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 Genetics Gregor 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.
Courtesy of Wikipedia 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
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.
Image courtesy of Wikepedia. 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.
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. Photo from IMS. USDA photo from Wikipedia.
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 Segregation When 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