Transcript Fundamentals of Genetics

Fundamentals of
Genetics
Chapter 9
Mendel’s Legacy
Section 9.1
Genetics
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The field of biology dedicated to
understanding heredity…
Heredity- the passing of traits from parents to
offspring
Was founded by the work of Gregor Johann
Mendel, the “Father of Genetics”
Gregor Mendel
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Austrian monk &
science teacher
Studied heredity and
statistics
Worked with pea
plants in his garden
Mid 1800s
Mendel’s Peas:
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Studied 7 characteristics of
peas
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Each characteristics had 2
different traits
ex) flower color
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Mendel’s Experiments:
1. Started off with 2 pure (true breeding) plant
groups for a particular characteristic
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P generation (parent)
2. Cross pollinated these plant groups (sexual
reproduction) to produce the next generation of
plants
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F1 generation (first filial)
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he counted & recorded the traits of the offspring
3.
Then he allowed the F1 generation plants self pollinate
(sexual reproduction with self) to produce the next
generation of plants
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F2 generation (second filial)
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he counted & recorded the traits of the offspring.
Mendel’s Results:
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The F1 generation always showed only 1 of the 2 traits for
the characteristic
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Mendel named this the dominant factor
The F2 generation always showed a 3:1 (or a 75% to a
25%) ratio between the 2 traits
–
Mendel named the second the recessive factor
Purple is dominant
White is recessive
F1
F1
F2
Notice that the predicted ratios & the actual ratios
are not exact!
Mendel’s Laws of Inheritance
1) Law of Segregation
•Mendel stated that… a
pair of factors is
segregated, or separated,
during the formation of
gametes.
•So, What does this mean?
Each egg and sperm
receives only one factor
from each parent.
2. Law of Independent Assortment
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Mendel also stated that … factors for
different characteristics are distributed to
gametes independently.
So, What does this mean?
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The factors for different characteristics are not
connected.
Molecular Genetics
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The study of the structure & function
of chromosomes & genes
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Allele: alternate form of a gene
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Mendel called them “factors”
Abbreviations:
 Dominant
allele = capital letter (B)
 Recessive allele = lower case letter (b)
Genetic Crosses
9.2
Vocab:
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Genotype: gene combination for a trait (BB, Bb,
bb)
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Phenotype: the physical feature resulting from a
genotype (Black, white)
Genotype Vocab:
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Homozygous: “same genes”
When the organism has the same alleles for the characteristic
(also called pure)
Ex.) BB = homozygous dominant
bb = homozygous recessive
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Heterozygous: “different genes”
When the organism has different alleles for the characteristic
(also called hybrid)
Ex.) Bb
–
Examples:
Genetic Probability:
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The likelihood (probability) of offspring of known
parents can be determined by Punnett squares
Types of Genetic Crosses:
1.
Monohybrid Crosscross involving a
single trait
ex.) flower color
2) Dihybrid Cross
Cross involving 2
traits
Let’s Practice Punnett Squares!
1.
2.
Monohybrid Cross
Dihybrid Cross
How do you find an unknown genotype?
Testcross
 Example) Is the red flower a pure
(RR) or hybrid (Rr) ?
 Cross the unknown flower with a
pure recessive flower (rr).
 This will allow the recessives to show
up in the next generation.
The results:
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If the flower is hybrid, the
Result if flower is hybrid
offspring will be 50% red and
50% white.
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If the flower is pure, the
offspring will be all red.
Result if flower is pure
Complications!
Incomplete Dominance and Codominance
1)
Incomplete Dominance: occurs when the F1
offspring has a phenotype between that of the
parents.
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neither allele is completely dominant
The flowers may be red or
white in the purebred
form, but the hybrid
form of the flowers is
pink.
Q- What would happen if
you crossed a red four
o’clock flower with a
white four o’clock
flower?
A- a pink flower!
2) Codominance
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Occurs when both alleles are expressed in a
heterozygous offspring.
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Neither allele is dominant or recessive
Example)
Q- What would happen if you crossed a horse with a white
coat (rr) with a horse with a red coat (RR)?
A- A horse with a roan coat (Rr) ~ both red and white hairs
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Question:
In rabbits, the allele for black coat color (B) is
dominant over the allele for brown coat color
(b). Predict the results of a cross between a
rabbit heterozygous for black coat color and a
rabbit homozygous for brown coat color.
What is the genotypic ratio?
What is the phenotypic ratio?
Question:
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Albinism is a recessive disorder (aa) that results
in a lack of pigment in the eyes, skin, and hair.
What are the genotypes and phenotypes of the
parents if they have 2 children with albinism and
2 children normally pigmented.
Use a Punnett square to complete this problem
Brown hair color is dominant over blonde hair,
and brown eyes are dominant over blue eyes.
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Q- If 2 heterozygous parents for both traits
mate, what is the phenotypic ratio of their
offspring?
__________ Brown hair, Brown eyes
__________ Brown hair, Blue eyes
__________ Blonde hair, Brown eyes
__________ Blonde hair, Blue eyes
Incomplete Dominance Q:
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Scottish fold cats are heterozygotes whose ears
fold down onto their heads several days after
birth. The homozygote genotype, rr, for this trait
causes death. Construct a Punnett square to
determine the genotypic and phenotypic ratios of
a cross between 2 cats with folded ears.
B. Complications: Multiple alleles
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ABO blood groups
Dominant
Dominant
Codominant
Recessive
C. Complications: Pleiotropy
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- One gene affects
many characters
- Sickling allele of
hemoglobin
D. Complications: Polygenic Inheritance
and Quantitative Characters
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- One trait
determined by
multiple genes
- Converse of
pleiotropy
- e.g., skin color: at
least 3 genes
Complications:
Epistasis
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- Expression of one
gene depends on
another
- Mouse coat color:
B - black coat
b - brown coat
C - pigment
c - no pigment
What are my chances?
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3.
Obtain 2 pennies.
Flip each coin simultaneously 50 times.
Record the results of each set of flips.
Total the number of combinations of
your results:
__ Heads-Heads
__ Heads-Tails
__ Tails-Tails
4.
Record your results on the board.
5.
Calculate your percent chance of each
combination.
(# of combos)
50
6.
Calculate the class average percent
chance of each combination.