Complex Inheritance Patterns
1. Incomplete Dominance – phenotype of
heterozygous is intermediate of the 2
homozygous phenotypes
- results in the blending of traits
- example: snap dragons
• Alleles:
R = red
R’ = white
• Genotypes/Phenotypes
red
RR = __________
white
R’R’ = _________
pink
RR’ = __________
Example Problem:
Red snapdragon crossed with a pink snapdragon
RR
RR’
_______
X _______
Results:
2
50
___
of 4 red = _____%
2 of 4 pink = _____%
50
___
0 of 4 white = _____%
0
___
human example of incomplete dominance:
sickle cell anemia (on the clinical level)
• most common in Africans/African Americans
• 1 in 12 black people are carriers
• causes red blood cells to form sickle shape
• sickled cells block blood flow
• symptoms: shortness of breath, pain in joints, anemia,
fatigue
• being a carrier is an advantage against malaria
– alleles:
A = normal
S = sickle cell
genotypes/phenotypes:
normal
AA = _________________
(no sickling)
carrier
AS = _________________
(few cells will sickle;
extremely mild symptoms)
sickle cell anemia (distinct sickling;
SS = _________________
full-fledged symptoms)
Problem: 2 carriers have a child. What is the
chance that the child will have SCA?
AS
AS
_______
X _______
25%
% chance ______
2. Codominance – both phenotypes of
homozygous individuals show up in
heterozygous
-
Results in the expression of both traits
Example: chickens
alleles:
B = black
W = white
genotypes/phenotypes:
black
BB = __________
white
WW = _________
checkered
BW = __________
Problem: Cross 2 checkered chickens.
What % of the offspring will be white?
BW
BW
_______
X _______
1 of 4 is white = ______%
25
___
white
Another Codominance Example
• Roan cow – result of cross between a red cow
and a white cow
3. Multiple alleles – there are more than two
forms (alleles) for the trait
Human example: ABO blood typing
alleles:
IA , IB , i
IA and IB are codominant to each other
IA and IB are dominant to __i__
genotypes/phenotypes:
IAIA and IAi = _______________
type A blood
type B blood
IBIB and IBi = _____________
type AB blood
IAIB = ____________
type O blood
ii = ___________
Importance of blood typing:
• parentage
• blood transfusions
incompatible blood can result in blood clotting,
then death
Problem: A woman with blood type O has a
baby with blood type O. Her husband has
blood type AB. Could the baby be his?
Problem: A woman with blood type A has a
baby with blood type O. Her husband has
blood type B. Could the baby be his?
• Transfusion compatibility:
• A and B are surface proteins on red blood cells
• You CANNOT accept blood from someone who has a
protein that you do not have!
Acceptor
A
D
o
n
o
r
A
B
AB
O
B
AB
O
Rh factor – another surface protein
_+_ = has Rh protein
_-_ = does not have Rh protein
(-) can give to (+)
(+) cannot give to (-)
So, O- can give blood to anyone = universal donor
AB+ can get blood from anyone = universal acceptor
If a woman is Rh - and is pregnant with a baby
who is Rh + , her body can attack her next Rh+
child.
Problem for 2nd child!!
Test for Blood Typing
4.
Sex Determination
Humans have 22 pairs of autosomes
(chromosomes shared by males and females)
and 1 pair of sex chromosomes (X and Y) =
__23__ pairs = __46__ total per cell.
Humans:
XX = female
XY = male
Problem:
– A man and a woman have 5 sons. What is the
chance their next child will be a girl?
ALWAYS __50%__ chance!!!
• FYI: The man “chooses” the sex of the child.
Women only have X’s to give. Man gives X to
daughters and Y to sons.
5. Sex-Linked Traits/Diseases
– controlled by genes on the X or Y
chromosomes
If trait is on:
– X and is dominant – more females have it (ex.
scoliosis)
– X and is recessive – more males have it (ex.
baldness, colorblindness, hemophilia)
– Y – only males have it
Red-green colorblindness (recessive on X)
normal
colorblind
N =______________
n = _______________
female normal
female colorblind
XNXN = ___________
XnXn = ___________
male colorblind
female carrier
XNXn = ___________
XnY = ___________
male normal
XNY = ___________
A man who is not colorblind has a son with a
woman who is a carrier for colorblindness.
What is the chance that the son will be
colorblind?
Who did the son inherit the colorblindness from?
Can a man that is not colorblind have a daughter that is?
Hemophilia – disease that interferes with blood clotting
• Treated with clotting enzymes or transfusion
• Recessive on X
Famous People with Hemophilia
Albert and Victoria –
English royal family
Ryan White and mom
Romanoff Family
normal
hemophilia
N = ____________
n = _____________
female normal XnXn = ____________
female hemophilia
XNXN = ___________
male
hemophilia
female carrier
XNXn = ___________
XnY = ____________
male normal
XNY = ___________
A woman who is a carrier for hemophilia and a man who has
hemophilia want to have a child. They want to know the
chance of their child having hemophilia.
Sex-Linked Recessive Pedigrees
• Males have it; females are carriers
6.
Polygenic Traits – traits controlled by
many genes; difficult to predict exact
outcome
These traits show broad range of phenotypes
Ex. hair color, eye color, skin color, human height