Chapter 3 Section 1
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Transcript Chapter 3 Section 1
Chapter 3 Section 1
Mendel’s Work
Gregor Mendel
Young priest that worked in the
garden at a monastery in
Vienna.
Considered the “Father of
Genetics”
Observed traits in hundreds of
pea plants.
Mendel’s Work - Vocabulary
Traits
Heredity
Physical characteristics of organisims
Passing of traits from parent to offspring
Genetics
Scientific study of heredity
Mendel’s Peas
Why was Gregor Mendel lucky he chose
peas to study?
Many traits exist only in two forms
Peas produce many offspring in one
generation.
How did Mendel make his
crosses?
Tall Plant
Short Plant
Mendel’s Experiments
Purebred Plant
One that always produces offspring with the
same form of a trait as the parent
i.e. purebred short plants will only produce
short offspring.
Purebred pink plants will only produce pink plants
X
White flower would mean the parents are not purebred
X
Mendel’s Experiments
Mendel crossed plants with opposite forms of
a trait
i.e. short plants crossed with tall plants
Look at Figure 2 on page 82
Fig 2 page 82
What did Mendel get
when he crossed
purebred tall plants with
pure bred short plants?
All Tall plants
What did Mendel get
when he let the F1
plants self pollinate?
75% tall and 25%
short.
Mendel’s Experiments
Mendel studied stem height followed by six
other “opposite” traits.
Look at Figure 3 on page 83.
What other “opposite” traits did Mendel
study?
Genes and Alleles
Genes
Factors that control traits
Alleles
Different forms of a gene
One allele is inherited from each parent.
Alleles can be dominant or recessive
Dominant and Recessive Alleles
Dominant allele
Always seen if present
Indicated by a capitol letter (T)
Recessive allele
Masked or hidden if dominant allele is present
Indicated by a lower case letter (t)
Hybrid vs Purebred
Hybrid
Organism has two different alleles for the trait – one
dominant and one recessive.
Purebred
Organism has two identical alleles for a trait – two
recessive OR two dominant.
Alleles are represented with capitol and
lower case letters.
T = Tall allele (dominant)
t = Short allele (recessive)
Purebreds
TT – two dominant alleles – plants will be tall
tt – two recessive alleles – plants will be short
Hybrid
Tt – one dominat allele and one recessive allele –
plants will be tall.
Phenotypes and Genotypes
Phenotype
Genotype
Physical appearance or visible trait
Example is flower color, stem height etc.
The genetic makeup or allele combination
Examples are TT or Tt
Genotypes determine phenotypes
Homozygous and Heterozygous
Homozygous
An organism that has two identical alleles for a trait
TT – homozygous dominant
tt – homozygous recessive
Heterozygous
An organism that has two different alleles for a trait
Tt – heterozygous – will show the dominant trait
Consider a rabbit with black fur and the allele combination (Bb).
What is the rabbit’s phenotype for fur color?
Black
_________
What is the rabbit’s genotype for fur color?
(Bb)
__________
Is the rabbit heterozygous or homozygous for the
heterozygous
fur color trait? _______________
If black is dominant over white, what genotype
(bb)
would produce a white rabbit? _______
If fur color in rabbits was a codominant trait
instead of dominant/recessive trait, what color fur
White and black
would the (Bb) rabbit have? ________________
Probability and Genetics
Chapter 3 - Section 2
Probability
Probability
The likelihood that a particular event
will occur
Example: coin toss
Punnett Squares
Punnett Squares
chart that shows all possible
combinations of alleles that can
result from a genetic cross. Used
to determine the probability of a
particular outcome.
Using a punnett square
Tt
(Tall)
t
T
T
Tt
(Tall)
Tall
Tall
TT
Tt
Tall
Short
t
Tt
tt
Mendel’s Work Review Worksheet
What trait in pea plants is being studied in
the cross above?
Plant height (short/tall)
What are the two alleles for this trait?
Tall (T)
Short (t)
Which allele is the dominant allele? Explain
how you know.
Dominate allele is Tall (T).
Know because all offspring in F1 are tall.
Mendel’s Work Review Worksheet
Which allele is the recessive allele? Explain
Recessive is short (t)
Know because this trait is hidden in the F1
What alleles do the F1 offspring have? Explain
which allele was inherited from which parent.
All F1 offspring are hybrids with the alleles
(Tt)
(T) was inherited from the tall parent and
(t) was inherited from the short parent.
Codominance
Codominance – alleles are neither
dominant or recessive therefore both
alleles are expressed in the offspring
Codominance
The heterozygous offspring will express (show) both phenotypes