- Discontinuous variation
Discontiuous variation refers to dominant-recessive
allele interaction; it's one or the other.
- Simple dominant-recessive gene
interactions in humans
- Examples
- Normal vision dominant over near-sightedness
Though not all near-sightedness is tied to this genetic
interaction, there is a near-sighted, recessive allele.
- Normal color vision dominant over
colorblindness
- Rh positive is dominant over Rh
negative
Another example of discontinous variation (dominant-recessive
interaction). If you are AB+ (blood type) then you are Rh+: you produce
the Rh factor protein. (O- blood is a universal donor blood, which means
the blood people may hound O- people, like Dr. St. Clair.)
- Continuous variation: polygenic
inheritance in humans
There are gradations of differences: a variety of
phenotypes. Example: height: small #s at the extreme short and tall ends,
more in between: a bell-shaped curve.
- Characteristics of polygenic traits
- Continuous variation or gradation
with small differences
- Variation within a population based
on a polygenic trait yields a normal bell-shaped curve
- Human examples
Dr. St. Clair's friends: Art (about 5' 5") and
Donna (under 5' tall) had fraternal twins: one was their size (Reed), while
the other (George?) grew to be 6'2". Art and Donna's 'taller' genes,
though they may have had few, must have come together in the taller boy's
case.
- Height
(above example)
- Intelligence
There's no such thing as a dumb allele and a smart
allele, but there are genetic factors: each is endowed with different types
of intellectual abilities. (Environment is also a large factor in human
intelligence.) So even Dan isn't a complete numbskull.
- Skin color
Another example of polygenic inheritance. Dr. St.
Clair's slide showed the family of a lighter-skinned father, a darker-skinned
mother, and two kids: each with a different skin color. These differences
are a product of various alleles and their interactions (polygenic inheritance).
- Human genetic potential
Remember from earlier: 1 - For normal, proper development
to take place, an intact genetic blueprint must be present, and 2 - the
environment must provide the freedom and materials for that blueprint to
be expressed in full. Thus, an expectant mother carries a lot of weight
when it comes to the proper nourishment and care of herself and her child.
- Inherited potential
The potential contained within the genetic blueprint
(DNA).
- Interactions between inherited
potential and environmental influences
Many non-genetic factors are involved in human development:
nourishment, disease, etc.. What else?
- Mutagenic substances
Mutagenic substances are those that adversely effect
the genetic blueprint.
- Nutritional influences
- Protein deficiency
Every 24 hours: 45,000 children under the age of
5 die of starvation and starvation related disease (13-14 million children
annually.) The problem is a complex one, as there are political, logistic,
economic, etc. problems, but if individuals contributed $10-15/month, much
could be done to resolve this problem: this is not an inevitable pattern.
- Poverty cycle
 15.1
Poverty Cycle
The genetic potential of third-world people is no less than ours, but
often that potential cannot be met due to poverty. Poverty and its effects
tend to cycle from parents to children: it is often an inherited and self-perpetuating
condition.
- Sex-linked inheritance
X-linked inheritance. Women have two X chromosomes:
XX. Men have one X and a Y: XY, so any gene on the male X chromosome will
be fully and completely expressed. This is because there is no corresponding
allele, as would be the case with women, who have two X chromosomes. Example:
A Royal Pedigree: Queen Victoria was a carrier. (Qualifications of a 'carrier':
1 - defective allele is in the genotype; 2 - the defective allele is not
expressed in the person's phenotype; 3 - the person may pass the defective
allele on to offspring, and it may be expressed in them. All three conditions
must be met for a person to qualify as a carrier.) So, by these qualifications,
when it comes to X-linked inheritance, males cannot be carriers.
- Examples
Sonia Bake's pedigree: each in her family took a
taste-test (PTC paper): 'tasters' were able to taste something bitter (recessive)
on the test paper; 'non-tasters' (dominant) couldn't taste the bitter on
the test paper. The Father Bake proved to be a taster, while Mother Bake
proved to be a non-taster. Some of the kids were 'tasters' and other were
'non-tasters' so we can conclude that Mother Bake was a carrier. Otherwise,
every child would have received a dominant (non-taster) allele from mother's
X chromosome: so none--boy (X from mother, Y from father) or girl (X from
mother, X from father)--would have been a taster.
Be able to predict the (average) ratios of genotypes and phenotypes when
it comes to sex-linked inheritance. Also, be able to determine the possibilities
of parents' genotypes and phenotypes from that of their offspring.
- Interactions between inherited
potential and environmental influences
- Colorblindness
Colorblindness is a sex-linked, recessive trait.
- Hemophilia
Hemophilia is also a sex-linked (X-linked) trait.
- Nondisjunction
Abnormal separation of homologous chromosomes or
sister chromatids caused by their failure to separate properly during cell
division.
- Sex chromosomes
Nondisjunction of the sex chromosomes in humans results
in the following:
- Turner's syndrome
XO: no second sex chromosome. Results in a sterile
female; abnormal sexual characteristics
- Klinefelter's syndrome
XXY. Occurs in 1 of 700 live male births. Sterile
male. Partial breast development. Reduced development in testes. Effects
intelligence.
- XXY karyotype
XYY: fertile male; few symptoms; (thought to predispose
such men to crime, but unsupported).
- Autosomes
All chromosomes that are not sex chromosomes are
autosomes.
- Down syndrome
Three #21 chromosomes (instead of two): trisomy 21.
(Trisomy may occur with other chromosomes.) Down syndrome is highly variable
in effects; appearance, intelligence, etc.. 70% of the time, Down syndrome
is related to the male gamete (sperm), especially with men over 55. The
likelihood of nondisjunction increases with age in both men and women.
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