Blending Theory – People believed that offspring were a
blending of the two parent’s genetic material.
Gregor Mendel (1865) – An Austrian Monk disproved this
theory by performing experiments on pea plants.
He found that when you mixed a pure breeding tall plant
with a pure breeding short plant, all the offspring were
tall plants.
Then when he crossbred the first generation, he found
that he got tall plants and short plants in a 3:1 ratio.
Mendel suggested that the traits were contained in gene
pairs called Alleles. In breeding, each parent would
contribute 1 allele. Usually, there was a dominant allele
for one trait and a recessive allele for the alternative
trait.
Codominance – Mendel also noticed that in some species
of flowers, the alleles shared their dominance, rather
than one dominating the other.
Vocabulary
Allele: One alternative of a pair or group of genes (TT or Tt)
Dominant: An allele producing the same phenotypic effect
whether inherited heterozygously or homozygously; an allele that
"masks" a recessive allele.
Gene: A unit of genetic information that occupies a specific
position on a chromosome and comes in multiple versions called
alleles.
Genotype: The genetic constitution of an organism.
Heterozygous: Having a genotype with two different and
distinct alleles for the same trait.
Homozygous: Having a genotype with two of the same alleles
for a trait.
Phenotype: The physical or observable characteristics of an
organism.
Recessive: An allele producing no phenotypic effect when
inherited heterozygously and only affecting the phenotype when
inherited homozygously; an allele "masked" by a dominant allele.
Problem #1
The ability to roll the tongue is dominant over the inability to do so in humans.
1. If two heterozygous tongue-rollers have children, what genotypes could their
children have?
2. If a non-tongue-roller has children with a homozygous tongue-roller, what will their
children's genotypes be?
Problem #1: Basic Monohybrid Cross
The ability to roll the tongue is dominant over the inability to do so in humans.
(A) If two heterozygous tongue-rollers have children, what genotypes could their
children have?
Using T=tongue-rolling and t=non-tongue-rolling, the genotypes and ratios are - 1 TT : 2 Tt : 1 tt. See
Punnett square below.
T
t
T
TT
Tongue-roller
Tt
Tongue-roller
t
Tt
tt
Tongue-roller Non-tongue-roller
(B) If a non-tongue-roller has children with a homozygous tongue-roller, what will their
children's genotypes be?
All the children will be tongue-rollers. See Punnett square below.
T
T
t
Tt
Tt
Tongue-roller Tongue-roller
t
Tt
Tt
Tongue-roller Tongue-roller
Problem #2
In an alien species, eye colors can be black, white, or gray. A white-eyed mother and a
black-eyed father have all gray-eyed children.
1. What genetic pattern is most likely demonstrated here?
2. A gray-eyed couple has two babies: one white-eyed and one black-eyed. Does this
confirm or deny your answer to part A? If necessary, revise your hypothesis.
3. A gray-eyed alien and her black-eyed husband want a white-eyed baby. Is this
possible?
Problem #2
In an alien species, eye colors can be black, white, or gray. A white-eyed mother and a
black-eyed father have all gray-eyed children.
(A) What genetic pattern is most likely demonstrated here?
The genetic pattern most likely demonstrated in this example is that of codominance, or incomplete
dominance.
(B) A gray-eyed couple has two babies: one white-eyed and one black-eyed. Does this
confirm or deny your answer to part A? If necessary, revise your hypothesis.
This new evidence confirms the hypothesis of incomplete dominance. See Punnett square below (W=black
eyes, w=white eyes).
W
w
W
WW
Ww
Black eyes Gray eyes
w
Ww
ww
Gray eyes White eyes
(C) A gray-eyed alien and her black-eyed husband want a white-eyed baby. Is this
possible?
This is not possible. See Punnett square below (W=black eyes, w=white eyes).
W
w
W
WW
Ww
Black eyes Gray eyes
W
WW
Ww
Black eyes Gray eyes
Problem #3
A mother has blood type A and genotype AO. A father has blood type B and genotype
BO. What blood types are possible for the offspring?
Problem #3
A mother has blood type A and genotype AO. A father has blood type B and genotype
BO. What blood types are possible for the offspring?
The genotypes AO (type A blood), BO, (type B blood), AB (type AB blood), and OO (type O blood) are all
possible. See Punnett square below.
A
O
B
AB
BO
Type AB Type B
O
AO
OO
Type A Type O
Problem #4
Pattern baldness is sex-linked, but cannot be expressed normally in females.
1. If a carrier female marries a pattern-bald man, what are the possible genotypes of
their children?
2. If female is homozygous for pattern baldness (but does not express the trait due to
hormones), and she marries a normal man, what percentage of her sons will be
pattern-bald?
Problem #4
Pattern baldness is sex-linked, but cannot be expressed normally in females.
(A) If a carrier female marries a pattern-bald man, what are the possible genotypes of
their children?
Using XB as bald and X as normal, the genotypes possible are: XBXB, XXB, XBY, XY.
XB
XB
Y
X
XBXB
XXB
Affected female
Carrier female
(not expressed)
XBY
Affected male
XY
Normal male
(B) If female is homozygous for pattern baldness (but does not express the trait due to
hormones), and she marries a normal man, what percentage of her sons will be patternbald?
Using XB as bald and X as normal, the genotypes possible are: XBX, XXB, XBY, XBY. Therefore her sons will
all be pattern-bald.
XB
XB
X
XBX
XBX
Carrier female Carrier female
Y
XBY
XBY
Affected male Affected male