ANSWERS

1. Define genetics.
   Genetics is the science of animal heredity and the study of how traits and characteristics are transmitted from generation to generation.
2. Name the scientist who discovered genes.
   Gregor Mendel (1822-1884). Mendel was an Austrian monk and a biologist.
3. State the First Law of Genetics.
   Genes exist which influence characteristics. They exist in pairs and retain their identity from generation to generation (ie. alleles separate to different gametes).
4. What is the exception to Mendel's First Law of Genetics?
   Mutation.
5. 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.
6. Describe the base pairs that form the structure of DNA molecules.
   Adenine (A) and thymine (T) or guanine (G) and cytosine (C). A always links with T, and G always links with C.
7. What are chromosomes?
   Special structures within the nucleus that control the cell. They contain chromatin fibres that hold the genetic information.
8. What is the centromere?
   The region of the chromosome which becomes allied with the spindle fibres at mitosis and meiosis.
9. Why is it that chromosomes are only visible under the microscope when the cell is dividing?
   During interphase (the non-dividing state) the chromatin fibres are elongated and form a confused network.
10. State the specific number of chromosomes in domestic dogs and cats.
    • Dogs - 78 chromosomes (38 pairs of autosomal chromosomes and 2 sex chromosomes).
    • Cats - 38 chromosomes (18 pairs of autosomal chromosomes and 2 sex chromosomes).
11. Describe the sex chromosomes.
    Sex chromosomes are called the "X" or "Y".
    • Female - XX.
    • Male - XY.
12. Since females possess 2 X chromosomes, it follows that they must therefore receive twice the number of genes carried on the X compared to males. In order to compensate for this, only one X chromosome in each cell of a female is activated and contains functional genes. The other X chromosome becomes contracted and contains predominantly inactivated and non-functional genes. This contracted X chromosome may be visible in the nucleus; what is it called?
    The Barr body (or sex chromatin).
13. What are autosomes?
    Any chromosome other than the sex chromosomes. They are composed of pairs of chromosomes which are alike.
14. What are homologous chromosomes?
    Those that pair during meiosis and contain an identical arrangement of genes in the DNA pattern.
15. Where are genes located?
    Each pair of genes has its own allocated place on a particular chromosome which is known as the gene locus.
16. What are alleles or allelomorphs?
    All genes that occupy the same gene locus.
17. What are dominant genes?
    Genes that can suppress the effect of the allelomorphic partner. Dominant genes are denoted by a capital letter.
18. What are recessive genes?
    Genes whose effects are masked by that of their allelomorphic partner. Recessive genes are denoted by a small letter.
19. Give an example of co-dominant genes.
    Genes coding for blood groups (both alleles are expressed).
20. What is meant by a phenotype?
    The outward physical appearance of an animal (eg. colour or coat type).
21. What is meant by a genotype?
    The genetic constitution of an animal as received from its parents. The genotype may be different from the phenotype due to the presence of recessive genes.
22. What is the difference between homozygous and heterozygous?
    Homozygous describes an animal's genes when both allelomorphic partners are the same (ie. identical alleles in regard to a given characteristic). Heterozygous describes an animal's genes when the allelomorphic partners are dissimilar.
23. What is meant by "incomplete dominance"?
    Incomplete dominance describes alleles which are neither completely dominant nor recessive, and the result is a mixture of each.
24. What are sex-linked genes?
    Genes 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.
25. What are sex-limited genes?
    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.
26. What is meant by "lethal factors"?
    Genes or gene combinations not compatible with life. Lethal mutations are an exception to Mendel's First Law of Genetics. The gene fails to duplicate exactly and is thus unable to function resulting in the death of the individual.
27. What is meant by "gene mutation"?
    Gene mutation 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.
28. What is meant by "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.