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’

P1 RR X R’R’

R R

R’

R’

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.

RR’ RR’

RR’ RR’

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’

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.

RR RR’

RR’ R’R’

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 BB = Blue bb = Chocolate

BAB = Red Bb = Blue

BAb = Red

4. The Rabbit Example:

Coat color:

C = dark gray fur

cch = chinchilla fur

ch = himalayan fur

c = white fur

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 x Cch

cch ch

C

ch

1 himalayan: 2 dark gray : 1 chinchilla

Ccch

dark gray

Cch

dark gray

cchch

chinchilla

chch

himalayan

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 XY

X Y

X

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.

XX XY

XX XY

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 Y

XR

XR

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

Xr

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

XRXR

Red-eyed female

XRY

Red-eyed male

XRXr

Red-eyed female

XrY

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 LengthSay 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 areresponsible 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) temperatureb) nutritionc) lightd) chemicalse) 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:

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

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

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 O

O O

*There is no way this man could have had an AB child with this woman.

AO

Type A

OO

Type O

AO

Type A

OO

Type O

AO

Type A

AO

Type A

AO

Type A

AO

Type A

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 Xh XH Xh

Xh XH

Y Y

XHXh

Normal Female

XhXh

Hemophiliac

female

XHYNormal male

XhYHemophiliac

male

XHXH

Normal Female

XHXh

Normal Female

XHYNormal male

XhYHemophiliac

male

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 pairSex 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.