Transcript Document

Chapter 9, Section 1: Pre-Mendel
Early Ideas about Heredity
• The Blending Theory:
– Main theory of inheritance until the late 19th
century
– Each parent contributed “factors” that were
blended in the offspring
• What was the problem with this theory?
– All individuals of a population would
eventually look the same.
– Once blended, traits would never separate
and show up in later generations.
Chapter 9, Section 1: Mendel
Gregor Mendel (1822 – 1884)
• Augustinian monk who
studied pea plants
• Established the
particulate theory of
heredity
• Significance:
– Developed pure lines
– Counted results and kept
statistical notes
• His work remained
undiscovered until 1903.
Mendel’s Research
Character
Dominant Trait
Why did he research
pea plants?
Flower Color
1. They are normally
self-pollinated,
but can be
cross-pollinated.
Flower Position
2. They have several
qualitative traits
that are easy to
distinguish
Seed Shape
i.e.,Tall vs. Short
Purple
Recessive Trait
White
Side
Top
Yellow
Green
Round
Wrinkled
Inflated
Constricted
Seed Color
Pod Shape
Pod Color
Green
Yellow
Stem Length
Tall
Dwarf
Mendel’s
Research
1.
2.
3.
4.
5.
Removed stamens from
purple flower.
Transferred pollen from
stamens of white flower
to pistil of purple flower.
Pollinated flower matured
into a pod.
Planted seeds from pod.
Examined offspring:
All purple flowers…
Watch this video clip, and see
if you can explain why…
Mendel’s Conclusions
1. Law of Segregation
a. Factors (genes) for a particular trait occur in
pairs
b. For each trait, an organism inherits two
genes, one from each parent.
c. Dominant alleles mask recessive ones
i. Exception 1: Incomplete Dominance
ii. Exception 2: Co-dominance
d. Two alleles for each trait segregate during
gamete production
Law of Segregation:
Factors for a particular
trait occur in pairs
Homologous pair of
Chromosomes
Genes:
The “factors” that
control traits.
Alleles:
Different forms of a
gene.
Allele for White
Flowers
Locus
for
Flower
Color
Gene
Allele for Purple
Flowers
Back to Mendel’s
Conclusions
W
or
w
Ww
w
or
w
ww
• Each individual is diploid
– Diploid: Containing a double-set of chromosomes (2n)
• Each gamete is haploid
– Gamete: Reproductive cell (egg or sperm)
– Haploid: Containing a single-set of chromosomes (n)
Law of Segregation:
One version of each
gene is inherited from
each parent
Back to Mendel’s
Conclusions
Dominant
alleles
mask
recessive
ones
P (Parent) Generation
True-breeding parents
705 plants had
purple flowers
224 plants had
white flowers
F1 (1st Filial) Generation
Hybrid Offspring
What happened to the
recessive traits?
F2 (2nd Filial) Generation
What is the F2 ratio?
705:224  3:1
Dominant
alleles
mask
recessive
ones
P Generation
Phenotype (Appearance)
Genotype (Genetic Makeup)
Gametes
Purple Flowers
PP
P
White Flowers
pp
p
F1 Generation
Phenotype (Appearance)
Genotype (Genetic Makeup)
Gametes
Purple Flowers
Pp
p
P
Punnett Square
F2 Generation
What is the Genotypic Ratio
of the F2 Generation?
PP:Pp:pp  1:2:1
P
P
PP
p
Pp
p
Pp
pp
What is the Phenotypic Ratio
of the F2 Generation?
3:
1
Dominant
alleles
mask
recessive
1
ones
Genotype
Phenotype
PP
(homozygous)
Purple
Pp
(heterozygous)
Purple
Pp
(heterozygous)
Purple
pp
(homozygous)
White
Ratio = 1:2:1
Ratio = 3:1
3
2
1
1
Dominate
alleles
mask
recessive
ones
Dominant phenotype,
unknown genotype:
PP or Pp?
Recessive phenotype,
known genotype:
pp
What would your
If you have a dominant
phenotype
hypothesis
be if the
genotype was Pp?
(like purple flowers) how would you
design an experiment to determine
P
p (PP)
P
p
if it was
homozygous
Pp
Pp
P
p
p
p
or heterozygous (Pp)?
What would your
hypothesis be if the
genotype was PP?
If PP,
then all offspring purple:
Pp
Pp
If Pp,
then ½ offspring purple
and ½ offspring white:
pp
Pp
pp
Pp
This is called
a test cross
Back to Mendel’s
Conclusions
P Generation
Phenotype(s): Red and White
Genotype(s): CRCR and CWCW
Gametes: CR and CW
CR
CW
F1 Generation
Phenotype: Pink
Genotype: CRCW
Gametes: CR and CW
½ CR ½ CW
F2 Generation
½
½ CR
CRCR
Exception to Dominant Alleles
Masking Recessive Alleles:
Incomplete Dominance:
Pink Snapdragons
½ CW
CRCW
CR
½
CRCW
CWCW
CW
Exception to Dominant Alleles
Masking Recessive Alleles:
Co-dominance: Blood Types
Back to Mendel’s
Conclusions
Law of Segregation:
Two alleles for each
trait segregate during
gamete production
Back to Mendel’s
Conclusions
This is a process called meiosis:
Specifically it is “crossing over,” which occurs
very early during Prophase I of Meiosis
Mendel’s Conclusions
2. Law of Independent Assortment
The law of segregation followed one single
trait at a time, such as flower color.
What if two traits were followed?
Two hypotheses were written and
experimented on to determine if traits were:
–
–
Dependent (linked), or
Independent (not linked)
If the genes are not connected, then they
should segregate independently.
P
Following two traits:
Seed shape and seed color
Gametes
YR
F1
Ova
yr
F2
yr
YyRr
YR
YR
Sperm
yr
Hypothesis:
If the genes for seed
shape and color are
connected in some way,
then the dominant R and
Y alleles and the recessive
r and y alleles will be
segregated as matched
sets in the gametes.
What are the genotypes of
the P Generation?
YYRR
What does the P
Generation pass on to F1?
yr
YR
F1 Phenotype?
F1 Genotype?
What does the F1
Generation pass on to F2?
Combine the sperm and
ova to produce the
offspring in F2
yr
What do you expect the
phenotype will be when YR
and YR are combined?
What will the Genotype be
when YR and YR are
combined?
Law of Independent
Assortment
yyrr
Gametes
YyRr
Ova
YR
Yr
yR
YR
Yr
YYRR
YYRr
YyRR
Sperm
yR
YYRr
yr
YyRR
YYrr
Phenotypic Ratios
YyRr
YyRr
Yyrr
yyRr
9/16
Yellow & Round
3/16
Green & Round
3/16
Yellow & Wrinkled
1/16
Green & Wrinkled
Yyrr
yyRR
Now try to do it on your own!
What are the phenotypic ratios?
YyRr
YyRr
yyRr
yyrr
Significance of Mendel’s Research
1. Probability was introduced as a biological
concept:
The number of times an event occurs
The number of trials
2. Punnett Square was introduced as a tool
Probability:
Segregation of
alleles into ova
Pp
x
Pp
The number of times an event occurs
The number of trials
½
½
P
P
½
Segregation of
alleles into sperm
p
P
P
½
½x½=
¼
p
P
P
¼
p
How do you get
a genotypic ratio of
1:2:1 from these numbers?
p
¼
p
p
¼
Back to Mendel’s Principles
Punnett Square Exercise
One-trait cross
The allele for the hairy
trait is “H,” while the
allele for the hairless
trait is “h.”
Make a cross between
two homozygotes for
each of these traits.
Punnett Square Exercise
One-trait cross
Henry VIII divorced six of his
wives (two of whom
were
executed) for not
bearing him
any sons.
Use Mendelian Genetics to prove to King
Henry that it wasn’t his wives’ fault.
Remember:
Females are XX and Males are XY.
Punnett Square Exercise
One-trait cross
In a populations of hamsters, brown fur is
dominant (B), and white fur is recessive (b).
Predict the offspring from
the cross of a white
hamster and a brown
hamster if the
brown
hamster's
mother was
white.
Punnett Square Exercise
Two-trait cross
Following two traits:
Kernel shape: Plump vs. withered (P and p)
and Kernel color Red vs. yellow (R and r)
Predict the cross between a homozygous
recessive corn plant and a homozygous
dominant corn plant.
Punnett Square Exercise
Two-trait cross
Now take the offspring from that cross,
and self-pollinate that plant.
What is the phenotypic
ratio of this cross?
Punnett Square Exercise
Two-trait cross
My pet guinea pigs (Joni
and Chachi) are going to
have little guinea pigs.
What is the possibility that
their offspring will have long hair, if I know that
Joni and Chachi are heterozygous for the
following traits?
Black fur (B)
White fur (b)
Long hair (L)
Short hair (l)