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:-
- 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.
- 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,
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. (This
is sometimes known as the ‘47 XYY Karotype’.)
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. Attempts have been made to link the XYY
karotype to criminal behaviour - though the results are far from conclusive. See
Chromosomes, Genes and Neurotransmitters.
- 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.
