- GENES
| Polled: lacking horns, either naturally or because they have been removed. |
From the last lecture, we learned that the cell nucleus in both plants and animals contain threads of protein called chromosomes and that sections of the chromosomes are made up of 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 seeds for example, a soya bean plant will if they are planted and grown produce soya beans and not groundnuts. A Brahman bull mated to a Brahman cow will give you a Brahman calf and not a Hereford
calf. It is nature’s way of making sure that 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 of thousands of genes which control all the aspects of the plant or animal but each gene controls only one particular factor. Aberdeen Angus cattle all carry a gene which gives their black coat color, but 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 a red coat and a white face, but no gene for a black coat.
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 a plant or animal and these genes are usually given symbols, letters of the alphabet, in order to make things a bit simpler. For example, the gene for the colour of a red coat is written as R, and that for black coat as B, and the gene for polling in cattle (lack of horns) as P.
Now that we can identify certain genes by symbols, we can consider their behavior 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 which is a mixture of red and white. If the chromosomes in a normal cell of a Shorthorn with a red coat are examined, there will be 2 genes for coat colour, which we will call R.
Figure 1: A Pair of chromosomes
During the process of Meiosis, the cell division of the sex cells in the testes of the Bull or the ovaries of the Cow, the pair of chromosomes will be divided up, and only one of the chromosomes will be left in the sex cell with one gene for the red coat colour, R.
Figure 2: A parent cell
When fertilization 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 fertilized egg will have the normal number of chromosomes and two genes for red coat – RR.
Figure 3: Fertilization in a bull and a cow containing dominant genes.
In this way, a red Shorthorn Bull mated with a red Shorthorn Cow will produce a calf with a red coat.
Let us now consider the two other colours which appear in Shorthorn cattle, the white and the roan. We have given the gene for red coat the symbol R, and will give the gene for white coat, the symbol r, and any Shorthorn cow or bull would have one of the following genes in its cells:
RR – would have a red coat
Rr – would have a Roan coat (Red with some 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. In the case of a roan bull with Rr genes mated to a roan cow with Rr genes, the following possibilities could happen:
Figure 4: A bull and a cow with dominant and recessive genes.
| 8 Rr Roan | The colour would depend on |
| which egg. You could get a Red, | |
| 4 rr White | Roan or White calf and this would |
| Depend entirely on chance. The | |
| 4 RR Red | Ration would be: 4RR, 8Rr, 4rr |
| or 1 Red, 2 Roan, 1 White |
In every cell of a Shorthorn bull or cow there are the two genes for coat color situated on two chromosomes, but you can see that these could be RR, rr or Rr. These genes which control something like coat colour, but in different ways, are called alleles, so that Rr are alleles. The plants or animals which are produced from these genes are called allelomorphs, so that a Red Shorthorn and a Roan Shorthorn are Allelomorphs.
Let us now consider the case of polled cattle and horned cattle, and use the symbols P for polled, and p for horned. In any naturally polled animal, such as an Aberdeen Angus, each cell will carry two genes for polled. PP, and in any naturally horned animal such as a Friesland, each cell will carry two genes for horns, pp. If you cross a polled bull with a horned cow, you will get the following result:
Figure 5: A bull which has a polled dominant gene and a cow with a recessive gene for horns.
The result of this cross is a calf with the gene Pp. The question to ask is, ‘when this calf grows up will it be polled or have horns’, or will it only have one horn. Have you ever seen a cow with only one horn? The answer is that this calf will grow up polled and the reason for this is that Polled is what we call 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’ of a herd by using a polled bull in the herd. The polled Hereford cattle were produced by using Red Poll bulls on the horned Hereford cows.
Now consider the case of the calf we have just been talking about, with the Pp genes. What will happen if we mate two animals with these Pp genes? The following will happen:
Figure 6: breeding with animals that contain both dominant and recessive genes.
Say that all the sperm fertilizes all the eggs; we get:
1 & 3 = PP )
- & 4 = Pp ) Polled
- & 3 = pP (written Pp) )
2 & 4 = pp Horned
We would end up with 2 x 2 = 4 animals. How many of these would be polled and how many horned’. The answer is 3 polled and 1 horned, because all the PP and Pp animals are polled and only the pp animals are horned.
This leads us to another interesting point, Animals that have the genes PP are polled and ones that have the genes Pp are 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 two animals with Pp are mated the chances are that 1 calf out of 4 mating, or to make it simpler, one-quarter of the calves will be horned because both parents carry a gene for horns and if the sperm with that gene (p) fertilizes an egg with the same gene (p) the calf will have horns.
So we have the situation where a bull and a cow are both polled but may be carrying a gene for horns and this introduces new terms to you. An animal that looks polled is said to have a phenotype for polled. If it is an animal carrying the PP genes, it is said to have a genotype for polled, but if it is carrying the Pp genes it has a Genotype for Polled or Horned. In other words, Phenotype is what an animal looks like and Genotype is what the animal breeds like. Two other terms are Homozygous and Heterozygous. The animal that carries PP is Homozygous for Polled and will always produce polled offspring. The animal with Pp is Heterozygous for polled because although it looks polled it carries a gene for horns and can produce horned calves.
Two other terms to remember are dominant and recessive. The gene for polled P is a dominant gene and the gene for horns p is a recessive gene because when they are together in an animal as in Pp, the result is a Polled animal. Another dominant gene is the one for black coat colour B. If a black animal is mated with an animal of another colour, say red, the offspring will be black.
The Aberdeen Angus breed, therefore, carries two dominant genes, one for Polled, P, and one for black, B, so that if an Aberdeen Angus bull is used in a herd of, say, Friesland’s all the calves will be Polled and Black.
- SUMMARY OF TERMS
In the last two lectures you have been given several new terms, so it might be useful to summarise these:
- 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 the 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, e.g. PP
- Heterozygous – a plant or animal carrying two genes which affect the same characteristics (e.g./ Polled or Horned), but which are not the same – Pp. These are often called hybrids.
- Allele – two genes which affect the same characteristics but in different ways – Pp
- Allelomorph – the plants or animals produced by crossing parents containing Alleles
– Red and Roan Short horns are Allelomorphs.
- Dominant – a gene which is dominant over another gene.
- Recessive – a gene which is carried by a plant or animal, which is overcome or dominated by another gene, carried by the same plant or animal.
