Principles of Genetics and Breeding

12.1.4 Inbreeding depression

As inbreeding increases, it often causes a decrease in productivity which is termed “inbreeding depression”.

• Inbreeding depression is a decrease in growth rate, fecundity, etc. that is observed in the inbred group when it is compared to a control population where three is no inbreeding.
• The severity of inbreeding depression depends on the level of inbreeding, the phenotype in question, and the population.

There are several explanations for why inbreeding depression occurs, and what are all contribute to inbreeding depression.

a) Qualitative genetic explanation

i) The most common explanation is that inbreeding depression occurs because of the pairing and expression of detrimental recessive alleles.

• Although the terms “dominant” and “recessive” do not meant that one allele is good and the other undesirable, most of the alleles that produce abnormal phenotypes or that lower viability are recessive.
• Even though mutation rates are usually only one in 10,000 to one in 100,000 replications per gene, because each fish can produce many gametes (up to 500,000,000 sperm per spawn) and because each fish has tens of thousands of genes in its genome, each fish produces dozens to hundreds of gametes that contain a mutant copy of one or more genes.
• Most of these mutant alleles are recessive.
• Since recessive alleles can be expressed only when a fish is homozygous, these mutations tend to accumulate in a population.
• Consequently, many fish carry “hidden” copies of these mutant alleles in the heterozygous state.
• Each fish carries a number of these detrimental recessive alleles, and inbreeding uncovers them.
• When relatives mate, they produce offspring with an increased level of homozygosity; consequently, some of the detrimental recessive alleles that the parents carry in the unexpressed heterozygous state are paired and expressed in the offspring.
• Detrimental recessive alleles can also be paired and expressed when unrelated fish mate. The difference is one of magnitude, and that is what causes inbreeding depression.
• It has been estimated that each individual carries dozens of mutant alleles that lower viability.
• Most of these mutant alleles will produce only a small reduction in fitness, but several will produce phenotypes so abnormal that they are lethal or cause premature death.
• Even though the probability of producing a defective offspring is the same for each recessive allele (25% if both parents are heterozygote), if both parents carry a large number of identical defective recessive allele, the chance of producing defective offspring increase dramatically.
• The more closely related the parents, the more alike they are genetically, so it is more likely that defective offspring will be created.

ii) A second explanation for inbreeding depression is that by decreasing heterozygosity, inbreeding reduces what is called “over dominance”.

• Over dominance occurs when the heterozygous genotype produces a phenotype that is superior to the two homozygous loci.
• The best known example of over dominance occurs in humans. In many populations, the gene which produces haemoglobin exists in two allelic forms: one produces normal haemoglobin, while the other produces sickle-cell haemoglobin.
• Individuals who are homozygous normal are susceptible to malaria and get quite ill and can die when infected. The heterozygote are superior, even though they are slightly anaemic, because they are resistant to malaria.

b)Quantitative genetic explanation

Since inbreeding increases homozygosity, it also decreases heterozygosity. By doing this,

• inbreeding affects the interactions between alleles at each locus, which means that it affects V_D, and dominance effects can play a major role in the production of many quantitative phenotypes.
• When h² is small (<0.15), as it often is for viability and fecundity, dominance effects can be quite significant.
• The creation of excess homozygosity tends to disrupt dominance effects, which tend to be more unidirectional than the additive effects.
• Inbreeding can be considered to be opposite of crossbreeding, which is used to exploit V_D.
• Consequently, if crossbreeding maximizes exploitation of V_D, inbreeding minimizes exploitation, which means that inbreeding makes phenotypes controlled by V_D worse (inbreeding decreases phenotypic mean).

This genetic explanation makes sense, because inbred lines are often used in crossbreeding programmes. The creation of inbred lines is done to produce opposite types of homozygosity that will be maximized when hybrids are created, which is how V_D is exploited. This explanation is further supported by the fact that crossbreeding, the breeding programme that is used to exploit V_D, can also be used to eliminate inbreeding and inbreeding depression.