7.1. Heritability
Semyonova states,
“Probably most people recognize that every dog breed results from human manipulation of inherited physical traits.
Until recently, most people probably also recognized that much dog behavior is also a result of manipulating inheritance: if you want to do sheep trials, you get a border collie. If you get a beagle, he will likely become instantly deaf to your calls if he picks up a scent to track.
But after discussion started about perhaps banning breeds who often attack and kill, defenders of these breeds began to dispute the heritability of any kind of dog behavior.”
and
“Heritability of behavior
Another researcher, Linda van den Berg, investigated specifically the heritability of impulsive aggression among golden retriever, a breed rarely involved in fatal and disfiguring attacks. The goal was find out whether impulsive aggressive behavior was inherited in those few golden retrievers who exhibit it, and if so, to isolate the gene responsible for the behavior. van den Berg found high heritability of impulsive aggression, but did not succeed in isolating the responsible gene(s).
The heritability of abnormal aggression in certain breeds of dogs can no longer be denied. The bodies of these dogs have been selected to execute a killing bite more efficiently than other breeds. These dogs share physical conformation to the task of killing, including exaggerated jaw muscles, heavy necks and shoulders, and body mass that makes defense against an attack much more difficult. Among people who want dogs who can kill, these are the breeds of choice because they are physically more fit for it than other breeds.”
On Page 31 of van den Berg’s paper, it is stated,
“Heritability estimations
Heritability (h2) describes the contribution of additive genetic variance to phenotypic variance (Bourdon 1997b; Nicholas 2003). Several reviews discuss heritability estimations of behavioural traits in dogs (Houpt and Willis 2001; Mackenzie et al. 1986). Some early studies of aggression-related traits in police or military dogs failed to produce h2 [heritability] estimates higher than zero (Barlett 1976; Pfleiderer-Hogner 1979; Reuterwall and Ryman 1973; Willis 1976).”
On page 31 of the van den Berg study, it is stated,
“Three recent studies obtained h2 [heritability] estimates between 0.06 and 0.33 for aggression scores in behavioural tests (see Table 2). It thus seems that the heritability of aggression is low but significant in the general dog population.”
On page 10, of the van den Berg study, it is stated,
“Based on the results, we made a selection of dogs for the molecular genetic studies presented in chapter 3. We used a candidate gene approach in our search for molecular genetic variants that contribute to aggression in Golden Retrievers. Four genes of the canine serotonergic system were cloned, characterised, and studied in Golden Retrievers (sections 3.1, 3.2, and 3.3). We studied the serotonin receptor genes 1A, 1B, and 2A (htr1A, htr1B, htr2A) and the serotonin transporter gene (slc6A4). In section 3.4, we describe the analysis of these genes in related Golden Retrievers. We conclude that the candidate genes are not likely to play a major role in the variation in aggression in the Golden Retrievers.).”
Heritability is an estimate of the influence of nature (genes) and nurture (environment or how the animals or plants are raised or grown) on a group.
Heritability (h2) of zero means that genes have no influence on a particular trait. The trait is wholly the result of environment.
On Page 33 of van den Berg’s paper, it is stated,
“There have been no studies linking specific genes to aggression in dogs.”
How can you have “high heritability” if you can not accurately determine the genetic contribution?
What are the preceding quoted Semyonova passages supposed to mean?
What are the citations for the scientific evidence or scientific studies that form a basis for the preceding Semyonova passages?
Every living thing has heritability, i.e., characteristics that result from genetics or environment. The heritability may be as low “0.0” meaning that genetics makes no contribution or as high as “1.0” meaning that genetics accounts for all of the characteristic or it can be anything in between.
In addition, van den Berg’s study was done on Golden Retrievers that were selected because they had already exhibited aggressive behavior. The dogs were not randomly selected from the entire population of Golden Retrievers. Consequently, it is not valid to claim that whatever behavior was found in the Golden Retrievers would be found in ALL Golden Retrievers, and it would definitely be incorrect to attribute any findings from the van den Berg study to be applicable to any other breed of dogs.
Semyonova states,
“the assertion that this aggression is not heritable is no longer tenable.”
What is this suppose to mean?
Where are the citations for studies and/or objective data that support this statement?
According to Gerhard Adam of Science 2.0. at http://www.science20.com/gerhard_adam/what_heritability-93424
“”Heritability” is a term used in many articles and through much of the scientific literature and invariably promotes the idea that it relates specifically to inherited traits. As a result, it is often assumed that the heritability of a particular trait relates to how much influence genetics has on the trait manifesting in an individual.
However, that isn’t what it means.
Heritability attempts to address the relationship between nature (genetics) and nurture (environment), so that as each changes, the variation between individuals within a population can be estimated based on these influences. In this context, “environment” simply represents everything external to the genome that could effect expression.
Therefore the first significant aspect of heritability that must be understood is that it tells us nothing about individuals. It is strictly an estimate of the variations that occur within populations. If heritability is applied to an individual it is a meaningless concept [since an individual cannot be said to vary with anything].
It also doesn’t tell us anything about the specific influence of genes on any particular trait, since that would be the result of inheritance. We also need to understand that a trait is something that is “selectable”. In other words, there exists a possibility that outcomes can vary in the expression of a particular trait.
Assume that we can create an environment that is identical in every aspect for a particular population of organisms. They develop and grow and as they reach adulthood we observe differences in the traits that they manifest. Since the environment exerts an identical influence on the organisms, then we can rule out the environment as being a factor [in other words, it will affect them all equally]. From this we can conclude that whatever differences exist between organisms [variance] must be the result of genes alone.
In this case, the heritability would be 1.0 or 100%, indicating that only the genes are responsible for the variation we see between individuals. Similarly we could conduct another thought experiment where we take individuals that are genetically identical [clones] and subject them to various environments. When we examine the traits, whatever variation exists cannot be due to the genes [because they are identical] therefore, the variation is solely affected by the environment. In this case the heritability would be 0.0 or 0% (2).
What this particular designation provides is a means by which we can begin to estimate or assess the influences that may be present in particular traits. So while genes are responsible for the expression of a trait, heritability is used to determine how specific such an expression is to the genes alone.
As a result, heritability is often used in artificial selection to establish which traits are the most likely to be successfully selected for. The higher the heritability of a trait, the more influence one has in obtaining that trait by selecting the best breeding pair.
One difficulty that arises with heritability is that any considered trait must be demonstrably linked to genetic transmission. This can become problematic when heritability is used to evaluate behavioral traits where the genetic link may be tenuous. In an effort to measure heritability, there is often a reliance on twin studies under the assumption that variances between them must be accountable to environment since they are effectively genetically identical. However, as previously mentioned, this can result in difficult interpretations when the traits in question are purely behavioral. Until such time as behavioral traits can be explicitly linked to genes, any statement regarding heritability must be considered suspect.
[According to dictionary.com, “tenuous” means,
1. thin or slender in form, as a thread.
2. lacking a sound basis, as reasoning; unsubstantiated; weak: a tenuous argument.
3. thin in consistency; rare or rarefied.
4. of slight importance or significance; unsubstantial:
He holds a rather tenuous position in history.
5. lacking in clarity; vague:
He gave a rather tenuous account of his past life.]
(1) Heritability is ultimately a proportion which is expressed as a value between 0 and 1. As a result, we can find heritability numbers that indicate 0.30 or 0.60 or some such proportion to indicate the influence of the genes in the variability of a particular trait. In short, heritability can be defined as the ratio of variance due to genes to total variance in a population.
It is given that genes are responsible for the trait itself. Heritability attempts to establish the influence of nature (genes) versus nurture (environment) on the variability in a population. Therefore the variation of a trait that has a heritability of .30 can be said to be influenced by genetics at about 30% and the environment at 70%. In addition, such traits must actually be selectable.”
Note that when you are talking about a dog’s “environment” you are talking about the way a dog is raised and socialized.
Consequently, if Semyonova is claiming that heritability establishes anything in dogs or anything else, then she is wrong because as Gerhard Adam of Science 2.0. at doc57heritability-a-primer states,
“One difficulty that arises with heritability is that any considered trait must be demonstrably linked to genetic transmission. This can become problematic when heritability is used to evaluate behavioral traits where the genetic link may be tenuous.”
and
“Until such time as behavioral traits can be explicitly linked to genes, any statement regarding heritability must be considered suspect.”
On Page 33 of van den Berg’s paper, it is stated,
“There have been no studies linking specific genes to aggression in dogs.”
Currently, as a practical matter, heritability is virtually meaningless as it is an “estimate” of the effects of genetics and environment on the behaviour of living things. There is no way to definitively or precisely determine the heritability of any living organism, i.e., the genetic cause of a given trait. In theory, it is someone’s “best guess”. Since there is no way to accurately measure the effects of genes and behavior, one person’s “guess” of heritability is as “valid” as the next. In reality, neither is based on any concrete or reproducible measurements or evidence. To suggest otherwise cannot be supported by facts.
In addition, heritability estimates are for a group and NOT an individual. Theoretically, it is not unlike a “stereotype”, i.e., ascribing traits to a group based on a few within the group: however, heritability is based on more scientific studies or facts. It is done but that doesn’t mean it is correct, accurate or right. It is not that different than the weather reports, i.e., the best guess of whether it is going to rain today somewhere in the area, but it says nothing about what is going to happen or whether it is going to rain where you are at.
