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Given one specific genetic defect in the grandparent (common ancestor), the inbred child is far more at risk of expressing it than an outbred child.
Given one specific genetic defect in the grandparent (common ancestor), the inbred child is far more at risk of expressing it than an outbred child. Remember, however, that the genome has many (usually) unexpressed defective genes. Inbreeding increases the risk of expressing all the genetic defects that are scattered throughout the genome. The actual increase in expressed genetic defects in an offspring produced by incest has to be calculated by adding together all the individual effects taking place at the fraction of these loci that have been rendered homozygous by inbreeding. So, if there are, in the inbred offspring, 20 additional loci with defective alleles that have been made homozygous by incest, then the estimated injury to the offspring is calculated by summing all of their effects together. Because a haploid genome never meets a perfect copy of itself, even under close inbreeding only some of the genetic defects will be expressed.
8. So, if there are 64 defective alleles at 64 different loci in the grandparent, how many do you expect to be expressed in a grandchild produced by brother-sister incest?
9. If there are 64 defective alleles at 64 different loci in the grandparent, how many of these do you expect to be expressed in a grandchild produced by outbreeding (parents degree of relatedness = 0)? We are assuming the background rate for each of these genes in the population is 1/1000.
Recall that r is the notation used for “degree of relatedness.” The degree of relatedness is the probability that a randomly sampled gene found in one individual will be found in another because they were copied and transmitted during reproduction from the same common ancestor—i.e., that they are “identical by descent.”
10. If two parents are related by .5 (father-daughter, full-brother-sister, mother-son), what is the chance that, in forming the offspring, a randomly sampled gene existing in one parent will be matched with its copy?
The general rule is: r/2 = the probability that a randomly sampled gene existing in one parent will be matched with its copy. This is the same as the proportion of the offspring’s genome that has been rendered homozygous by being inbred.
11. What is the proportion of the child’s genome would be rendered homozygous if her parents were full brother and sister?_____ What is the proportion of the child’s genome that would be rendered homozygous if her two parents were mother and son?_____ What is the proportion of the child’s genome that would be rendered homozygous if her two parents were father and daughter?_____ What is the proportion of the child’s genome that would be rendered homozygous if her two parents were half-brother and sister?____ What is the proportion of the child’s genome that would be rendered homozygous if her two parents were uncle and niece?____ What is the proportion of the child’s genome that would be rendered homozygous if her two parents were full cousins?____ What is the proportion of the child’s genome that would be rendered homozygous if her two parents were second cousins (r = 1/32 for second cousins)?_____
12. So, how many times worse is it (in negatively affecting the offspring produced) for a person to mate with a parent than with a second cousin?
The unit for measuring the potential negative effects of the defective recessive genes in the genome is the “lethal equivalent.” A single gene made homozygous can lead to the death the individual before she could reproduce. The allele responsible for terminating a life before reproducing is called a lethal. Many genetic defects are less serious than that. So, if two different defects at two different loci would together be lethal, then they add up to one lethal equivalent. A single lethal equivalent may be one gene, 10 that reduce fitness by 10%, 20 that reduce fitness by 5%, and so. An organism with a fitness of zero contributes no genes to the next generation—death before reproducing and a lifetime of nonreproduction are the same thing from a Darwinian point of view. The number of lethal equivalents in humans is not known exactly, but is estimated to be between 2 and 4. The minimum cost of incest in the nuclear family is believed to be in excess of 1/3 (that is, a child of incest is likely to produce less than two thirds of the offspring that an outbred child would produce). This is considered to be a large underestimate, since it is based on deaths only, and deaths after birth but before reproductive maturity. It would not count lack of success in reproducing, or deaths later in life, such as in adolescence or early adulthood.
13. Now, imagine a gene that caused a mating psychology that differentially found close relatives more sexually attractive than non-relatives. What kinds of bodies would it be more likely to find itself in?
14. Imagine a gene that caused a mating psychology that was averse to mating with close genetic relatives. Would it find itself in healthy bodies more often than the gene for incestuous desires? Would it increase or decrease in frequency, compared to the design that favors
15. Discussion Question: What do you think of Freud’s claim that family remembers, including parents and their children, and brothers and their sisters, find each other the primary objects of sexual desire?
