Biology 101

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Biology 101
Fall, 2007
Week 4 Genetics Inherited traits
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GENETICS - before and after Mendel
Josef Kölreuter discovered in the 1760s that
offspring could have features of only one parent,
or could be intermediate between both.
Karl Friederich von Gaertner did gt10,000
hybridization experiments in the 1820's. Some of
these identified the traits in peas (purple
flower color, pod color and seed shape) that were
subsequently used by Mendel in his "laws" of
inheritance.
Gregor Mendel (1822-1884) is generally regarded
as the father of genetics.
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The textbook (Stern) accentuates the work of
Barbara McClintock.
This was certainly important McClintock was
awarded a Nobel Prize in 1983
However, the transposable elements she discovered
relate more directly to epigenetics than to
genetics.
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Mendelian Genetics
Gregor Mendel was an Augustinian monk with
training in agricultural science and mathematics.
Several individuals had studied inheritance of
traits, but Mendel applied what is now known as
the Scientific Method to address what was a
puzzling situation. In particular, he
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1. Tested a specific hypothesis and planned his
experiments carefully, using clear examples.
2. Obtained pure-breeding lines for starting his
experiments.
3. Followed not only the offspring of the first
cross, but also those of subsequent crosses.
4. Counted offspring from each cross and analyzed
the results mathematically.
5. Kept accurate records of his experiments and
results, enabling others to repeat them.
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Mendel derived two basic laws

• SEGREGATION
• Hereditary traits are determined by discrete
  factors (genes) that occur in pairs, one from
  each parent.

• INDEPENDENT ASSORTMENT
• The inheritance of a pair of factors for one
  trait is independent of that for other factors.

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In fact, various situations cause variations in
the above events so that many scientists no
longer consider the above situations laws.
Importantly
(a) The involvement of a gene pathway (polygenes)
for a trait means that it may not be governed by
a single, discrete, factor.
(b) Genes are present in linear arrays on
chromosomes. If two genes are close together on
the same chromosome (i.e., linked), there is
relatively little chance that they will assort
independently.
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Segregation Paired factors segregate during the
formation of reproductive cells (meiosis I) so
that each cell gets one of the factors.
Dominance Sometimes one factor dominates the
other factor. A dominant trait masks/suppresses
the alternative (recessive) trait for a
particular feature. Conversely, a recessive
trait is masked or suppressed by the dominant
trait for the feature in question.
Independent assortment When considering two or
more pairs of traits, the factors for each pair
of traits assort independently to the
reproductive cells.
Gene is the modern term for one of Mendel's
paired "factors".
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Alleles are genes at the same position (locus) on
homologous chromosomes (i.e. chromosomes that
carry the same genes and that pair up early in
meiosis I).
Homologous chromosomes in an individual may carry
the same or different alleles at a given locus.
A plant is homozygous for a given gene if it has
identical alleles for that gene on both
homologous chromosomes bearing the gene.
A plant is heterozygous for a given gene if it
has different alleles for that gene on the two
homologous chromosomes bearing the gene.
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Genotype the genetic constitution of an organism.
Phenotype the physical form or appearance of an
organism. This may differ from the genotype
because of dominance and other regulatory events
that mask the full expression of the genotype.
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Punnett square is a useful diagram for
determining the predicted ratios of offspring
resulting from a genetic cross. (see Stern, p.
246). Make one yourself at http//www.usoe.k12.
ut.us/curr/science/sciber00/7th/genetics/sciber/pu
nnett.htm
Progeny ratios of phenotypes often reveal the
genetic state of plants that were crossed
Thus a phenotypic ratio of 31 is typical for
the progeny of a monohybrid cross between two
parents heterozygous for a dominant trait.
The genotypic ratio for the same progeny is 121
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W. Bateson (worked with Punnett)
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A phenotypic ratio of 9331 is typical for the
progeny of a dihybrid cross between parents that
are both heterozygous for two dominant/recessive
traits.
Gametes are reproductive cells, e.g. egg cells
sperm.
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MUTATIONS
Mutations can arise in many ways. Most are point
mutations, consisting of single base
substitutions, insertions or deletions that occur
as nucleotide typographical errors during DNA
replication
single base substitution error CCTGAGG
GGACACC CCTGTGG
single base deletion error CCTGAGG GGAC CC
CCTG GG
single base insertion error CCTGAGG
GGACTTGG CCTGAAGG
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• Sickle cell anemia is a clear example of the
  effects that can arise from what appears to be a
  very minor change in DNA a single base
  substitution error
• mRNA sequences CCUGAGG
  CCUGUGG
• amino acids coded for
  Glu  Val

In normal ß-globin (one of the two polypeptides
in hemoglobin), the sixth amino acid is Glutamic
acid, a charged amino acid residue.
As a result of the error, the codon for the sixth
amino acid is changed so that Valine, a nonpolar
amino acid, is incorporated into ß-globin instead
of glutamic acid, during messenger translation.
As a result, ß-globin does not assume its correct
tertiary structure, hemoglobin function is
deficient, and the affected erythrocytes are
deformed, with a sickle shape.
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• Exposure to chemicals and radiation can give rise
  single base changes. Exposure to sunlight
  radiation can lead to methylation or other
  chemical modifications of DNA that prevent proper
  expression of genetic information, e.g., cytosine
  methylation
• CCTGAGG CmCTGAGG

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Since the process of selection of the fittest has
optimized most systems, the vast majority of
mutations are harmful.
However, some will be beneficial, and the cell
with the new genetic information resulting from
the mutation will be able to outperform other
cells.
This enhanced fitness at the cellular level may
increase the survival and reproductive
performance of the organism, and in that case the
mutation will be conserved.
Changes that occur during chromosomal
segregation, and especially during synapsis,
involve large segments of genetic information.
Large changes are typically lethal and are
therefore not propagated/conserved, but many
heritable defects result from such events.
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Transposition occurs when genetic information is
moved from one chromosomal location to another.
This can result from
movement of transposable elements,
chromosome breakage,
such as
incorrect pairing of chromosomes (namely pairing
of non-homologous chromosomes instead of
homologous ones) in meiosis I.
or errors during meiosis
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Translocations are large transposition events,
usually by chromosome breakage and incorrect
reattachment
Note that chromosome 1 has become longer (gained
genes) and chromosome 2 has lost genes and is
shorter
Inversion Genes are rearranged, by processes
such as those given above
ABCDEF AEDCBF
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Duplication Segments of a chromosome are
duplicated
ABCDEF
ABCDEFDEF
Polyploidy Multiple copies of all chromosomes
are present. This can result from failure of
chromosomes to separate after crossing-over.
Aneuploidy Certain chromosomes are present in
extra copies or are deficient in number.
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COMPLEX INHERITANCE
Mendel's observations are only accurate for
dominant/recessive genes.
There are many ways in which progeny ratios can
differ from those obtained for a single dominant
monohybrid cross
Incomplete dominance
Codominance
Multiple genes
Serial genes
Polygenic inheritance
Pleiotropic genes
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Incomplete dominance
...results when the expression of a gene is
additive rather than dominant
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Codominance
...similar to incomplete dominance, but genes may
encode a noncompeting phenotype, as in the case
of allozymes (i.e., slightly different functional
enzymes made by different genes at the same locus
on homologous chromosomes).
Multiple genes
May encode the same or similar protein (trait).
They are not allelic, as each of the multiple
genes is at a different locus (not all of the
loci necessarily in the nucleus). Isozymes are
the products of such multiple genes.
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Serial genes
...often act in concert to produce a phenotype.
Thus, a series of genes may be needed to complete
a metabolic pathway, e.g. the development of
flower color.
Such interaction of two or more serial genes is
epistasis.
Polygenic inheritance
Complex traits such as yield (e.g. tons of wheat
per hectare) and plant height often involve the
interaction of several genes.
Pleiotropic genes
...may affect more than one phenotypic
characteristic. For example, the purple flowers
and seed coat of peas is probably pleiotropic.
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In tobacco the sizes and shapes of leaves,
flowers, anthers and fruits are controlled by the
S gene.
Plants with at least one dominant S allele (SS or
Ss) grow longer and narrower organs
(ss plants have short, broad structures).
In tobacco, many genes are involved in the
development of inflorescence and leaf color and
shape. However, their effect may be overriden by
S, a pleiotropic gene.
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NON-MENDELIAN INHERITANCE
Linkage
When genes are located in close physical
proximity they do not assort independently.
Most of Mendel's traits are on separate
chromosomes, or are on distant parts of the same
chromosome.
However, pod shape and plant height are linked
traits in pea.
Mendel did not report results for hybrids
involving these traits. They did not conform to
his laws of inheritance.
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Cytoplasmic inheritance
This results when a trait is entirely or
partially encoded by an organelle (e.g.
chloroplast or mitochondrion) genome.
Transposons
Transposable elements have the ability to move
from one place in the genome to another.
Typically, a transposon contains only a few
genes. However, a gene that is interrupted by the
presence of a transposon can be inactivated or
changed in its function.
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Epigenetics
Because the nucleotides of DNA can be modified,
e.g. by methylation, the same sequence may or may
not be available for transcription
Further, the association of DNA with chromatin
depends on the chemical status of the histones
Histones can be modified in several ways,
including acetylation, methylation,
phosphorylation, ubiquitination, glycosylation,
and ADP ribosylation.
These modifications alter the availability of
DNA, and hence genes, for expression
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Histone proteins (blue and yellow) form the core
of a nucleosome.
One end (tail) of the histones can wrap round the
DNA (blue nucleosomes), making it inactive, or be
in an open and active configuration (yellow
nucleosomes).
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The Hardy-Weinberg Equilibrium
The study of events that occur in gene pools that
modify gene frequencies is known as Population
Genetics.
The mathematical model developed by G.H. Hardy
and W. Weinberg predicts that the proportional
frequencies of dominant and recessive alleles
will be maintained from generation to generation
in a randomly mating population.
This holds good when
(1) The population is large
(2) Individuals do not move in or out of the
population
(3) Mutations do not occur
(4) Reproduction is random, not selective
(5) All alleles and combinations of alleles have
equal fitness i.e. there is no natural selection.