Observing Patterns in
Inherited Traits
Chapter 10
Before you go on…
Review the answers to the following questions to test your
understanding of previous material.
1.
2.
3.
4.
5.
6.
Most organisms are diploid. What does this mean in terms of
inheritance?
What is the smallest unit of inheritance, and how do they
function?
What do you call alternate forms of a gene? How do these
arise? Can there be more than two?
How are gametes produced, and how do you describe the
genetic information that is normally contained in each?
Explain Independent Assortment, which describes the
behavior of genes (on different chromosomes) during Meiosis.
Discuss linked genes, and the significance of crossing over.
When and how does this happen?
Learning Objectives
1. Explain the process Mendel used in his experiments with garden peas,
and what his results added to our understanding of inheritance.
2. Construct a Punnett square demonstrating a monohybrid cross
between one homozygous dominant individual and one homozygous
recessive individual. Then use this method to predict the genotypes
and phenotypes of a cross between 2 of these offspring (F1 = first filial
generation). Use correctly the terms: parental generation, first filial (F1)
generation and second filial (F2) generation. What is the predicted
phenotypic ratio in the F2 generation?
3. Construct a Punnett square demonstrating a dihybrid cross between
heterozygotes for both traits. Predict the phenotypes that would result.
What is the predicted phenotypic ratio? Is this a good illustration of the
principle of independent assortment?
4. Discuss the effect on the phenotype for traits governed by codominant
alleles, alleles which demonstrate incomplete dominance, and multiple
allele systems. Provide some examples of each.
5. Differentiate between polygenic inheritance and pleiotrophy, with
examples.
6. How is gender inherited, and what is the predicted phenotypic ratio?
What are sex-linked genes: how are they inherited, how does one
become a “carrier” for a sex-linked trait, and how do you predict the
inheritance pattern for these genes?
Model Organisms
• Rapid Reproduction
• Produce large # of offspring)
• Few chromosomes
• Ease of propagation
Model Organisms
Organism
N
# genes
bacteria
NA
4288
yeast
16
6,022
roundworm
5
14,000
fruit fly
4
15,000
dicot plant
5
25,000
mouse
40
100,000
human
23
100,000
pea
7
30,000
*
* Although not all of these code directly for the production of
proteins.
p. 152
Mendelian
Genetics
Gregor Mendel
(1822-1884)
A botanist and
• Described the variation
mathematician
To this day,
Mendel’s reproducing
in sexually
principles accurately
species.
describe
the
inheritance of traits.
The Garden Pea Plant
• Self-pollinating
• True-breeding (normally selfpollinating, so different alleles not
introduced)
• Can be experimentally crosspollinated
p. 153
Mendel’s Peas
p. 154
Mendel studied these 7 characters.
How many traits were studied?
What do these results tell us about
allele dominance for these
characteristics?
Independent Assortment
• Mendel recognized “units” of inheritance that
governed specific traits
p. 156
– e.g. pink or white flowers
– We know these units as alleles
• Mendel concluded that the two “units” for the first
character were to be assorted into gametes
independently of the two “units” for the other
character
– This is true if the genes for the two characters are located on
different chromosomes
– e.g. flower color and pod shape are not linked
– In other words, homologous chromosomes separate and are
sorted into gametes at random during meiosis
p. 153
Genetic Terms
Dominant allele (Y)
will mask a recessive
allele (y) that it is
paired with as a
result of fertilization.
A pair of homologous
chromosomes
(one from egg; one from sperm)
A gene locus (i.e. location)
A pair of alleles
(Yy, YY, or yy)
Three pairs of genes
Genotypes:
If both alleles the
same: homozygous
If alleles different:
heterozygous
What is the difference between
the terms phenotype and
genotype?
Monohybrid Cross:
What did Mendel learn from this experiment?
p. 154
Generation
Parental (P)
True-breeding
yellow pea
(pollen)
True-breeding
green pea
(eggs)
x
grow plants,cross
pollinate
allow to self-fertilize
First Filial (F1)
all yellow
6022 yellow : 2001 green
Second Filial (F2)
3:1
What color are the peas
in this plant?
Alleles…
When this diploid plant
produces a gamete,
what allele(s) might the
gamete contain?
Pea color alleles
Y
y
…on Homologous
Chromosomes
If allowed to selfpollinate, and
fertilization is random,
what are the possible
combinations of alleles
in the resulting
offspring, and what will
their peas look like?
p. 153
Genotype
Phenotype
Punnett Square of a
Monohybrid cross
Yellow x Yellow
During gamete formation, the
alleles segregate from each other
YY x Yy = ?
Y
Y
Y
YY
YY
y
Yy
Yy
Genotypic ratio? 1:1, YY:Yy
Phenotypic ratio? 4:0, yellow: green
p. 155
Punnett Square of a
Monohybrid Cross
Aa X Aa
Female gametes
A
A
a
AA
Aa
Male
gametes
1:2:1
a
p. 155
Genotypic ratio?
Aa
aa
Phenotypic ratio?
3:1
Test Cross
Pea with
unknown
genotype
Y? x yy
• the phenotype of the offspring
will reveal the genotype of the
unknown parent.
Y
y
y
Yy
yy
y
Yy
yy
• If offspring result in ½ yellow
and ½ green what is the
parental genotype?
p. 155
Complete Dominance
• Unattached
earlobes
• Widow’s peak
• Long eyelashes
• Freckles
• Cleft chin
• Certain diseases:
e.g. cystic fibrosis
p. 153
Many traits are not expressed
via complete dominance.
• Sex linked traits
Which chromosome(s) carries these alleles?
• Incomplete dominance (p. 158)
Is this the same as “equally dominant” aka co-dominant?
• Polygenic inheritance (p. 158)
“poly” means….
• Pleiotrophy & Epistasis (p. 159)
Differentiate between these forms of gene expression.
• Environmental controls (p. 162)
Provide at least one example of how the environment can
affect gene expression.
Sex linked genes
• Fathers pass sex linked alleles to all of
their daughters but none of their sons.
Mothers pass alleles to both.
b
XY
b
–Color blindness
–Hemophilia
–Male pattern baldness
XX
b
p. 169, 175
Flower Color in Snapdragons:
Incomplete Dominance
Pink-flowered plant X Pink-flowered plant
(heterozygote)
(heterozygote)
White-, pink-, and red-flowered plants
in a 1:2:1 ratio
p. 158
Incomplete Dominance
Hair texture in
Caucasians
Curly hair is shaped like an elongated
oval and grows at a sharp angle to the
scalp.
RR: curly
Rr: wavy
rr: straight
ABO Blood Type:
Co-dominance in a
Multiple Allele System
• Type A - IAIA or IAi
Identify the blood type known as the:
Universal donor…
• Type B - IBIB or IBi
• Type AB - IAIB
• Type O - ii
p. 158
Universal recipient …
Polygenic
Inheritance &
Continuous
Variation
• Genes A, B, C each
contribute a unit of
darkness.
AABBCC: very dark
AaBbCc: intermediate
darkness
aabbcc: very light
p. 160
The cumulative effect of
multiple genes on one
phenotype.
The cumulative effect
of multiple genes on
one phenotype.
Pleiotropy
• Alleles at a single
locus may have
effects on two or
more traits
• Classic example is
the effects of the
mutant allele at the
beta-globin locus
that gives rise to
sickle-cell anemia
– Cell shape and
resistance to malaria
p. 159
Why has this
disease not been
eliminated by
natural selection?
Epistasis:
gene expression
dependent upon two or more genes.
Melanin
• BB = black lab
• Bb = black lab
• bb = brown lab
How much melanin?
• EE = full deposition
• Ee = full deposition
• ee = no deposition
p. 160
Epistasis cont.
• BBEE, BbEE, BBEe,
or BbEe = black
Must have at least one dominant
allele at both loci for black
pigment to be deposited
• BBee, Bbee, or bbee
= yellow
Pigment is not deposited
• bbEE = chocolate
• bbEe = chocolate
Brown pigment is deposited
Don’t for get the
world around you
Acidity of the soil changes the
color of hydrangea flowers.
The expression
of some genes
is heavily
influenced by
environmental
factors.
p. 162
Don’t for get the world
around you
• Being in the sun darkens
our skin
• Ratio of red : white blood
cells are affected by
exercise, injury, and illness.
• Siamese cat and Himalayan
rabbit: fur color is
dependent upon
temperature.