18 Questions About Genetics That Responsible Breeders Should Know The Answers To

Genetics is the science of animal heredity and the study of how traits and characteristics are transmitted from generation to generation. It is a complex subject, but a good understanding by breeders is important as the correct knowledge may help prevent the breeding of unsuitable animals and unhealthy offspring.

 

1.  Why is it advantageous for breeders to be able to identify an animal carrying a recessive gene?

Recessive genes (genes whose effects are masked by that of their allelomorphic partner). Recessive genes are often unwanted, and therefore identification of animals heterozygous for a recessive gene means that breeding from such animals can be avoided. Identification of a recessive carrier can be established by means of test mating to a homozygous recessive animal or a known heterozygous recessive carrier. Homozygous describes an animal's genes when both allelomorphic partners are the same (i.e. identical alleles in regard to a given characteristic), whilst heterozygous describes an animal's genes when the allelomorphic partners are dissimilar.

2.  What is DNA?

Deoxyribonucleic acid. DNA governs the activities of the cytoplasm and is the material controlling heredity from parents to offspring. DNA is a double stranded molecule; each strand consisting of 2 long complementary chains held together by base pairs. This type of structure is known as a double helix. Genes are made of DNA.

3. What are chromosomes, and how many do domestic dogs possess?

Chromosomes are special structures within the nucleus that control the cell. They contain chromatin fibres that hold the genetic information. Domestic dogs possess 78 chromosomes (38 pairs of autosomal chromosomes and 2 sex chromosomes).

4.  What is meant by the term "backcross to the recessive"?

A mating to a homozygous recessive animal or a known heterozygous animal. A backcross to the recessive mating is used to reveal the presence of a recessive gene in an individual whose phenotype is the same as an animal homozygous for a dominant gene.

5.  What is meant by the "first filial generation"?

This term is used when describing a test mating, and refers to the offspring produced by the parent generation. The first filial generation may be abbreviated to the F1 generation. The F2 generation are the offspring produced by the F1 generation.

6.  Who was Mendel, and what are Mendel's First and Second Laws of Genetics

Gregor Mendel (1822-1884) was an Austrian monk and biologist who discovered genes. His first law states that genes exist which influence characteristics. They exist in pairs and retain their identity from generation to generation (i.e. alleles separate to different gametes). Mendel’s second law state that each pair of alleles separates independently of every other pair of alleles.

7.  What may cause exceptions to Mendel's First Law?

Gene mutation; which may occur when the sequence of base pairs is repaired in a different sequence following damage to a chromosome at the site where the gene is located. Gene mutation is not always disadvantageous, since a slight mistake in gene structure may provide another means by which offspring can differ from the parent generation. Lethal mutations however are a different matter, and here the gene fails to duplicate exactly and is thus unable to function resulting in the death of the individual.

8.  What may cause exceptions to Mendel’s Second Law?

Linkage; which describes 2 pairs of genes occurring on the same chromosome, which do not assort independently.

9.  What is meant by "multifactoral inheritance"? Give an example

Multifactoral inheritance describes a characteristic which is influenced by a number of genes, rather than 1 gene or its alleles. An example is hip dysplasia – where a number of genes interact with themselves and the environment.

10.  What is the difference between a genotype and a phenotype?

A genotype is defined as the genetic constitution of an animal as received from its parents. A phenotype is the outward physical appearance of an animal (e.g. colour or coat type).The genotype may be different from the phenotype due to the presence of recessive genes.

11.  What is the difference between a congenital and an inherited defect?

Congenital defects are present at birth and may be inherited or due to environmental factors. Inherited defects may not be evident until some time into the life of the animal. An example of a congenital defect is cleft palate.

12.  What is meant by "inbreeding"?

Selective breeding between close family members in order to increase homozygosity and thus fix desired characteristics.

13.  What is meant by "line breeding"?

A mating within a family maintaining a relationship with a particular ancestor in order to preserve desired characteristics.

14.  What is the name of the term given to the breeding of animals less closely related than those picked at random, and why might this be beneficial?

Outbreeding which increases heterozygosity and introduces new genes into a population. Outbreeding masks the effects of recessive genes and results in better offspring since undesirable recessive genes are not expressed.

15.  What is the main disadvantage of outbreeding?

The offspring of an outcross will not breed true since they are heterozygous and not homozygous for their alleles.

16.  What is a hybrid?

The result of a cross conveying superiority on the heterozygote (i.e. increases vigour when the parents come from different strains or species).

17.  What is epistasis?

Epistasis describes the suppressive effect of some genes expressed upon other genes which are not their alleles. This is seen in albino animals which are homozygous for a recessive gene c which prevents coat colour from being expressed. A cc animal will be albino irrespective of any other coat colour genes it may possess.

18.  What is the difference between sex linked and sex limited genes?

Sex linked genes are located on the sex chromosomes. There are many more genes on the X chromosome than on the Y chromosome and thus sex-linked genes are more likely to be located on the X. One important disease controlled by sex-linked genes is haemophilia. Sex limited genes are genes that can only be expressed in one sex, such as those associated with the volume of milk production. The gene is present in both the male and the female, but is only expressed in the female.