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genetics

Question	Answer

GENETICS AND HEREDITY Gregor Mendel’s Discoveries
• Mendel discovered the particulate behavior of genes
• brought an experimental and quantitative approach to studying
genetics
• Mendelian inheritance reflects rules of probability
Introduced fundamental laws of genetics:
1. Unit character
2. Dominance
3. segregation
4. independent assortment
• ~1857, Mendel began breeding peas to study inheritance
• Peas have several advantages for studying genetics
– many varieties available with distinct heritable features
(characters e.g. color) with different variants (traits, e.g. pink)
– strict control over which plants mate with which others
– Each plant has male
(stamen) and female
(carpel) sexual organs
– In nature, pea plants typically
self-fertilize, fertilizing ova
with their own sperm cells
– However, Mendel could also
move pollen from one plant to
another to cross-pollinate
plantsGENETICS AND HEREDITY Gregor Mendel’s Discoveries
• Mendel discovered the particulate behavior of genes
• brought an experimental and quantitative approach to studying
genetics
• Mendelian inheritance reflects rules of probability
Introduced fundamental laws of genetics:
1. Unit character
2. Dominance
3. segregation
4. independent assortment
• ~1857, Mendel began breeding peas to study inheritance
• Peas have several advantages for studying genetics
– many varieties available with distinct heritable features
(characters e.g. color) with different variants (traits, e.g. pink)
– strict control over which plants mate with which others
– Each plant has male
(stamen) and female
(carpel) sexual organs
– In nature, pea plants typically
self-fertilize, fertilizing ova
with their own sperm cells
– However, Mendel could also
move pollen from one plant to
another to cross-pollinate
plants
• In a typical experiment, Mendel cross-pollinated two
different parents (P), true-breeding pea varieties to produce
an F1 (1 st familial generation)
- then allowed the offspring to self-pollinate and produce 3rd
generation (F2)
• It was analysis of 3rd generation plants that revealed
principles of heredity:
F 1 hybrids all TALL
- short gene seemed to have vanished
Allowed to self-fertilize and new seed
grown
F 2 generation included both TALL and
short plants (short trait reappeared)
• Using large sample size recorded
ratio as 3 tall:1 short
Example
P. Parents: TALL and short
• Reappearance of short plants in the
F 2 generation indicated that the
heritable factor for “short” was not
diluted or “blended”or disappeared
but coexisted with the TALL factor
in F 1 hybrids
• Mendel reasoned that the heritable
factor for “short” must be
present in F1 plants, but not
affect height
• Mendel found similar results with crosses for six other characters
• Mendel developed hypotheses to explain these results
consisting of 4 related ideas:
1. Law of unit characters: f actors (alleles), which always
occur in pairs, determine the inheritance of
characteristics
– genes are always at the same position (locus) on chromosomes
- alternative versions of genes (ALLELES) account for variations
in inherited characters
• purple and white alleles are variants of the same gene
For each character an organism inherits two alleles, one
from each parent
– each diploid (2n) organism has a pair of homologous chromosomes
and therefore 2 copies of each allele
– one set of chromosomes is inherited from each parent
– may be identical, as in the true-breeding plants of the P generation
(homozygous)
– Or, the alleles may differ
• in the flower-color example, the F 1 plants inherited a purple-
flower allele from one parent and a white-flower allele from the
other (heterozygous)
2. Law of dominance: f or any given pair of alleles, one may
mask expression of the other
- the DOMINANT ALLELE, always determines the
organism’s appearance (or behaviour)
- the other, RECESSIVE ALLELE may have no noticeable
effect on appearance (when combined with a dominant allele)
– Mendel’s F 1 plants had purple flowers because the purple-flower
allele is dominant and the white-flower allele is recessive
3. The 2 alleles for each character segregate (separate)
during gamete production
• segregation of alleles corresponds to the distribution of homologous
chromosomes to different gametes in meiosis
– if an organism has identical alleles for a particular character, all gametes will
have identical alleles
– but, if different alleles are present, 50% will receive one allele and 50% the
other
The overall hypothesis is summarized in:
Mendel’s 3. LAW OF SEGREGATION: every individual carries pairs
of factors (alleles) for each trait and the MEMBERS OF A PAIR
SEGREGATE INDEPENDENTLY during the formation of gametes
• Mendel’s law of segregation provides a mechanism to get
the 3:1 ratio he observed in the F 2 generation
• F 1 hybrids will produce two classes of gametes, 50% the
purple allele and 50% with the white allele
• The gametes of the 2 classes unite randomly
– produces 4 equally likely combinations of sperm and egg
In the F 2 population:
– ¼ will inherit 2 p alleles and
produce white flowers
– ½ will inherit 1 p allele and 1 P
allele and produce purple
– ¼ will inherit 2 P alleles and
also produce purple flowers
Overall: 3 purple : 1 white ratio
A PUNNETT SQUARE ANALYSIS of the flower-color example
demonstrates Mendel’s model
• In the F 2 it is not possible to predict the genotype of an
organism with a dominant phenotype
– must have one dominant allele, but could be homozygous
dominant or heterozygous
• A test cross, breeding a
homozygous recessive
with dominant phenotype,
but unknown genotype,
can determine the identity
of the unknown allele
Segregation of 1 trait –like this is
called a MONOHYBRID CROSS
What happens when you look
at parents differing in two
traits?
Dihybrid cross
• F 1 generation composed of
dihybrids
• Produces 4 kinds of
gametes
– Punnett square used to
determine genotypes of
zygotes
– Dihybrid cross produces
9:3:3:1 phenotypic ratio
• Mendel repeated the dihybrid cross experiment for other
character pairs and always observed a 9:3:3:1 ration in the
F 2 generation
• i.e. each character appeared to be inherited independently
Mendel’s LAW OF INDEPENDENT ASSORTMENT
The 2 alleles of a gene assort (segregate) independently
of the alleles of OTHER GENES
- if you follow just one character, you will observe a 3:1 F 2
ratio for each, just as if this were a monohybrid cross
We now know that this is caused
by both independent
assortment of chromosomes (for
genes on different chromosomes)
and recombination (for genes on
the same chromosome) in meiosis
Mendelian inheritance reflects rules of probability
• Mendel’s laws of reflect the laws of probability that apply
to a coin toss
Mendel discovered the particulate behavior of
genes: review
• While we cannot predict with certainty the genotype or
phenotype of any particular seed from the F2 generation
of a dihybrid cross, we can predict the probabilities that it
will be a specific genotype or phenotype
• Mendel’s experiments succeeded because he counted
enough offspring to discern this statistical feature of
inheritance
• Mendel’s laws apply to all diploid organisms that
reproduce by sexual reproduction
• Mendel’s studies of pea inheritance endure not only in
genetics, but as a case study of the power of logical
scientific reasoning (remember he knew nothing about
DNA etc)
EXTENDING MENDELIAN GENETICS:
In reality the relationship between genotype and
phenotype is rarely simple
• While Mendel's studies hold true in many cases there are
more complex situations which do not precisely follow
Mendel's Laws
• In fact, Mendel was lucky to choose a system that was
relatively simple genetically
– characters controlled by a single gene
– each gene has only two alleles, one completely dominant to the
other
Allele Dominance is NOT always complete
• The heterozygous F 1 offspring of Mendel’s crosses
always looked like one of the parental varieties because
one allele was dominant to the other
• However, some alleles show incomplete dominance
- heterozygotes show a distinct intermediate phenotype,
not seen in homozygotes
– offspring of a cross between heterozygotes will show 3
phenotypes: both parental + the heterozygote
– the phenotypic and genotypic ratios are identical, 1:2:1
• A clear example of incomplete dominance is seen in
flower color of snapdragons
– a cross between a
white-flowered plant
and a red-flowered
plant produces all
pink F 1 offspring
– self-pollination of the
F 1 offspring produces
25% white, 25% red,
and 50% pink offspring
• It is in the biochemical pathways from genotype to
phenotype where dominance and recessiveness come into
play
– e.g. wrinkled seeds in pea plants with two copies of the recessive
allele are due to the accumulation of monosaccharides and excess
water in seeds because of the lack of a key enzyme
• seeds wrinkle when they dry
– both homozygous dominants and heterozygotes produce enough
enzymes to convert all the monosaccharides into starch and form
smooth seeds when they dry
Dominance/recessiveness relationships have 3 important
points
1. They range from complete dominance, though degrees of
incomplete dominance, to codominance
2. They reflect the biochemical mechanisms
by which specific alleles are expressed in the
phenotype
3. They do not determine, or correlate with, the relative
abundance of alleles in a population
• If 2 genes are located close together on the same chromosome they
DO NOT segregate independently
– they are said to be LINKED
• In general the chance of recombination between 2 genes decreases
the closer together they are
Genetic Linkage • Genetic linkage can be used to
make LINKAGE MAPS
• Segregation analysis is used to
follow the inheritance of traits
relative to each other
– can map genes relative to how
frequently recombination occurs
between them
– used in conventional plant breeding
and is used extensively in identifying
genes that control particular traits
Partial Genetic
Map of the Pea
Interactions Between Genes
• Genes may affect each others action
• Some genes interfere with, or contribute to the effects of
others - known as EPISTASIS
• Many traits are controlled by multiple genes
– a trait controlled by multiple genes does not show a clear
difference between groups of individuals they show continuous
variation (quantitative traits)
e.g. Ear length in Corn
• Even if only 2 genes are involved, 5 different phenotypes
are possible
• When more genes are involved it becomes impossible to
draw boundaries between phenotypes - continuous variation
Cytoplasmic Inheritance
• Plastids have their own genomes which encode for many
of their functions
• Plastids are only inherited maternally
(mitochondria and chloroplasts have genomes)
• A number of important plant traits are
associated with plastids:
- cytoplasmic male sterility (a mitochondrial trait)
used for hybrid seed production in maize, onions, carrots, beets
and petunias
-leaf variegation (often a chloroplast trait): variegation in Coleus
and Hostas is a result of mutant (non-green) chloroplasts
• Emphasizing single genes and single phenotypic characters
is an inadequate perspective on heredity and variation
– A more comprehensive theory of Mendelian genetics must view
organisms as a whole
– “Phenotype” has been used to this point in the context of single
characters, but is also used to describe all aspects of an organism
– “Genotype” can refer not just to a single genetic locus, but also to
an organism’s entire genetic makeup
An organism’s phenotype reflects its overall genotype + unique
environmental history
- especially important in plants!
Phenotype is a result of the genotype interacting
with the environment
• Phenotype depends on genes + the environment
– a single tree has leaves that vary in size, shape, and greenness,
depending on exposure to wind and sun
– important factors include nutrition, temperature, ion concentrations
Extreme example: the water buttercup
2 forms of leaf genetically identical
"normal" leaves on the surface
divided almost root-like under water
Key Points
1. Mendel’s Laws:
• Law of segregation
• Law of independent assortment
2. Understand the meaning of the terms:
• Allele, recessive, dominant, phenotype, genotype, homozygous,
heterozygous, genetic linkage, epistasis
3. Punnet Square Analyses
• Understand how they work and are used to calculate genotype probabilities
4. Inheritance of cytoplasmic genomes - how is it different?
5. Understand why genotypes and phenotypes are different things?

Created by: abigail.phipps

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