- THE WORK OF ABBE MENDEL
The foundations 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, of which he became an abbot, near the town of Brunn. Modern genetic scientists have checked and verified his work 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, and 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 an example, Figure 1 on Page 3 shows the results of one of his experiments where a tall variety of pea was crossed with a dwarf variety. Let us call the gene for tallness T and the gene for the 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. He knew that the tall variety looked and bred tall and so the cells must have been carrying the genes TT; similarly, with the dwarf variety, the cells must have been carrying the genes tt. Referring back to the last lecture, we can say that the tall variety was homozygous for tallness and the dwarf variety homozygous for dwarfness. When he crossed the tall pea with the dwarf pea, the offspring were all tall peas, and he concluded that tallness was dominant.
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 got a ratio of 3 tall peas to 1 dwarf pea. This is a 3 to 1 ratio in favor of the dominant gene, as in the case of the polled and horned cattle.
Figure 1: Crossing a tall variety of peas with a dwarf variety &
Figure 2: Crossing two plants from the F1 generation.
THE IMPORTANT FACTS WHICH WERE FIRST DISCOVERED BY MENDEL WERE:
- The inheritance of characteristics is dependent on factors called genes, which are passed on from the parent to their offspring;
- The genes controlling a characteristic are present in pairs. We now know that the genes are on pairs of chromosomes in the nucleus of cells;
- Only one gene of a pair can enter a gamete – this is what happens during the process of cell division by meiosis; and
- In any pair of genes affecting one character, there are two possibilities:
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, or Phenotype. This is the case with a polled bull carrying the PP genes; it looks polled and will always breed polled calves.
|Inherit: derive (a quality, characteristic, or predisposition) genetically from one’s parents or ancestors.|
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 or 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 gene is recessive. The external appearance of the organism, the Phenotype, is determined by the dominant gene while the recessive is completely hidden.
Sometimes different genes have equal effects. When the Red Shorthorn is crossed with the White Shorthorn it produces a Roan Shorthorn which is an equal mixture of red and white.
LINKAGE 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 and such genes are said to be ‘linked’. An example in man is that blue eyes and baldness often go together in European males.
As we have said earlier, cattle have 60 chromosomes, made up of 30 pairs, and of these 29 pairs are identical but the last pair do not match and these are called the sex chromosomes. The larger of the two is called the ‘X’ chromosome and the shorter the ‘Y’ chromosome.
Figure 3: X and Y chromosomes
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, or gametes, produced by meiosis in the female will carry one ‘X’ chromosome, while the male will produce one gamete with an ‘X’ and one gamete with a ‘Y’
chromosome. See Diagram below:
Figure 4: Shows gametes of a male and female.
From every mating, half the offspring should be males and half females, but other factors can alter this.
- SEX LINKAGE
The male offspring receives its X chromosome from its mother and the Y chromosome from its father and the other genes on these chromosomes are called sex-linked genes. The Y chromosome is shorter than the X chromosome and 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 and this is the case with haemophilia, the bleeding disease which affected the Czars of Russia. The disease is passed on from one generation to the next generation by the females, carried on the X chromosome, but only the male children are affected and actually suffer from the disease. The females are ‘carriers’ but do not suffer from it. Other sex-linked characteristics are baldness and color 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 and this can account for the higher mortality rate in male offspring among mammals. This is shown in the table below:
Table 1: Sex Ratio in man at different ages
|Still births, abortions etc.||110 – 120||100|
|At birth||103 – 107||100|
|Below 20 years of age||100||100|
|Below 50 years of age||80||100|
|Below 80 years of age||50||100|
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 chickens when they are hatched.
If a Rhode Island Red hen, a breed which is pure breeding for red feathers is mated with a Light Sussex cockerel which is pure breeding for white feathers, all the Chicks hatched will have white feathers. From this we know that white feather color is dominant over red feather color. See Figure below:
Figure 5: Breeding with a red hen and a white cockerel.
However, the gene controlling feather color 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 chromosome carries no gene color.
If a Rhode Island Red cockerel is mated with a Light Sussex hen all the pullet chicks will be red and the cockerel chicks white. See Figure below:
Figure 6: Breeding with a white hen and a red cockerel.