(picture of pairs of nucleotide bases making up the rungs of the DNA ladder like that in atlas)
(picture of banded chromosome 19 similar to that in tactile atlas)
(picture of a pair of chromosomes, each with the locus of the “flower color gene” labeled. One chromosome contains a gene for purple flowers, the other a gene for white flowers, so this individual plant is “heterozygous” for flower color.
*Punnett’s square is a square or rectangle divided into 4 quadrants with one parents genotype written above the left and right quadrants – for instance Br above the left side and bl above the right side if Mom had brown eyes but we knew she carried a blue gene too. The other parent’s genotype is recorded outside the square on the left, next to the upper and lower quadrants. E.g. maybe Dad has 2 blue genes. By looking at the genetic contributions possible from each parent, each quadrant within the square can be filled in
with the possible genotypes of any offspring of the couple. For example the upper left quadrant (just under the label indicating Mom has a Br gene) would be filled in with one genotype possible – that the child will get a Br from Mom and 1 of the bl from Dad.
The quadrant below, in this example would look the same – a Br from Mom and a bl from Dad (but representing the other of his bl genes, even though the effect or outcome will be the same.). The top right quadrant (just under the label indicating Mom carries a blue gene) would show the child possibly getting that bl gene as well as a bl gene from Dad (thus having blue eyes). The lower right quadrant would look the same (bl – bl) but would again represent the child getting Mom’s bl gene and the other of Dad’s blue gene.
Looking at the total square we can see these parents have a 50-50 chance of having a Br-bl child and 50-50 chance of having a bl-bl child each time they get pregnant.
http://www.uni.edu/walsh/genetics.html
Genetics 2
Human
genome contains about 3 billion of these nucleotide base pair “rungs” which make up about 30,000 genes.
We share
99.8% genes in common with other humans, 98% genes in common with chimps, and
lots of genes in common with all living things (e.g. ½ the banana genome found
in the human genome)
Shuffling the Deck
After
the initial joining of 23 chromosomes from Mom and 23 from Dad (when the egg is
fertilized), the resulting 23 pairs of chromosomes in the new individual are
going to replicate each time the developing cells divide. In these later cellss
the chromosomes of offspring don’t always have one that’s completely from dad
and one completely from mom because
*during
replication there may be “crossing over”
of the “arms” of a pair of chromosomes and exchange of DNA (e.g. the end of the
arm from the point where Dad’s chromosome crosses Mom’s changes places with the
end of the arm on Mom’s chromosome. Now each member of the pair contains some
genetic material from each parent.
In the
case of genes located on the X chromosome, females (who have 2 X’s) would have
the usual 2 alleles for each gene, one on each X.
Males, however, only have 1 X chromosome and a
much smaller Y containing less than 1/4 as many genes(mostly related to testes
& body size). So many genes on the X do not have a corresponding allele on
the Y. If a male inherits a recessive gene at one of these positions on his X,
it will be expressed because he will ONLY have the recessive allele. *Example –
X-linked recessive red/green color blindness gene, X-linked hemophilia
Remember
– genes provide the templates for building proteins
If
females have functioning genes on both X’s and males only have the genes on 1
X, you might expect half as much of the X related proteins to be produced in
males. But in females there is random inactivation of 1 X in each cell!
Types of Genes
Structural
genes - contain the instructions for building proteins (via RNA &
ribosomes)
Operator
genes - determine whether particular structural genes will be active or “expressed”
or not
Operator
genes can be influenced by signals from the bodily or external environment.
This is
one example of regulation of gene expression.
Regulation
of Gene Expression
All
cells of an individual contain the same chromosomes, yet the genes that “play
off” or are expressed vary in each cell and at different times in the
individual’s life. This is why, for example, despite the fact that they contain
the same chromosomes and genes, skin cells are different from brain cells which
are different from muscle cells, etc. A different combination of genes triggers
differences in the structure and function of each due to various types of gene
regulation. Some examples: genes may be
blocked from being expressed until a certain compound or environmental
condition is present, genes may be switched on or off at different times.
Ease of RNA binding may be increased or decreased.
Some
genes, present in both sexes, are only expressed if exposed to particular sex
hormone.
Example:
Any of us may carry the gene for male pattern baldness, but it is only
expressed in the presence of significant levels of androgens. Usually only
those with testes will have these levels (later in life) and then the gene will
be switched on and expressed. But female bodybuilders taking anabolic steroids
can inadvertantly switch on this gene and then show male pattern baldness if
they have that gene.
don’t
usually “see” the effects of a recessive
gene unless you have a pair of them;
a sex limited gene will only be expressed
under certain hormonal conditions;
expression
of many genes is “regulated” (turned on or off) by other factors
An
estimate of the amount of the variance in the behavior/characteristics of a
specific group/population is due to genetic variation.
Heritability
ranges from 0 (none of variance due to heredity) to 1 (all of the variance due
to heredity)
Heritability
depends on the group or population studied
Comparing
the similiarity of pairs of identical twins vs. the similarity of pairs
fraternal twins
Comparing
the similarity of child/biological parent vs child/adopted parents
BUT:
these studies sometimes overestimate heritability
Field of
research focused on understanding the relative contributions of heredity and
environment (and their interaction) on behavior and psychological functioning
Other
research methods used: inbred strains, selective breeding/artificial selection
, genome mapping and manipulation
Identical
twins not only share their genetic makeup in common: they often share the same
blood supply & chorion in utero, whereas fraternal twins typically have
separate chorions.
Identical
twins may be raised & reacted to more identically than fraternal twins.
Even in
studies looking at twins raised apart, adoptive parents are screened to have
similar characteristics, so “environments” not that different
So some
portion of twin similarity may be due to environment
A
change, over generations, in the frequencies of various genes in a population
Evidence
of the gradual change in species over time
Striking
similarities between species suggesting common ancestry, yet differences
between related species adapting to different environments
Major
changes in species via selective breeding
Selective
breeding ( or artificial selection) can
lead to dramatic behavioral or physical differences in different strains of the
same species.
But in
some cases the interaction of genetics and environmental factors can cause the
differences to disappear- e.g. Tryon maze-bright and maze-dull rats.
Metabolic
disorder transmitted by recessive gene on Chromosome 12, causing a lack of the
enzyme phenylalanine hydroxylase and a toxic buildup of phenylalanine.
This
impairs brain development & causes retardation, hyperactivity,
hyperirritability, possible seizures. Also low levels of dopamine.
About 1
in a 100 Caucasians (especially Scots and Irish), fewer Asians, and very few
Blacks carry the recessive gene.
1 in
10,000 babies born with PKU (those who get the recessive gene from each of
their parents).
Even in
genetic disorders “Nature” may interact with “nurture”
PKU’s
effects depend on the environment (diet consumed)
With a
special diet most of PKU’s effects are prevented
Most
states require screening of newborns
Controlling
diet (low phenylalanine) greatly alters effects of PKU.
A more
normal diet may be eaten more safely after the “sensitive period”*
*But:
PKU kids show “frontal lobe” type cognitive deficits (like impulse control)
despite diet - may be due to higher than
normal phenylalanine levels even on the diet or low DA.
Huntington’s Disease
DNA mutation produces excess “CAG repeats” in the
gene’s “instructions”
36-250 instead of
usual 29 or fewer, resulting in an abnormal form of protein known
as huntingtin.
The more repeats, the earlier symptoms appear.
# of repeats can increase across generations,
especially in kids inheriting gene from father
Brain damage may be due to decrease in normal protein
+ adverse effects of abnormal protein on critical growth factors keeping cells
alive