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B65 Animal Genetics
No two animals are exactly alike. Even with twins one may be taller, one may be heavier, or grow faster.
The two main factors that contribute to these differences in animals are:
The genetic make up of the animal.
The phenotype is the physical appearance of the animal.
The genotype is the genetic make up of the animal
Both the environment and the genetic make up effect the physical appearance of the animal.
The quantity and quality of the feed
Exposure to parasites and Diseases
The type of terrain (Steep Mountains, desserts, irrigated pasture)
The producer has a lot of control over the animal's environment.
A producer can also influence, to a lesser degree, the genetic make up of an animal.
In nature, genetics are passed on through the process of natural selection. The strongest, healthiest, most powerful animal generally spreads its genetics. Animals that are weak may have a poor immune system and are diseased, or may have conformation problems. Generally these animals do not survive long enough to pass on their genetics.
A producer crosses two parents based on a desired outcome.
A tough, dominant, alpha male may not be a desirable trait for domestic animals.
Agriculture producers select for traits that have economic importance, such as low birth weight, growth rate, feed efficiency, mothering ability, carcass traits.
The economically important traits are influenced by both the environment and the genetic make up of the animal.
Controlled Breeding Programs
Outcrossing: Breeding purebred animals with unrelated purebred animals.
Cross Breeding: Breeding animals of the same species but of a different breed.
Hybrid Vigor or Heterosis:
Purebred Bulls X Purebred Cows of another breed
8-10% increase in weaning weight
Two-Breed Backcross or Crisscross
Breed A X Breed B = Crossbred calves
Crossbred X Breed A or B
Charolais Bull X Hereford Cow = Cross
Cross X Charolais
Yields 67% of maximum heterosis
Three Breed Rotation Cross
3 Breeds (Angus, Simmental, Charolais)
Crossbred females bred to purebred bull of breed A
Resulting cross mated to purebred bull of breed B
Resulting cross mated to purebred bull of breed C
87% of maximum heterosis
All selection is based on the concept that desired characteristics are passed on from the parents to the offspring.
Humans have 46 chromosomes. Each parent contributes 23.
A chromosome is a long protein strand on molecules called DNA.
DNA is made up of segments called genes.
Each gene is responsible for a particular trait.
Genes form a code or a blueprint for how the animal is to be formed.
One chromosome (strand of DNA) will attach to another forming a spiral shape called a double helix.
Each half is bound together by substances called nucleotides.
There are four main nucleotides.
Nucleotides are shaped so that each substance can pair with one particular nucleotide.
Adenine can only pair with thiamine.
Cytosine can only pair with guanine.
When cells undergo mitosis and divide, each half replicates itself so two strands exactly alike are formed. (DNA replication)
The genetic sequence on the DNA is used as a pattern for how the animal is to be constructed. RNA (Ribonucleic acid) reads the pattern and transfers the information to the rest of the cell.
Genetic engineering is a technology that allows specific genetic information or traits to be built into or engineered into the genes of a species.
In genetic engineering, segments of DNA are cut and spliced into existing DNA, placing new genetic information into the existing DNA.
As the embryo begins to grow and develop, cells differentiate. Some cells develop into muscle and bone, some into skin and some into internal organs.
The process of how cells differentiate is not fully understood.
Sex cells called gametes undergo meiosis and only carry one strand of DNA.
At conception, chromosome halves from each parent combine to form a paired chromosome.
There is almost an infinite number of ways that the genes can be arranged on a strand of DNA. This arrangement determines the make up of the new animal.
Each male gene that controls a specific trait combines with the female gene for the same trait.
A pair of genes that control a specific trait are called alleles.
If both genes that control a specific trait are alike, they are said to be homozygous.
For example, if the male gene for hair color is black and the female gene that controls hair color is also black.
If they are different (black & red) they are said to be heterozygous.
In this case, one gene will be dominate and determine coat color.
Dominant gene = trait overpowers others
Recessive gene = must be accompanied with another recessive gene to express trait
P = polled
p = horned
Genotype is the genetic make up of the animal.
Phenotype is the physical appearance of the animal.
If a homozygous horned cow (pp) is mated to a homozygous polled bull (PP), what will the genotypic and phenotypic ratio of the calves be?
A monohybrid cross is an estimation of a predicted outcome for a single trait.
If a homozygous horned cow (pp) is bred with a heterozygous polled bull (Pp), what percent of the calves will be polled?
What results if two heterozygous animals are mated.
A dihybrid cross is a estimation of a predicted outcome for two traits.
Mate an Angus bull that is homozygous black and polled (BBPP) to a red shorthorn cow which is homozygous red and horned (bbpp).
The bull BBPP can be simplified to BP (black & polled is the only possible contribution for the bull).
The cow bbpp can be simplified to bp (red & horned is the only possible contribution for the cow).
Now mate two of the offspring which are heterozygous for black/red and polled/horned (BbPp)
How do you do a Punnett square for two heterozyous animals?
Use all possible gene combinations.
Both the bull and cow are BbPp.
What are the possible contributions?
BP, Bp, bP,bp for both animals. (4 x 4 grid)
Black Polled = 9 out of 16 or 56.25%
Black Horned = 3 out of 16 or 18.75%
Red Polled = 3 out of 16 or 18.75%
Red Horned = 1 out of 16 or 6.25%
If a heterozygous bull (BbPp) is mated with a homozygous cow (BBPP)
What are the outcomes?
If a (BbPp) bull is mated with a (BBPp) cow.
What are the outcomes?
Paint color is a desirable characteristic of paint horses and is dominate to solid color.
If a Homozygous dominate stallion and is bred with a solid colored mare, how likely is it that a paint foal will result?
What if the stallion is heterozygous for paint color?
Some alleles may have two dominate genes.
Shorthorn cattle are red, white or roan.
Red shorthorns carry the gene for red coat color RR
White shorthorns carry the gene for white coat color WW
Cattle that are roan or spotted carry a gene for red and a gene for white RW
Both are dominant, creating a spotted or roan colored animal.
Shorthorn: Red X White
The Additive Expression of Genes
Instead of a single pair, a number of genes may be added together to produce a single trait.
Milk production is controlled by several genes.
Size and body capacity of the female
Mammary size and function
Rate of gain
Occasionally a defect will happen and genetic traits are not passed on as intended.
Example: two headed calves
An abnormality is similar to a mutation, only it is caused by something in the environment
Sometimes genetic mutations can be used to introduce new kinds of species.
Polled Hereford Cattle
Determining an Animal's Sex
Whether a mammal is a male or a female is determined upon conception.
Gamete (sex cell) contains one half of the sex chromosome from the parent.
The female chromosome is referred to as XX.
When the chromosome divides and half goes to the offspring, each half is the same.
The male chromosome is referred to as XY. When divided, a gamete will be either X or Y.
When the male and female gamete combine they will either be XX Female or XY male.
What is the probability of a male being conceived over a female child?
What sex will the offspring be?