Chapter 11
MENDELIAN PATTERNS OF INHERITANCE
Early Ideas about Heredity
People knew that sperm and eggs transmitted
information about traits
 Blending theory
 Problem:



Would expect variation to disappear
Variation in traits persists
Gregor Mendel
 1st
to apply mathematics
to biology
 Careful, delibrate
 Well prepared
 Garden Pea




Easy to grow
Self/cross pollination
Many varieties
Used discrete traits, easily
observed
Tracking Generations

Parental generation
P
mates to produce

First-generation offspring
F1
mate to produce

Second-generation offspring
F2
Monohybrid Crosses
 Experimental intercross between
 two F1 heterozygotes
AA X aa
Aa (F1 monohybrids)
Aa X Aa
?
Mendel’s
Monohybrid
Cross Results
F2 plants showed
dominant-torecessive ratio that
averaged 3:1
5,474
round
1,850
wrinkled
6,022
yellow
2,001
green
882
inflated
299
wrinkled
428
green
152 yellow
705
purple
224
white
651 long
stem
207 at
tip
787
tall
277
dwarf
Fig. 11-6, p. 172
Genes
Units of information
about specific traits
 Passed from parents
to offspring
 Each has a specific
location (locus) on a
chromosome

Allele
Different molecular
forms of a gene
 Arise by mutation
 Dominant allele
masks a recessive
allele that is paired
with it

Allele Combinations

Homozygous




having two identical alleles at a locus
AA or aa
true-breeding
Heterozygous



having two different alleles at a locus
Aa
hybrids
Genotype & Phenotype

Genotype refers to particular genes an individual
carries

Phenotype refers to an individual’s observable
traits

Cannot always determine genotype by observing
phenotype
Check Progress
 B = black, b= white
 If a heterozygous rabbit reproduces with one of its
own kind, what phenotypic ratio do you expect
among the offspring?
 If there are 120 rabbits, how many expected to be
white?
11.2 Mendel’s laws
Law of Segregation
Law of Independent
Assortment
 Each indv has 2 alleles
 Each pair of alleles
for each trait
 Alleles separate when
gametes form
 Gametes have 1 allele
for each trait
 Fertilization gives new
indv 2 alleles
assorts indepently
 All possible combo of
alleles can occur
 Applies to alleles on
diff c-somes
 Expected ratio 9:3:3:1
Dihybrid Cross
 Experimental cross between individuals that are
homozygous for different versions of two traits
 In fruit flies, L = long wings and l = short wings; G =
gray body and g = black body. List all possible
gametes for a heterozygote
Dihybrid Cross: F1 Results
purple
flowers,
tall
TRUEBREEDING
PARENTS:
AABB
GAMETES:
AB
x
white
flowers,
dwarf
aabb
AB
ab
ab
AaBb
F1 HYBRID
OFFSPRING:
All purple-flowered, tall
meiosis,
gamete
formation
1/4
AB
1/4
Ab
aB
1/4
ab
1/4
AB
1/4
aB
1/4
Ab
1/4
ab
1/16
AABB
1/16
AABb
1/16
AaBB
1/16
AaBb
1/16
AABb
1/16
AAbb
1/16
AaBb
1/16
Aabb
1/16
AaBB
1/16
AaBb
1/16
aaBB
1/16
aaBb
1/16
AaBb
1/16
Aabb
1/16
aaBb
1/16
aabb
Fig. 11-9b, p.175
Laws and Meiosis
 Doesn’t matter which way the homologous pair faces
during metaphase I (2nd law)
 Metaphase II cell has one chromosome (1st law)
 Gametes can have all possible combinations of alleles
(both laws)
OR
 See p. 196
Test Cross

Individual that shows dominant phenotype is
crossed with individual with recessive phenotype

Examining offspring allows you to determine the
genotype of the dominant individual
Autosomal Recessive
Inheritance Patterns
If parents are both
heterozygous, child
will have a 25%
chance of being
affected
Fig. 12-10b, p. 191
Examples
 Methemoglobinemia
 Cystic Fibrous
 Niemann-Pick
Autosomal
Dominant Inheritance
Trait typically
appears in every
generation
Fig. 12-10a, p. 190
Achondroplasia
Autosomal dominant
allele
In homozygous form
usually leads to
stillbirth
Heterozygotes
display a type of
dwarfism

Have short arms and legs relative
to other body parts
Huntington
Disorder
Autosomal dominant
allele
Causes involuntary
movements, nervous
system deterioration,
death
Symptoms don’t usually
show up until person is
past age 30
People often pass allele
on before they know
they have it
ABO Blood Type
Range of genotypes:
IAIA
IBIB
or
or
IAi
Blood
Types:
A
IAIB
AB
IBi
ii
B
O
Fig. 11-10a, p.176
Codominance: ABO Blood Types
Gene that controls ABO type codes for enzyme that
dictates structure of a glycolipid on blood cells
 Two alleles (IA and IB) are codominant when
paired
 Third allele (i) is recessive to others
 Also known as Multiple Alleles

Incomplete
Dominance
X
Incomplete Homozygous
parent
Dominance
Homozygous
parent
•Intermediate phenotype
All F1 are
heterozygous
•Familial hypercholesterolemia
•CF heterozygous
•Incomplete pentrance
•polydactyly
X
F2 shows three phenotypes in 1:2:1 ratio
 The allele for albinism (c) is recessive to allele for
normal pignmentation (C). A normally pigmented
woman whose father is an albino marries a man
whose parents are normal. They have 3 children, 2
normal and 1 albino. Give the genotypes for each
person.
 In garden peas, one pair of alleles controls the height
of the plant and a second pair of alleles controls
flower. The allele for tall (D) is dominant to allele for
dwarf (d), and the allele for purple (P) is dominant to
the allele for white (p). A homozygous tall purple
plant is crossed with a dwarf white plant.
 What is the appearance of F1 and F2?
Epistasis
Interaction between the products of gene pairs
 Common among genes for hair color in mammals

Examples
 Antocyanin pigments
 Start molecule colorless Intermediate colorless  Purple
 Lab coat color
 Start molecule melanin  deposition
 Eye color humans
 Blue 1  Blue 2  Green  Brown
Coat Color in
Retrievers
BBEE
X
bbee
F1 puppies
are all BbEe
F2 puppies
BE
Be
bE
be
BE
BBEE
BBEe
BbEE
BbEe
Be
BBEe
BBee
BbEe
Bbee
bE
BbEE
BbEe
bbEE
bbEe
be
BbEe
Bbee
bbEe
bbee
black
brown
yellow
Pleiotropy
Alleles at a single locus may have effects on two or
more traits
 Marfan syndrome - Mutation in gene for fibrillin
affects skeleton, cardiovascular system, lungs,
eyes, and skin

Polygenic

A more or less continuous range of small
differences in a given trait among individuals

The greater the number of genes and
environmental factors that affect a trait, the more
continuous the variation in versions of that trait
Continuous
Variation
Variation in human
eye color
Eye color Calculator
Fig. 11-18, p.180
Temperature Effects
on Phenotype
Rabbit is homozygous for an
allele that specifies a heatsensitive version of an enzyme in
melanin-producing pathway
 Melanin is produced in cooler
areas of body

Figure 11.16
Page 179
Environmental Effects on Plant
Phenotype
Hydrangea
macrophylla
 Action of gene
responsible for floral
color is influenced by
soil acidity
 Flower color ranges
from pink to blue

Elevation and the yarrow
Fig. 11-17b, p.179
X-Linked Recessive Inheritance
Males show disorder
more than females
Son cannot inherit
disorder from his
father
Fig. 12-10, p.194
Thomas Morgan
1900’s
Red female x white
male = all red
F2 = 3 red: 1 white
male
Supported
chromosome theory
of inheritance
Males = hemizygous
Fig. 12-9, p.193
Examples of X-Linked Traits
Color blindness
Hemophilia
Menkes syndrome
Muscular Dystrophy
Adrenoleukodystrohy (ALD)
Color Blindness
Fig. 12-12, p.195
Color Blindness
Fig. 12-12, p.195
1 XrXr
2 XRY
3 XRY
4 XRXr
5XrY
6 XRXr
7 XrY
8 XrXr
9 XrY
10XRX
11 XRY
12 XRX
13 XrY
14 XrXr
15 XrY
Hemophilia
Fig. 12-11, p.194