Now that the process of cell division (mitoses and meiosis) is understood, it can be seen that the process of meiosis is the reason we study genetics.
The effect of additive and non- additive genes
This basic introduction ton genetics– the cell, genes and DNA; how RNA and the cell ribosomes are responsible for forming different types of protein structures that develop in different types of tissue which grow into organs through the process ofmitosis,and how meiosis determines the sex of the animal — we can look at the application of this information to breeding better animals.
When a flock of farm animals (herd of cattle) is observed, it is clear that there are differences in characteristics, such as body conformation, height, coat colour, horns or pole, etc.
There are 2 factors responsible for the genetic variation between individual animals in the same herd: No-additive gene effect, and additive gene effect.
NON- Addictive Gene effect
The Non-additive gene effect occurs when a single pair of genes is responsible for a specific trait or character in an animal, for example, coat colour. The non-additive gene pairs contribute to the genotype of the animal.
We also refer to the gene effect as being homozygous as explained in more detail later in this unit.
Addictive gene effect
The effect of Additive genes refers to many genes that are coded together for expressing the same trait. An example of a function of additive genes is eye colour and milk and wool production. Additive genes together with the influence of environment contribute to the phenotype.
We also refer to the gene effect as being heterozygous as explained in more detail later in this unit.
As the number of additive genes increases, the distribution of phenotypes becomes more continuous. In addition, as already stated, most quantitative traits are also affected by the environment. Environmental effects may obscure genetically-caused differences between phenotypic classes.
For example, nutrition affects adult size in livestock. Although carcass weight and size are determined through an additive gene effect, the feeding and nutrition (environment) will contribute up to 80% of the animals’ weight (phenotype).
The following additive genes have been found to all contribute together to milk production:
Genotypes and Phenotypes
Genotype is a set of genes in the DNA responsible for particular traits. Genotype is the genetic makeup of an organism and it results in some of the physical characteristics of that organism. These are the genes responsible for coat colour, size, body conformation, etc.
Phenotype is the physical expression (characteristics) of those traits that was also influenced by the environment; those traits that you can see such as coat colour, the size, and body conformation.
To clarify further:
Qualitative Characteristics(Traits)
Qualitative characteristics (traits) – dominant and recessive traits are controlled by 1 pair of genes. An example of qualitative traits is the single pair of genes responsible for influencing coat colour, or horns versus polled.
Very few traits of economic importance in farm animals are inherited by qualitative traits- simply put – coat colour or the polled condition does not contribute much to economic performance.
Example: A Milking Shorthorn having two genes for red (RR) is red in colour, while an animal having two genes for white (rr) is white in colour. A Milking Shorthorn that has one gene for red (R) and one for white (r) is neither red nor white but roan (Rr), which is a mixture of red and white. Thus, red and white mating in Milking Shorthorn cattle usually produce roan offspring. Likewise, white and white mating generally produce white offspring, even though white Milking Shorthorn is seldom pure because the face bristles, eyelashes, and ears usually carry red hairs. Roans, having one gene for red and one for white on the paired chromosomes, never breed true and when mated together produce calves in the proportion of one red, two roans, and one white. The most certain way to produce roan Milking Shorthorns, is to mate red cows with a white bull, or vice versa – this produces roan calves. If a roan animal is bred to a red one, one-half of the offspring will be red, and the other half will be roan. Likewise, when a roan animal is bred to a white one, approximately an equal number of roan and white calves will be produced.
You may be confused as to why the offspring present as mixed colour in the F1 generation, even though they carry the dominant gene (R) for red colour coats (further explained under Dominant vs Recessive genes). This is because, contrary to the example given, in nature Genetics can be more complicated and this occurrence is explained by Co-dominance. Thus, the Heterozygote (Rr) will exhibit both the red trait and the white trait.
Some alleles though dominant, do not independently present the phenotype, but rather in conjunction with the recessive geneare presented in the phenotype, thus resulting in a mixed phenotype of both red and white hair being expressed. This occurrence is termed Co-dominance.
Quantitative Traits
Most phenotypic traits in animals are affected by many genes (size, weight, shape, lifespan, physiological traits and fecundity). Often, it is not feasible to determine the number of genes affecting a particular trait, and the individual effects of genes on the phenotype. Many of these traits can be measured on a quantitative, rather than a qualitative scale. This is where the terms quantitative trait and quantitative genetics come from.
As where qualitative traits are controlled by 1 pair of genes, Quantitative traits are a measurable phenotype that depend on the cumulative actions of many genes and the environment.
These traits can vary among individuals, over a range, to produce a continuous distribution of phenotypes. Examples include height, weight and blood pressure.
Most traits of economic importance, such as milk yield and composition, conformation, feed efficiency, disease resistance, are controlled by multiple genes.
Estimates of the number of pairs of genes affecting each economically important characteristic vary greatly, but the majority of geneticists agree that ten or more pairs of genes are involved for most such traits. In addition to being influenced by many pairs of genes, quantitative traits differ from qualitative traits because they are frequently strongly influenced by the environment.
Quantitative traits have:
- continuous distributions, not discrete classes
- Are usually affected by many genes (polygenic)
- Are also affected by environmental factors
Dominant and Recessive
A dominant gene is when one gene (the dominant gene) completely overrules the effect of another gene – the recessive gene. Black coat colour is for example dominant over red coat colour in Angus cattle.
A dominant trait is indicated by a capital letter, for example “B” for the black coat colour. The degree of dominance depends on the animal’s entire genetic makeup, together with environmental factors to which the animal is exposed.
A recessive gene will not show its effects in the presence of a dominant gene. Red coat colour is in the same example recessive in Angus cattle to the dominant gene for black coat colour. Recessive is represented with a lower-case “b” for red coat colour in the same example.
Homozygous And Heterozygous
Heterosis
Heterosis is also referred to as “Hybrid vigour”. This refers to the tendency for a cross-bred individual to show qualities or traits that are superior to the traits of both parents. Heterosis (or hybrid vigour) is one of the most widely utilised phenomena in animal breeding.
Heterosis is a result of crossbreeding. The purpose of cross breeding is to:
- Cash in on the advantages of heterosis
- To make use of average breed effects
- To breed a cow herd based on hybrid vigour
- To target specific markets
- To create a specific breeding plan for a herd/flock of animals
Heterosis can be defined as the difference in the value of a trait compared to the average value of the parents for the trait.
For example: The average value for the weaning weight of a breed ( Breed A) is 225 Kg, and the average value for another breed (Breed B) is 270 kg. After mating Breed A with Breed B, the calf of these two breeds averages 260 Kg. The heterosis for weaning weight is 12.5 Kg, or 5.05%. This extra weaning weight is free because nothing more was done than using a different breed.
Heterosis = Average weight between breed A and B – weigh of new offspring
= (225 + 270) / 2 – 260
= 247.5 – 260
= 12.5 Kg (12.5 / 247.5 = 5.05%)
Heterosis is not consistent from one breed to another. Breeds that differ genetically, will exhibit more heterosis than genetically alike breeds.
Example: Boss indicus (Brahman) x Bos taurus (Hereford) differ genetically more than a Bos taurus (Simmental) x Bos taurus (Limousine), and the heritability between Boss indicus x Boss taurus will be much more.
Heritability
Heritability is a statistical reference of the ratio of variation that occurs in a breeding programme, due to the differences between genotypes, in contribution to phenotype variation for a character or trait in a population. (Population refer to, for example, a specific herd of cattle or flock of sheep, used as a population in a breeding programme)
The heritability estimates, therefore express the estimate likelihood of a specific trait to be transferred from the 2 parents to the offspring. (What are the chances for a trait in selecting 2 parents to breed for that specific trait, such as early weaning weight?)
Some statistical examples of heritability estimates:
- Heritability estimates of some important traits in beef cattle. *
- Heritability estimates of some important traits in sheep*
