Transcript Mendelian Genetics

Genetics
Gregor Mendel
Gregor Johann Mendel
►
Gregor Mendel In 1865 turned the study
of heredity into a science
►
His work was so brilliant and
unprecedented at the time it appeared
that it took thirty-four years for the rest
of the scientific community to catch up to
it.
►
His short monograph, "Experiments with
Plant Hybrids," in which Mendel described
how traits were inherited, has become
one of the most enduring and influential
publications in the history of science.
Theory of Genetic Blending
► Before
Mendel’s work, plant and animal breeders
based their work on the theory of “Blending”.
► This
theory states that parents with different traits
produced offspring of intermediate appearance.
 Example: Red X White flowers = Pink flowers
► When
red or white flowers reappeared in future
generations it was due to the instability in the
genetic material
Mendel’s Experimental Approach
► Mendel
experimented
with pea plants
► This was a great model
to do genetics on




Self fertilizing
Cross fertilize
Breed true
Clear and contrasting
traits
Genetic terms
► Genes:
segments of
DNA that provide the
information for traits
and characteristics
► Locus: The location of
a specific gene on a
specific Chromosome
► Allele: alternative
forms of a gene.
Homologuous
chromosomes
Locus, the location
for a specific gene
A pair of alleles
Three pairs of genes
or
Three pairs of alleles
Terms cont.
► Diploid
is a 2n organism with two complete
sets of genetic information.
► Haploid is a 1n organism or cell with a
single complete set of genetic information
► Homozygous is when both alleles for a trait
are the same
► Heterozygous is when the alleles for a
particular trait are different.
Terms cont.
►
Dominant alleles: An allele that is expressed and
hides or masks “Recessive” alleles.
 Symbolizes as a capital letter.
 Example “AA” in homozygous dominant.
►
Recessive allele: An allele that is expressed in the
homozygous form only.
 Symbolized as a lower case letter
 Example: “aa” in homozygous recessive.
Terms cont.
► Genotypes:
The actual alleles that are present in
an individual.
 Example:
► Homozygous
dominant = AA
► Heterozygous = Aa
► Homozygous recessive = aa
► Phenotype:
is the observable characteristic. It is
the combination of genes and environment
Terms cont.
►P
=
Parental generation
► F1
=
The first generation
► F2
=
The off-spring from the mating of
the first generation together
Mendel’s Monohybrid experiments
► Realizing
that the blending theory didn’t fit
with what he was observing Mendel
developed an alternative hypothesis
► Hypothesis:
He said “Genes are particulate
factors that passes unchanged from parent
to progeny unchanged”.
► Some traits mask or hide other traits
Mendel’s Theory of Segregation
►
Monohybrid cross
 Crossing parents to
observe a single trait in
the off-spring
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Gene segregation
 Each parent randomly
contributes one set of
genetic information to the
offspring
Mendel’s Monohybrid Cross
Mendel’s Monohybrid Cross
Probability and Punnet Squares
►A
possibility of outcomes and crosses
► Test crosses
 Unknown genotype crossed with homozygous
recessive
Independent Assortment
►
Do genes assort randomly in the gamets?
 Do some genes prefer the company of other
genes?
► Example:
If one is good at music are also
likely to be good a math as well?
AABB
purpleflowered
tall parent
(homozygous
dominant)
AB
aabb
whiteflowered
dwarf parent
(homozygous
recessive)
ab
X
F1 OUTCOME: All F1 plants purple-flowered, tall
(AaBb heterozygotes)
AaBb
AaBb
meiosis, gamete formation
1/4
AB
1/4
Ab
1/4
aB
1/4
ab
1/4
AB
1/4
Ab
1/4
aB
1/4
ab
ADDING UP THE F2 COMBINATIONS POSSIBLE:
1/16
1/16
1/16
1/16
9/16 or 9 purple-flowered, tall
AABB AABb AaBB AaBb
1/16
1/16
1/16
1/16
AABb AAbb AaBb Aabb
1/16
1/16
1/16
3/16 or 3 purple-flowered, dwarf
3/16 or 3 white-flowered, tall
1/16 or 1 white-flowered, dwarf
1/16
AaBB AaBb aaBB aaBb
1/16
1/16
1/16
1/16
AaBb Aabb aaBb aabb
Possible outcomes of cross-fertilization
Fig. 11.9, p. 181
Mendel’s Theory of Segregation
► Diploid
cells have pairs of genes or
homologous chromosomes
► During
meiosis the two genes segregate and
wind up in different gametes
Independent Assortment
Theory in Modern Form
► Independent
Assortment
 Gametes require genes independently of
how other pairs of genes were sorted out
► Variety
of Offspring
Dominance Relation
► Incomplete
Dominance
 Red snapdragon crossed with white snapdragon
---------> Pink F1
► Codominance
 Multiple allele system
►ABO Blood types