Genetics

Plant and animal tissues are composed of cells. These cells have an outer membrane, cytoplasm, and nucleus. The nucleus contains the chromosomes (rod like bodies). The chromosomes are contained in pairs, and each chromosome has genes. Genes are the functional components of inheritance. When cells divide to produce more body cells, the chromosomes replicate as well by a process called mitosis. During mitosis, each chromosome divides into two identical (daughter) cells.

Each species has a characteristic number of chromosomes. Poultry have the most (39 & 41) and pigs the fewest (19) Humans have 23.

 Gametogenesis

The production of sex cells, occurs in the testicle or ovary. Gametes from the testicle are sperm (spermatogenesis), and from the ovary are eggs or ova (oogenesis). The cell division is accomplished through meiosis. Each newly formed gamete contains only one of the original chromosome pairs.

Meiosis occurs in the primordial germ cells near the outer wall of the seminiferous tubules of each testicle, and near the surface of each ovary. Initially, meiosis is similar for both male and female. The chromosomes replicates so that the pair is doubled, then each pair comes together through synapsis. After the synapsis, the cell is called a primary (spermatocyte, or oocyte).

Spermatogenesis

The primary spermatocyte contains two bodies or structures formed by four parts. Two rapid cell divisions occur, where no chromosomes are replicated, and result in four cells, each contains two chromosomes. The spermatid cell looses much of their cytoplasm and develop a tail. Four sperm are produced for each Primary spermatocyte. So the net result is the number of chromosomes is half of the primordial germ cell.

Oogenesis

The primary (like the male) contains a tetrad. The first (maturation) division after the primary oocyte is formed produces a larger nutrient containing cell (secondary oocyte) and a smaller cell (first polar body). Each of these cells contains a dyad. The second maturation division produces the ovum, and the second polar body. Each contains two chromosomes. The first polar body may also divide, but all polar bodies die and are reabsorbed. The ovum, like the sperm contains only half of the original chromosome pair (one chromosome).

Fertilization

When a sperm and ovum unite, each contributes one chromosome to the resulting pair. When this occurs, the ovum is fertilized, and referred to as a zygote. Fertilization is the union of the sperm and ovum along with the establishment of the paired condition of the chromosomes. The zygote is termed diploid (double or paired chromosomes), whereas the gametes are haploid (one single un-paired chromosome).

Gametogenesis reduces the number of chromosomes in a cell to half the diploid number, and fertilization reestablishes the normal diploid number.

Genes & Chromosomes

Chromosomes are in pairs, therefore genes are in pairs. The location of the gene on the chromosome is called the locus (loci = plural). When the chromosomes are paired in diploid cells they are said to be homologous chromosomes. The genes at each loci on one chromosome strand matches to the corresponding loci on the other chromosome. The transmission of genes to offspring depends entirely on the transmission of these chromosomes to the offspring.

A special pair of chromosomes, the sex chromosome (X & Y), exist as a pair in which one of the chromosomes does not correspond entirely in terms of where the loci are present. The Y chromosome is much shorter than the X.

The X & Y chromosomes determine the sex (gender) of an animal. A female has two X, while a male has an X and Y. The female being XX can contribute only X’s to the genetic make up of the offspring. The male being XY can contribute either the X or the Y to make the offspring XX or XY, respectively. So it is the male that determines the sex of the offspring.

In all bird species, including chickens and turkeys, it is the female that determines the sex of the offspring. The chromosomes are identical in the sperm (XX) but in the egg are different (XY).

The genes at a corresponding loci may correspond to each other in a way that they control a trait, or they may contrast. If they correspond, they are said to be homozygous, and if the differ, they are said to be heterozygous. Those genes that occupy corresponding loci in homologous chromosomes, but that affect the same character in different (black or red color) ways are called alleles. Genes that are alike and that affect the character developing in the same way are called identical alleles. Alike genes in homologous chromosomes are identical alleles.

In genetics, chromosomes are illustrated by lines, and genes are indicated by letters. When genes of a corresponding loci on homologous chromosomes differ, one of the genes usually overpowers the expression of the other. This allele is called dominant. The allele whose expression is prevented is called recessive. The dominant allele is symbolized by a capital letter, while the recessive is symbolized by a lower case letter. EXAMPLE: In cattle black hair color is dominant to red hair color, so B = black and b = red. There are three combinantions possible then, BB, Bb, bb. Both BB and bb are homozygous for the genes that determine hair color, but one is homozygous dominant (BB) while the other is homozygous recessive (bb). The animal that is Bb is heterozygous (allelic genes).

Six Fundamental Types of Mating

With three kinds of individuals (homozygous dominant, homozygous recessive, and heterozygous) and one pair of genes being considered, six types of mating are possible. Using genes designated as B = black, and b = red in cattle, the six mating possibilities are BB X BB, BB X Bb, BB X bb, Bb X Bb, Bb X bb, and bb X bb.

1. Homozygous dominant X Homozygous dominant (BB X BB). Each can only produce one kind of gene, B. Therefore the union of the gametes, from two homozygous dominant parents will be B X B, resulting in a BB offspring. Therefore homozygous dominant parents produce homozygous dominant offspring.

2. Homozygous dominant X Heterozygous (BB X Bb). The ration of the offspring will be 1:1 for homozygous dominant : heterozygous.

3. Homozygous dominant X Homozygous recessive (BB X bb). All offspring will be heterozygous Bb).

4. Heterozygous X Heterozygous (Bb X Bb). Each parent will produce either a B or b. Four chances, with three outcomes. The genotypic ratio is 1:2:1 (25% BB, 50% Bb, and 25% bb). And the appearencee of the offspring will be 3 dominant:1 recessive. This 3:1 ratio is the phenotypic ratio.

5. Heterozygous X Homozygous recessive (Bb X bb). 1 heterozygous:1 homozygous recessive. Genotypic same as phenotypic ratio.

6. Homozygous recessive X homozygous recessive (bb X bb). All offspring will be homozygous recessive.

Multiple Gene Pairs

Consider there are two gene pairs to be considered, each independently affecting a particular trait. One determines coat color, the other determines polled or horned (B = black, b = red, P= polled, p = horned). If a bull is heterozygous for both traits, (BbPp) and mates heterozygous cows, we can calculate the expected phenotypic and genotypic ratios. Crossing Bb X Bb = 3:1 black to red. Pp X Pp gives 3:1 polled to horned., Combined BbPp X BbPp:

Get 9 black polled. 3 black horned, 3 red polled, and 1 red horned.

	9 genotypes	4 Phenotypes
1BB X	1BBPP	black, polled
	2BBPp	black, polled
	1BBpp	black, horned
2Bb X	1BbPP	black, polled
	2BbPp	black, polled
	1Bbpp	black, horned
1bb X	1bbPP	red, polled
	2bbPp	red, polled
	1bbpp	red, horned

GENE INTERATIONS

A gene may interact with another gene in the same chromosome (linear interaction), with it’s corresponding gene in a homologous chromosome (allelic interaction), or with genes in nonhomologous chromosomes (epistatic interaction). Environmental factors may interact with genes internally (hormones) or externally (nutrition, temperature, amount of light). Linear interactions are known to exist in lower animals (Drosophila – fruit fly), but have not been demonstrated in farm animals.

Allelic interactions

Each gene occupying the same locus on homologous chromosomes may influence the trait individually, but the effect depends on dominance or recessive. Allelic interactions may also be called dominance interactions. When unlike genes occupy corresponding loci, complete dominance may exist. In this situation, only the dominant gene is expressed. A good example is the polled dominance in cattle discussed previously. The heterozygous animal is indistinguishable from the homozygous dominant phenotypically.

There may also be a lack of dominance, in which the heterozygous animal shows a different phenotype than either homozygous animal (dominant or recessive). A good example is sheep ear length. LL have long ears, and ll are earless. The Ll heterozygous sheep have short ears, being intermediate.

Lack of dominance can also be considered additive gene action. Additive gene action occurs when each gene has an expressed phenotypic effect. EXAMPLE: if D gene and d gene influence rate of gain (D = 0.1 lb/d and d = 0.05 lb/d). DD increases gain by 0.2, Dd increases gain by .15, and dd increases gain by .10 lb/d. There is evidence that many genes influence many production parameters, like rate of gain.

EXAMPLE: the D and d gene as before also may work with the N and N gene in the same respect (N = .1 lb/d and d = .05 lb/d).

DDNN	= .40
DDNn & DdNN	= .35
DdNn, DDnn, & ddNN	= .30
Ddnn & ddNn	= .25
ddnn	= .20

Overdominance is a condition where the heterozygous animals are superior to either of the homozygous conditions. The heterozygous animals show better vigor, or are more desirable in other ways (milk, fertility, etc.). Heterozygotes of breed crosses are more vigorous, than strightbred parents, the are said to possess heterosis. This greater vigor or productivity of crossbreds is also said to be an expression of hybrid vigor (same as heterosis).

The bar graphs may represent livestock production (fertility, milk, growth, feed efficiency, carcass merit, etc.).

1. Lack of dominance, homozygous is superior, heterozygous is intermediate, and the other homozygous is inferior.

2. Overdominance, heterozygous is superior to either homozygous.

Epistatic Interaction

When a gene or gene pair alters or masks the expression of genes on another chromosome, this is Epistatic Interaction. In other words, a gene that interacts with a gene that is not allelic to it, is said to be epistatic.

Several coat colors in horses are due to epistatic interactions. For example: Two basic coat colors are black and chestnut. The recessive gene, b, produces chestnut. Thus BB and Bb are black and bb are chestnut. Some horses pssess a gene for white color that masks all other genes for color that exist in the genotyoe. This gene is called the dominant white gene W. The recessive gene w allows other color genes in the genotype to express themselves. Thus the geno and pheno types are:

BBWW or BWw = White

BbWW or BbWw = White

BbWW or bbWw = White

BBww = Black

Bbww = Black

Bbww = Chestnut

Epistasis can result from a dominant or recessive gene or combination of both. Another EXAPMLE: White Rock chickens, C = color, c = albino, W = color, and w = white. Homozygous cc prevents W from showing and ww prevents CC from showing. If the mating CcWw X CcWw was made, there would be only two phenotypes (colored or white). This contrasts with four phenotypes if C and W showed complete dominance, or nine phenotypes if there was no dominance expressed in the two pairs of genes. Thus epistasis changes the phenotypic ratio, but the number of different genotypes remains the same.

INTERACTIONS BETWEEN GENES AND ENVIRONMENT

Genes interact with both internal and external environments. External includes temperature, light, altitude, humidity, disease, and feed supply. Some breeds (Brahman) can withstand high temperatures better than others, while some breeds (Scotch Highland) can withstand the extreme cold better than others.

One of the most important (if not the most important) is feed supply. Some breeds can survive when feed is in short supply for long periods of time, and they may eat just about anything they can. Other breeds select only palatable feed, and therefore have poor production when feed is in short supply.

Allelic, epistatic, and environmental interactions all influence the genetic improvement that can be made through selection. When the environment has a large effect on production traits, genetic improvement is low. EXAMPLE: If animals in a population are maintained at different nutritional levels, those that are consuming better diets perform better. Much of the growth difference is due to the environmental factor nutrition. The producer has two alternatives, 1) standardize the environment so that it causes less variation, or 2)maximize the expression of the production trait by improving the environment. He first approach is geared to increase genetic improvement so that selection is more effective. These improvements are permanent. The second approach does not improve genetic qualities of animals, but it allows the expression of the animals genetic potential. The most sensible approach is to expose the animals to an environment similar to that which commercial animals are expected to perform economically.