What about genes, the environment and development? - Chapter 3

What does evolution have to do with heredity?

As people we have similarities in the way we develop (we all have two eyes, we all get wrinkles when we get old etc.) These are a product of species heredity which is the genetic endowment that all members of a certain species have (thus one reason why certain development patterns are universal). 

Darwin's theory of evolution was meant to explain how characteristics of species change and how new species are formed. It has 3 main arguments:

1. There is genetic variation in a species: some members have different genes than others. 
2. Some genes aid good adaption more than others: genes that contribute to strength make adaptation better than genes that contribute to weakness. 
3. Genes that do aid, will be passed on to the offsping more often: thus the principle of natural selection that makes sure the "best" specie members survive. 

Evolution is about the interaction between genes and environment. The environment influences which genetic aspects are useful for the species. Cultural evolution is the process of inheriting a human environment and the ways of adapting to it, inventing better ways and then passing those on again (e.g. smoke signals turning into text messages). 

Evolutionary psychology is when the theory of evolution is used to understand why humans think/behave as they do. 

How does individual heredity work?

A zygote is a fertilized egg and it is given 23 chromosomes by each the sperm and the ovum, so 46 in total, organized in 23 pairs. Chromosomes are bodies in the nucleus of each cell and contain genes, a.k.a. the units of heredity. 

Sperm and ova have only 23 chromosomes since they are produced through meiosis, a process of cell division: a reproductive cell containing the usual 46 chromosomes splits to form two 46-chromosome cells and then these two split again into four cells, where each cell gets only 23 chromosomes, and results in either one egg or four sperm which have just one member of each of the parent's 23 pairs of chromosomes. Ova are formed prenatally and later ripen one by one during menstruational cycles and sperm production begins in puberty and continues. 

After a sperm penetrates an ovum, the sperm cell disintegrates and releases its genetic material. The nucleus of the ovum releases genetic material too and then a single-celled zygote is created from this combination. This is conception. The zygote becomes multiple-celled through mitosis: a process of cell division in which a cell divides to produce two identical cells, both containing the same 46 chromosomes. Then they become four, then eight, and so on. All human cells contain copies of those 46 chromosomes: except the sperm and ova. Mitosis continues over a life span, creating new cells that support growth or replace damaged cells. 

Both members of one chromosome pair (1 is from the mother, 1 from the father) influence the same characteristics. A chromosome consists of deoxyribonucleic acid (DNA). DNA is made up of different sequences of 4 chemicals: adenine, cytosine, guanine and thymine. Some sequences form genes and there are 20000-25000 genes. Each one (there can be 2 or more different versions) provides instructions for the production of certain proteins (which build all tissues and substances like hormones, neurotransmitters and enzymes).

In the important Human Genome Project the sequence of the chemical letters that make up the DNA were mapped, in a complete set of human chromosomes. It was found only 3 % of the human genome consists of genes (DNA that transcribed into ribonucleic acid (RNA) and then helps produce certain proteins). The other DNA helps regulate the activity of genes and thus, along with environment, regulates when genes turn on and off in cells. 

A quirk of meiosis is crossing over, which captures how when chromosome pairs line up before separation, they can cross and partly exchange. This quirk combined with the fact that a single parent can produce 2^23 genetically different ova/sperm, makes sure it's almost impossible that there would be a human genetically like you. The exception is identical twins (or monozygotic twins) because then one fertilized ovum divides to form two or more genetically identical individuals. Fraternal (or dizygotic) twins come about when two ova are released at approx. the same time and they are both fertilized by sperms. Fraternal twins are as genetically alike as normal siblings. 

Of the 23 chromosome pairs, 22 are similar for males and females (autosomal chromosomes). The 23d pair contains the sex chromosomes (male XY, females XX). So, the father with his different Y chromosome determines the gender of the child. 

In sum: an individual has a genome in each of its cells, with 20000-25000 protein-coding genes & lots of regulatory DNA, on 46 chromosomes in 23 pairs. 

Genotype (the genetic makeup of a person) does not equal phenotype (characteristics of a person). For instance: a child with a genotype that supports exceptional height may not really grow tall when he's malnourished. So environment plays a role as well. And genes influence and are influenced by their biochemical environment: a cell can become part of an eyeball or a kneecap, depending on what cells surround it and what they do. All of a person's cells have the same genes on the same chromosomes, so it's not about the genes that are there; it's about which of those genes are expressed. Gene expression stands for the activation of certain genes in certain cells at certain times, and only "turned on" genes are influential. And this activation can be caused by genetics (regulatory DNA) and by the environment. 

There are three major mechanisms of inheritance:

• single gene-pair inheritance. Thus, a characteristic is influenced by just one pair of genes (one from the mother, one from the father). There is a dominant gene and a recessive one and the dominant is expressed. However the dominant can also dominate incompletely so the two traits "blend" (red and white flowers can give pink), or codomination can exist (red and white flowers can give flowers with red and white streaks). 
• sex-linked inheritance. Then a trait is influenced by genes on the sex chromosomes, instead of the autosomal chromosomes. This could also be named X-linked since the Y chromosome is shorter and contains fewer genes. For instance: males experience color blindness more often than females since colour blindness is a recessive gene on the X chromosome, so girls have a big chance that the other X chromosome has a dominant one to take charge but boys do not get another "colour blindness" chromosome. 
• polygenic inheritance. Thus, a characteristic is influenced by multiple pairs of genes that interact with multiple environmental factors. Examples are weight, intelligence and personality. When a trait is influenced by many genes, many degrees of the trait are possible; it depends on how many of the associated genes an individual inherits. 

Sometimes a new gene appears out of the blue and neither parent has it. This is a mutation: a change in structure/arrangement of a gene. The chances of mutation are increased by environmental hazards like radiation or toxic waste, but most mutations are just spontaneous faults in cell division. New research shows that fathers contribute to new mutations more than mothers and that as they grow older the odds of mutations increase since more errors are made during sperm production. 

Another discovery by the Human Genome Project is the importance of copy number variations (CNV's): another type of inherited or spontaneous errors in which part of the genome is deleted or duplicated. CNV's are more extensive than mutation: they can influence multiple genes and they can cause a person to have only one gene (deletion) or three or four genes (duplication). CNV's can increase risks of some polygenic disorders like autism and ADHD. 

Also, chromosome abnormalities can occur (too many or too few chromosomes) through errors in meiosis. This is the main cause of pregnancy loss. Down syndrome (also called trisomy 21, because it goes with three instead of two 21st chromosomes) is one form. Chance is partly responsible for this, but chances of having a child with Down increase as the parent's ages increase and/or the parent is damaged by environmental hazards as well. Examples of sex chromosome abnormalities are: - Turner syndrome: a female is born with just one X chromosome. Results in small, underdeveloped girls with usually lower-than-average spational and mathematical skills. - Klinefelter syndrome: a male gets one or more extra X chromosomes. Usually comes with long limbs/faces, more feminine characteristics and sometimes language learning disabilities. - XXY syndrome: male gets an extra Y chromosome. Results in tall, strong males with often learning disabilities. 

Examples of genetic diseases:

• Sickle-cell disease. This is a blood disease in which red blood cells take on a sickle instead of round shape, become entangled and then distribute less oxygen causing breathing problems and pain. It's common among African people since it probably started as a mutation, which protected from malaria and so was passed on. Affected people face loads of treatment. Genetic mechanism: recessive gene pair.
• Huntington's disease. This disease typically strikes in middle age and disrupts the expression of genes in the nervous system, creating issues like motor problems, personality changes and loss of cognitive skills. An awful, incurable disease caused by a dominant gene, CNV also involved. 
• Phenylketonuria (PKU). A metabolic disorder resulting in brain damage and intellectual disability, since a critical enzyme needed to metabolize phenylalanine (component of many foods) is lacked and therefore it can turn into a harmful acid attacking the nervous system. Now infants are screened for it and affected children immediately get a diet. Genetic mechanism: recessive gene pair. 
• Cystic fibrosis. A disease common among Caucasions in which a glandular problem results in mucus buildup in lungs, complicating breathing and shortening life. Genetic mechanism: recessive gene pair.
• Fragile X syndrome. A disease with a literally fragile looking X chromosome, causes intellectual disability. Genetic mechanism: dominant gene on X chromosome. A CNV in an important gene for brain development is involved. 
• Hemophilia. A disease more common in males featuring a deficiency in the blood's ability to clot. Genetic mechanism: gene on X chromosome. 
• Tay-Sachs disease. A metabolic defect results in accumulation of fat in the brain, degeneration of the nervous system and often death in childhood. Genetic mechanism: a recessive gene pair. 

What are techniques of prenatally detecting abnormalities?

• Ultrasound: easiest and most common technique, using sound waves to scan the womb and create a visual image of the fetus. 
• Amniocentesis: for detecting chromosome abnormalities and checking the presence of certain genes. Procedure: sample of amniotic fluid is taken from the abdomen and analyzed. Relatively safe from the 15th week of pregnancy.
• Chorionic villus sampling: features conserting a cathether through the vagina and cervix and then taking tiny hair cells from the chorion (the membrane surrounding the fetus) to analyze. Rest is equal to the amniocentesis, however slightly less safe and possible from the 10th week. 
• Maternal blood sampling: testing the mother's blood for chemicals that can indicate fetus abnormalities, sometimes obtaining embryonic DNA that went loose. Noninvasive, safe and usable early in pregnancy, but is usually followed up with something else to create certainty. 
• Preimplantation genetic diagnosis: costly procedure for parents who know they have high risk. Using IVF only fertilized eggs without the tested issues are placed. 

However many disorders are polygenic, so it's sometimes hard to be certain. And what if the technique shows a "damaged" baby..is it ethical to abort all those babies?

What are genetic and environmental influences?

Behavioral genetics comprises the study of how genetic and environmental differences create physical and psychological trait differences. It now is possible to estimate the heritability of traits (thus the proportion of variability in the trait in different people that can be linked to genetic differences). Experimental breeding research in animals has shown genes contribute to e.g. learning ability, aggressiveness and sex drive. For humans the research is mostly about determining whether the genetic similarity is associated with the degree of physical/psychological similarity, for instance in twin studies (identical vs fraternal twins, raised apart vs raised together). There are some limitations to twin studies: the prenatal environment could be involved and identical twins being treated more similarly than fraternal ones could interfere, A second method is adoption study to compare genetics and environment. Useful but limitations as well: again, the prenatal environment is involved as well, there is a tendency to place children in similar homes as they were adopted from, and adoptive homes are generally above-average environments. Nowadays complex family studies are being used, including all different kinds of siblings to compare and contrast, and longitudinal twin studies are assessed. 

After such a study statistics are used to estimate the degree of heredity or environmental influences. Concordance rates are calculated (percentage of pairs of people studied in which both members display the same trait, e.g. smoking) and if they are higher for more genetically related pairs, heredity is at work. For a trait that can vary in degree correlation coefficients are calculated. Concluding, three factors that contribute to individual differences can be looked at: heritability, shared environmental influences, and nonshared environmental influences. Important: when genetically identical twins reared together do not have a correlation coefficient of a perfect 1, nonshared experiences made them unique.

Molecular genetics is the analysis of specific genes and their effects. To find which genes contribute, people's entire genomes need to be analyzed to figure out which genes distinguish individuals with or without the specified trait. 

What are some behavioral genetics findings?

• Intelligence is quite a heritable trait. From infancy to adulthood, individual differences in intelligence more strongly reflect genetic makeup and nonshared environmental influences, whereas the influences of shared environmental influences wane. SES also has influence on the estimates of heritability and environmental influences for intelligence. Thus herability of a trait can differ depending on age, SES, culture..
• Living together generally does not make children more alike in temperament and personality, nonshared influences is more important than shared. Genetics also matter.
• Children can inherit predispositions to develop disorders, but experiences will interact with their genetics to determine how it turns out.
• Genes contribute to almost all traits, however some are more heritable than others. Physical and physiological characteristics are typically strongly heritable, intelligence is moderately heritable. Temperament/personality less heritable, in psychological disorders it varies from disorder to disorder. Genetics even modestly contribute to differences in attitudes and interests. 

How does the interplay between genes and the environment work?

How do heredity and environment work together to make us us? There are three important forms of gene-environment interplay:

• Gene-environment interactions: the effects of our genes depend on the environment we experience, and how we react to that environment depends on what genes we have. And it often takes a combination of high-risk genes and a high-risk environment/bad experiences to trigger psychological issues. The diathesis-stress model proclaims that psychological problems comes from an interaction of a person's vulnerability to problems, and the experience of stressful events. But some of the genes that are "risk genes" also seem to make people benefit more than others from nurturing environments. This is the differential susceptibility hypothesis: some people are more reactive to environment than others, based on their genetics. 
• Gene-environment correlations: how are genes and environment experiences interrelated? While "interactions" tell us genetics cause people to react differently to experiences, "correlations" show people with different genes have different experiences. This can happen in three ways: passive, evocative and active. Passive has to do with, for instance, parents providing their offspring with genes as well as a certain environment that's compatible with those genes: so the environments are correlated with the genotypes (e.g. social parents create a social environment, both have influence). In evocative correlations, a child's genotype evokes certain environmental reactions: a social baby gets more social attention in return. And active correlations is about genotypes influencing which environments are looked for: a genetic social kid will seek parties. In sum: in environmental influences, heredity still plays some part! Reciprocal influences are at work, meaning for example: negative parenting can contribute to antisocial children, but genetical predispositions to antisocial behavior in children also bring out the worst in parents. Genetically informed studies try to see whether genetics explain apparent environmental effects, to understand true environmental effects (e.g. comparing adoptive children and biological children, comparing identical twins). 
• Epigenetic effects on gene expression: epigenesis stands for the process through which nature and nurture co-act on development. And epigenetic effects are ways in which environment influences the expression of certain genes, found by analyzing RNA. So there is an epigenome for each genome, with epigenetic markings that alters gene expression. Example of an epigenetic effect: with rats, a nurturing mother causes genes that regulate stress response to be turned on, and a neglecting mother causes those genes to be turned off, so those rat pups can't handle stress as good (this is social transmission). And overweight fathers can pass on epigenetic marks through their sperm to create a bigger obesity chance in their kids: thus, traits acquired during life can also be passed on (this is epigenetic inheritance)! 

Gene therapy stands for altering someone's genetic makeup, brings ethical discussion and usually most issues are too complex (polygenic, epigenetic effects) to solve with gene therapy. Another thing to keep in mind: should we even be trying to separate genetic and environmental influences, when interplay makes them inseperable?