads of protein called chromosomes and that chromosomes contain genes. These genes are responsible for passing the characteristics of a plant or animal from the parents to the offspring (from one generation to the next). In this way, if seeds from a corn plant for example are planted and grown, they will produce corn and not potatoes. In the same way, a Simmental bull mated to a Simmental cow will produce a Simmental calf and not a Hereford calf. It is nature's way of making sure that the physical characteristics of each species of plants and animals are carried on from generation to generation without being altered. The cell nuclei of all plants and animals carry hundreds or thousands of genes that control all the aspects of the plant or animals, but each gene controls only one particular factor. For example, Aberdeen Angus cattle all carry a gene that gives their black coat colour. They have no gene for white coats because a pure bred Angus has no white on its coat. In the same way, Hereford cattle all carry a gene for red coat and a white face but no gene for a black coat. 7

8 Because there are so many genes carried in the cells of plants and animals, breeders and farmers are concerned only with a few genes - the ones that control the economic characteristics of plants and animals. These genes are usually given symbols (letters of the alphabet) in order to make things a bit simpler. For example, the gene for red coat colour is written as R while that for black coat colour as B. The gene for polling in cattle (lack of horns) is given the symbol P. Now that we can identify certain genes by symbols, we can consider their behaviour in more detail. Take the case of the Shorthorn breed where individual animals can have either a dark red coat, a white coat or what is called a roan coat (red and white hairs mixed). If the chromosomes in a normal cell of a Shorthorn with a red coat are examined, there will be two genes for coat colour which we will call R (see Figure 9). When sex cells divide in the testes of the bull or the ovaries of the cow (by meiosis), the pair of chromosomes will be divided up. Only one chromosome will be left in the sex cell with one gene for the red coat colour, R (see Figure 10 below). When fertilisation takes place, the sperm cell from the bull (carrying one R) will join with the egg cell of the cow (the ovum) and the resulting fertilised egg will have the normal number of chromosomes and two genes for red coat, RR (see Figure 11 below). In this way, a red Shorthorn bull mated with a red Shorthorn cow will produce a calf with a red coat. 8

9 Let us now consider the two other colours which appear in Shorthorn cattle - the white and the roan. We have given the symbol R to the gene for red coat and we will call the gene for white coat r. As roan is a mixture of red and white, we will call this gene Rr. Any Shorthorn cow or bull will have one of the following genes in its cells: RR - would have a red coat Rr - would have a roan coat (red and white) rr - would have a white coat We have seen what happens when a bull with RR genes is mated with a cow with RR genes - we get a calf with RR genes and a red coat. In the same way, a bull with a white coat and rr genes mated to a cow with rr genes would give you a white calf with rr genes. We can work out the possible coat colours of calves from the mating of a roan bull with Rr genes to a roan cow with Rr genes. Start off like this: R(Cow) r(cow) R(Bull) R(Bull) 9

10 To work out these possibilities, you take the R gene of the bull and cross it first with the R gene of the cow to give RR. You then cross the R gene of the bull with the r gene of the cow to give Rr. (By convention, the big R is always put first so you always write Rr and never rr). You then take the r gene of the bull and cross it first with the R gene of the cow to give Rr (remember, big R first). Finally, you cross the r gene of the bull with the r gene of the cow to give rr). We can fill in the chart as we go along. It should look like this: R RR Rr BULL r Rr rr The possibilities we have from this crossing are as follows: 1 RR - red coated calf 2 Rr - roan coated calves 1 rr - white coated calf The colour of the calf will depend which sperm fertilises which egg. You could get a red, roan or white calf depending entirely on chance. However, there is more possibility of getting a roan calf. For every one chance of getting a red or white calf there are two chances of the calf being roan. In every cell of a Shorthorn bull or cow there are two genes for coat colour situated on two chromosomes. These genes could be RR, Rr or rr. These genes which control something like coat colour, but in different ways, are called alleles. RR, Rr and rr are all examples of an allele. The plants or animals that are produced from these genes are allelomorphs. Examples of an allelomorph are a Red, White or Roan Shorthorn. We now need to consider the case of polled cattle and horned cattle. We will use the symbol P for polled and p for horned. In any naturally polled animals such as the Aberdeen Angus, each cell will carry two genes for polled, PP. By contrast, in any naturally horned animal such as the Friesian, each cell will carry two genes for horns, pp. If you cross a polled bull with a horned cow, you will get the following results: P(Bull) P(Bull) p(cow) Pp Pp p(cow) Pp Pp You can see there is a hundred percent chance of the calf carrying Pp genes. This means the calf will carry the genes for polled and horns. The question to ask is "When this calf grows up, will it be polled or will it have horns?" It can't have just one horn! The answer is that the calf will grow up polled. The reason for this is that polled is dominant over horned. If a polled animal is crossed with a horned animal, the offspring will always be polled. In this way, the horns can be "bred out" or a herd by using a polled sire in the herd. For example, the polled Hereford cattle were produced by using Red Poll bulls on the horned Hereford cows. Now let us reconsider the case of the calf we have just been talking about with the Pp genes. What will happen if we mate two animals with Pp genes? Try working it out yourself. Draw up a chart and fill it in using the method that we used with the red, roan and white coats. Did you arrive at the same answers as given in the chart below? 10

11 P(Bull) P(Bull) P(Cow) PP Pp p(cow) Pp pp The possibilities are as follows: 1 PP - polled calf 2 Pp - polled calf (polled is dominant) 1 pp - horned calf In this example, there is 1 chance of having a horned calf and three chances of the calf being polled. This leads us to an interesting point. Animals that have the genes PP are polled but animals that have the genes Pp are also polled. If two animals with PP are mated, all their offspring must be polled because neither of the animals carries a gene for horns. If, however, two animals with Pp are mated, the chances are that 1 calf out of 4 matings (or one-quarter of the calves) will be horned. This is because both parents carry a gene for horns. If the sperm with that gene (p) fertilises the egg with the same p gene, the calf will have horns. We have, therefore, the situation where a bull and cow are both polled, but may be carrying a gene for horns. This introduces new terms to you: Phenotype: what an animal looks like. Genotype: what an animal breeds like. To explain these terms more fully, an animal that looks polled is said to have a phenotype for polled. If it is an animal that carries PP genes, it is said then to also have a genotype for polled because it will produce polled offspring. If, however, it is carrying the Pp genes, it has a genotype for polled or horns because there is a chance that the offspring will have horns. Two other terms for you to know are: Homozygous: the animal that carries the PP gene is homozygous for polled and will always produce polled offspring. Heterozygous: the animal that carries the Pp gene is heterozygous for polled offspring because, although it looks polled, it carries a gene for horns and can produce horned calves. In addition, you need to be sure about the terms dominant and recessive. The gene for polled, P, is a dominant gene while the gene for horns, p, is recessive because when they are together in an animal as Pp, the result is a polled animal. Another example of a dominant gene is black coat colour, B. If a black animal is mated with an animal of another colour, the offspring will be black. The Angus breed of cattle carries two dominant genes, one for polled, P, and one for black coat colour, B. If an Angus bull is used in a herd of Friesian for example, all the calves will be polled and black because these two genes are dominant. 11

12 SUMMARY OF TERMS In this lesson you have been given several new terms so it might be useful to summarise them. Make sure you are familiar with all these terms as they will be used in the following lessons: Chromosome: the protein strands in the nucleus of a cell which play an important part in growth and reproduction. Gene: part of a chromosome which passes on a particular characteristic of a parent to its offspring. Mitosis: the process whereby a cell divides to form two new cells and which is used in normal growth. Meiosis: the process whereby a sex cell divides to form two new sex cells but which halves the number of chromosomes in the new sex cell. Diploid: a cell containing the normal number of chromosomes for the species, e.g. 60 for cattle. Haploid: a sex cell containing half the number of chromosomes for the species, e.g. 30 for cattle. Phenotype: the external appearance of a plant or animal. The phenotype of an Aberdeen Angus is polled and black. Genotype: the genetic make up of a plant or animal; the genes that it carries in its cells. Homozygous: a plant or animal carrying two similar genes, eg. PP. Heterozygous: a plant or animal carrying two genes which affect the same characteristic but which are not the same, e.g. Pp (polled or horned). These plants or animals are often called hybrids. Allele: two genes which affect the same characteristic but in different ways, e.g. Pp. Allelomorph: the plants or animals produced by crossing parents containing alleles, e.g. Red and Roan Shorthorns. Dominant: a gene which is dominant over another gene. Recessive: a gene which is carried by a plant or animal, but which is overcome or dominated by another gene carried by the same plant or animal. THE WORK OF MENDEL The foundation of the science of genetics were laid by an Austrian scientist named Gregor Mendel (born 1822; died 1884). Mendel became a monk and lived and worked at a monastery near the town of Brunn. He later became an abbot. Modern genetic scientists have checked and verified the work of Mendel, but when he first published the results of his experiments, the importance of his scientific work was not realised. Mendel did all his work with the ordinary garden pea plant. The characteristics which he used were tall or dwarf peas, peas with yellow pods, peas with smooth coated-seeds and those with wrinkled seeds. Mendel carried out hundreds of experiments, crossing one type of pea with another. To give you just one example, Mendel crossed a tall variety of pea with a dwarf variety. We will call the gene for tallness, T, and the gene for dwarf factor, t. Before Mendel crossed the two varieties, he made sure that they were pure bred by selecting seed from plants that had been tall or dwarf for several generations. In other words, he knew that the tall variety looked tall and bred tall so the cells must have been carrying the genes TT. Similarly, the dwarf variety must have been carrying the genes tt. 12

13 Using the terms used previously, we can say that the tall variety was homozygous for tallness and the dwarf variety was homozygous for dwarfness. When he crossed the tall pea with the dwarf pea, the offspring were all tall peas so Mendel concluded that tallness was dominant over dwarfness (see Figure 12). The offspring obtained by crossing two pure bred lines like this is called the first filial generation or the F1 generation. Mendel then took the tall peas of the F1 generation and crossed them with each other and he got a ratio of three tall peas to one dwarf pea. This is a 3 to 1 ratio in favour of the dominant gene as in the case of the polled and horned cattle (see Figure 13). 13

14 The important facts which were first discovered by Mendel were: (a) (b) (c) (d) The inheritance of physical characteristics is dependant on factors, called genes, which are passed on from parents to their offspring. The genes controlling a physical characteristic are present in pairs. We now know that the genes are on pairs of chromosomes in the nucleus of the cells. Only one gene of a pair can enter a gamete - this is what happens during the process of cell division by meiosis. In any pair of genes affecting one character, there are two possibilities: (i) (ii) The genes may be alike with both having the same effect on the character. The organism is then pure breeding and said to be homozygous for that character. An important result of this is that the internal genetic structure of the organism (the genotype) must be the same as the external appearance (the phenotype). This is the case with a polled bull carrying the pp genes; it looks polled and will always breed polled calves. The genes may be different, in which case the organism is not pure breeding and is said to be heterozygous for that character. The internal genetic structure of the organism (the genotype) is different from the external appearance (the phenotype). This is the case with a polled bull carrying Pp genes; it looks polled but can produce horned calves. If the genes in a pair are not equal, the gene with the stronger effect is said to be dominant and the weaker one recessive. The external appearance of the organism, the phenotype, is determined by the dominant gene while the recessive is completely hidden. Sometimes different genes do have equal effect. As you learnt earlier, when the Red Shorthorn is crossed with the White Shorthorn it produces a Roan Shorthorn which is an equal mixture of red and white. Linkages of Genes Because of the way the chromosomes in a sex cell divide during meiosis, genes that are close together on the chromosome are kept together and are inherited as a group by the offspring. Such genes are said to be linked. An example of linked genes is the blue eyes and baldness that often go together in European males. SEX DETERMINATION As we have said earlier, cattle have 60 chromosomes made up of 30 pairs. 29 of these pairs are identical to each other, but the last pair does not match. These are called the sex chromosomes. The larger of the two is called the X chromosome and the shorter is called the Y chromosome (see Figure 14). The body cells of all females contain two X chromosomes while the cells of males contain one X and one Y chromosome. Many different genes are carried on these sex chromosomes. All the sex cells (gametes) produced by meiosis in the female will carry one X chromosome while the male will produce one gamete with an X chromosome and one gamete with a Y chromosome. From every mating, half the offspring should be males and half should be females but other factors can alter this (see Figure 15). 14

15 Sex Linkage Earlier in this lesson we spoke of the linkage of genes - the way that clusters of genes are kept close together on the chromosome and inherited in a group by the offspring. This happens with the X and Y chromosomes too. The male offspring receives its X chromosome from its mother and Y chromosome from its father. The genes that make these sex chromosomes are called sex-linked genes. The Y chromosome is shorter than the X chromosome and it does not carry all the genes found on the X chromosome. It is possible for a male to have an undesirable gene carried on the extra length of the X chromosome. This is the case with haemophilia (the bleeding disease) which affected the Czars of Russia. The disease is carried on the X chromosome and passed on from one generation to the next by the females. However, only the male children are affected and actually suffer from the disease. The females are the carriers of the disease but do not suffer from it. Other sex linked characteristics are baldness and colour blindness found in men. Because of the difference in length between the X and Y chromosomes, other dangerous recessive genes can be carried on the X chromosome. This accounts for the higher mortality rate in male offspring among animals. This is shown clearly on the following table. The number of human males and females at different ages Time of life (Age) Male Female Fertilisation Still births, natural abortions At birth Below 20 years of age Below 50 years of age Below 80 years of age In birds, including poultry, there is a difference from mammals in that the female sex chromosome has no active gene on it so that any sex linked characteristics are taken from the male parent. This fact can be used to decide the sex of some chicken breeds when they are hatched. For example, if a Rhode Island Red hen (which is pure breeding for red feathers) is mated with a White Sussex cockerel (which is pure breeding for white feathers) all the chicks hatched will have white feathers. From this we know that white feather colour is dominant over red feather colour (see Figure 16 below). 15

16 However, the gene controlling feather colour is sex linked, so that a white feathered hen carries only one gene for white feathers. The other gene, because of the special female sex chromosomes, carries no gene colour. Therefore, if a Rhode Island Red cockerel is mated with a White Sussex hen (white feathers) all the pullet chicks will be red and the cockerel chicks will be white (see Figure 17 below). This allows male and female chicks to be sorted from birth. 16

17 SELF ASSESSMENT Perform Self Assessment Test 1.1 If you answer incorrectly, review the notes and try the test again. SET TASK Talk to an animal breeder or farmer and ask them what particular characteristics they are looking to promote in their animals e.g. a dairy farmer might want to promote cows which produce large quantities of milk OR they may feel high butterfat production is more important from their cows. Are there certain characteristics that are unwanted? Write a half page report on your findings. Additional research may be required on your behalf to fully answer some of these questions. Try to look for animal biology or physiology text books from a local library. If the question is on genetics, look for a book on genetics from a college or university. Try an internet search it is a wonderful source of information. ASSIGNMENT Complete Assignment 1 17