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
• Create and complete two testcross Punnet squares:
(assume G=green and g=yellow)
gg x Gg
G
gg x GG
g
g
Gg
gg
g
Gg
gg
½ green
½ yellow
G
G
g
Gg
Gg
g
Gg
Gg
all green
• Why homozygous recessive for testcross?
–Clear and easy determination of unknown’s genotype: 1:1 = heterozygote; all
dominant = homozygote
Today’s Agenda:
• Mendel and multiple characters
•Exceptions to Mendel
• Sex-linked traits
• Gene linkage
How To Punnet Squares
https://www.youtube.com/watch?v=Y1PCwxUDTl8
14-8
2. Fig.
Probability
and genetic outcomes
What about multiple characters? Are they inherited
together or separately?
For the purposes of example, consider the following two characters:
1. Seed color:
1 Generation
• FPossible
phenotypes = Yellow
YyRr OR green
Hypothesis of
• Yellow is dominant
to greenHypothesis of
dependent
assortment
Predictions
independent
assortment
Predicted
2. Seed shape:
Sperm
offspring of
Sperm
or
• FPossible
/ YR / yr = Round OR/ wrinkled
2 generation phenotypes
YR / Yr / yR
/ YR
• Round is dominant to wrinkled YYRR YYRr YyRR
1
2
1
1
2
1
1/
2
4
1
4
1/
4
YYRR
YyRr
4
1/
4
YyRr
Yr
Eggs
yr
YyRr
3/
4
yyrr
1/
4
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
yR
1/
4
Phenotypic ratio 3:1
1/
4
yr
9/
16
3/
16
3/
16
1/
16
Phenotypic ratio 9:3:3:1
RESULTS
yr
YR
Eggs
1/
2
1
4
14-8
2. Fig.
Probability
and genetic outcomes
What about multiple characters? Are they inherited
together or separately?
YYRR
P Generation
Gametes
yyrr
YR
F1 Generation

EXPERIMENT
yr
YyRr
Hypothesis of
dependent
assortment
Predictions
Predicted
offspring of
F2 generation
Hypothesis of
independent
assortment
Important
Sperm
Sperm
or Vocab Note:
/ yR / yr
YR / yr cross deals /with
YR /one
Yr gene
A MONOHYBRID
/ YR
e.g. Aa x Aa YYRR YYRr YyRR YyRr
/ YR
1/
2
1
1
2
1
1
1
4
4
1
4
1
4
4
2
YYRR
Eggs
YyRr
1/
4
Yr
YYRr
YYrr
Yyrr
YyRr
A DIHYBRID cross deals with two genes
YyRr
yyrr
/ yR
e.g. AaBb x AaBb
YyRR YyRr yyRR
1/
2
Eggs
yr
1
3/
4
4
yyRr
1/
4
Phenotypic ratio 3:1
1/
4
yr
YyRr
9/
16
3/
16
Yyrr
yyRr
3/
16
yyrr
1/
16
Phenotypic ratio 9:3:3:1
RESULTS
Fig. 14-8
P Generation
YYRR
Gametes
yyrr
YR

EXPERIMENT
yr
F1 Generation
Suppose that two F1 individuals are crossed. Consider two mutually exclusive
hypotheses about inheritance:
1. Strict dependent assortment = inherited allele
ALWAYS preserved in the gametes an individual produces
combinations are
2. Independent assortment = all possible
combinations of inherited
alleles of different genes are equally likely in an individual’s gametes
14-8
2. Fig.
Probability
and genetic outcomes
YYRR
P Generation
Gametes
yyrr

YR
F1 Generation
EXPERIMENT
yr
YyRr
Hypothesis of
independent
assortment
Hypothesis of
dependent
assortment
Sperm
or
1/
4
Sperm
1/
2
YR
1/
2
1/
2
1/
4
yR
1/
4
yr
YR
YYRR YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
YR
YyRr
YYRR
Eggs
1/
2
Yr
yr
1/
4
Predicted
offspring of
F2 generation
YR
1/
4
1/
4
Yr
Eggs
yr
YyRr
3/
4
yyrr
1/
4
yR
1/
4
Phenotypic ratio 3:1
1/
4
yr
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
14-8
2. Fig.
Probability
and genetic outcomes
YYRR
P Generation
Gametes
yyrr

YR
F1 Generation
EXPERIMENT
yr
YyRr
Hypothesis of
independent
assortment
Hypothesis of
dependent
assortment
Sperm
or
1/
4
Sperm
1/
2
YR
1/
2
1/
2
1/
4
yR
1/
4
yr
YR
YYRR YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
YR
YyRr
YYRR
Eggs
1/
2
Yr
yr
1/
4
Predicted
offspring of
F2 generation
YR
1/
4
1/
4
Yr
Eggs
yr
YyRr
3/
4
yyrr
1/
4
yR
1/
4
Phenotypic ratio 3:1
1/
4
yr
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
Fig. 15-2b
F1 Generation: 2 possible
arrangements of
0.5 mm
chromosomes
All F1 plants produce
yellow-round seeds (YyRr)
R
r
LAW OF SEGREGATION
The two alleles for each
gene separate during
gamete formation.
LAW OF INDEPENDENT
ASSORTMENT Alleles of
genes on nonhomologous
chromosomes assort
independently during
gamete formation.
R
y
y
r
Y
Y
Meiosis
r
R
r
R
Y
y
Metaphase I
Y
y
1
1
r
R
r
R
Y
y
Anaphase I
Y
y
r
R
Metaphase II
R
r
2
2
Gametes
y
Y
Y
R
R
1
4
r
1
YR
3
4
yr
Y
Y
y
r
y
Y
y
Y
r
r
14
Yr
y
y
R
R
14
yR
3
2. Probability and genetic outcomes
• Mendel’s “law” of independent assortment = alleles
for each character segregate independently during
gamete formation
• Given what YOU know about the relationship
between genes and chromosomes (which Mendel
did NOT), when would this “law” be violated?
2. Probability and genetic outcomes
R
Y
r
y
YR
yr
Today’s Agenda:
• Mendel and multiple characters
• Exceptions to Mendel
• Sex-linked traits
• Gene linkage
If only it were all so simple…
The view provided by (my simplified presentation of)
Mendel’s pea experiments:
• one gene  one character
(e.g., flower color gene  color of flower)
• one allele  one phenotype
(e.g., P allele  purple flower)
• two alleles of each gene, one completely
dominant, the other recessive
(e.g., P dominant to p)
3. Extending the Mendelian model
• Patterns of inheritance different from those
discussed so far can be caused in many ways. Just
to name a few:
a)
b)
c)
d)
e)
Lack of complete dominance by one allele
A gene has more than two alleles
A gene produces multiple phenotypes
Multiple genes affect a single phenotype
Environmental circumstances affect the phenotype
To learn more about all of these, take EBIO 2070!
For now, the simplest exceptions:
1. Genes on sex chromosomes
2. Gene linkage
X
Y
REMINDER:
A complete single set of
human chromosomes
includes:
• 22 autosomes (non-sex
chromosomes)
• 1 sex chromosome
(diploid cells have
44 autosomes and
2 sex chromosomes)
Humans and many other
species have
chromosomal sex
determination
In the human system,
females have two “X”
chromosomes, males
have one “X” and one “Y”
X
Y
Fig. 15-5
Other forms of chromosomal sex determination in the animal
kingdom…
76 +
ZW
The Z-W system
76 +
ZZ
Fig. 15-6c
What consequences might sex chromosomes have for
patterns of inheritance and gene expression?
22 pairs of
chromosomes
+
22 pairs of
chromosomes
+
X
X
Y
X
Who determines the sex of our offspring?
Diploid
Parent Cell
Gametes
XX
X
XY
X
X
Y
Dad determines a
child’s sex!
Patterns of inheritance in mammals (and other XY systems)
from female parent
from male parent
allele on X
chromosome
(“X-linked”)
passed on to either sons
or daughters with
probability ½
passed on ONLY to
daughters with
probability 1
Diploid
Parent Cell
XAXa
XAY
Gametes
XA
Xa
XA
Y
Patterns of inheritance in mammals (and other XY systems)
allele on Y
chromosome
(“Y-linked”)
from female parent
from male parent
typically not possessed
by females
passed on ONLY to sons
with probability 1
Diploid
Parent Cell
Gametes
Dad is who
determines
a child’s
sex
XX
X
X
X
XYA
YA
Patterns of gene expression in mammals (and other XY systems)
expression in females
expression in males
dominant Xlinked allele
yes
yes
recessive Xlinked allele
ONLY if present with
other recessive allele
yes
never present
(never expressed)
yes
Y-linked allele
Male-pattern baldness
SRY gene:
Testes formation
Santhi’s Story
http://www.ibnlive.com/videos/28851/
how-are-athletes-gender-tested.html
Santhi Soundarajan won the silver
medal in the 800-meter race at the 2006
Asian Games in Doha, Qatar.
Following her silver medal performance,
she was stripped of her medal.
Santhi has female genitalia but her
genotype is XY.
Speakequal.com
Patterns of gene expression in mammals (and other XY systems)
expression in females
expression in males
dominant Xlinked allele
yes
yes
recessive Xlinked allele
ONLY if present with
other recessive allele
yes
never present
(never expressed)
yes
Y-linked allele
Genes on chromosomes
b) Sex-linked traits
• Breeding fruit flies
(Drosophila melanogaster)
– Rapid breeders
– Males = XY; Females = XX
– For Drosphila:
recessive alleles = “mutant” (b)
dominant alleles = “wild type” (b+)
News.wisc.edu
Fig. 15-3
One of Morgan’s experiments (think back to Mendel’s peas):
• Character: eye color
• Phenotypes: red or white
P Generation
(true breeding)
F1 Generation
All offspring had
red eyes
 Is the allele for white eyes dominant or
recessive?
Then, cross the F1 offspring with each other, and what does the
F2 generation look like?

3:1
ratio of red : white
2:1:1
ratio of red female : red male : white male
The best explanation for the pattern of inheritance seen in the F2
generation is:
a)The eye color gene is on an autosome
b)The eye color gene is sex-linked, on the X chromosome
c)The eye color gene is sex-linked, on the Y chromosome
d)There is not enough information to discriminate between
hypotheses (a) through (c)
b) Original discoveries
Genes on chromosomes
Fig. 15-4c
CONCLUSION
P
Generation
R
X
X

X
Y
r
r
r
Sperm
Eggs
F1
Generation
R
R
R
All females XRXr
All males XRY
r
R
Sperm
Eggs
F2
Generation
R
R
R
r
r
r
R
r
b) Original discoveries
Genes on chromosomes
Fig. 15-4c
CONCLUSION
P
Generation
R
X
X

X
Y
r
r
r
Sperm
Eggs
F1
Generation
R
R
R
All females XRXr
All males XRY
r
R
Sperm
Eggs
F2
Generation
R
R
R
r
r
r
R
r
Females all red:
½ XRXr
½ XRXR
Males half red (XRY)
and half white (XrY)
For now, the simplest exceptions:
1. Genes on sex chromosomes
2. Gene linkage
X
Y
Gene Linkage and Fruit Flies
https://www.youtube.com/watch?v=-_UcDhzjOio
Fig. 15-2b
All F1 plants produce
yellow-round seeds (YyRr)
LAW OF INDEPENDENT
ASSORTMENT Alleles of
genes on nonhomologous
chromosomes assort
independently during
gamete formation.
R
y
r
Y
Meiosis
r
R
Y
y
Metaphase I
1
r
R
Y
y
Anaphase I
Metaphase II
R
r
2
y
Y
Y
Y
r
r
14
Yr
y
y
R
R
14
yR
3
Fig. 14-8
 In crosses involving two characters, sometimes
you get outcomes that were intermediate between
these two hypotheses.
F1 Generation
YyRr
Hypothesis of
independent
assortment
Hypothesis of
dependent
assortment
Sperm
or
1/
4
Sperm
1/
2
YR
1/
2
1/
2
1/
4
yR
1/
4
yr
YR
YYRR YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
YR
YYRR
Eggs
1/
2
Yr
yr
1/
4
Predicted
offspring of
F2 generation
YR
1/
4
YyRr
1/
4
Yr
Eggs
yr
YyRr
3/
4
yyrr
1/
4
yR
1/
4
Phenotypic
ratio 3:1
1/
4
yr
9/
16
3/
16
3/
16
1/
16
Phenotypic
ratio 9:3:3:1
Example
• Morgan crossed flies to study the characters of
body color and wing size
 Genes for both are located on autosomes
Fig. 15-9-1
EXPERIMENT
P Generation (homozygous)
Wild type
(gray body,
normal wings)
b+ b+ vg+ vg+

Double mutant
(black body,
vestigial wings)
b b vg vg
F1 generation
?
Fig. 14-8
F1 dihybrid
(wild type phenotype)
F1 dihybrid
(wild type phenotype)
x
b+ b vg+ vg
b+ b vg+ vg
Hypothesis of
dependent
assortment
Hypothesis of
independent
assortment
b+b?v+v? bbvv
b+b?v+v?
3 : 1
Observed (approx.):
9 :
8 :
b+b?vv
bbv+v?
3 : 3
2 : 2 : 4
bbvv
: 1
Why would some genes be inherited neither completely together nor completely
independently?
 Gene linkage
• Each chromosome has hundreds or thousands of genes
• Genes located on the same chromosome that tend to be inherited
together are called linked genes
• Occasional crossing over leads to occasional, but not common,
recombinant chromosomes
crossing over
Sources of genetic variation
Recombination of Linked Genes: Crossing Over
Prophase I
of meiosis
Pair of
homologs
Nonsister
chromatids
held together
during synapsis
Chiasma
Centromere
Anaphase I
 Crossing over during Prophase I of meiosis is the
mechanism for recombining alleles
Fig. 15-UN1
Gene
linkage
b+ vg+
Parents
in testcross
Most
offspring
b vg

b vg
b vg
b+ vg+
b vg
or
b vg
b vg
3. Gene linkage
Recombinant
chromosomes
Fig. 15-10b
b+ vg+
b vg
b+ vg
b vg+
944
Wild type
Black(gray-normal) vestigial
206
Grayvestigial
185
Blacknormal
Eggs
Testcross
offspring
965
b+ vg+
b vg
b+ vg
b vg+
b vg
b vg
b vg
b vg
Recombinant offspring
Parental-type offspring
5
:
5
:
1
:
1
b vg
Sperm
Today’s Exit Ticket
An x-linked recessive allele b produces red-green color blindness in humans. A
normal-sighted woman whose father was color-blind marries a color-blind man.
1.What genotypes are possible for the mother of the colorblind man?
1.What are the chances that the first child from this marriage will be a color-blind
boy?
1.Of the girls produced by these parents, what proportion can be expected to be colorblind?
1.Of all the children (sex unspecified) of these parents, what proportion can be
expected to have normal color vision?