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Mendel’s Legacy
Genetics is everywhere these days – and it will continue as a dominant force in
biology and society for decades to come.
Wouldn’t it be nice if people understood it better?
The Fundamental Question
What is the relationship between genes (genotype) and observable
characteristics (phenotype)?
The answer?
Phenotype = Genotype + Environment.
Genes and Environment Determine Characters
Genetically identical hydrangeas growing in soils of different acidity
(different environments).
The phenotype = genotype + environment principle applies equally to
human traits.
A Mendelian Genetic Primer
Genes come in pairs that separate in the formation of gametes.
The members of the pair may be identical (homozygous) or non-identical
(heterozygous).
Each form of a particular gene is an allele.
A Mendelian Genetic Primer
One allele is dominant over another (or so Mendel
believed).
Only two alleles of a given gene are possible in an
individual although many alleles of a gene are possible
within a population.
How did Mendel assist us in coming to these
underatandings?
Why was Mendel’s use
of the Garden Pea ideal?
• They were
commercially
available.
• Easy to grow and
matured quickly.
• Enclosed sex organs
in the flower.
• ***Seven different
traits with ONLY two
possible variations.
The Reality of “Round and Wrinkled” – Two Alternative Traits of
the Seed Shape Character
Note: each seed is a
new individual of a
different generation
– seeds are not of
the same generation
as the plant that
bears them.
Mendel’s Monohybrid Cross
– P to F1
• Mendel created purebred
plants for each of the traits he
studied and called them the
Parent generation (P).
• Crossing these parents he
created the First Filial (F1)
generation, that all possessed
the same trait variation!!!
• WHY??????
• Mendel then continued crosses
to the F2 (second filial, the
grandchildren).
What was learned?
The green trait was not lost or
altered, even though it
disappeared in the F1.
What this meant???
• One trait must have been
dominant to the other in
the expression of its trait.
What do the numbers tell???
• Calculating the reappearance of the recessive trait in
only ¼ of the F2 suggests that the genes must come in
pairs which separated in the formation of sex cells.
• These genes from different sex cells must then reform a
new pair of genes in the new individual.
• NOTE: During Mendel’s time there was no knowledge
of DNA/genes, so Mendel called them inheritance
factors.
Monohybrid Crosses and the Principle of Segregation
A cross between individuals differing
in a single character is a monohybrid
cross.
The analysis of monohybrid
crosses allowed Mendel to deduce
the Principle of Segregation ....
Genes come in pairs that separate
in the formation of sex cells (and
these sex cells unite randomly at
fertilization).
Principle of Segregation Demystified
NOW WE KNOW:
The principle of segregation is explained by the
behavior of homologous chromosomes at meiosis.
Segregation
A Punnett Square is a Handy Way of Analyzing Crosses
In a Punnett square for a monohybrid cross,
the Principle of Segregation is applied.
Possible offspring
Consistency
is Good
Characters
investigated by
Mendel
No matter what
the character,
Mendel observed
a 3:1 ratio of
characters in the
F2.
Monohybrid Crosses Yielded Consistent Results
Therefore, the Principle of Segregation indeed
is a general principle of genetics.
What Works for Peas Also Works for Humans
An albino woman
In the cross Aa x Aa, where A is a
dominant allele for wild type (standard)
pigmentation and a is a recessive allele
for no pigmentation (albinism), ¾ of
offspring will be wild type and ¼ will be
albino.
Do this monohybrid cross
• In pea plants, round seeds are dominant to
wrinkled seeds. If a homozygous round seed
plant is crossed with a heterozygous round
seed plant, what is the expected phenotypic
and genotypic ratios in the F1 ?
Answer
R R
R
r
RR RR
Rr Rr
Phenotypic Ratio: All Round
Genotypic ratio: 2 RR : 2 Rr
P – RR x Rr
Alleles produced for gametes are R or R from one parent
and R or r from the other parent
Try This one !
Fruit fly wing length is controlled by a
dominant allele for long wings (L), short is
recessive. If a heterozygous long winged fly
is mated to a homozygous short winged fly,
what is the expected phenotypic and
genotypic ratio in their offspring?
• Do both sheets from webpage.
– Monohybrid Practice
– Spongebob genetics
Then Pg, 535 # 10,11,12 and 14.
Understand these concepts
pg 529-530
• Principle of Dominance –
• Law of Segregation –
• Unit Theory of Inheritance –
Probability & Product Rule
• Probability – is the chance of an event
occurring, usually represented by a ratio.
• Product Rule – states the chance that two or
more independent events will occur
TOGETHER.
– This is calculated by finding the product of
their individual probabilities occurring alone.
Flip a coin
• Probability of Heads?? - -
• ½ (50%)
• Probability of flipping Two (2) heads Together?
Try this
• What is the probability of rolling a regular die
and getting a 6?
• What is the probability of rolling a pair of dice
and getting “snake-eyes” (a pair of 1’s)?
Test Cross
• To determine the UNKNOWN genotype of
an organism that shows the dominant
phenotype, we perform a TEST CROSS.
• This is done by taking the organism with
the unknown genotype (TT or Tt) and cross
it with a purebred (homozygous) recessive
and look at the results.
• If any of the
offspring show
the recessive
condition then
the unknown
parent MUST
have been
heterozygous.
Extensions to Mendel’s analysis
• In Single-gene inheritance: – Sometimes pairs of alleles show deviations from complete
dominance and recessiveness
– Sometimes more than two different variations (alleles) exist
for a gene
– Sometimes one gene may determine more than one trait
• In Multifactorial inheritance:
• the phenotype expressed arises from the
interactions between one or more genes, the
environment, and chance events.
Dominance is not always complete
• Crosses between true-breeding strains can produce hybrids with phenotypes different from both parents
• Ex.
• Incomplete Dominance
• Co-Dominance
• Multiple Alleles
Incomplete dominance
• This occurs in the case where two pure breeding (homozygous) parents that show two completely different phenotypes have F1 hybrids that show a completely different phenotype from either parent.
• The F1 expresses an intermediate phenotype. Neither allele is dominant or recessive to the other.
• Phenotypic ratios are same as genotypic ratios
Codominance
• Co-Dominance occurs when parents have different traits and the alleles for these traits are equally dominant to the other. The F1 hybrid offspring of a cross between two purebreds will express the phenotype of both parents equally.
• Phenotypic ratios are same as genotypic ratios
Multiple Alleles
• In many species a gene may have more than
two possible alleles for any given gene.
– Ex. Human Blood Groups (A, B, and O)
– (A & B are co-dominant and O is recessive)
There Are Often More Than Two Alleles of a Gene
Blood groups are determined by one gene with three alleles.
Also that the blood groups show both complete dominance and
codominance.
Multiple Alleles Can be Grouped into a Dominance Series
Dominance series for lentil bean coat color.
Try:
-Practice Problem from Pg 542
-Thinking Lab pg 543.
- Then pg 544 - #2, 5, 6 a & b,
Do variations on dominance relations
negate Mendel’s law of segregation?
• Dominance relations only affect the relationship between genotype and phenotype
• Alleles still segregate randomly and unite randomly
• Gene products (plus external factors) control the expression of phenotypes
What about Multiple traits?
• Does being tall (dominant) give you more
of a chance at also being yellow (dominant).
• Mendel performed a series of experiments
like before, but tracked two different traits
at the same time.
• Two purebred
parents when
crossed still
produced a
heterozygous F1
• Note: Notice the
numbers for the
phenotype and
genotype of the
F2
• Anything similar?
Step by step!!
• #1 - Make a legend
• #2 – Show parents 4 dashes times 4 dashes.
• ____ ____ ____ ____ x ____ ____ ____ ____
– NOTE- Remember two dashes for each trait!!
• #3 Perform “FOIL” on each parent to make
gametes.
• #4 make your Punnett square.
• Tip- Keep Same letter together in the boxes
and always put the capital letter first (when
possible)