Children get 50% of their genes from their father and 50% from their mother. The results of what they do may influence and may, in some instances, determine behaviour.

An example of research into this is that of James Olson, Philip Vernon, Julie Harris & Kerry Lang (2001) who used questionnaires to investigate attitudes to a range of variables such as the death penalty for murder, crossword puzzles, loud music, sweets, exercise and their own attractiveness. The researchers found evidence for genetic effects on family members in 26 of the 30 variables, with monozygotic twins being the most similar.

A gene consists of a long strand of DNA (deoxyribonucleic acid). A chromosome is a double chain of DNA and has a structure that looks like a twisted ladder. One of the functions of DNA is to control gene activity.

Genes contain bases (chemicals) called guanine (G), cytosine (C), adenine (A) and thymine (T). The coding sequence (3-letter combinations of G, C, A or T, each coding for an amino acid) contains the instructions for what the gene will produce. The sequence is copied to produce an RNA (ribonucleic acid) molecule. RNA organises the synthesis of proteins which act according to the genetic instructions. Transfer RNA (tRNA) transports amino acids to the ribosomes of the cell and messenger RNA (mRNA) acts as a model to form proteins which dictate how the organism develops.

The human genome (all the genes in a cell) has now been mapped. It is estimated that the human genome has just under 3 billion base pairs and around 20,000-25,000 genes. The genetic message carries millions of combinations of base pairs in DNA carried on chromosomes. Humans have 23 pairs of chromosomes in each cell - other than egg and sperm cells. These have only one strand of 23 chromosomes. When sperm fertilises an egg, their two strands combine to make 23 pairs and this is a new life. A male baby (XY) receives an X chromosome from his mother and a Y chromosome from his father; a female baby (XX) receives an X chromosome from each parent. The ‘default setting’ is for all foetuses to develop into females unless there is a Y chromosome.

Genes can be physically linked so that, if one gene is inherited, the other is inherited too.

However, although all our genes have been identified, this does not mean all their functions are understood. It is often the position of a gene or a combination of genes which leads to certain behaviours.

Dominant genes, which only need to be on one of a pair of chromosomes, always lead to certain characteristics. Recessive genes have to be on both chromosomes of a pair for the characteristic to occur. However, recessive genes can be passed on so the effect might appear in a future generation.

How much our behaviour is influenced by genes and how much it is shaped by the environment is the subject of the Nature-Nurture Debate. The genotype is an individual’s genetic constitution; another way of defining it is as the raw genetic potential at the time of conception. The phenotype is what the individual becomes after that, from the micro-environment of the womb through to the act of dying. It is the result of genes interacting with each other and the environment.

It is important, though, to remember that genes do not directly cause behaviour. Rather it is the proteins that they produce - as in the graphic below:-

indicating environmental factors play a role.  

Epigenetic modification is the way environmental factors influence which genes are turned on and off. 50-year-old MZ twins have over three times the epigenetic differences of young MZ twins.

Twin studies compare the concordance rates of MZ and DZ twins to evaluate how much a characteristic might be genetic. (A concordance rate is the statistical measure of two or more people in a classification  having the same characteristic.) However, there are problems in making assumptions on this basis....

• Sometimes twins are treated as just that - twins - by parents, family, friends and other important socialisers, regardless of whether they are identical or not. In which case, it might be safe to assume their environment is similar enough to accept that genetics would determine any differences between MZ and DZ twins. However, people often do treat identical twins will be treated as more alike than non-identical twins, meaning MZ twins will share the same environment more than DZ twins.
• The effects of epigenetic modification need to be considered. Even in the micro-environment of the womb, there may be significant differences - eg: they may share the placenta or they may not. Since some genes require environmental triggers, even small environmental differences may result in one MZ twin being ‘triggered’ while the other isn’t. Gert von Syndow & Aime Rinne (1958) are just one team of researchers to have found significant differences in MZ twins at a very early age. In their case they found significant differences in weight and haemoglobin values during the first 10 months of life.
• No study has ever found a 100% concordance rate for a behavioural characteristic - therefore, environmental factors must play a part.

Adoption studies have the advantage that the environment of adopted children is different from that of their biological families, yet they have genes in common with them. Therefore, if they have characteristics in common with their biological relatives but not their adoptive relatives, it can be argued  strongly that the effect is due to genes rather than the environment. The case for genetic influence is particularly strong when MZ twins are reared apart. They do not share the same environment, so they tend to develop differently in spite of their identical genetic heritage.  There is no other ethical way of studying individuals with identical DNA.

This argument for genetic influence can fall down if the environment of the adoptive family is similar to that of the biological family. In the Western world there are usually tight criteria applied to who is allowed to adopt - ie: people who will produce a certain kind of environment. So, environments of adoptive families may not always be that different from those of biological families. In many cases there is selective placement - a deliberate aim of matching the adoptive family as close as possible to the biological family.

Additionally the likes of Nancy Newton Verrier (1993) have put forward the case that adoptive children are much more vulnerable to developing attachment disorders. If so, then this vulnerability might skew any data collected from adoptive children in relation to mental health matters. (This vulnerability can be interpreted as the vMEME harmonic of BEIGE and PURPLE at birth and shortly after not having its needs of survival and safety met through bonding with the birthmother. See also The Biological Impetus to Attachment.)

A limitation on studies on MZ twins reared apart is small sample size since this simply does not occur that often. (Where MZ twins do get split up, they are often cared for by relatives who are likely to have some of the same environmental features in their household(s).)

Genes and Gender Development

In the very early stages of development an embryo starts to generate hormones and genes start organising according to gender. Studies of mice - eg: Eric Valian (1964) - have identified 54 genes where activity levels vary by gender.

The development of sex organs is governed by DNA in the genes and is processed through RNA and proteins.

Genes influence sex differentiation in humans through 2 main stages after fertilisation before hormones take over:-

1. The fertilised egg divides to form a large number of identical cells. During the development of the of the foetus, the cells differentiate to form the various body organs, including the sex organs. At this stage both males and females have a gonadal ridge and, after 6 or 7 weeks of gestation,  2 sets of internal ducts: the Műllerian (female) and the Wolffian (male). The external genitalia appear female.

1. The gonadal ridge becomes either an ovary or a testis. In an XY the gonadal ridge develops into testes because the SRY gene on the Y chromosome produces the testis-determining factor protein. In an XX the male elements spontaneously disintegrate while the female ones thicken and grow into a womb

From this point on genes have no further effect on gender development.

Twin and Adoption Studies

Monozygotic (MZ, from the same fertilised egg, so-called ‘identical’ twins have 100% the same genes whereas dizygotic (DZ, from two fertilised eggs, non-identical) twins are the same as all siblings, sharing on average around 50% the same genes. Monozygotic twins are always the same sex. However, from the start there will be some small physical differences - eg: fingerprints.

If a characteristic is genetic, then both MZ twins should share that characteristic. Eg: if intelligence is genetic, then MZ twins should have the same IQ; they almost always don’t,

Chromosomal Problems in Gender Development

Problems with sex differentiation can occur at any time during the first few months of life. For example:-

• A gonad might not develop into a testis or an ovary. Eg: if the SRY gene is absent or deficient, then, despite the presence of the Y chromosome, the XY foetus does not receive the SRY signal to develop testes.

• Incorrect Műllerian or Wolffian duct development also causes problems. For example, Műllerian Inhibiting Substance (MIS) might not be accompanied by androgens or the foetus might not respond to androgens, in which case the foetus will have neither male nor female internal duct structures. Lack of MIS but with androgen secretion can lead to a foetus having both male and female duct structures.

• Men with Klinefelter’s Syndrome - it affects about 1 male in every 500 to 1,000 - have an extra X chromosome - ie: XXY. Their testes are small and their fertility is usually very weak - many are effectively sterile. Small, low functioning testes means they have low testosterone levels. Usually the penis is small too and they may have slightly increased breast tissue. There is no growth of body hair.
  They may experience other difficulties - eg: learning language. At 3 years of age the child still may not talk. These poor language skills typically affect the development of reading ability - with all the problems that will bring during schoolhood.
  Temperamentally Klinefelter’s babies are often described as passive and co-operative. This calmness and shyness usually stays with them throughout life.
  It seems that the extra X chromosome is caused by a complex fault in one of the parents’ sex cells, either the ovum or the sperm.
  There is a ‘mosaic’ form of Klinefelter’s in which some cells have the XXY pattern and some the standard XY pattern.
  XXY males can be treated using testosterone supplementation (starting at around the age) of 11-12 and other therapies, although the genetic variation cannot be reversed.

• Turner’s Syndrome occurs in about 1 in every 2,500 births of girls. It is more common in pregnancies that do not go to term. The condition is caused by a missing X chromosome - ie: there are only 45 chromosomes and the designation is X0. It is the only known survivable condition where there is a single rather than a pair of chromosomes. Girls with

Klinefelter’s Syndrome - photo copyright © 2009 Chafer Zorbun

Turner’s Syndrome - photo copyright © 2010 Pregnancy-bliss.co.uk

Turner’s Syndrome are short and their ovaries do not work properly, with the result that most are infertile. They do not usually produce oestrogen and progesterone at puberty and usually require hormone supplementation from the age of 12-13 to develop breasts and pubic hair and to have periods. However, since the womb is intact, in-vitro fertilisation is possible using donor ova.
The SHOX gene, which occurs on the X chromosome, s important for growth and development. It is thought that missing one copy of this gene is the cause of girls with Turner’s Syndrome being shorter than average. They typically don’t exceed 4’8” but growth hormone supplementation can help.
In addition to being very noticeably short, girls with Turner’s Syndrome often have webbing of the neck.
As with Klinefelter’s, there is a mosaic variation of this syndrome, with some cells having the standard XX pattern. Some with the mosaic form do conceive.
Girls with Turner’s Syndrome often have higher than average verbal ability but lower than average spatial ability, visual memory and maths skills. They also tend to have difficulties with social adjustment and generally have poor relationships with their peers.
It is incurable but the condition is not thought to be genetically inherited.

• Super Male Syndrome affects about 1 in 1,000 boys and was first identified by Avery A Sandberg, G F Koepf, T Ishihara & T S Hauschka in 1961. It is thought that an error in chromosome separation results in sperm cells with an extra copy of the Y chromosome, meaning that a male foetus from such a fertilisation has the designation XYY.
  XYY boys experience an increased growth velocity during earliest childhood, with an average final height approximately 7 cm above expected final height. have an increased risk of learning difficulties (up to 50%, compared to an estimated average of 10% in standard XY boys) and delayed speech and language skills. Delayed development of motor skills (such as sitting and walking), weak muscle tone (hypotonia) and hand tremors or other involuntary movements (motor tics) These characteristics vary widely among affected boys and men.
  Behavioural and emotional problems are also possible - though many studies indicate not usually more so than standard XY boys
  In a 1965 study of difficult-to-manage, retarded men in prison Patricia Jacobs, Muriel Brunton & Marie Melville found a surprising number had XYY chromosome patterns. They attributed their aggressive, violent nature to this extra Y chromosome. However, later studies - eg: Clive Hollin (1989) - found this kind of genetic abnormality to sufficiently prevalent in the general population to suggest there is  no significant correlation between the XYY pattern and greater male aggression.

• Females with XXX may have menstrual irregularities and, although rarely exhibiting severe mental impairments, have an increased risk of learning disabilities, delayed speech and deficient language skills. It results during division of a parent's reproductive cells and occurs about once in every 1,000 births.

• XXYY affects one in every 18,000-40,000 male births. Common features include tall stature, gynecomastia, truncal obesity, skin ulcers, and a craniofacial dysmorphism described as a ‘pugilistic’ facial appearance.