Transcript Ch8IntrotoGenetics

Chapter 8
Introduction to Genetics
The Work of Gregor
Mendel
 What is Genetics? the
study of heredity
Gregor Mendel’s Peas
 Pollen: plant’s sperm
 Egg Cells: plants
reproductive cells
 Fertilization: joining of
pollen + egg cells 
develops into embryo in a
seed
Born in 1822.
His work with pea
plants laid the
foundation for
Genetics.
Working with pea plants…
 Self-pollinating: pollen fertilizes egg cells in the
SAME flower (single parent reproduction)
 True-breeding: offspring genetically identical to
parents due to self-pollination
 Cross-pollination: combining reproductive cells
from 2 DIFFERENT parent plants
Mendel could crossbreed a purple flower
with a white flower…
What do you think is
the color of the
offspring?
Genes and Dominance
 TRAIT: specific characteristic (seed
color, plant height, etc)
What did Mendel do in his pea plant
experiments?
 Studied 7 different traits each with
contrasting characters (ex) Height, short
or tall
 He crossed the plants (with contrasting
characters) and looked at their offspring
P = parental generation = original pair of plants
F1 = first filial generation= first generation
Hybrids: offspring from parents with different
traits
Tracking Generations
 Parental generation
P
mates to produce
 First-generation offspring
F1
mate to produce
 Second-generation offspring
F2
CROSS-POLLINATION:
Mendel cut the male parts of one
flower (ouch!) and dusted the
female parts with pollen from
another flower.
P GENERATION: purple x
white flowers
F1 GENERATION: all
purple flowers
HYBRID
PLANTS
What happened in Mendel’s crosses?
All the offspring only had one of the
parent’s characters…the other parent’s
character disappeared!!
Mendel’s Conclusions:
1.
2.
3.
Inheritance is determined by factors that
are passed down
GENES: the factors that determine traits
Contrasting characters are different
forms of a gene called ALLELES
Mendel’s Principle of Dominance
 some alleles are dominant, some are recessive
 DOMINANT ALLELE: form of trait that will
always be exhibited; usually expressed in
capitals
 RECESSIVE ALLELE: form of trait is only
exhibited when the dominant allele is NOT
present
 (ex) Allele for tall is dominant for and the allele
for short is recessive
What happened to the
recessive allele?
 Mendel wanted to
know if the
recessive allele
disappeared from
the F1 plants.
 F1 CROSS: He
self-crossed the F1
generations to
make F2 offspring
THE F2 GENERATION…
 The recessive traits reappeared!!
~¼ plants had white flowers, the recessive trait
 Summary of Crosses:
tall plants X short plants  tall plants
P
P
F1
tall plants self-pollinating  ¼ short, ¾ tall
F1 Cross
F2
Explaining the F1 Cross
 Why did the recessive allele reappear? At some
point, the recessive allele had to separate from the
dominant allele. This is called…
 SEGREGATION: separation of alleles 
occurs during formation of gametes (eggs & sperm) in
anaphase II of meiosis
 F1 plants inherited 1 tall allele & 1 short allele from parents
 When gametes are formed, the two alleles segregate from
each other  each gamete has 1 copy of each gene
 So, 2 different types of gametes are formed (one w/ tall
allele, one w/ short allele)
SEGREGATION
Probability & Punnett Squares
 Mendel realized that the principles of
probability can explain the results of
genetic crosses.
 PROBABILITY: likelihood an event
will occur (ex) Flip coin 3x in a row,
1/8 chance it will be heads all 3 times
(½x½x½)
 The pattern in which alleles segregate
is random…just like a coin flip! So
which ever allele gametes receive is
also random.
Punnett Squares
What is a Punnett Square?
 A diagram showing the possible genetic
combinations from a particular cross
 Can be used to predict and compare the genetic
variations that will result from a cross
What do the letters represent in a punnett square?
 Letters represent alleles: capital = dominant
lowercase = recessive
 Homozygous: has two identical alleles for a trait
(ex) TT or tt
 Heterozygous: has two different alleles for the
same trait (ex) Tt
Punnett Square
for TT x Tt
Punnett Square
for YY x yy
Genotype vs Phenotype
 GENOTYPE: the genetic
makeup of an organism
(ex) TT
 PHENOTYPE: the
physical characteristics
exhibited (ex) tall plant
In the Punnett Square shown
What is the genotype of the
offspring?
What is the phenotype?
Probability and Segregation
 F2 generation from
Tall F1 plants 
¾ tall, ¼ short
 3:1 ratio of tall to
short plants
Punnet squares work
to predict outcomes,
so Mendel’s ideas
about segregation are
accurate!
Exploring Mendelian Genetics
 Does the gene that determines flower color
have anything to do with the gene for
height?
(ex) Do all tall plants have purple flowers?
 Mendel performed TWO-FACTOR CROSSES:
crossing 2 different genes and following traits
as they pass from one generation to the next
Two-Factor Cross: F1
 Two Genes: shape of pea & color of pea
 The Cross: Round yellow peas x wrinkled green peas
RRYY




x
rryy
What are the possible alleles parent 1 can pass? RY
What are the possible alleles parent 2 can pass? ry
Draw a Punnett Square for this cross.
All F1 were RrYy (round and yellow) or HYBRIDS
This cross does not answer question, but provides
hybrids for next cross
Two-Factor Cross: F2
 F1 Generation = RrYy
 How would these alleles segregate when F1 self-
pollinated?

RrYy x RrYy
 Do the two dominant alleles stay together? NO
 What are the possible alleles each parent
can pass on?

There are 4 possible combinations: Ry, RY,
rY, ry
 Draw a Punnett Square for this cross.
INDEPENDENT
ASSORTMENT
 The F1 Hybrid cross
produces a 9:3:3:1
phenotype ratio
 Mendel found that
the 2 alleles (seed
shape & seed color)
don’t influence each
other’s inheritance
 This is called the
principle of
Independent
Assortment: genes
for different traits can
segregate
independently during
the formation of
gametes
Independent Assortment
OR
Metaphase I:
A
A a
a
A
A a
a
B
B b
b
b
b B
B
Metaphase II:
Gametes:
A
A
a
a
A
A
a
a
B
B
b
b
b
b
B
B
B
A
B
A
1/4 AB
b
a
b
a
1/4 ab
b
A
b
A
1/4 Ab
B
a
B
a
1/4 aB
Summary of Mendel’s
Principles
A.
Inheritance of characteristics is determined by
genes which are passed to offspring
B.
If 2+ alleles of a trait exist, some alleles may be
dominant, others may be recessive
C.
Sexually reproducing organisms have 2 copies of
each gene which segregate during gamete
formation
D.
Alleles for different genes segregate
independently
Beyond Dominant and Recessive
Alleles
 Genetics is more complicated
 Some alleles are neither dominant
nor recessive
 Many traits are controlled by
multiple alleles or multiple genes
Other Inheritance Patterns…
1. Incomplete Dominance
2. Codominance
3. Multiple Alleles
4. Polygenic Traits
Incomplete
Dominance
Homozygous
parent (RR)
X
Homozygous
Parent (rr)
 When one allele is not
completely dominant;
recessive allele is not
totally masked
All F1 are
heterozygous
 Heterozygous phenotype
is in between the two
homozygous phenotypes

X
(ex) Red snapdragon
flowers (RR) X
snapdragon white (rr)
flowers  pink hybrid
flowers (Rr)
F2 shows three phenotypes in 1:2:1 ratio
Incomplete
Dominance
homozygous parent X homozygous parent
All F1 offspring
heterozygous for
flower color:
Cross two of the F1
plants and the F2
offspring will show
three phenotypes in
a 1:2:1 ratio:
Codominance
 Both alleles contribute to the phenotype
 Heterozygous genotype expresses both
phenotypes


(ex) Feather colors in chickens: white feathers X
black feathers  speckled chicken
(ex) Horse coats: red X white roan coat
Codominance:
ABO Blood Types
 Alleles that controls blood type
are codominant
 Two alleles A & B are both
exhibited when paired, a third
allele (i) is recessive to others




AA or Ai = Type A Blood
BB or Bi = Type B Blood
AB = Type AB Blood
ii = Type O Blood
Multiple Alleles
 > 2 possible alleles for a gene
 Individuals can still only have 2 alleles each
but more than 2 alleles exist in a population


(ex) coat color in rabbits  lots of options
due to 4 different alleles
(ex) blood type is determined by multiple
alleles
Polygenic Traits
 Traits controlled
by the interaction
of 2+ genes

(ex) Fruit fly eye color
(3+ different genes)

(ex) Skin color in
humans (4+ different
genes), eye color,
height, weight