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Modern Genetics
Part 5
History
 Gregor Mendel: “Father of Genetics” Austrian Monk
(1822-1884)
 Published the results of scientific research on Garden
Peas (Pisium sativum) in 1865.
Why study peas?
 Great natural variation- stem length, seed color, pod
shape ,pod color, small, edible, easy to grow, many
offspring, easy to cross fertilize
Useful Terms
 Trait: characteristic of an organism.
 Gene: piece of DNA that codes for a protein.
 Allele: different forms of a gene.
 Genotype: The allele combination of an individual.
 Phenotype: The visible characteristics that result
from a genotype.
Chromosomes and Human Genetics
I.
Human Chromosomes
A.
Types
1.
Sex Chromosomes – 1 pair – carry the genes that
determine male and female features (also some
non-sex traits)
1.
X and Y do not look alike but behave as a
homologous pair at meiosis
2. XX = female, XY = male
2.
Autosomes – non-sex chromosomes (22 pairs) –
genes are unrelated to sex determination
PUNNETT SQUARE
Method for determining possible allele combination for the offspring
Gametes outside
Offspring Inside
Determining Sex…
X
X
MOM
X
X X
X X
X Y
X Y
DAD
Y
One Example of Mendel’s Work
P
Tall
DD
x
Dwarf
dd
Homozygous
Dominant
Homozygous
Recessive
All Tall
Dd
F1
Clearly Tall is Inherited…
What happened to Dwarf?
Heterozygous
1.
Tall is dominant to Dwarf
2.
Use D/d rather than T/t for
symbolic logic
F1 x F1 = F2
Punnett Square:
F2
possible
gametes
Phenotype
Genotype
possible gametes
D
d
D
Tall
DD
Tall
Dd
d
Tall
Dd
Dwarf
dd
3/
Tall
1/ Dwarf
4
4
Dwarf is not missing…just masked
as “recessive” in a diploid state…
there IS a female contribution.
Sample Problem
 In hamsters, white fur color (W) is dominant to brown
fur color (w). If you cross a heterozygous female with
white fur color (Ww) with a male that has brown fur
(ww), what genotypes and phenotypes would you
see and in what ratios?
Genetics After Mendel
P
F1
Red
PRPR
Yellow
PYPY
x
When these alleles go walking, they both do
some talking (codominance)!
All Orange
PRPY
OK, so we cannot use R/r nor Y/y so we pick
a third letter…P for the petal color gene.
Notice: we do NOT mix R/Y or r/y!
F1 x F1 = F2
Punnett Square:
F2
possible
gametes
After 1900 several scientists tried to
replicate Mendel’s crosses using other
species including snapdragon.
possible gametes
PR
PY
PR
Red
PRPR
Orange
PRPY
This F2 will NOT have a 3:1 ratio
of phenotypes.
PY
Orange
PRPY
Yellow
PYPY
Instead it shows a 1:2:1 ratio!
The exception here proves the rule.
Sample Problem
 In horses, black and white coat colors are codominant.
Heterozygous horses have gray coats.
Black horse genotype: HBHB
White horse genotype: HWHW
Gray horse genotype: HBHW
What would be the possible genotypes and phenotypes of
the following crosses?
Black x White
Gray x Gray
Blood Types
Blood Type:
A
B
 AB
O
Genotype
 I AIA , IAi
 I BIB , I Bi
 IA IB
 ii
1.
Try These
If a male is homozygous for blood type B and a female is
heterozygous for blood type A, what are the possible blood
types in the offspring?
2)
Is it possible for a child with Type O blood to be born
to a mother who is type AB? Why or why not?
Chromosomes and Human
Genetics
I.
Human Chromosomes
A.
Types
1.
Sex Chromosomes – 1 pair – carry the genes that
determine male and female features (also some
non-sex traits)
1.
X and Y do not look alike but behave as a
homologous pair at meiosis
2. XX = female, XY = male
2.
Autosomes – non-sex chromosomes (22 pairs) –
genes are unrelated to sex determination
B. Determining Sex…
X
X
MOM
X
X X
X X
X Y
X Y
DAD
Y
II. Gene Location
A.
Linked – Linkage Groups – genes located so close together on a
chromosome that the traits always seem to appear together
Ex. Red hair and freckles
Ex. Colorblindness and Hemophilia
XX
B.
Sex-linked Traits – genes on the sex chromosomes
-
Expression of certain genes often appears more in one sex than
the other
Males require only one copy of a gene since they only have one X
chromosome
See Royal Families of Europe Pedigree
Ex. Eye color in fruit flies, hemophilia, color-blindness
Colorblindness Tests
X-Linked/Sex Linkage – do not
write
 Genes present on the X chromosome exhibit unique
patterns of inheritance due to the presence of only
one X chromosome in males.
 X-linked disorders show up rarely in females
 X linked disorders show up in males whose mothers
were carriers (heterozygotes)
Practice Sex-linked Problems….
 What will the result of mating between a normal (noncarrier) female and a hemophiliac male?
 A female carrier who is heterozygous for the recessive, sex-linked
trait causing red-green colorblindness, marries a normal male.
What proportions of their MALE progeny will have red-green
colorblindness?
 Hemophilia is inherited as an X-linked recessive. A woman
has a brother with this defect and a mother and father
who are phenotypically normal. What is the probability
that this woman will be a carrier if she herself is
phenotypically normal?
C. Gene Interactions
1.
2.
Polygenetic trait – many genes influence a single
trait (ex. Height, intelligence)
Pleiotropic effect – one gene having many effects
(ex. Gene to make testosterone)
Pleiotropy
 Expression of a single gene has
multiple phenotypic effects
 Marfan Syndrome – abnormal gene
that makes fibrillin (important in
connective tissues)
?
III. Genetic Analysis
A.
Karyotype – visualized chromosomes stained,
squashed, and photographed at metaphase
- They are characteristic of the species or individual
B. Pedigree – chart showing family
relationships (see worksheet)
Pedigree Analysis
 Method of tracking a trait
through generations within a
family.
 Good method of tracking
sex-linked traits as well as
autosomal traits.
Sex-Linked Pedigree
 Shows gender bias with
males exhibiting the
trait more often than
females
Autosomal Dominant Pedigree
 Autosomal dominant traits
do not skip a generation
 Autosomal dominant traits
do not show gender bias
Autosomal Recessive Pedigree
 Autosomal recessive
traits skip a generation
 Autosomal recessive
traits do not show
gender bias
IV. Non-Mendelian Genetics
1.
Incomplete Dominance – blended inheritance
-
Neither form of the gene is able to mask the other
Ex. Snap dragon petal color
R1R1 – RED
R1R2 – PINK
R2R2 - WHITE
Incomplete
Dominance
 Neither allele is dominant
 Heterozygotes are a blend of
homozygous phenotypes = no
distinct expression of either
allele
Try these
 In a plant species, if the B1 allele (blue flowers) and
the B2 allele (white flowers) are incompletely
dominant (B1 B2 is light blue), what offspring ratio is
expected in a cross between a blue-flowered plant
and a white-flowered plant?
•What would be the phenotypic ratio of
the flowers produced by a cross between
two light blue flowers?
2. Codominance
 No dominance and both alleles are completely
expressed
 Ex. Cat color
 C1C1 – Tan
 C1C2 – Tabby (black and tan spotted)
 C2C2 - Black
Try These
1.
Cattle can be red (RR = all red hairs), white (WW
= all white hairs), or roan (RW = red & white hairs
together.
a.
Predict the phenotypic ratios of offspring when a
homozygous white cow is crossed with a roan bull.
b.
What should the genotypes & phenotypes for parent cattle
be if a farmer wanted only cattle with red fur?
1.
A cross between a black cat & a tan cat produces a
tabby pattern (black & tan fur together).
a. What pattern of inheritance does this illustrate?
b. What percent of kittens would have tan fur if a
tabby cat is crossed with a black cat?
3. Multiple Alleles
 More than 2 alleles for one trait
 Ex. Eye color, hair color, blood type, guinea pig fur color
 ABO blood groups




Each individual is A, B, AB, or O phenotype
Phenotype controlled by marker on RBC
IA and IB alleles are dominant to the i allele
IA and IB alleles are codominant to each other
Blood Types
Blood Type:
A
B
 AB
O
Genotype
 I AIA , IAi
 I BIB , I Bi
 IAIB
 ii
1.
Try These
If a male is homozygous for blood type B and a female is
heterozygous for blood type A, what are the possible blood
types in the offspring?
2)
Is it possible for a child with Type O blood to be born
to a mother who is type AB? Why or why not?
3.
A child is type AB. His biological mother is also type
AB. What are the possible phenotypes of his biological
father?
Human hair color follows a similar pattern:
Alleles: HBn = brown HBd = blonde hR = red hbk = black
HBnHBn = dark brown HBdHBd = blonde
HBnHBd = sandy brown HBdhR = strawberry
HBnhR = auburn
blonde
HBnhbk = dark brown HBdhbk = blonde
Dominant does NOT mean frequent!
hRhR = red
hRhbk = red
hbkhbk = black
Recessive can
be common!