Vet Science

Genetics

Definition of Genetics A branch of biology which studies

heredity and variation in organisms. Genetics studies the transmission of

genes from one generation to another. A blueprint of traits and characteristics is

established for the new offspring from the genes transferred from both parents.

The genotype is the genetic makeup while the phenotype is the physical makeup.

Genes and Chromosomes

Chromosomes (found in pairs) are contained in the nucleus of every cell. Within the chromosomes are smaller units called genes.

Genes contain the information that control all of the biochemical processes (life processes) of the cell.

DNA – Deoxyribonucleic Acid

Genetic codes of species consist of distinct DNA.

DNA is a complex molecule composed of nucleotides joined together with phosphate sugars.

Simply, it is a nucleic acid which contains the genetic instructions used in the development and functioning all living organisms other than viruses.

Nucleotide is building block of nucleic acids. Each nucleotide is composed of sugar, phosphate, and one of the four bases.

DNA Cont’d

The main role of DNA molecules is long term storage of information.

The segments of DNA which carry the genetic information are called genes.

Other DNA sequences have structural purposes or are involved with the use of the genetic information.

Double Helix of DNA

Consists of two long polymers of simple units called nucleotides with backbones made of sugars and phosphate groups held together by ester bonds.

The two strands run opposite directions to each other and are said to be anti-parallel.

Attached to each sugar is one of 4 types of bases. It is the sequence of these bases along the backbone of the helix which encodes the information.

The 4 bases are divided into two groups: purines, (adenine, A and guanine, G) and pyrimidines, (thymine,T and cytosine, C)

Helix Cont’d The DNA is a sequence of the base pairs

that represents the code for a specific gene.

The sequence of the 4 bases arranged in pairs with a the pairs of the allele provides 256 combinations.

Consider the many different traits and the multiple genes and alleles make for exponentially of the complexity of DNA.

Chromosomes

Organized structures of DNA. Contained in the nucleus of the cell. Found in pairs. Numbers of pairs are specie specific. Humans – 46, Cattle – 60, Swine – 38,

Sheep – 54, Goats – 60, Horses – 78 Dog – 78, Cats – 38,

Genes

Genes contain the information that controls all of the biochemical processes of the cell.

The gene codes are for the synthesis of specific proteins of the cell.

Genome

Definition - The complete genetic material of an organism.

Genome bases are build to preserve species in as pure a state as possible.

DNA Replication

Definition – the process of making a copy of the DNA molecule.

DNA Replication

Principles of Inheritance Gregor Mendel discovered the principles of

inheritance using peas as a subject. Alleles affect the same trait, but each

allele causes the production of a different protein and how the trait is expressed.

The locus of alleles on a chromosome is said to be homozygous if the alleles are identical. Unlike alleles at the locus on the chromosome are called heterozygous.

Definition of Inheritance The method by which alleles are

passed from one generation to another is called inheritance.

Gametes are produced from reproductive cells by the parent.

Each gamete contains a single allele for each gene or ½ of the genetic code of the parent.

Genotypes and Phenotypes

When gametes are combined during fertilization, a complete set of genetic code is present. This complete set contains the genetic traits of the new individual is called the genotype of the animal.

The physical traits which are expressed from the genetic code present or the physical appearance of the animal is called its phenotype.

Two Principles of Genetics

Principle of segregation – Mendel’s first law states that when gametes are formed, the genes at given locus separate so that each is incorporated into different gametes.

Alleles separate so that only one is found in any particular gamete.

Principles Cont’d Principle of independent assortment

– states that in the formation of gametes, separation of a pair of genes is independent of the separation of other pairs.

When the two principles are applied together, they provide a means for randomization of alleles within the gametes. Thus, our understanding of how variation exists within a population.

Gametes Male gametes are the sperm cells. Female gametes are the egg cells. They are the germ cells of

reproduction. Each normal body tissue cell (somatic

cell) has 1 pair of sex chromosomes. The other chromosomes within the somatic cell are called autosomes.

Haploid and Diploid Numbers

All somatic cells contain a diploid (2n) number of chromosomes.

The germ cells, sperm and egg, contain a haploid (1n) number.

Types of Cell Division Mitosis is the process of somatic cell

division. It is replication of cells and responsible

for the maintenance of body cells, tissues and organs. In addition, mitosis is responsible for growth of young animals.

The cell contains a diploid number of chromosomes and through the replication process two cells emerge with diploid numbers.

Stages of Mitosis

Prophase Metaphase Anaphase Telophase YouTube video

Meiosis

Meiosis is responsible for the process of gametogenesis or the formation of gametes.

Two processes of gametogenesis are: Spermatogenesis – sperm cells Oogenesis – egg cells

YouTube video

Gene Expression

Dominant genes – one member of each gene pair is expressed to the exclusion of the other.

Recessive genes – one member of each gene pair is only expressed when the dominant allele is absent from the animal genome.

Genetic Expression Possibilities

Homozygous dominant (RR) Homozygous recessive (rr) Heterozygous dominant (Rr) Codominance – both alleles are expressed in the

phenotype when present in a heterozygous state. Roan shorthorns

Incomplete dominance – no dominance and the heterozygous phenotypic state is an intermediate. Gray horse, palomino horse

Epistasis – when gene expression is influenced by the presence of another. Bay horse (brown with black tail), chestnut horse

Sex Linked or Related Inheritance

XX chromosomes are female XY chromosomes are male The passage of Y with appropriate gene

expression provides easy determination of the phenotype of the male offspring.

The passage of X masks the genotype of the offspring and expression phenotypically is seen but not necessarily projectable for future offspring.

Sex Influenced Inheritance Best example: Polled and Horned

Dorset sheep HH or homozygous dominant – both

male and female have horns. Hh or heterozygous individuals –

male has scurs while the female is polled.

hh or homozygous recessive individuals – all are polled.

Population Genetics Is the study of how gene and genotypic

frequencies change within a given population. Gene frequency is defined as the proportion of

loci in a population that contain a particular allele.

Phenotypic frequency refers to how often we see the particular allele. It is expressed as a % or as a decimal.

What we see is usually an indicator of the genetic make up of the individual. Remember that what we see may or may not be what we have present genetically.

Making Genetic Change to the Population

Mutations and genetic drift can be bring about change in the population.

Mutations are changes in the chemical composition of gene that alters the DNA of the individual.

Genetic drift is a change in the gene frequency owing to chance. An inverse relationship occurs relating to the size of the population.

Neither one of the above can be used effectively to make direct change to the gene frequency of a population. Too much is left to chance.

Genetic Improvement by Migration and Selection

Migration is the process of bring new breeding stock into a population. Wholesale change is made in the influx of new genes to the existing population.

Migration success in a breeding program is accomplished due to the new genes dramatically changing the gene frequency within a herd or flock.

Selection

Is the process of allowing certain animals to be parents while other are not bred or are used as slaughter individuals.

Two types of selection: Natural Artificial

Natural Selection Using multiple males in herd or flock

results in certain males becoming dominant in passing along their genetic makeup along to offspring as compared to other males in the herd or flock.

Physical traits of the male: size, dominant personalities, mobility and reproductive efficiency can lead to changes in the genetic frequency of the offspring.

Carload sales of bulls in the Western states are an example.

Artificial Selection

Management determines the use of males or females in the breeding herd.

The managerial decision is based on the desired outcome from the offspring of particular matings, i.e. breeding for carcass vs breeding for reproductive efficiency by using selected males and females.

Quantity vs Quality Traits Qualitative traits are defined as those

phenotypes that are classified into groups rather than numerically measured. Examples given: color of hair coat, horns, white faces, black hooves

Quantitative traits are numerically measured and are controlled by many genes which individually have small influence. Examples are: ADG, FE, pigs weaned/litter, #of milk

Remember – environment will affect the quantitative trait expression. Some environment may aid while other may hinder the positive effect of selecting quantitative traits for herd improvement.

Heritability Definition – it is a measure of the

proportion of phenotypic variation that can be passed on to offspring.

It is used as an indicator of the amount of genetic progress that can be achieved by choosing superior sires and dams.

It is the proportion of the difference between individuals that is due to additive gene effects.

Additive gene action or effect is when the total phenotypic effect is the sum of the individual effects of the alleles.

Improvement from Heritability Understanding

Selection differential is the phenotypic advantage of the parents chosen to provide offspring. The difference in the mean of the parents compared to the mean of the population.

Example from text: Herd average is 7 pigs/litter born. Producer chooses sires and dams who average 11 pig/litter. Difference is 4 pigs. Litter size is 10% heritable thus 4(10%) = .4 pigs. We can expect litters from gilts produced from the crossing to produce an average of 7.4 pigs/litter born.

Percents of Heritability

Reproductive traits are low in heritability. <20%

Growth related traits are moderate in heritability. 20-40%

Carcass traits are considered high in heritability. >40%

Systems of Mating

Inbreeding Linebreeding Outbreeding Crossbreeding

Inbreeding The mating of closely related

individuals, i.e. father to daughter, son to daughter.

Inbreeding decreases the variation in the genes existing in a herd, thus increasing the homozygosity of desired traits as well as some undesired traits.

Undesired performance is called inbreeding depression.

Linebreeding A form of inbreeding where the

concentration of genes centers around a common ancestor, i.e. grandson to granddaughter.

Also used to concentrate genes for phenotypic expression.

Common among companion animals, dogs in particular.

Outbreeding or out crossing Heterosis is defined as the improvement

in performance by individual offspring from the crossing over the performance of the parent population.

Increased heterozygosity or hybrid vigor results.

Do not confuse heterosis with in purebreds with the heterosis resulting from crossbreeding.

Crossbreeding

The mating of individuals from two different breeds.

Maximization of heterosis occurs with the two breed cross.

Breed rotations are common methods.