Family and Parenting 3-Book Bundle. Michael Reist

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СКАЧАТЬ called X and Y, and they determine a person’s gender. Women get two copies of the X chromosome, while men get one X and one Y. During meiosis (the creation of gametes, or sperm and eggs), one chromosome of each type is selected at random. The same goes for sperm. During reproduction or fertilization (acts instigated by dopamine, the great motivator!), the two payloads combine, creating a new organism with a full set of chromosomes, a quarter of which (give or take a chromosome or two) came from each of its four grandparents.

      Think of your genes as a deck of playing cards. Each deck contains 46 cards divided into two different suits. One suit came from your mother — call it clubs — and the other from your father — let’s say spades — meaning you got 23 cards from each of them. When creating reproductive cells, your body sorts through your genetic deck, selecting one card from each of the 23 pairs at random. The result is a half-deck containing one copy of cards 1 to 23, some of them clubs and some of them spades.

      Now say you meet a partner of the opposite sex, hit it off, and reproduce.[13] His (or her) half deck combines with yours, contributing its own set of cards 1 to 23, compiled at random from his own maternal and paternal suits (call them diamonds and hearts). The resulting child has a full deck of 46 cards, 23 from you and 23 from your partner. Technically, the 23 cards your child inherited from you are themselves of two different suits — clubs and spades from your mother and father, respectively — but they have become, for all practical purposes, one suit. Genetic inheritance is thus the piecemeal combination of traits from various ancestors. With each subsequent generation, the level of genetic relatedness between elder and younger is roughly halved. After as few as six generations, there is a decent chance that a person doesn’t have a single chromosome in common with their ancestor, despite the order of their nucleotides remaining, as with all humans, 99.9 percent identical.

      Though we have two copies of each autosomal chromosome, the pairs are by no means identical. If they were, the entire process of chromosomal shuffling would be useless. Every functioning chromosome is largely similar to others of its type, in that it codes for the same genes, is essentially the same length, and for the most part contains the same nucleotide sequences. However, certain genes have mutations that vary from person to person, causing them to function in a different manner. For example, almost everyone has the gene necessary to determine eye colour, but not everyone’s eyes are the same shade. Some have genes that code for brown eyes, while others have genes that code for green, or hazel, or blue. Genes with multiple derivatives are called polymorphisms, and they are responsible for the astounding variety of traits between humans. Since human beings have two copies of each chromosome, they have two copies of each polymorphic gene. The precise type (or types) they possess is called an allele.

      Sometimes humans have two copies of the same allele, in which case they are homozygous. In other cases, they have two different alleles of the same gene, making them heterozygous. In these cases, the two alleles can interact in a number of ways, the most famous of which was discovered by an Austrian monk named Gregor Mendel. Mendelian genetics divides alleles into dominant and recessive types. When an individual possesses both a dominant and a recessive allele of the same gene, the result is not a compromise between the two. An individual with one brown eye allele and one blue eye allele will not develop murky blue irises, or one brown eye and one blue eye.[14] Rather, the dominant trait supersedes the recessive trait, which lies unexpressed in the gene, awaiting the opportunity to perhaps show itself in a subsequent generation. In the case of eye colour, blue eyes are recessive and brown eyes are dominant.[15] A man with one brown-eye allele (symbolized by a capital B) and one blue-eye allele (symbolized by a lower case b)[16] will have brown eyes. Say that man meets a brown-eyed woman who also has a recessive blue-eye allele, and together they have a child. The child’s eye colour genes could look one of four ways. She could be homozygous for the brown-eye allele (BB), heterozygous for the brown and blue-eye alleles (Bb or bB, depending on which allele comes from which parent), or homozygous for the blue-eye allele (bb). In the first three cases, the child, like her parents, will have brown eyes, as the presence of the dominant allele (B) overpowers that of the recessive allele (b). In the latter case, though, the dominant allele is not present, and so the child will have blue eyes.

      Very few traits (including the one in our example) truly fit the Mendelian mould of single-gene origin and dominant/recessive binary.[17] Most traits require multiple genes to develop, and some single gene traits vary in their degrees of expressivity, or the extent to which the “dominant” trait dominates. Still others are co-dominant, meaning that neither allele overwhelms the other. Nevertheless, Mendel’s insights were remarkable, considering all he had to work with were a few pea plants and his own powers of observation. With these humble tools, Mendel documented the first evidence of genetic inheritance, paving the way for what would become arguably the biggest scientific undertaking of the 20th century: mapping the human genome.

      DRD4

      As we’ve already learned, DRD4 (the gene) codes for DRD4 (the receptor), and DRD4 allows the human brain to dole out jolts of positive reinforcement in the form of dopamine. Its connections to drug addiction and depression seem obvious — drugs being a pharmacological shortcut to euphoria, and depression being a chemical imbalance precluding one’s ability to experience pleasure — but what links DRD4 to ADHD, heart disease, or any of the other conditions to which it is accused of contributing? Moreover, why DRD4 and not DRD3 or DRD5? The answer lies in the allele.

      Within the third exon (or section of codeable, non-“junk” DNA) of DRD4 sits a nucleotide sequence 48 base pairs long. This sequence repeats from 2 to 10 times, depending on the allele, contributing to DRD4’s reputation for being one of the most variable genes in the human genome. The 48 base-pair repeat is not the only repeated sequence in DRD4, nor is it the longest, but it is nevertheless the focus of a great deal of scientific scrutiny.

      The most common number of repeats found in DRD4 are 3 and 4, but for susceptibility to depression, addiction, and a host of other maladies, 7 seems to be the magic number. For reasons that continue to elude us, the 7-repeat allele increases a person’s predisposition toward risk-seeking behaviour, which includes typical “high-risk” activities, such as drug use, illicit sex, and gambling, but also extends to extreme sports and high-pressure business decisions. This creates an odd schism in public opinion on the 7-repeat allele. While considered an albatross around the necks of junkies and problem gamblers, it can be seen as a positive attribute when possessed by athletes and successful business people, both of whom thrive in high-risk environments.

      5-HTTLPR

      We’ve already thanked dopamine for our ability to experience pleasure; it’s only fair we now give serotonin its due.

      Though a neurotransmitter much like dopamine, serotonin is principally found in the gut, where it regulates intestinal movements. In the brain, it serves a very different function, facilitating feelings of happiness and well-being. The link between digestion and contentment may seem tenuous, but from an evolutionary standpoint, it’s actually quite logical. If one considers pleasure outside its cultural trappings, it’s ultimately an incentive for continued survival. Pleasure has become far more decadent in modern society, where basic necessities are freely available. But at its humble roots, pleasure is derived from activities necessary for the propagation of our species: sex, warmth, sleep, and, most importantly, food. As a result, we are genetically inclined to feel a sense of contentment when these needs are met, and a drive to meet them when they’re not. Serotonin helps us achieve this end.

      Studies have linked serotonin levels to food availability, which, in social animals, also relates to one’s place in the social hierarchy. When injected with excess serotonin, animals with diminutive statuses in the hierarchy display uncharacteristically aggressive behaviour. In normal circumstances, a crayfish, when faced with a bigger opponent, will perform a supplicating tail-flip gesture that forces it backward, allowing it to flee. However, when injected with serotonin, it becomes more aggressive and attacks its opponent. Curiously, the opposite is true of dominant crayfish. When they receive a boost of serotonin, their behaviour becomes more СКАЧАТЬ