Transcript Document 7375951

Fundamentals of
Genetics
Chapter 9
Genetics
 The field of biology dedicated to
understanding how characteristics are
transmitted from parent to offspring.
 Studied by Gregor Mendel
 Heredity- the transmission of
characteristics from parents to offspring.
Gregor Mendel
 Austrian monk &
science teacher in the
mid 1800’s
 Worked with pea
plants in his garden
 Studied 7
characteristics of peas
 Each characteristics
had 2 different traits
EX) tall/short,
purple/white
 Page 176
Mendel’s Experiments:
1. Started off with 2 pure plant groups for
a particular characteristic
 P1 generation
2. Cross pollinated these plant groups
(sexual reproduction) to produce the
next generation of plants
 F1 generation
3. When the seeds matured from the F1
generation, he counted & recorded the
traits of the offspring.
4. Then he allowed the F1 generation
plants self pollinate (sexual
reproduction with self) to produce the
next generation of plants

F2 generation
(notice that the predicted ratios & the
actual ratios are not exact!)
Mendel’s Results:
 The F1 generation always showed only 1
of the 2 traits for the characteristic
 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
Example:
Purple is dominant
White is recessive
F1 generation:
all purple
F2 generation:
75% purple
25% white
The 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.
The Law of Independent
Assortment
 Mendel also stated that … factors for
different characteristics are distributed to
gametes independently.
 So, What does this mean?
 The factors for different characteristics are
not connected.
Human traits
 link
Molecular Genetics
 The study of the structure & function of
chromosomes & genes
 Allele: alternate form of a gene
 Mendel called them “factors”
 Abbreviations:
 Dominant allele = capital letter
 Recessive allele = lower case letter
(B)
(b)
9.2 Vocabulary:
 Genotype: the genetic makeup of the
organism (BB, Bb, bb)
 Phenotype: the physical appearance of
the organism (Black, white)
Genotype Vocabulary:
 Homozygous: “same genes”
 When the organism has the same alleles for
the characteristic
 BB = homozygous dominant
 bb = homozygous recessive
 Heterozygous: “different genes”
 When the organism has different alleles for
the characteristic
 Bb
Genetic Probability
 The likelihood
(probability) of
offspring of
known parents
can be
determined by
Punnett squares
Punnett Squares: diagrams biologists use to
predict the probability that certain traits will be
inherited by offspring.
3 Types:
Monohybrid Cross – One characteristic
Dihybrid Cross – Two characteristics
Trihybrid Cross – Three characteristics
Monohybrid Cross
A cross between
individuals that
involves one pair
of contrasting
traits.
 1 characteristic
 2 contrasting traits
 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 homozygous for black
coat color and a rabbit homozygous for
brown coat color.
What is the genotypic ratio?
What is the phenotypic ratio?
 Brown hair color is dominant over blond
hair, and brown eyes are dominant over
blue eyes.
 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
__________ Blond hair, Brown eyes
__________ Blond hair, Blue eyes
Dihybrid Cross
A cross between
individuals that involves
2 pairs of contrasting
traits.
• 2 characteristics
• 4 contrasting traits
•Assumes the genes
aren’t linked
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:
 If the flower is hybrid, the
offspring will be 50% red
and 50% white.
Result if flower is hybrid
 If the flower is pure, the
offspring will be all red.
Result if flower is pure
Complications!
1) Incomplete Dominance: occurs when the
F1 offspring has a phenotype between
that of the parents.
 neither allele is completely dominant
Example) Incomplete Dominance
 Four o’clock flowers
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
 Occurs when both alleles are expressed in
a heterozygous offspring.
 Neither allele is dominant or recessive
Q- What would happen if you crossed a horse
with a white coat with a horse with a red coat?
A- A horse with a roan coat (both red and white
hairs) !
Codominance
Ex) ABO blood groups
Dominant
Dominant
Codominant
Recessive
3) Epistasis
 occurs when the alleles of one
gene cover up or alter the
expression of alleles of
another gene
Ex) Mouse coat color:
 B - black coat
 b - brown coat
 C - pigment
 c - no pigment
4) Polygenic Inheritance
 One trait
determined by
multiple genes
 Ex) skin color: at
least 3 genes