Download Lab 9: Mendelian Genetics

Lab 9: Mendelian Genetics
BIOL 1107
Spring 2015
Japhia Jacobo
Mendel used the scientific approach to identify
two laws of inheritance
• Mendel discovered the basic principles of
heredity by breeding garden peas in carefully
planned experiments
Figure 14.2
TECHNIQUE
1
2
Parental
generation
(P)
3
Stamens
Carpel
4
RESULTS
First filial
generation
offspring
(F1)
5
Figure 14.2a
TECHNIQUE
1
2
Parental
generation
(P)
Stamens
3
Carpel
4
Figure 14.2b
RESULTS
First filial
generation
offspring
(F1)
5
• Mendel chose to track only those characters
that occurred in two distinct alternative forms
• Mendel mated two contrasting, true-breeding
varieties - hybridization
• The true-breeding parents - P generation
• The hybrid offspring of the P generation - F1
generation
• F2 generation- When F1 individuals self-pollinate
or cross-pollinate with other F1 hybrids
Figure 14.3-1
EXPERIMENT
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
Figure 14.3-2
EXPERIMENT
P Generation
(true-breeding
parents)
F1 Generation
(hybrids)
Purple
flowers
White
flowers
All plants had purple flowers
Self- or cross-pollination
Figure 14.3-3
EXPERIMENT
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
F1 Generation
(hybrids)
All plants had purple flowers
Self- or cross-pollination
F2 Generation
705 purpleflowered
plants
224 white
flowered
plants
• purple flower color: dominant trait
• white flower color: recessive trait
Mendel’s Model
• Mendel developed a hypothesis to explain the
3:1 inheritance pattern he observed in F2
offspring
• First: alternative versions of genes account for
variations in inherited characters
• These alternative versions of a gene are now
called alleles
• Each gene resides at a specific locus on a
specific chromosome
Figure 14.4
Allele for purple flowers
Locus for flower-color gene
Pair of
homologous
chromosomes
Allele for white flowers
• Second: an organism inherits two alleles, one
from each parent
• Alternatively, the two alleles at a locus may
differ, as in the F1 hybrids
• Third: if the two alleles at a locus differ,
– The dominant allele - determines the organism’s
appearance
– The recessive allele - has no noticeable effect on
appearance
Mendel’s Laws: #1 The law of segregation
• Fourth: The law of segregation
– the two alleles for a heritable character
separate (segregate) during gamete formation
and end up in different gametes
• This segregation of alleles corresponds to the
distribution of homologous chromosomes to
different gametes in meiosis
Figure 14.5-1
P Generation
Appearance:
Purple flowers White flowers
Genetic makeup:
pp
PP
p
Gametes:
P
Figure 14.5-2
P Generation
Appearance:
Purple flowers White flowers
Genetic makeup:
pp
PP
p
Gametes:
P
F1 Generation
Appearance:
Genetic makeup:
Gametes:
Purple flowers
Pp
1/
1/
2 p
2 P
Figure 14.5-3
P Generation
Appearance:
Purple flowers White flowers
Genetic makeup:
pp
PP
p
Gametes:
P
F1 Generation
Appearance:
Genetic makeup:
Gametes:
Purple flowers
Pp
1/
1/
2 p
2 P
Sperm from F1 (Pp) plant
F2 Generation
P
Eggs from
F1 (Pp) plant
p
3
P
p
PP
Pp
Pp
pp
:1
Useful Genetic Vocabulary
• Homozygous - An organism with two identical
alleles for a character
• Heterozygous - An organism that has two
different alleles for a gene
• Unlike homozygotes, heterozygotes are not
true-breeding
• An organism’s traits do not always reveal its
genetic composition
• Therefore, we distinguish between an organism’s
– Phenotype - physical appearance
– Genotype - genetic makeup
Figure 14.6
3
Phenotype
Genotype
Purple
PP
(homozygous)
Purple
Pp
(heterozygous)
1
2
1
Purple
Pp
(heterozygous)
White
pp
(homozygous)
Ratio 3:1
Ratio 1:2:1
1
The Testcross
• The answer is to carry out a testcross
– breeding the mystery individual with a homozygous
recessive individual
• If any offspring display the recessive phenotype,
the mystery parent must be heterozygous
Mendel’s Law - #2 The Law of Independent
Assortment
• Mendel derived the law of segregation by
following a single character
• A cross between such heterozygotes is called
a monohybrid cross
Probability and Punnett Squares
• Punnett Square – A chart that shows how
parents’ alleles might combine in an offspring
– a tool that can help you understand the patterns of
heredity
– Geneticists use Punnett Squares to show all the
possible outcomes of a genetic cross and to
determine the probability of a particular outcome
Probability and Punnett Squares
• What is the probabilty
of…..
– The offspring having a
yellow seed color?
2 in 4
– The offspring having a
green seed color?
2 in 4
Using a Punnett Square
• In rabbits, black fur
color is dominant to
white. What is the
probability of
producing a white
rabbit if two
heterozygous rabbits
mate?
Using a Punnett Square
• Step 1: figure out the
genotype of the parents.
• Black is dominant to
white.
– B - black
– b - white
• Parents are
heterozygous.
– Bb - dad
– Bb - mom
Using a Punnett Square
Bb
• Step 2: set up a
Punnett Square.
B
B
b
b
Bb
Using a Punnett Square
• Step 3: Fill in the
Punnett Square.
B
B
b
b
Using a Punnett Square
• Step 3: Fill in the
Punnett Square.
B
B BB
b
b
Using a Punnett Square
• Step 3: Fill in the
Punnett Square.
B
B BB
b
b
Bb
Using a Punnett Square
• Step 3: Fill in the
Punnett Square.
B
B BB
b Bb
b
Bb
Using a Punnett Square
• Step 3: Fill in the
Punnett Square.
B
b
B BB
Bb
Bb
bb
b
Using a Punnett Square
• Step 4: Count the
results.
B
b
B BB
Bb
Bb
bb
b
• Genotypes
– 1 homozygous
dominant
Using a Punnett Square
• Step 4: Count the
results.
B
b
B BB
Bb
Bb
bb
b
• Genotypes
– 1 homozygous
dominant
– 2 heterozygotes
Using Punnett Square
• Step 4: Count the
results.
B
b
B BB
Bb
Bb
bb
b
• Genotypes
– 1 homozygous
dominant
– 2 heterozygotes
– 1 homozygous
recessive
Using a Punnett Square
• Step 4: Count the
results.
B
B BB
b
Bb
b
Bb
bb
• Genotypes
– 1 homozygous
dominant
– 2 heterozygotes
– 1 homozygous
recessive
• Phenotypes
Using a Punnett Square
• Step 4: Count the
results.
B
B BB
b
Bb
b
Bb
bb
• Genotypes
– 1 homozygous
dominant
– 2 heterozygotes
– 1 homozygous
recessive
• Phenotypes
– 3 black fur
Using a Punnett Square
• Step 4: Count the
results.
B
B BB
b
Bb
b
Bb
bb
• Genotypes
– 1 homozygous
dominant
– 2 heterozygotes
– 1 homozygous
recessive
• Phenotypes
– 3 black fur
– 1 white fur
Probability and Punnett Squares
• In rabbits, black fur
color is dominant to
white.
• What is the
probability of
producing a white
rabbit if two
heterozygous rabbits
mate?
– 1 in 4 or 25%
• Mendel identified his second law of inheritance by
following two characters at the same time
• A dihybrid cross, a cross between F1 dihybrids,
can determine whether two characters are
transmitted to offspring as a package or
independently
Figure 14.8
EXPERIMENT
YYRR
P Generation
yyrr
yr
Gametes YR
F1 Generation
Predictions
YyRr
Hypothesis of
dependent assortment
Hypothesis of
independent assortment
Sperm
or
Predicted
offspring of
F2 generation
1/
Sperm
1/
2
YR
1/
2
2
YR
YyRr
YYRR
Eggs
1/
2
1/
4
YR
4
Yr
4
yR
4
yr
Eggs
yr
YyRr
3/
yyrr
1/
4
YR
1/
4
1/
Yr
4
yR
1/
4
yr
yr
1/
1/
4
1/
YYRR
YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
4
Phenotypic ratio 3:1
1/
9/
16
3/
16
3/
16
1/
16
Phenotypic ratio 9:3:3:1
RESULTS
315
108
101
32
Phenotypic ratio approximately 9:3:3:1
Mendel’s Law - #2 The Law of Independent
Assortment
• Each pair of alleles segregates independently of
each other pair of alleles during gamete
formation
• Law applies only to genes on different,
nonhomologous chromosomes or those far apart
on the same chromosome
Figure 14.9
Rr
Segregation of
alleles into eggs

Rr
Segregation of
alleles into sperm
Sperm
1/
R
2
2
Eggs
4
r
2
r
R
R
1/
1/
r
2
R
R
1/
1/
1/
4
r
r
R
r
1/
4
1/
4
Degrees of Dominance
• Complete dominance occurs when phenotypes
of the heterozygote and dominant homozygote are
identical
• In incomplete dominance, the phenotype of F1
hybrids is somewhere between the phenotypes of
the two parental varieties
• In codominance, two dominant alleles affect the
phenotype in separate, distinguishable ways
Figure 14.10-1
P Generation
White
CWCW
Red
CRCR
Gametes
CR
CW
Figure 14.10-2
P Generation
White
CWCW
Red
CRCR
Gametes
CR
CW
F1 Generation
Gametes 1/2 CR
Pink
CRCW
1/
2
CW
Figure 14.10-3
P Generation
White
CWCW
Red
CRCR
CR
Gametes
CW
F1 Generation
Pink
CRCW
1/
Gametes 1/2 CR
2
CW
Sperm
1/
F2 Generation
1/
2
CR
1/
2
CW
Eggs
2
CR
1/
2
CW
CRCR CRCW
CRCW CWCW
The Behavior of Recessive Alleles
• Recessively inherited disorders show up only in
individuals homozygous for the allele
• Carriers are heterozygous individuals who
carry the recessive allele but are phenotypically
normal
– most individuals with recessive disorders are
born to carrier parents
Figure 14.16
Parents
Normal
Aa
Normal
Aa
Sperm
A
a
A
AA
Normal
Aa
Normal
(carrier)
a
Aa
Normal
(carrier)
aa
Albino
Eggs