- Mutations: a heritable change in
DNA
A RANDOM, heritable change in the DNA. The likelihood
of making a random change (mutation) and having whatever it is (a Mercedes
Benz, a living organism) come out for the better is highly improbable.
(But obviously not impossible, as mutations are the major source of genetic
variation.)
- Types of mutations
- Point mutations
A random, heritable change in a single nucleotide
unit of the DNA. This most frequently occurs during replication/ the being
in interphase, the DNA in chromatin form. (This figure is not in the syllabus:
 14.x Point Mutations)
Types of point mutations: deletion: a nitrogen-base pair is left out, which
produces a different mRNA which will then code differently in protein synthesis;
- Chromosomal mutations
14.1-4
Chromosomal Mutations
Chromosomal mutations are like point mutations but involve larger segments
of DNA (on the chromosomes); whole genes are being changed in some fashion:
four different patters:
- Duplication
Part of the chromosome is duplicated: one or more
genes is repeated.
- Deletion
Part of the chromosome is deleted, left out, skipped,
lost.
- Translocation
Part of the chromosome is, having being deleted from
another, is incidentally translocated (added) to another chromosome.
- Inversion
Part of the chromosome is inverted: the sequence
on the chromosome has flipped, reversed itself; oops.
Deletion,
translocation, and inversion mutations are examples of transposons, as
discussed in a previous lecture. Remember?
- Frequency of mutations
An average, eukaryotic frequency: one random, heritable
change (mutation) per every 200,000 cell divisions.
- Significance of mutations
What is the biological importance? MUTATIONS ARE
THE ULTIMATE SOURCE OF GENETIC VARIATION in the biological world. Mutations
are the most important source of genetic variation. Random, heritable changes
are the ultimate source of genetic variation. Why is genetic variation
important?
- Non-Mendelian genetics
- Incomplete dominance
An example of paired allele interaction (example:
dominant-recessive interaction is a paired allele interaction). Incomplete
dominance: it's not one or the other, as in dominant-recessive, but an
intermediate between the two. (example: a red-flowered and a white-flowered
generating a pink-flowered. So if you had a Rr x Rr with (incomplete dominance),
what would be the phenotypic ratio? (1 red, 2 pink, 1 white) Right? (Review:
what would the phenotypic (what shows) ratio be of the same cross but with
a dominant-recessive interaction?)
- Co-dominance
Example: blood type: we use 'I' and 'i' for denoting
blood type. And there are three different alleles for blood type: the A
allele, the B allele, and the O allele. The O allele is recessive to the
A and B. (To have type O blood you'd need the two recessive O alleles.)
Type A blood (phenotype) could result from either genotype: AO or AA paired
alleles. (Same with type B blood: BO, BB.) Co-dominance is demonstrated
in type AB blood: BOTH alleles are expressed completely: there is no blending,
they are both fully and completely expressed. (A + or - after the blood
type (A+) refers to the RH factor, which is determined by different genes,
and is basically a dominant-recessive interaction: RH+ is dominant to RH-.)
- Polygenic inheritance
('Poly'=many, 'genic'=genes.) Many characteristics
we assume as the result of dominant-recessive interactions are actually
polygenic: they are determined by the interactions of a number of alleles:
eye color, height, body shape, hair color, skin color, etc.. Several pairs
of alleles influence a single trait. When looking statistically at polygenic
inherited traits, we see a bell-shaped curve.
- Pleiotropy
One pair of alleles influences many traits (the flip-side
of polygenic inheritance). Example: sickle-cell anemia: cause by a rather
miniscule change in the DNA that causes a malformation of one (of the four)
hemoglobin proteins. With the abnormal structure (of the red blood cells),
there is abnormal function: they don't last as long as normal RBCs, which
causes anemia; the sickle cells tend to clump in small-diameter capillaries,
cutting off circulation, which causes a variety of serious problems. So,
the allele that results in sickle cell anemia influences many traits: blood
circulation, kidney function, capillary action, etc..
- Sex-linked inheritance
14.5
Gene Linkage
Inheritance having to do with sex hormones. In humans we are talking
about genes on the X chromosome (female: XX; male: XY). All alleles on
both chromosomes are expressed in males. Example: colorblindness: an X-chromosome,
recessive allele: much more common in males because they only have one
X chromosome. Females may be carriers without being colorblind because
they have another X chromosome with an non-colorblind allele that is dominant
over the recessive one. But in males, there is only one X chromosome, so
its alleles are always expressed, even if recessive. Females must have
two recessive alleles to be coloblind.
- Linkage and crossing-over
Linked genes are genes on the same homologous chromosomes.
For example, all the genes on any individual chromosome are linked. Crossing-over
can change linkage patterns relative to expression: so if alleles are exchanged
(in crossing-over during prophase I of meiosis) new patterns are formed.
By "relative to expression" we mean that though alleles may be
swapped, the linkage of traits remains intact: simplified, hypothetical
example: Supposing that the gene for earlobe attachment and the gene for
tongue-curling are on the same chromosome: whether or not crossing-over
occurs, the alleles for these two traits will be linked--inherited together.
The effect of crossing-over would only be between combinations of these
alleles: you could start with AATT (homozygous attached, tongue-curling)
and AaTt, and crossing-over could result in any number of recombinations
(example: say the 'A' alleles are crossed-over--> AaTT). This changes
the genotype and phenotype, and thus, 'linkage patterns relative to expression,'
but NOT the fact that the 'A' alleles ('A' or 'a' for earlobe attachment)
and 'T' alleles ('T' or 't' for tongue-rolling) are on the same chromosome:
linked.
- Genetic potential and environmental
influences
Three very important points: For development to occur
completely and accurately: 1- Must begin with an intact, accurate blueprint
(DNA): half from mother, half from father; 2- Need access to sufficient
resources: you can have a perfect blueprint for a 5,000 sq. foot house,
but if you only have the materials for a 500 sq. foot house, you can't
fulfill the blueprint; 3- A healthy environment: unhealthy environmental
factors can harm the (figurative) blueprint and/or the materials needed
for development.
- Mutagenic factors
Factors that can be harmful to the genetic make-up
and development of an organism. Example: deformed frogs in Minnesota (Wisconsin?).
- Interactions between genetic information
and the environment
Life needs the information (blueprint-DNA) and the
materials (environment) to build and maintain life.
- Nature
Constant interaction.
- Significance
Very.
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