[Heredity]
[Narrator:] It has long been observed that like tends to beget like.
However, there are variations within most animal groups and some of these have a hereditary basis.
Let us follow a simple case of inheritance in short-horns.
A pure-breeding red male produces sperms,
each of which carries in one of its chromosomes a hereditary unit or gene for red coat.
This, we shall indicate by a black dot.
A pure-breeding white short-horn female, produces eggs,
each of which carries in one of its chromosomes a gene for white coat.
This we shall indicate by a white dot.
Thus, from a mating between a pure-breeding red male, and a pure-breeding white female,
every fertilized egg or zygote, will have one gene for red coat, and one gene for white coat.
A zygote divides by a type of cell division called mitosis.
In this type of division, the chromosomes arrange themselves along the centers,
then the chromosomes and their genes duplicate themselves.
Of the two sets of chromosomes formed, one set moves to each daughter cell.
Note that each daughter cell contains the same hereditary materials as did the mother cell.
Since all cells of the body are formed in this way, each cell of the offspring contains a red gene and a white,
as did the zygote from which they came.
Due to the influence of both genes, the calf will be roan.
Let us now cross two roan animals.
When a body cell gives rise to sperms or eggs, the chromosomes pair off instead of lining up individually.
One chromosome of each pair goes to each daughter cell.
Thus, a roan male produces two kinds of sperm.
...gene for red coat, and one a gene for white.
Similarly, a roan female produces two kinds of eggs.
In mating, there's a random combination of sperms and eggs.
This sperm may unite with this egg to form a red-red zygote,
which will give rise to a red calf.
Or, it may unite with this egg to form a red-white zygote, which will give rise to a roan calf.
This sperm may unite with this egg to form a white-red zygote, which also will develop into a roan calf.
Or, it may unite with this egg to form a white-white zygote,
which will give rise to a white calf.
Thus, from a roan by roan cross, we may expect 1/4 of the offspring to be red, 2/4 roan, and 1/4 white.
Guinea pig crosses gives slightly different end results.
A pure-breeding black male Guinea pig produces sperms, all of which carry a gene for black coat.
A pure-breeding white female produces eggs, all of which carry a gene for white coat.
Hence, all zygotes resulting from such a cross will have a gene for black coat and a gene for white.
Since the zygote divides by mitosis, all the body cells will have both a black gene and a white.
In the determination of coat color, the black gene dominates over the white.
Consequently, the individual will be black.
Now, let us cross two such black Guinea pigs.
This black male with cells like this produces sperms, half of which carry the black gene,
and half of which carry the white gene.
Similarly, the black female produces two kinds of eggs.
In mating, this sperm may unite with this egg, or it may unite with this egg.
This sperm may unite with this egg, or it may unite with this egg.
Of the zygotes produced, three will carry at least one black gene.
One will lack a black gene.
Due to dominance of black, 3/4 will be black, and 1/4 white.
Rough coat in Guinea pigs is also inherited.
When a pure rough-coated male is crossed with a pure smooth female,
every zygote will have a rough gene and a smooth gene.
Rough is dominant over smooth.
Consequently, all the immediate offspring will be rough.
A mating between two such rough Guinea pigs will give three rough to one smooth.
Let us now cross a pure-breeding black rough male and a pure-breeding white smooth female.
Each sperm of a black rough male will carry a black gene, and a rough gene.
Each egg will carry a white gene and a smooth gene.
Consequently, every zygote from such a cross will carry each of the four genes:
black, white, rough, and smooth.
Due to the dominance of the black gene and the rough gene,
every zygote from this cross will develop into a black rough Guinea pig.
Let us now cross two such black rough Guinea pigs.
When a black rough male produces sperm, the chromosomes may orient themselves in two different ways.
In one, the black gene and the rough gene may go to one sperm, while the white gene and the smooth gene go to the other.
In the other possible way, the black gene and the smooth gene may go to one sperm,
while the white gene and the rough gene go to the other.
Thus, a black rough male will produce four times the sperms in equal numbers.
Similarly, a black rough female will produce four kinds of eggs in equal numbers.
The black rough sperm may unite with each of the four kinds of eggs.
The black smooth sperm may unite with each of the four kinds of eggs.
The white rough sperm may unite with each of the four kinds of eggs.
The white smooth sperm may unite with each of the four kinds of eggs.
Of the 16 possible combinations, nine will have at least one black gene and one rough gene.
Due to dominance, these offspring will be black rough.
Three will have at least one black gene, but no rough gene.
These will be black smooth.
Three will have no black genes, but at least one rough gene.
These will be white, rough.
One will have no black and no rough gene.
And this will be white, smooth.
Only when many litters are combined may one expect this theoretical 9:3:3:1 ratio.
In this film, we have shown schematically the mode of transmission of the hereditary materials
which are responsible for the inherited characters of all plants and animals.