Unit 3: Genetics
•
•
•
•
The Cell Cycle + DNA structure/function
Mitosis and Meiosis
Mendelian Genetics (aka - fun with Punnett squares)
DNA replication
Yesterday’s Exit Ticket
MITOSIS
MEIOSIS
preceded by replication of
chromosomes?
yes
yes
# of rounds of cell division
1
2
# of daughter cells
2
4
same as parent cell
half of parent cell
daughter cells genetically identical to
parent cell?
yes
no
sister cells thus produced identical to
one another?
yes
no
happens in diploid cells, haploid cells,
both, or neither?
both
diploid
# of chromosomes in daughter cells
compared to parent cell
crossing over (synapsis)?
(depending on organism)
no
yes
Today’s Agenda
• Where does variation come from?
• Mendelian Genetics, Part One
Sources of genetic variation
• Mutations (changes in an organism’s DNA) are the
original source of all genetic variation
• Mutations create different versions of genes called
alleles
Clarity check: homologous chromosomes
SAME gene, different ALLELES
Gene for hair color;
Allele for blonde hair
Gene for hair color; allele
for brown hair
Sources of genetic variation
• The behavior of chromosomes during meiosis and
fertilization reshuffles alleles and chromosomes
every generation
• Three mechanisms contribute to genetic variation:
a) Independent assortment of chromosomes
b) Crossing over
c) Random fertilization
Fig. 13-8b
Sources of genetic variation
a) Independent assortment
• Homologous pairs of chromosomes orient randomly during Meiosis I
 maternal and paternal homologs assort into daughter cells independently of
the other pairs
Metaphase I
of meiosis I
Blue can be
on top or
bottom
Fig. 13-11-2
Sources
of genetic variation
a) Independent assortment
Possibility 2
Possibility 1
with n = 2
there are
4 possibilities
for the
lineup
during
Meiosis II
4 possible assortments of
chromosomes in the gametes
Fig. 13-11-3
Sources
of genetic variation
a) Independent assortment
Possibility 2
Possibility 1
Metaphase II
Daughter
cells
Combination 1
Combination 2
Combination 3
Combination 4
Sources of genetic variation
a) Independent assortment
• “2n rule”: the number of possible chromosome
sorting combinations = 2n
 For humans (n = 23), there are 223 = 8,388,608
possible combinations of chromosomes based on
independent assortment alone!
Sources of genetic variation
b) Crossing over (Prophase of Meiosis I)
• homologous chromosomes pair up gene by gene and exchange homologous
segments
• This combines alleles that originated from two (grand)parents into a single
chromosome
blond hair
from G’pa
blue eyes from
G’pa
red hair
from G’ma
red hair from
G’ma
red hair
from G’pa
brown eyes
from G’ma
brown eyes
from G’ma
blue eyes from
G’pa
Mom’s ovary
cell
b) crossing over
Sources of genetic variation
Early in
Meiosis I
Pair of
homologs
during Meiosis I
(at anaphase I)
Nonsister
chromatids
held together
during synapsis
A single crossing
over event leads
to 4 genetically
unique daughter
cells!
during Meiosis II
(at anaphase II)
Daughter
cells
Recombinant chromosomes
Human cells → n = 23
What is n for the cells shown here?
A.1
B.2
C.3
D.4
E.5
1
2
Which cells in this picture are
haploid?
A.all
B.none
C.those above line #1
D.those below line #1
E.only those below line #2
A detailed look at meiosis
FIRST CELL
DIVISION =
“MEIOSIS I”
2nd CELL
DIVISION =
“MEIOSIS II”
Sources of genetic variation
c) Random fertilization
8.4 million
possible
gametes
8.4 million
possible
gametes
> 70 trillion possible
offspring!!!
Today’s Agenda
• Where does variation come from?
• Mendelian Genetics, Part One
Foundations of Genetics
Chapter 14
Outline
1. The work of Gregor Mendel
2. Probability and genetic outcomes
3. Ah, if only it were so simple: complications
on genes and traits
14-2a
1. Fig.
Mendel
a) The scientific method
1
TECHNIQUE:
“crossing” or
“hybridizing”
true-breeding
varieties
2
Parental
generation
(P)
Stamens
Carpel
3
4
14-3-3
1. Fig.
Mendel
EXPERIMENT
P Generation
(true-breeding
parents)
F1 Generation
(hybrids)
Purple
flowers
White
flowers
All plants had
purple flowers
F2 Generation
705 purple-flowered
plants
224 white-flowered
plants
1. Mendel
Making sense of the data:
Why were ALL the F1 flowers purple?
Why were some F2 flowers white?
Why was the ratio in the F2 generation 3:1?
 To explain the data, Mendel developed a model
1. Mendel
Mendel’s explanatory framework
Mendel’s Model: 4 related hypotheses
(remember, DNA had not yet been discovered!)
1. Alternative versions of heritable “particles”
(i.e., different alleles of the same gene)
1. Mendel
Mendel’s explanatory framework
Mendel’s Model: 4 related hypotheses
1. Alternative versions of heritable “factors” (i.e., alleles)
2. For each character an organism inherits
two alleles, one from each parent
14-4
1. Fig.
Mendel
Mendel’s explanatory framework
Diploid organisms
Allele for purple flowers
Location of lower color gene
Allele for white flowers
Homologous
pair of
chromosomes
1. Mendel
Mendel’s explanatory framework
Mendel’s Model: 4 related hypotheses
1. Alternative versions of heritable “factors” (i.e., alleles)
2. For each character an organism inherits
two alleles, one from each parent
(i) all F1 purple
(ii) some F2 white,
(iii) F2 purple:white ratio 3:1
1. Mendel
Mendel’s explanatory framework
Mendel’s Model: 4 related hypotheses
1. Alternative versions of heritable “factors” (i.e., alleles)
2. For each character an organism inherits two alleles, one
from each parent
3. If the two alleles at a locus differ, then one (the dominant allele)
determines the organism’s appearance, and the other (the
recessive allele) has no noticeable effect on appearance
(i) all F1 purple
(ii) some F2 white,
(iii) F2 purple:white ratio 3:1
1. Mendel
Mendel’s explanatory framework
Mendel’s Model: 4 related hypotheses
1. Alternative versions of heritable “factors” (i.e., alleles)
2. For each character an organism inherits two alleles, one
from each parent
3. Some alleles are “dominant”, others “recessive”
4. “Law of segregation” = the two alleles for a character are
separated (segregated) during gamete formation and
end up in different gametes
1. Mendel
b) Mendel’s explanatory framework
Mendel’s Model: 4 related hypotheses
1.
Alternative versions of heritable “factors” (i.e., alleles) account for
variations in inherited characters
2. For each character an organism inherits two alleles, one from each
parent
3. Some alleles are “dominant”, others “recessive”
4. “Law of segregation”
(i) all F1 purple
(ii) some F2 white,
(iii) F2 purple:white ratio 3:1
Outline
1. The work of Gregor Mendel
2. Probability and genetic outcomes
3. Ah, if only it were so simple: complications
on genes and traits
2. Probability and genetic outcomes
F1 individuals and their gametes
EXPERIMENT
P Generation
(true-breeding
parents)
F1 Generation
(hybrids)
RR
rr
homozygous

Purple
flowers
White
flowers
All plants had
purple flowers
2. Probability and genetic outcomes
F1 individuals and their gametes
F1 Generation
(hybrids)
All plants had
purple flowers
Possible gamete types
(with respect to flower color)?
Fig. 14-5-3
R
P Generation
Appearance:
Genetic makeup:
Purple flowers
RR
Gametes:
White flowers
rr
r
Rr
r
Rr
F1 Generation
Purple flowers
Rr
1/
2
Gametes:
1/
2
R
R
r
RR
Rr
Rr
rr
R
Eggs
r
3
1
r
heterozygous
Sperm
F2 Generation
Rr
r
R
Appearance:
Genetic makeup:
R
Rr
Fig. 14-5-3
Mendel’s “Law” of segregation is used to construct a
“Punnett square”
 this simple square tells you the expected frequencies of
genotypes and phenotypes from a particular cross
Fig. 14-5-3
P Generation
Appearance:
Genetic makeup:
Purple flowers
RR
Gametes:
White flowers
rr
r
R
F1 Generation
Appearance:
Genetic makeup:
Purple flowers
Rr
1/
2
Gametes:
1/
2
R
Sperm
F2 Generation
R
r
RR
Rr
Rr
rr
R
Eggs
r
3
1
r
Reviewing the numbers with respect
to this flower color gene:
 2 alleles x 2 alleles = 4 outcomes
 only 3 distinct genetic types, or
genotypes, 1:2:1
 only two distinct traits, or
phenotypes, 3:1
Testcross: a useful tool
How can we figure out the GENOTYPE of a purple flower?
 could be PP or Pp
Testcross: a useful tool
How can we figure out the GENOTYPE of a purple
flower?
(A)
PP
x
(B)
Pp
PP or Pp?
What do we cross the purple
flower with?
(C)
pp
Today’s Exit Ticket
• Create and complete two Punnet squares:
1) A testcross of a heterozygote (rr x Rr)
2) A testcross of a homozygous dominant individual
(rr x RR)
• Explain why using a homozygous recessive
individual is useful for distinguishing between
Rr and RR.