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Chapter 14
158
Mendel and the Gene Idea
Homologous chromosome:
Heterologous chromosome:
Haploid cell:
Diploid cell:
Allele:
Dominant gene:
Recessive gene:
Phenotype:
Genotype:
158
Gene:
Homozygous:
Heterozygous:
Hybrid:
Monohybrid:
Dihybrid:
159
Mendel’s First Law--- Law of Segregation:
Genes exist in pairs, and in the formation of
gametes, each gene separates or segregates
from the other member of the pair, and
passes into a different gamete so that each
gamete has only one of each kind of gene.
P
F1
PP x pp
|
Pp
Fig. 14-1
159
Pp x Pp
P
P PP
p Pp
p
Pp
pp
Phenotypic ratio: 3 purple : 1 white
Genotypic ratio: 1PP : 2Pp : 1 pp
Fig. 14-3-3
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
160
The Testcross:
It is a cross between an organism with
homozygous recessive and an organism
with an unknown genotype to find out the
genotype of the test organism. If the
offspring of the cross yields only dominant
types, the test organism must be a
homozygous dominant. It the offspring
are all recessive, the organism must be a
homozygous recessive. It the offspring are
half recessive and half dominant, the
organism must be a heterozygote.
160
Mendel’s Second Law: Law of Independent
Assortment (or Segregation):
The members of one pair of genes separate
(or segregate) at random from each other in
meiosis independent of the members of
other pairs of genes.
YYRR x yyrr
|
F1
YyRr (yellow and round)
160
F2
YR
YR YYRR
yR YyRR
Yr YYRr
yr YyRr
YyRr x YyRr
|
yR
Yr
YyRR
YYRr
yyRR
YyRr
YyRr
YYrr
yyRr
Yyrr
yr
YyRr
yyRr
Yyrr
yyrr
Fig. 14-8a
EXPERIMENT
YYRR
P Generation
Gametes
yyrr
YR

F1 Generation
yr
YyRr
Hypothesis of
independent
assortment
Hypothesis of
dependent
assortment
Predictions
Sperm
or
Predicted
offspring of
F2 generation
1/
4
Sperm
1/
2
YR
1/
2
1/
4
Yr
yR
1/
4
yr
YR
YYRR YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
YR
YYRR
YyRr
1/
4
Eggs
1/
2
1/
4
yr
1/
4
1/
2
YR
Yr
Eggs
yr
yyrr
YyRr
3/
4
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
Fig. 14-8b
RESULTS
315
108
101
32
Phenotypic ratio approximately 9:3:3:1
161
Rule of Multiplication:
The chance of coming up with a head or a tail in
tossing a coin is ½ or 50%. So, the chance of
coming up with two heads when two coins are
simultaneously tossed is ½ x ½ = ¼ or 25%
chance.
Rule of Addition:
The probability of an event that can occur in two
or more alternative ways is the sum of the separate
probabilities of the different ways. The
probability of F2 heterozygote is 1.4 + ¼ = ½ or
50%.
Fig. 14-9

Rr
Segregation of
alleles into eggs
Rr
Segregation of
alleles into sperm
Sperm
1/
R
2
R
1/
2
r
R
R
Eggs
1/
4
r
2
r
2
R
1/
1/
1/
4
r
r
R
r
1/
4
1/
4
161
Partial, intermediate or incomplete
dominance: The traits are not completely
dominant. An example is the color of the
flowers of snapdragons. A cross between a
homozygous red and a homozygous white
yields all pink snapdragons. When these
pink snapdragons are crossed among
themselves, they yield 1 red, 2 pink and 1
white (Fig. 14.9, p. 256).
Fig. 14-10-1
P Generation
Red
CRCR
Gametes
White
CWCW
CR
CW
Fig. 14-10-2
P Generation
Red
CRCR
White
CWCW
CR
Gametes
CW
Pink
CRCW
F1 Generation
Gametes
1/
2
CR
1/
2
CW
Fig. 14-10-3
P Generation
Red
CRCR
White
CWCW
CR
Gametes
CW
Pink
CRCW
F1 Generation
Gametes
1/
2
CR
1/
CW
2
Sperm
1/
2
CR
1/
2
CW
F2 Generation
1/
2
CR
Eggs
1/
2
CRCR
CRCW
CRCW
CWCW
CW
162
Multiple Alleles:
Some genes exist in more than two allelic
forms. An example is the human ABO
blood types. The genes are said to be
codominant as they both express in
heterozygous individuals.
Fig. 14-11
Allele
IA
Carbohydrate
A
IB
B
i
none
(a) The three alleles for the ABO blood groups
and their associated carbohydrates
Genotype
Red blood cell
appearance
Phenotype
(blood group)
IAIA or IA i
A
IBIB or IB i
B
IAIB
AB
ii
O
(b) Blood group genotypes and phenotypes
162
Blood Agglutinogens
(Antigens)
A
A
B
B
AB
A&B
O
none
Agglutinins Genotypes
(Antibodies)
B
AA or AO
A
BB or BO
none
AB
A&B
OO
162-163
Pleiotropy:
The ability of a single gene to have multiple
effects. An example is sickle-cell anemia.
It is caused by a single gene which causes
complex sets of symptoms. The genes that
control fur pigmentation may also cause
cross-eye in animals.
Epistasis:
It is a condition in which a gene at one
locus alters the phenotypic expression of a
gene at a second locus.
163
B: able to produce pigment
b: unable to produce pigment
C: able to store pigment
c: unable to store pigment
P
F1
F2
B/B C/C x b/b c/c
|
B/b C/c x B/b C/c
|
9 B/- C/- 3 B/- c/c 3 b/b C/(black)
(brown) (white)
1 b/b c/c
(white)
Fig. 14-12
BbCc

BbCc
Sperm
1/
4 BC
1/
4 bC
1/
4 Bc
1/
4 bc
Eggs
1/
1/
1/
1/
4 BC
BBCC
BbCC
BBCc
BbCc
BbCC
bbCC
BbCc
bbCc
BBCc
BbCc
BBcc
Bbcc
BbCc
bbCc
Bbcc
bbcc
4 bC
4 Bc
4 bc
9
: 3
: 4
163
Polygenic Inheritance:
Polygenic traits are influenced by many genes in an
additive way. They are qualitative traits as they
vary in a continuous way. The examples are
human height, skin pigmentation, and IQ.
Phenotype is determined by genes and
environmental factors. The phenotypic range is
called the norm of reaction. Height, skin color
and IQ show a continuous variation. Human ABO
blood group does not show a continuous
phenotypic expression. A person can be one of
the four blood types.
Fig. 14-13

AaBbCc
AaBbCc
Sperm
1/
Eggs
1/
8
1/
8
1/
8
1/
8
1/
1/
8
1/
1/
8
8
1/
8
1/
64
15/
8
1/
1/
8
8
8
1/
8
1/
8
1/
8
8
Phenotypes:
Number of
dark-skin alleles:
1/
64
0
6/
64
1
15/
64
2
20/
3
64
4
6/
64
5
1/
64
6
164-165
A family tree or pedigree describes the
interrelationships of parents and children
across the generations.
Recessively Inherited Disorders:
Cystic fibrosis:
Tay-Sachs disease:
Sickle-cell anemia:
Dominantly Inherited Disorders:
Achondroplasia:
Huntington’s disease
Fig. 14-17
Parents
Dwarf
Dd

Normal
dd
Sperm
D
d
d
Dd
Dwarf
dd
d
Dd
Dwarf
Eggs
Normal
dd
Normal
166
Fetal Testing:
Amniocentesis: 14th-16th weeks of pregnancy
Chorionic villi sampling (CVS): 8 to 10 weeks
Ultrasound:
Fetoscopy:
Newborn Screening:
Phenylketonuria (PKU): cannot break down
phenylalanine to tyrosine. The amino acid and its
by-product phenylpyruvic acid accumulate in
toxic levels in the blood, causing mental
retardation as the brain cells cannot develop.
Fig. 14-18
Amniotic fluid
withdrawn
Centrifugation
Fetus
Fetus
Placenta
Placenta
Cervix
Uterus
Suction tube
inserted
through
cervix
Chorionic
villi
Fluid
Fetal
cells
Biochemical
tests
Several
hours
Several
weeks
Several
weeks
(a) Amniocentesis
Karyotyping
Several
hours
Fetal
cells
Several
hours
(b) Chorionic villus sampling (CVS)
Fig. 14-18a
Amniotic fluid
withdrawn
Centrifugation
Fetus
Placenta
Cervix
Uterus
Fluid
Fetal
cells
Several
hours
Biochemical
tests
Several
weeks
Several
weeks
(a) Amniocentesis
Karyotyping
Fig. 14-18b
Fetus
Placenta
Biochemical
tests
Karyotyping
Suction tube
inserted
through
cervix
Chorionic
villi
Several
hours
Fetal
cells
Several
hours
(b) Chorionic villus sampling (CVS)