illustration of dna double helix from wikipedia. animal genetics agriscience 332 animal science...

Illustration of DNA Double Helix from Wikipedia.

ANIMAL GENETICS

Agriscience 332

Animal Science

#8406

TEKS: (c)(4)(B)


Introduction

Genetics is the science of heredity and variation.

It is the scientific discipline that deals with the differences and similarities among related individuals.


All animals have a predetermined genotype that they inherit from their parents.

However, an animal’s genotype can be manipulated by breeding and more advanced scientific techniques (genetic engineering and cloning).


For many years, managers of agricultural systems have manipulated the genetic makeup of animals to improve productivity and increase efficiency.

Successful manipulation of the genetic composition of animals requires an understanding of some fundamental principles of genetics.


Mendelian GeneticsGregor Mendel is recognized as the father of genetics.

Mendel, who was not scientifically trained, developed his theories in the 1850’s and 1860’s, without any knowledge of cell biology or the science of inheritance.

Photo courtesy of Wikipedia.


In later years, genes, chromosomes, and DNA were discovered and people began to understand how and why Mendel’s theories worked.


Mendel proposed three principles to describe the transfer of genetic material from one generation to the next.• The Principle of Dominance

• The Principle of Segregation

• The Principle of Independent Assortment


The Principle of Dominance – in a heterozygous organism, one allele may conceal the presence of another allele.


The Principle of Segregation – in a heterozygote, two different alleles segregate from each other during the formation of gametes.


The Principle of Independent Assortment – the alleles of different genes segregate, or assort, independently of each other.


Later studies have shown that there are some important exceptions to Mendel’s Principle of Independent Assortment, but otherwise, these principles are recognized as the basis of inheritance.


Mendel’s experiments dealt with the relationship between an organism’s genotype and its phenotype.

Genotype – the genetic composition of an organism.

Phenotype – the observable or measurable characteristics (called traits) of that organism.


Two organisms may appear to be similar, but they can have different genotypes.

Similarly, two animals may have the same genotypes, but will appear to be different from each other, if they have been exposed to different environmental conditions throughout their lives.


The relationship between phenotype and genotype is expressed as the following equation:

P = G + E

P = phenotype,

G = genotype, and

E = environment.


If two individuals with identical genotypes are exposed to the same environmental conditions, such as nutrition, climate, and stress levels, their phenotypes (measurable and observable characteristics) should be the same.


To understand Mendel’s principles and the relationships between phenotype and genotype, it is necessary to understand what makes up the genetic material of animals and how this is transferred from one generation to the next.


Genetic Material

The body is made up of millions of cells which have a very complicated structure.

These cells are made up of many parts that have specialized roles.


1. Nucleolus 5. Rough Endoplasmic Reticulum 9. Mitochondria

2. Nucleus 6. Golgi Aparatus 10. Vacuole

3. Ribosome 7. Cytoskeleton 11. Cytoplasm

4. Vesicle 8. Smooth Endoplasmic Reticulum 12. Lysosome

13. Centrioles

Courtesy of Wikipedia


The nucleus contains chromosomes that are visible under the microscope as dark-staining, rod-like or rounded bodies.


Chromosomes occur in pairs in the body cells.

The number of chromosomes in each cell is constant for individual species, but it differs among species.


Chromosomes are made up of tightly-coiled strands of DNA.

DNA is a complex molecule composed of deoxyribose, phosphoric acid, and four bases.

Individual genes are located in a fixed position (known as the loci) on the strands of DNA.


A Chromosome

A chromosome is made up of two chromatids and a centromere. The chromatids are formed from tightly coiled strands of DNA. If these strands of DNA are stretched out, individual genes can be identified.


A gene is made up of a specific functional sequence of nucleotides, which code for specific proteins.

A specific protein is produced when the appropriate apparatus of the cell (the ribosome) reads the code.

Image courtesy of Wikipedia.


The collection of genes that an organism has is called its genome.

Photo by Peggy Greb courtesy of USDA Agricultural Research Service.


In somatic cells (body cells), chromosomes occur in pairs, known as homologous chromosomes.

As a result, genes also occur in pairs.

Somatic cells are referred to as diploid, or 2n.


Gametes (reproductive cells) do not have paired chromosomes and are referred to as haploid, or n.


Cell Division

Cells must divide and increase in number so that animals can grow.

A new cell is formed when one cell divides.

Mitosis and meiosis are the two processes by which cells divide.


Mitosis is the type of cell division in which the genetic material in the parent cell is duplicated and then divides into two separate cells with identical genetic material.

Both new cells are diploid (2n) with a complete set of chromosomes identical to those in the parent cell.


Illustration showing stages of cell cycle:

Interphase – portion of cell cycle in which the cell grows then replicates DNA.

Mitosis – portion of cell cycle in which division of the cell takes place; includes Prophase, Metaphase, Anaphase, and Telaphase.

Image courtesy of Wikepedia.


Meiosis is the process of cell division that occurs in reproductive cells (sperm and egg).

In this type of division, the chromosome number is halved from the diploid number (2n) to the haploid number (n).


If gametes were diploid cells, the number of chromosomes would double with each generation.

Meiosis ensures that gametes receive only one-half the number of chromosomes that are present in parent cells.


Fertilization

Fertilization is the process of joining the male gamete with the female gamete.

Photo from Wikipedia.


All animals originate from the union of a single haploid cell from the female (ovum or egg) and a single haploid cell from the male (spermatozoa or sperm).

The result of this union is a zygote (diploid cell), which develops into a new animal with a full set of chromosomes.


When discussing different generations in genetics, the first generation is referred to as the parent or P generation.

Their offspring are referred to as the first filial or F1 generation.

P X P

F1 F1 F1 F1


When individuals from the F1 generation are mated with each other, their offspring are referred to as the F2 generation. F1 X F1

F2 F2 F2 F2


Principle of Dominance

In animals, chromosomes are paired and, therefore, genes are paired.

These paired genes code for the same trait, but they are not identical.

They can have different forms, known as alleles.


For example, sheep and cattle can be polled or horned.

One gene codes for this trait and the two possible forms (alleles) of the gene are polled or horned.

USDA photo from Wikipedia.

Photo from IMS.


A capital letter is used to denote the dominant form of the gene (P) and a small letter is used to denote the recessive form of the gene (p).

In the example, the polled allele is dominant and is, therefore, denoted by P, while the horned allele is recessive and denoted by p.


Because genes are paired, an animal can have three different combinations of the two alleles:

PP,

Pp, or

pp.


When both genes in a pair take the same form (PP or pp), the animal is referred to as being homozygous for that trait.

An animal with a PP genotype is referred to as homozygous dominant.

An animal with the pp genotype is referred to as homozygous recessive.


If one gene in the pair is the dominant allele (P) and the other gene is the recessive allele (p), the animal is referred to as being heterozygous for that trait and its genotype is denoted as Pp.


Genotype refers to the actual genetic makeup.

Phenotype refers to the physical expression of the genes.

If an animal has the allele combination PP, it will be polled.

If the combination is pp, the animal will be horned.


If it is a heterozygote, then genotypically the animal will have both traits (Pp), but phenotypically the animal will be polled because the polled allele (P) is the dominant form of the gene.


Mendel’s principle of dominance states that in a heterozygote, one allele may conceal the presence of another.


In this example, the polled allele is concealing the horned allele and, therefore, is referred to as the dominant allele.


Principle of SegregationWhen animals reproduce,

they only pass on half of their genetic material to their offspring because gametes, or reproductive cells, only have one chromosome from each pair.

The offspring will only receive one allele from each parent.


The Principle of Segregation explains some of the differences that are observed in successive generations of animals and can be used to predict the probability of different combinations of alleles occurring in offspring.


As previously discussed, three kinds of individuals are possible when describing a pair of genes:

• Homozygous dominant (PP),

• Homozygous recessive (pp), and

• Heterozygous (Pp).


Considering these three types of individuals, six combinations of the various genotypes are possible:

• PP x PP (both parents are homozygous polled),

• PP x Pp (one homozygous polled parent and one heterozygous polled parent),


• PP x pp (one homozygous polled parent and one homozygous horned parent),

• Pp x Pp (both parents are heterozygous polled),

• Pp x pp (one heterozygous polled parent and one

homozygous horned parent), and


• pp x pp (both parents are homozygous horned).

The genotypes of the parents can be used to predict the phenotypes of the offspring.


Predicting the Genotypes and Phenotypes of

OffspringA punnett square is a grid-like method that is used to display and predict the genotypes and phenotypes of offspring from parents with specific alleles.


The male genotype is normally indicated at the top and the female genotype is indicated in the vertical margin.


When crossing homozygous dominant parents (PP x PP), all offspring will be homozygous dominant polled individuals.


When crossing homozygous recessive parents (pp x pp), all of the offspring will be horned (homozygous recessive) individuals.


When crossing a heterozygous parent with a homozygous dominant parent (Pp x PP), the expected offspring would occur in a 1:1 ratio of homozygous dominant to heterozygous individuals.

Phenotypically, all offspring would be polled.


When crossing a homozygous dominant parent with a homozygous recessive parent (PP x pp), all offspring would be heterozygous and polled.


When crossing a heterozygous parent with a homozygous recessive parent (Pp x pp), the offspring would occur in a genotypic ratio of 1:1 for heterozygous to homozygous recessive.

About one-half of the offspring would be polled.


If two heterozygous parents are crossed (Pp x Pp), one can expect a genotypic ratio of 1:2:1, with one homozygous dominant polled, two heterozygous polled, and one homozygous recessive horned individuals.

The expected phenotypic ratio of offspring would be 3:1 (polled to horned).


Considering Multiple TraitsCommonly, there are

multiple traits that need to be considered when mating animals.

For example, consider that cattle can be horned or polled and white-faced or red-faced.

The horns and red-faced coloring are recessive traits.


If two individuals with two pairs of heterozygous genes (each affecting a different trait) are mated, the expected genotypic and phenotypic ratios would be:

Genotypes – 1 PPWW, 2 PPWw, 2 PpWW, 4 PpWw, 1 PPww, 2 Ppww, 1 ppWW, 2 ppWw, and 1 ppww;


Phenotypes – 9 polled, white-faced; 3 polled, red-faced; 3 horned, white-faced; and 1 horned, red-faced offspring.


The Law of Independent Assortment

When considering multiple traits, Mendel hypothesized that genes for different traits are separated and distributed to gametes independently of one another.


Therefore, when considering polled and white-faced traits, Mendel assumed that there was no relationship between how they were distributed to the next generation.


In most cases, genes do assort independently.

However, advances in genetics have shown that an abnormal situation, called crossing-over, can occur between genes for different traits.


Crossing-over is an exchange of genes by homologous chromosomes during the synapses of meiosis prior to the formation of the sex cells or gametes.

Thomas Hunt Morgan’s illustration (1916) courtesy of Wikipedia.


Other Concepts in Genetics

Non-traditional inheritance involves alleles that are not dominant or recessive.

Incomplete, or partial dominance, and co-dominance are two examples of non-traditional inheritance.


Partial (Incomplete) Dominance

Partial, or incomplete, dominance occurs when the heterozygous organism exhibits a trait in-between the dominant trait and the recessive trait.


Partially dominant alleles for color are seen in several species of flowers and in mice.

Ex. Homozygous mice are black (BB) or white (bb) and the heterozygous mice will be grey (Bb).


Sheep exhibit incomplete dominance in the trait for eye color.

When a pure, brown-eyed sheep is crossed with a pure, green-eyed sheep, blue-eyed offspring are produced.


Codominance

Codominance occurs when a heterozygote offspring exhibits traits found in both associated homozygous individuals.

An example of codominance is the feather color of chickens.


If a homozygous black rooster is mated to a homozygous white hen, the heterozygous offspring would have both black feathers and white feathers.


Roan is a coat color in horses (sometimes dogs and cattle) that is a mixture of base coat colored hairs (ex. black, chestnut) and white hairs.

Neither the base coat color or the white hairs are dominant nor do they blend to create an intermediate color.


The roan animal actually has both colored and white hairs.



Under these circumstances, neither allele is dominant and neither is recessive.

Therefore, each allele is denoted by a capital letter.


Epistasis (Polygenic

Inheritance)It is possible for more than one gene to control a single trait.

This type of interaction between two nonallelic genes is referred to as epistasis.


When two or more genes influence a trait, an allele of one of them may have an epistatic, or overriding, effect on the phenotype.

Comb shape in chickens is an example of an epistatic relationship.


Domestic chickens can have four different types of comb shapes: (a) rose, (b) pea, (c) walnut, and (d) single.


Comb shape is influenced by two independently assorting genes, R and P, each with two alleles.

Wyandotte chickens with rose combs have the genotype RRpp,while the Brahma chickens with pea combs have the genotype rrPP.


The F1 hybrids between these two varieties are RrPp; phenotypically, they have walnut combs.

If those hybrids are intercrossed with each other, all four types of combs appear in the progeny in a ratio of 9:3:3:1 for walnut:rose:pea:single.


Mutations and Other Chromosomal Abnormalities

Genes have the capability of duplicating themselves, but sometimes a mistake is made in the duplication process resulting in a mutation.


The new gene created by this mutation will cause a change in the code sent by the gene to the protein formation process.

Some mutations cause defects in animals, while others may be beneficial.


Mutations are responsible for variations in coat color, size, shape, behavior, and other traits in several species of animals.

The beneficial mutations are helpful to breeders trying to improve domestic animals.


Changes that can occur in chromosomes during meiosis include:

• Nondisjunction – chromosome number,

• Translocation or deletion – chromosome breakage,

and

• Inversion – the rearrangement of genes on a chromosome.


Changes in chromosomes are reflected in the phenotypes of animals.

Some chromosomal changes will result in abnormalities, while others are lethal and result in the death of an animal shortly after fertilization, during prenatal development, or even after birth.


Sex-Linked Traits

Sex-linked traits involve genes that are carried only on the X or Y chromosomes, which are involved in determining the sex of animals.

The female genotype is XX, while the male genotype is XY.


The X chromosome is larger and longer than the Y chromosome, which means a portion of the X chromosome does not pair with genes on the Y chromosome.


Additionally, a certain portion of the Y chromosome does not link with the X chromosome.

The traits on this portion of the Y chromosome are transmitted only from fathers to sons.

Sex-linked traits are often recessive and are covered up in the female mammal by dominant genes.


The expression of certain genes, which are carried on the regular body chromosomes of animals, is also affected by the sex of the animal.

The sex of an animal may determine whether a gene is dominant or recessive (Ex. Scurs in polled European cattle).


In poultry, the male has the genotype XX, while the female has the genotype Xw.

An example of a sex-linked trait in poultry is the barring of Barred Plymouth Rock chickens.


If barred hens are mated to non-barred males, all of the barred chicks from this cross are males, and the non-barred chicks are females.



Genetic Selection

Permanent improvements in domestic animals can be made by genetic selection through natural or artificial means.

Natural selection occurs in wild animals, while artificial selection is planned and controlled by humans.


Animals that exhibit desirable traits are selected and mated.

Animals that exhibit undesirable traits are not allowed to reproduce or are culled from the herd.“Geneticist Michael MacNeil examines results from genetic analysis of sires used in the Angus Sire Alliance program at Circle A Angus Ranch. Economic values for individual traits are used to sort bulls by potential profitability of their offspring” (USDA – ARS).

Photo by Peggy Greb courtesy of USDA Agricultural Research Service.


The goal of selection is to increase the number of animals with optimal levels of performance, while culling individuals with poorer performance.

Genetic improvement is a slow process and can take several generations to see an improvement in a trait.


Artificial insemination and embryo transfer are breeding methods that are commonly used to decrease the time taken to improve a trait.

“Angus surrogate mother nurses her Romosinuano embryo transfer calf. Initially, scientists are investigating the influence of surrogate breed on Romosinuano calf traits such as length of gestation and birth and weaning weights” (USDA-ARS)

Photo by Scott Bauer courtesy of USDA Agricultural Research Service.


Traits are passed from parents to offspring, but some traits are more heritable than other traits.

That is, the genotype of an individual will be expressed more strongly and environment will be less influential for particular traits.


Trait Sheep Swine Cattle

Weaning weight 15-25% 15-20% 15-27%

Post-weaning gain efficiency

20-30% 20-30% 40-50%

Post-weaning rate of gain 50-60% 25-30% 50-55%

Feed efficiency 50% 12% 44%

Loin eye area 53% 53% 56%

Heritability of Various Traits in Livestock


Several genes influence some traits.

For example, rate of growth is a trait that is influenced by appetite, energy expenditure, feed efficiency, and body composition.

Photo by Brian Prechtel courtesy of USDA Agricultural Research Service.


Breeding systems aim to improve a single trait or multiple traits.

Single trait selection – aimed at improving one trait in a breeding program with little or no regard for improvement in other (associated) traits.


Multiple trait selection – aims to simultaneously improve a number of traits.

Theoretically, multiple trait selection should result in a faster rate of gain toward a specific objective.


Most domestic species now have a recognized system in place that allows breeders to estimate the genetic merit of individuals.

In the United States, cattle, sheep, goat, and swine breeders use expected progeny differences (EPDs).


EPDs are used to compare animals from the same species and breed.

“Newly developed EPDs (expected progeny differences) make it possible to select for tenderness and carcass and beef quality traits in Brahman cattle, shown here at the ARS Subtropical Agricultural Research Station in Brooksville, Florida” (USDA-ARS).

Photo by David Riley courtesy of USDA Agricultural Research Service.


For EPD values to be used effectively, one needs to know the breed averages, the accuracy of the EPDs, and who estimated the EPDs.

A high EPD is not necessarily good; it depends on the trait being considered and breeding objectives.


Modern GeneticsIn recent years, traditional methods of improvement through selection and breeding have been superceded by genetic manipulation.

A substantial amount of research has focused on direct manipulation of genes and DNA.


Gene Transfer

Genetic engineering basically refers to transferring a gene from one individual to another.


Scientists are able to code genes for desirable compounds and insert them into other cells, such as microorganisms.

These microorganisms produce these desirable compounds on a large scale.


This area of genetic manipulation makes important contributions to domesticated animals in relation to immunology, vaccines, aging, and cancer.“Annie the cow: bioengineered to have a gene for mastitis resistance” (USDA-ARS).

Photo by Scott Bauer courtesy of USDA Agricultural Research Service.


The implications for introducing superior production, conformation, and disease-resistant traits into domestic animals through gene transfer hold considerable promise in the genetic improvement of animals.


Cloning

Embryonic cloning of animals involves the chemical or surgical splitting of developing embryos shortly after fertilization and, consequently, developing two identical individuals.


The separated embryos are allowed to culture, or grow, to a more advanced embryonic stage before they are implanted into the uterus of a recipient mother for full development.


Embryonic cloning has been performed successfully in several species of animals.


Nuclear Transfer

Nuclear transfer is another method of cloning that involves the microsurgical collection of nuclear material from a donor cell which is then transferred into an unfertilized ovum that has had its own nucleus removed.


The cells that develop successfully become identical individuals.

Dolly the Sheep (the first mammal cloned from adult cells) and many other species have been cloned this way.



Worldwide, the institute that has cloned the most species is Texas A&M University, College of Veterinary Medicine, which to date has cloned cattle, swine, a goat, a horse, deer, and a cat.


Nuclear Fusion

Another innovation in genetic engineering, called nuclear fusion, involves the union of nuclei from two gametes, male or female sex cells.


This fusion shows promise for the uniting of nuclei from two outstanding females, two outstanding males, or the normal outstanding male and female combination.


The possibility for selecting desired traits at the cellular level holds exciting implications for the genetic improvement of domestic animals.


ALL RIGHTS RESERVED

Reproduction or redistribution of all, or

part, of this presentation without

written permission is prohibited.

Instructional Materials Service

Texas A&M University

2588 TAMUS

College Station, Texas 77843-2588

http://www-ims.tamu.edu

2006

illustration of dna double helix from wikipedia. animal genetics agriscience 332 animal science...

Documents

c4b slide

photo courtesy of wikipedia

principle of segregation

principle of dominance

animals genotype

genetic composition

genetic makeup of animals

different alleles