Download Section11.3OtherInheritance

Section 11.3:
When Heredity Follows Different
Rules
*Some hereditary patterns do not follow Mendel’s Laws
I.
Complex Patterns of Heredity
*Mendel’s patterns of inheritance are referred to as
Simple Mendelian Inheritance - inheritance that is
controlled by dominant and recessive alleles.
*Some patterns of inheritance are Complex - they do not
abide by the laws of dominance and recessiveness.
A.
Incomplete Dominance: Appearance of a Third
Phenotype
1. Incomplete dominance - occurs when neither allele is
completely dominant, thus the phenotype of a
heterozygous individual is intermediate - or “in between” the homozygous dominant and homozygous recessive
individuals.
This defies the pattern of dominance in which the
heterozygote has the same phenotype as the
homozygous dominant individual.
2. Example: The Case of the “Snapdragons”
Given:
RR = Red Flowers
R’R’ = White Flowers
R’R = Pink Flowers
What would happen if a homozygous red flowered
plant is crossed with a homozygous white flowered
plant?
P1
RR x R’R’
RR
P1
X
R’R’
R
R
R’
RR’
RR’
R’
RR’
RR’
F1 - all are RR’; all are PINK
*Neither R nor R’ are totally dominant.
This is why a ( ‘ ) is used rather than R and r alleles.
How does this happen?
*The R allele codes for an enzyme that produces red
pigment, when the plant has two of these alleles it
expresses red flowers.
*The R’ allele codes for a defective enzyme which does not
produce pigment, therefore the flowers are white because
no color is produced.
*If you have a plant that is RR’ it has one pigment for red
and one for no color, therefore the flowers will be an
intermediate between red and white, thus they will appear
pink.
THIS IS NOT BLENDING!!!!!!
3. Crossing the F1 ‘s
*now cross the individuals of the F1 generation:
F1
R
R’
R
R’
RR
RR’
RR’
R’R’
F2 - 1 RED : 2 PINK : 1 WHITE
The alleles did not “blend” together because if they did
you would not have been able to separate them out.
Snapdragon flowers:
B. Codominance: Expression of Both Alleles
1. Codominant Alleles - causes the phenotypes of both
homozygotes to be produced in the heterozygous individual;
both alleles are expressed equally.
2. Example: Chicken Feathers
Given:
BB = Black Feathers
WW = White Feathers
*Two different uppercase letters are used to represent
the alleles of codominant inheritance.
According to Mendel, BW = Black
According to incomplete dominance BW = Gray
*This actually follows NEITHER!
BW = Checkered
Half the feathers are black, half are white
Can two checkered chickens mate and produce black chickens
or white chickens?
YES! Two checkered chickens can
Result in the following possibilities:
Other examples of Codominance:
C. Multiple Alleles: Making Multiple Phenotypes
1. Multiple Alleles - when more than two alleles control a
trait. Each trait only has two alleles, but in a population there
may exist more than two alleles for a trait.
2. Where do new alleles come from?
New alleles are often the result of a spontaneous
mutation in which one nitrogenous base is changed in the
DNA sequence.
3. The Pigeon Example:
There are three alleles in the pigeon population that
determine a pigeon’s feather color; Each pigeon in the
population will only receive 2 of the three alleles.
In Pigeons:
BA = dominant allele, ash-red colored feathers
B = blue feathers
b = chocolate colored feathers
Order Of Dominance:
BA > B > b
Therefore:
BABA = Red
BAB = Red
BAb = Red
4. The Rabbit Example:
Coat color:
C = dark gray fur
cch = chinchilla fur
ch = himalayan fur
c = white fur
BB = Blue
Bb = Blue
bb = Chocolate
Rabbit Example:
Order of Dominance:
C > cch > ch > c
Possible Genotypes:
a) dark gray rabbit: CC, Ccch, Cch, Cc
b) chinchilla rabbit: cchcch, cchch, cchc
c) himalayan rabbit: chch, chc
d) white rabbit: cc
What kind of bunnies would you get from the following cross?
cchch
cch
x
Cch
ch
C
Ccch
dark gray
Cch
dark gray
ch
cchch
chinchilla
chch
himalayan
1 himalayan: 2 dark gray : 1 chinchilla
D. Sex Determination
1. Autosomes - refers to all chromosomes other than the sex
chromosomes.
2. Sex chromosomes - chromosomes which determine the
sex of an individual ex) X, Y
In humans:
XX = females
XY = males
Punnett Square for determining sex:
XX
X
x
XY
X
Y
XX
XY
XX
XY
X
*Every time a man and woman reproduce, there is a 50%
chance that they will have a boy and a 50% chance they will
have a girl.
E. Sex-Linked Inheritance
1. Sex-Linked traits - traits that are controlled by genes
located on the sex chromosomes; X & Y.
The traits are written as superscripts ex) Xa
The Y chromosome has no corresponding allele to the
allele on the X chromosome, therefore no superscript is
used on the Y and the Y cannot “mask” or “hide” a bad X.
2. The Fruit Fly Example
*fruit flies, like humans have X & Y sex chromosomes.
a) Thomas Hunt Morgan - studied and
bred fruit flies. Noticed that white eyes
are more common with males than with
females.
b) Morgan’s Crosses:
r = white eyes
R = red eyes
P1
XRXR
x
XrY
(red-eyed female) (white-eyed male)
Xr
XR
XR
Y
XRXr
Red-eyed
female
XRY
Red-eyed
male
XRXr
Red-eyed
female
XRY
Red-eyed
male
F1 Results:
ALL offspring are red-eyed
half, 50% are red-eyed females
half, 50% are red-eyed males
What would you get if you crossed the F1 offspring with
each other?
F1
XRXr
x
XRY
Do the punnett square and record the phenotype and
genotype ratios!
F2 Results:
XR
Y
XR
XRXR
Red-eyed
female
XRY
Red-eyed
male
Xr
XRXr
Red-eyed
female
XrY
White-eyed
male
All females are red-eyed
males have a 50% chance of being white or red
1 red-eyed male : 2 red-eyed females : 1 white-eyed male
*Morgan surmised that the gene for eye color must be on the
X chromosome and not on the Y; therefore a male’s eye color
is determined by only ONE allele. If the allele is a recessive
one it cannot be masked because the Y cannot mask or
override the X.
3. X linked traits are passed on to both male and female
offspring, but tend to affect more often since they only
receive one X and whatever is carried on it is expressed.
Females may be carriers, and thus can have a normal allele
to mask a detrimental one.
Y linked traits are passed only from male to male (father to
son) and do not affect females.
F. Polygenic Inheritance
1. Polygenic inheritance - the inheritance pattern of a trait
that is controlled by 2 or more GENES. These genes may
be on the same chromosome, or they may be on different
chromosomes.
Each gene may have more than 2 alleles.
ex) skin color & height in humans
cob length in corn
2. Example: Corn Cob Length
Say that cob length is affected by three genes:
gene A, gene B, gene C
Each gene has two alleles that can be either uppercase,
or lowercase, therefore there are 6 alleles that are
responsible for determining cob length.
Each of the 6 alleles represent a certain cob length:
ex) aabbcc - shortest cob
(2 inches)
AABBCC - longest cob
(12 inches)
AaBbCc - may be 6 inches
II. Environmental Influences
*Remember that the genetic makeup of an organism at the
time of fertilization determines only the organisms’ potential
to develop and function; environment can play a key role in
this as well.
A. Influences of External Environment
1. External Factors that affect gene expression:
a) temperature
b) nutrition
c) light
d) chemicals
e) infectious agents/ pollutants
2. Example
Bacteria --> some bacteria change in color depending on
the temperature of their environment.
Ex) Serratia marcescens
brick red color @ 25 C
cream colored @ 30 C
B. Influences of Internal Environment
1. Internal environments of males and females are different
due to hormones and other structural differences.
*differences are controlled by different hormones that
are determined by different genes.
2. Examples:
coloring in birds,
male vs. female peacock
male pattern baldness
Section 12.3:
Complex Inheritance In Humans
I.
Codominance in Humans
*Phenotypes of homozygous dominant and homozygous
recessive individuals are both present in the
heterozygous individual in equal amounts.
A. Sickle Cell Anemia
1. Affects - African Americans whose families
originated in Africa, as well as some white
Americans whose families originated around
the area of the Mediterranean.
1/12 - 1/10 African Americans is a carrier
2. Homozygotes with the Sickle Cell Disease
*sickle = “half-moon shaped”
a) cause - the oxygen carrying protein of the RBC
(hemoglobin) has one single amino acid different than the
hemoglobin of normal individuals. (Refer back to 11.2/11.3
notes).
b) what happens - the defective hemoglobin causes a
change in the shape of the RBC, causing slow blood flow
because the sickle cells get stuck in the small capillaries
and blood vessels.
c) signs / symptoms:
1) tissue/ muscle
damage
2) organ damage
3) pain in joints
4) anemia
(low # of RBCs)
5) shorter life span
3. Heterozygous Carriers:
a) cause - carry one allele that codes for sickle cell, and also
have one allele that is normal.
b) what happens - produce both normal and sickle cell
RBCs. Luckily they produce enough normal RBCs and
hemoglobin to compensate for the sickle cells. Thus they
can live relatively healthy lives.
c) signs / syptoms only display traits when
oxygen levels are
reduced.
Alleles for Sickle Cell Anemia & Trait:
RR = ALL Normal RBCs
SS = ALL Sickle Shaped
RS = 1/2 Normal, 1/2 Sickle
II. Multiple Alleles in Humans:
*involves more than two alleles in the population for the trait.
A. Blood Typing (the ABO system)
one gene --> blood type
3 alleles --> A,B,O
1. Human Blood Typing Determination:
human blood type is determined by the presence or
absence of certain proteins on the surface of an individual’s
RBC (red blood cell).
2. Gene I:
Gene I is the blood type gene; it has three alleles: IA, IB, i
Each individual has two out of these three alleles.
Alleles can be written as IA, IB, i or as A, B, O.
3. Possible Blood Types:
Allelic Combinations
Blood Type
IAIA or AA
Type A
IA i or AO
Type A
IBIB or BB
Type B
IB i or BO
Type B
IAIB AB
Type AB
i i OO
Type O
Blood Type A has surface protein “A”
Blood Type B has surface protein “B”
Blood Type AB has surface proteins “A” and “B”
Blood Type O does not have surface proteins
Antigens and Antibodies for ABO Blood Typing:
Antigens = another word for the proteins located on the RBC
Antibodies = proteins found in the person’s plasma
AB = Universal recipient
O = Universal donor
B. Why Blood Typing Is Important
1. Transfusions - incompatible
blood types will clump together
and cause death.
EX) blood type A cannot be
given to blood type B
2. Blood typing CANNOT be used to determine the father of a
child, but it can be used to possibly rule a man out.
ex) If a child is type AB, and a mother is type A, if she is
accusing a type O man of being the father he can be ruled
out because it is not possible:
A
O
A
A
O
AO
Type A
OO
Type O
O
AO
Type A
OO
Type O
O
AO
Type A
AO
Type A
O
AO
Type A
AO
Type A
*There is no way this man could have had an AB child with
this woman.
III. Sex-Linked Traits in Humans
*Several human traits are determined by genes carried on
the sex chromosomes; mainly on the X chromosomes.
*When a son receives an X chromosome from his mother he
will express whatever is on that X chromosome. He cannot
hide it with his Y.
A. Red-Green Color Blindness
1. People with red-green color blindness cannot tell the
difference between these two colors when they are
placed next to each other.
2. Cause - color blindness is caused by the inheritance of
either of 2 recessive alleles at two gene sites on the X
chromosome. These genes affect the red and green
receptors in the cells of the eyes.
3. Problems - cannot
tell difference in colors.
Traffic lights, Christmas,
etc.
What someone who is color blind may see:
Normal
Color Blind
B. Hemophilia: An X-Linked Disorder
1. What is it?
Hemophilia A is a disorder in which blood will not clot. It
affects 1/ 10,000 males and only 1/100 million females.
2. Cause - X-Linked disorder in which the individuals
affected are missing a protein called Factor VIII, this protein
helps the blood to clot when there is an injury.
Males inherit the defective alleles from mothers who are
either carriers or have the disorder. Since the Y
chromosome does not have an allele for this protein the
male can never mask it, thus it only takes one “bad”
recessive allele in order for males to express the disorder.
Females need to receive two “bad” recessive alleles in order
to have the disease, therefore they are less likely to have
the disorder because if they have one normal X they can
mask a bad X.
Xh
Y
XH
Xh
XHXh
XhXh
Normal
Hemophiliac
Female
XH
Xh
XHXH
XHXh
Normal
Normal
female
Female
Female
XHY
XhY
XHY
XhY
Normal male
Hemophiliac
male
Normal male
Hemophiliac
male
XH
Y
3. Treatment - blood transfusions; injections of factor VIII.
Both types of treatment can be very expensive and risky.
IV. Polygenic Inheritance in Humans
*More than one gene with 2 or more alleles that determines
a single trait.
A. Eye Color
*blue, green, grey, hazel, brown, yellow
B. Skin Color: A Polygenic Trait
*There are a number of genes involved in skin color
*A wide range of skin color exists due to these many genes
that affect it.
Skin color map:
Skin colors:
V. Change in Chromosome Number
*Many abnormal phenotypes are the result of a change in
chromosome number.
A. Unusual Numbers of Autosomes
1. Normal Chromosome Number:
Human cell - 46 chromosomes; 23 pair
Autosomes =
44; 22 pair
Sex Chromosomes = 2; 1 pair
*unusual numbers result from nondisjunction - when
paired homologous chromosomes do not separate
properly during meiosis.
2. Identifying Abnormal Numbers of Chromosomes
a) Cell samples are taken from an individual or from a fetus.
b) Metaphase chromosomes are photographed using a
microscope.
c) The picture of the chromosomes is enlarged, cut apart,
and arranged according to length and location of the
cetromeres.
*Karyotype - a chart in which chromosomes are
arranged according to their length and centromere
location; helps to pinpoint unusual chromosome #’s.
Amniocentesis:
B. Down Syndrome: Trisomy 21
1. Down Syndrome - occurs when there is an extra #21
chromosome. Therefore, instead of having one pair of
homologous chromosomes, there are triplets.
2. Affects - 1/ 700 live births; chance increases as mother’s
age reaches 35+ at time of conception.
3. Results in mental
retardation.
C. Unusual Numbers of Sex Chromosomes
1. XO = one X chromosome is missing
2. XXX or XXY - one extra
X chromosome is added.
3. XYY - one extra Y
chromosome.