Genetics - Key Facts

The following information should be read in conjunction with the two page chart and glossary of terms which will help you to understand the complex and complicated subject of Genetics.

dna strands

DNA

DNA is a long fine fibre made up from two strands that stick together with a slight twist to form a helix shape.

DNA is found in cells and is organised into stretches of genes where the base proteins attach to coil the DNA to fit into each cell, giving rise to structures known as chromosomes.  

Along these stretches are instructions to ‘turn a gene on’ and ‘turn a gene off’; and large stretches whose purpose is not even known

Genes

Genes are made from Deoxyribonucleic Acid (DNA). DNA is made up of four nucleotides which are individual chemical structures known as bases. These four nucleotides are, Adenine ‘A’, Thymine ‘T’, Cytosine ‘C’ and Guanine ‘G’, they are joined end to end.  

Genes carry the instructions or ‘plans’, for the making of thousands of proteins that are found and deciphered by the cell. The random combination of these bases determines what the cell will look like and what job that cell will do and how the many different cells of the body will be arranged.

Each cell has a nucleus containing 78 chromosomes, the exception to this being red blood cells (which have no chromosomes) and the reproductive cells, eggs and sperm (which have 39 chromosomes each). For example, how to make haemoglobin; haemoglobin is the protein that carries oxygen around the bloodstream. The body needs to constantly make haemoglobin.

Chromosomes

A chromosome is made up of DNA and the proteins attached to it. Each chromosome has a thread of DNA running along its length and the genes are arranged along this thread. This resembles

chromosomes
beads on a string. Chromosomes are arranged in pairs and in each cell of a dog there are a total of 78 chromosomes; 39 from the sire and 39 from the dam. These are 38 pairs of autosomes and two chromosomes involved in specifying gender, i.e. X or Y chromosomes.

Gender determination in canines is exactly the same as in humans; bitches have two X chromosomes whilst the dog has one X and one Y chromosome.

Each set of 39 chromosomes contains approximately 20,000 genes, representing a sequence of 3 billion bases. These 20,000 different genes are required to specify the dog.

Gene Mutations

The ‘plan’ embedded in the gene can become altered by a process called mutation. This can involve change in the sequence of the bases by adding or removing some of the base sequence within the gene. Considering the times a gene has to copy and reproduce itself it not surprising that mistakes (mutations) can occur.

On the first ‘ladder’ is a ‘normal’ strand of DNA. The other three show various mutations, as indicated by the boxes. On the second strand, a substitution has occurred, changing a base pair. On the third strand, a deletion has occurred, removing a base pair. On the fourth strand, an insertion has occurred so there is an extra base pair in the sequence. These mutations can cause changes in amino acid sequences.

mutations 'ladder'
Consequences of gene mutations:

This depends on the gene in which the mutation has occurred. Some mutations are silent and have no consequences, others affect the gene so that the plan can no longer be used to make a functional protein.

In the Wheaten this could be the effect the mutated gene has on kidney formation, the consequence being Renal Dysplasia (RD). Once a mutation has occurred within a gene, it is fixed and cannot be reversed.

The dog carrying the mutation will pass this mutant gene onto its offspring, if the consequence of the mutation is a disease state, like RD, then this is an inherited disease.

Note: Not all mutations are bad (deleterious), occasionally, some mutations can be beneficial. This is how evolution has progressed to make the individual fitter and enabling them to have the advantage in their environment.

There are two types of mutation that can occur in genes and the different effects are determined by the fact that dogs have two copies of every gene.

If a recessive mutation occurs in a gene the effect is not initially noticed because the second, normal copy of the gene masks the presence of the recessive mutant gene. A disease caused by a recessive mutant will only be seen in a dog that has two copies of the recessive mutant.

If a dominant mutation occurs the consequences will be felt despite the fact that there will also be a normal gene present. An animal that inherits a dominant mutation will be affected.

Inheritance and genetic mutations

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Autosomal Dominant Trait

Both parents do not have to have the gene for the disorder to cause the trait to occur. However, since the trait is expressed in the heterozygous state, one parent must show the trait in order for it to occur among the offspring.

There are few exceptions to this rule.

At the present time it is not known why a dominant gene masks or hides the recessive alleles and it may be that the concept of dominance is operational and may not reflect any intrinsic property of the gene. Nevertheless, the fact that dominant traits are expressed in certain ratios can be easily demonstrated.

The general characteristics of an autosomal or simple dominant trait follow:

  • The gene is located on any one of the thirty-eight pairs of autosomes.
  • The gene is generally present in the heterozygous state.
  • At least one parent of an affected offspring must show the trait, unless a new mutation is involved.
  • The trait occurs in successive generations (no skipping).
  • About 50% of the offspring of an affected dam or sire will also be affected.
  • On the average males and females are equally affected.
  • Dogs that are phenotypically normal are also genotypically normal.

Autosomal Recessive Inheritance

The general characteristics of an autosomal simple recessive trait follow:

  • The gene is located on any one of the 38 pairs of autosomes.
  • To be expressed (to show the trait) the gene must be present in the homozygous state (both genes must be identical).
  • The trait tends to occur in one generation and then skips one or two generations until carrier descendants are again mated allowing the genes to be expressed.
  • Each of the parents of an affected puppy is a proven carrier (heterozygote) of the abnormal gene but generally show no phenotypic manifestation of the trait.
  • If the given trait is rare in a breed (one affected amongst 2,000 or 3,000 normal dogs) there may be increased inbreeding among the parents (increased consanguinity) of affected dogs.
  • Matings between heterozygotes (carriers), on average, produce 25% affected (homozygous recessive), 50% carriers (heterozygous) and 25% that do not have the mutant gene (homozygous dominant or wild type).
  • On the average males and females are affected equally.

Polygenic Trait

The general characteristic of a polygenic trait follow:

  • As with a recessive trait, both the sire and the dam must contribute one or more of the genes that cause the abnormal phenotype in the offspring.
  • Unlike recessive traits, the contribution from the sire and dam need not be equal.
  • Since we do not know the number or the specific effect the genes involved in polygenic traits have in dogs, no predictable Mendelian ratios are associated with these traits.
  • Both sexes are affected with polygenic traits (excluding sex-limited traits) but not necessarily in equal numbers.
  • The trait may skip generations and may appear to be erratic in occurrence.
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