Chapter 9.1~
Mendel &
The Gene Idea
Mendelian genetics
Character: heritable feature, i.e., fur
color
Trait : variant for a character, i.e.,
brown
True-bred: all offspring of same
variety
Hybridization: crossing of 2 different
true-breds
P generation: parents
F1 generation: first generation
Leading to the Law of Segregation
Alternative versions of genes
(alleles) account for variations
in inherited characteristics
Org inherits 2 alleles per
character
dominant allele - fully
expressed in orgs appearance.
recessive allele - no
noticeable effect on
appearance
The alleles for each character
segregate (separate) during
gamete production (meiosis).
Mendel’s Law of Segregation
Genetic vocabulary…….
Punnett square: predicts the
results of a genetic cross
between individuals of known
genotype
Homozygous: pair of identical
alleles for a character
Heterozygous: two different
alleles for a gene
Phenotype: an organism’s
traits
Genotype: an organism’s
genetic makeup
Testcross: breeding of a
recessive homozygote X
dominate phenotype (but
unknown genotype)
Practice monohybrid crosses
Reporting results… %, fraction, ratios…
Phenotypic results:
Genotypic results:
TRAIT KEY
Hairline:
Tongue:
Earlobes
Eye shape
Blood type
Hair
Freckles
Middle toe
Fingers
Lip size
Knuckle
Thumb
W= widow’s peak
R = can roll
F = free hanging
L = almond shaped
A = type A blood
C = curly
K = freckles
D = short
S = six
T = thick lips
H = hair on middle knuckle
B = hitchhikers
w= straight hairline
r = can’t roll
f = attached
= round shaped
B = type B blood
c = straight
k = no freckles
d = long
s = five
t = thin lips
h = no hair on middle knuckle
b = straight thumb
The Law of Independent Assortment
Law of Segregation involves 1 character. What about 2 (or more)
characters?
Monohybrid cross vs. dihybrid cross
The two pairs of alleles segregate independently of each other.
Mendel’s Law of Independent Assortment
Cross a
yR
with
YR
Yr
yR
YyRR
YyRr
yyRR
yr
yyRr
Results:
6/16
yr
YyRr
Yyrr
yyRr
yyrr
6/16
2/16
yR
YyRR
YyRr
yyRR
yyRr
yr
YyRr
Yyrr
yyRr
yyrr
2/16
Practice Slide
Results:
Using the rule of multiplication to find particular
genotypes or phenotypes
What is the probability of getting a yyRR if a
is crossed with a
?
Complete a monohybrid cross for each character &
then multiply the two fractions.
… so chance of getting a yy if cross a Yy & yy?
… chance of getting a RR if cross a Rr & Rr?
Using the rule of multiplication to find particular
genotypes or phenotypes
What is the probability of getting a green round if a
is crossed with a
?
Complete a monohybrid cross for each character &
then multiply the two fractions.
… so chance of getting a green if cross a Yy & yy?
… chance of getting a round if cross a Rr & Rr?
Extending Mendelian Genetics, I
Incomplete dominance:
appearance between
the phenotypes of the 2
parents.
Results in 3 possible
phenotypes – one being
intermediate.
Ex: snapdragons

Molecular explanation for incomplete dominance
One allele carries the
code for a functional
protein and the other
doesn’t. Since there’s
only one copy of the
functional allele, not
enough proteins are
synthesized which
results in the
intermediate
phenotype in the
heterozygote.
How is this different
from complete
dominance?
Extending Mendelian Genetics, II
Codominance: two alleles affect the phenotype in separate,
distinguishable ways.
Ex: human blood types
Multiple alleles: more than 2 possible alleles for a gene.
Ex: human blood types
Extending Mendelian Genetics, III
Pleiotropy:
genes with
multiple
phenotypic
effect. Ex:
sickle-cell
anemia
Extending Mendelian Genetics, IV
Epistasis: a gene at one
locus (chromosomal
location) affects the
phenotypic expression
of a gene at a second
locus. Ex: mice coat
color




B = black
b = brown
C = pigment production
c = no pigment production
Epistasis Problem
•
In a certain breed of plants, D produces dark red color and is dominant over d
which produces a light red color. Another gene determines in which cells the
pigment will be synthesized. Allele w allows synthesis of the pigment throughout
the petals but the mutant allele W prevents pigment production.
D = dark red
d = light red
w = pigment production
W = no pigment production
You cross two plants that are DdWw.
What will be the genotype and phenotype ratios of the crosses?
Possible phenotypes:
Dark red = 3/16
Light red = 1/16
DW
Dw
dW
White = 12/16 (3/4)
DW
DDWW
DDWw
DdWW
dw
DdWw
Dw
DDWw
DDww
DdWw
Ddww
dW
DdWW
DdWw
ddWW
ddWw
dw
DdWw
Ddww
ddWw
ddww
Possible genotypes:
DDWW = 1/16
DDWw = 2/16 (1/8)
DdWW = 2/16 (1/8)
DdWw = 4/16 (1/4)
DDww = 1/16
Ddww = 2/16
ddWW = 1/16
ddWw = 2/16 (1/8)
Ddww = 1/16
Same problem
continued…
DW
Dw
dW
dw
DW
DDWW
DDWw
DdWW
DdWw
Dw
DDWw
DDww
DdWw
Ddww
dW
DdWW
DdWw
ddWW
ddWw
dw
DdWw
Ddww
ddWw
ddww
Extending Mendelian Genetics, V
Polygenic
Inheritance: an
additive effect of two
or more genes on a
single phenotypic
character Ex:
human skin
pigmentation and
height
Human disorders
The family pedigree – trace a character through
generations

Square = male, circle = female, shaded = has trait
What is it about the recessive allele
that causes the disease
phenotype?
codes for malfunctional protein or none at
all
Hetero normal b/c dom allele produces
enough of the needed protein
Recessive Human Genetic Disorders
Tay-Sachs

fatal genetic lipid storage disorder in which harmful
quantities of a fatty substance called ganglioside
GM2 build up in tissues and nerve cells in the brain.
Cystic fibrosis

Build up of mucus in the lungs and digestive tract
due to the lack of a transport protein in the cell
membranes.
Sickle-cell
Substitution of single amino acid in
hemoglobin of RBC
1/400 African Americans
1/10 are hetero for it
Treatment = blood transfusions
Hetero have disease b/c alleles are codom
on molecular level.
High hetero b/c they are res to malaria!
Cystic fibrosis





Build up of mucus in the lungs and digestive
tract due to the lack of a transport protein in
the cell membranes.
Most lethal in US
1/2500 whites of Euro decent have it
1/25 are carriers
Antibiotic & pounding
Dominant Human Genetic Disorders
•Huntington’s

results from genetically programmed degeneration of brain cells,
called neurons, in certain areas of the brain. This degeneration
causes uncontrolled movements, loss of intellectual faculties,
and emotional disturbance.
Testing
•amniocentesis
•chorionic villus sampling
(aka: CVS)
Quiz tomorrow on ch 14
Get out snowflake problems, ch 12 review
sheet w/ aliens on it, pedigree practice
sheet… we’re going over them.
Photosnythesis labs due Fri.
Ch 9 The Chromosomal Basis of
Inheritance
Chromosomal theory in inheritance – mendelian genes
have specific loci on chromosomes & it is the
chromosomes that undergo segregation & independent
assortment
Morgan traced genes to specific chrom
 fruit flies
Prolific breeders
New gen every 2 wks
Only 4 pair chrom.
Normal (wild type) = lower case of mutant w/ +
Mutant = lower case
Use different symbols with
these problems…
Lower case of mutant
+ = dominant
So… b+ = gray
b = black
vg+ = normal wings
vg = vestigial wings
b+ b vg+ vg x bbvgvg
b+ vg+
b vg
b vg
b vg
b vg
b+ vg
b vg+
b vg
Recombination Frequency
Using recombination frequencies to construct a
genetic map – ordered list of the genetic loci
along a chromosome
Farther apart 2 genes are – higher
probability x-over will occur.
AP Bio 12/16 Do It Now…
What fraction of offspring would you expect
to be bald if you crossed a bald man with a
woman who is heterozygous for the bald
allele.
Sex Linked Traits – traits that are determined
by genes on the X chromosome
Unique patterns of inheritance

Why? Males need only 1 rec allele to have a sex linked disorder
Colorblindness, hemophilia (blood won’t clot), baldness, duchenne muscular
dystrophy
All recessive disorders
More men than women have sex linked traits.
Explanation:
XH XH = Homozygous Dominant Woman
XH Xh = Heterozygous Woman
Xh Xh = Homozygous Recessive woman so
hemophilia
XH Y = Dominant male so no hemophilia
Xh Y = Recessive male so hemophilia
So… males only need one recessive allele
to have a sex linked trait since they only
have one X
Women have 2 X chromosomes so they
have to inherit 2 recessive alleles to
have a sex linked trait.
Males inherit sex linked traits
from their mothers.
Why? B/c males inherit their X
chromosomes from their moms – their
dads give them their Y chromosomes.
So… men inherit baldness from their
moms!
Sex linked trait problems:
Cross a woman heterozygous for
baldness with a man who is not bald.
Hint: show chromosomes too
XB Xb
XB X X
X Y
Y
B B
B
XBXb
XbY
¼ chance that child will be bald. ½ chance that they have a boy
that’s bald.
Everyone should be able to see a 12
Nomal =5
color blind = 2
X inactivation in female mammals
1 of the 2 X chromosomes in each cell becomes almost
completely inactivated during embryonic development.
Barr body – condensed form of inactive X located
along inside of nuc mem
Random from cell to cell… mosaic of active X s
Ex: calico cats, human females lacking sweat glands
15.5 from
ch 15 study
guide
2 normal
colorsighted
individuals
produce the
following
children &
grandkids.
#1 from
ch 15
study
guide p
108
Scientists use recombination frequencies to determine
the order of genes on a chromosome. Try to figure out
the order of these 5 genes given the following
recombination frequencies.
D
16
B
4
C
10
A
6
E