Patterns of Heredity and Human Genetics

A Look at Genetic Complexities

Chapter 12 Notes

What happens when heredity follows different

rules? The Exceptions to Mendel’s Rules

Section 12.2

At the end of this lesson, YOU will be able to:◦ Distinguish between alleles for incomplete

dominance and codominance.◦ Explain the patterns of multiple allelic and

polygenic inheritance.◦ Analyze the pattern of sex-linked inheritance.◦ Summarize how internal and external

environments affect gene expression.

Section Objectives

Remember Punnett Squares

Heterozygous Chin Dimple (male)

XNo Chin Dimple

(female)

Dd

dd

D

d

d d

His 4 conclusions were:◦ The Rule of Unit Factors◦ The Rule of Dominance◦ The Law of Segregation◦ The Law of Independent Assortment

Remember Mendel?

http://www.pbs.org/wgbh/nova/orchid/images/amat_mendel.jpg

Sometimes, patterns of inheritance are not as simple as Mendel’s Rules imply.

The exceptions to Mendel’s Rules are when nature uses a different method of determining traits.

Exceptions to Mendel’s Rules

The Law of Independent Assortment can be broken when genes are found close together on the same chromosome.◦ The genes will appear

linked, or show up together.

◦ The closer the genes are to each other the more they will be inherited together.

Gene Linkage

Usually, a dominant gene produces a protein for the trait.

The recessive allele either produces a nonfunctional protein or no protein at all.◦ So we see the

dominant trait in hybrids because it is the only trait expressing a protein.

Breaking the Rule of Dominance

Protein

DNA

mRNAmRNA

When heterozygous individuals show an intermediate phenotype between the two homozygous phenotypes.◦ Having one copy of a

gene does not produce enough protein to completely mask the recessive allele.

Incomplete Dominance

http://www.miracosta.edu/home/rmooney/Mendelian%20genetics_files/slide0015_image033.jpg

Snapdragons◦ If you cross a red

flower and a white flower, the resulting hybrid will be pink. RR = red flower rr = white flower Rr = pink flower

◦ If you cross two pink flowers (Rr), you get: 25% Red Flowers 50% Pink Flowers 25% White Flowers

Incomplete Dominance

RR

r r

Rr

R r

RrRr

RrRr

RrRrRR rr

Hair◦ Straight (HH)◦ Wavy (Hh)◦ Curly (hh)

Incomplete Dominance

When a heterozygous individual shows the phenotypic traits of both alleles.◦ Both alleles produce

a protein, which are seen in the hybrids.

◦ The traits do not blend!

Codominance

Feather Color in Chickens◦ A black chicken

would be BB.◦ A white chicken

would be WW.◦ A hybrid, BW, would

have a checkered appearance. Both white and black

pigments are seen in the offspring.

CodominanceB B

W

W

WB

W

B

BW

BW

BW

BW

BW

BW

BB

WW

Sickle Cell

Codominance

When a trait is controlled by more than two alleles.◦ Each individual only

owns two alleles, but others in the population may possess different types.

Multiple Alleles

Fur Color in Rabbits◦ C = Dominant allele◦ ch = Himalayan fur◦ cch = Chinchilla fur◦ ca = Albino fur

Multiple Alleles

Cch

cchcacbcb- Bugs Bunny

Blood Types◦ IA, IB, or i

Multiple Alleles

IAIA or IAi IBIB or IBi

IAIB ii

When one trait is controlled by more than one gene.◦ The genes may be

on the same or different chromosomes.

◦ Both genes have a single phenotypic effect.

Polygenic Traits

Coat Color in Labrador Retrievers◦ Controlled by two

different genes, the B gene and the E gene.

◦ A dihybrid cross of two black labs (BbEe x BbEe) results in: 9 Black Pups 3 Chocolate Pups 3 Golden Pups 1 Golden Pup with a

brown nose and light eyes.

Polygenic Traits

http://www.oakhillkennel.com/library/color.html

Eye Color◦ Brown Gene◦ Green Gene

Polygenic Traits

When traits are determined by several factors from the genetic makeup and the organism’s environment.◦ The genes only

represent the potential.◦ Environmental

influences turn on the genes at different times and in different amounts.

Multifactorial Traits

Temperature, nutrition, light, chemicals, and infections can influence gene expression.◦ Arctic Foxes have

coats that change color due to temperature.

Multifactorial Traits

Height, Intelligence, Cholesterol, Weight, Mental Illness, etc.

Multifactorial Traits

How do our chromosomes determine our sex?

Sex Determination and Sex-linked Traits

Section 12.2

Humans have a total of 46 chromosomes, or 23 pairs:◦ 22 pairs of autosomes◦ 1 pair of sex

chromosomes Autosomes

◦ All of the chromosomes that determine the traits other than sex.

◦ Come in different sizes with different genes on them.

◦ Pairs 1-22

Sex Determination

Sex Chromosomes◦ Determine the sex of

the individual.◦ Pair 23

In females, these chromosomes match in the form of XX.

In males, these chromosomes are different, as in XY.

Sex Determination

The combination of sex chromosomes decides if you are a boy or a girl.◦ A mother (XX) can only

supply eggs that have an X chromosome.

◦ The father (XY) has some sperm with a X chromosome and some with a Y chromosome.

Sex Determination

XX

X

Y

XX XX

XY XY

G- 50% XX; 50% XY

P- 50% female; 50% male

Comparing the X and the Y

Genes that are located on the sex chromosomes.◦ The X chromosome

contain many important genes that are necessary for survival.

◦ The Y chromosome contains the SRY gene which determines maleness.

Sex-Linked Traits

First observed in fruit flies (Drosophila).

Fruit flies have either red or white eyes.◦ Thomas Hunt Morgan

noticed that all of the white-eyed flies were male.

◦ Therefore, eye-color in flies is a sex-linked trait.

Sex-Linked Traits

Because the X chromosome is much larger than the Y, most sex-linked traits are on the X.

When writing the alleles for these traits, you must include the chromosomes that the individual has:

Sex-Linked Traits

Y Y Chromosome

(no alleles)

XR

X Chromosome(red-eyed allele)

Xr

X Chromosome(white-eyed allele)

Because males have only one X chromosome, they are more likely to get a single defective copy.◦ XRY- red-eyed male◦ XrY- white-eyed male

Sex-Linked Traits

XRY

XrY

Because females receive two X chromosomes, they are more likely to get a dominant allele that can cover the effects of the recessive trait.

A carrier female has a recessive allele but does not show the trait (heterozygous)

Sex-Linked Traits

XRXR

Homozygous Red-eyed Female

XRXr

Carrier Female

XrXr

White-eyed female (rare)

Sex-Linked Punnett Squares

Homozygous Red-eyed Female x White-eyed MaleXRXR XrY

XR

XR

Xr Y

XR Xr

XR

Xr

XR Y

XR Y

G- 50% XRXr

50% XRY

P- 50% red-eyed female 50% red-eyed male

Sex-Linked Punnett Squares

Heterozygous Red-eyed Female x Red-eyed Male

XRXr XRY

XR

Xr

XR Y

XR XR

XR Xr

XR Y

Xr Y

G- 25% XRXR

25% XRXr

25% XRY 25% XrY

P- 50% red-eyed female 25% red-eyed male 25% white-eyed male

How do pedigrees show inherited traits within

families?Understanding Pedigrees

Section 12.1

A graphic representation of traits inherited within a family.◦ Allows scientists to trace

the history of a genetic disorder.

◦ Uses symbols to represent individuals. Circles represent females Squares represent males If the symbol is shaded, the

individual is affected by the trait.

Pedigrees

Normal Female

Normal Male

Affected Female

Affected Male

Inherited traits can be followed from generation to generation.◦ Horizontal lines

connect two individuals who have mated.

◦ Vertical lines represent the offspring of a union.

Pedigrees

Bb Bb

Bb bb BB Bb

I

II

1 2

1 2 3 4 5

Sex-Linked vs. Autosomal◦ If more males are

affected by a trait than females, it is probably sex-linked.

◦ If it affects males and females equally, it is probably autosomal.

Rules of Pedigrees

Dominant vs. Recessive◦ If a trait skips a

generation, it is recessive.

◦ If the trait is found in each generation, it is probably dominant.

Rules of Pedigrees

Identifying Genotypes◦ If any males are

carriers, the trait is autosomal.

◦ If a male has a sex-linked trait, his mother was probably a carrier.

Rules of Pedigrees

Affects males and females equally. Skips generations (appears in some

generations but not in others). Males can be carriers

Autosomal Recessive

Affects males and females equally Does not skip any generations.

Autosomal Dominant

Affects males more than females. Skips generations.

◦ Must use the chromosomes (XY or XX)

Sex-Linked Recessive

Example #1: AlbinismAre males affected more frequently that females?

NO Autosomal Disorder

Does the disorder skip generations?YES (P1)

Recessive Disorder

Autosomal RecessiveA- normala - albino

Example #2: HemophiliaAre males affected more frequently that females?

YES Sex-Linked Disorder

Does the disorder skip generations?YES Recessive Disorder

Sex-Linked Recessive+ = normalH = hemophilia

Pedigree PracticeGenetic Trait: ACHOO(Sneezes in response to light)

#1- Is this trait sex-linked or autosomal?

#2- Is this trait dominant or recessive?

#3- What is the genotype of individual A?

#4- What is the genotype of individual B?

#5- What is the genotype of individual C?

A

B

C

D#6- What is the genotype of individual D?

A picture of an individuals chromosomes.◦ Homologous

chromosomes are paired up.

◦ Pairs are arranged by size.

◦ Karyotypes can help diagnose chromosomal disorders.

Karyotype

When arranging the chromosomes:◦ Autosomal

Chromosomes are placed in order by size.

◦ The Sex Chromosomes are pair 23.

Karyotypes

What to look for:◦ The Sex

Chromosomes: Male (XY) or Female

(XX) Is there an odd number

(XXY, XYY, XO, XXX)

◦ The Autosomes Are there two or three

chromosomes for pair 21?

Trisomy 21 Down Syndrome

Karyotypes

Example #1

Number of Autosomes:

Number of Sex-Chromosomes:

Karyotype:

Phenotype:

44

1

45 (X)

Female-Turner Syndrome

Example #2

Number of Autosomes:

Number of Sex-Chromosomes:

Karyotype:

Phenotype:

44

3

47 (XYY)Male

Jacob’s Syndrome

Example #3

Number of Autosomes:

Number of Sex-Chromosomes:

Karyotype:

Phenotype:

44

3

47 (XXY)Male

Klinefelter Syndrome

What is the genetic basis for determining blood

types?

Understanding Blood Types

Your body produces antibodies that attack any foreign objects within you.◦ Usually, this fights bacteria,

viruses, or fungi. All of your cells have

antigens on their surface.◦ Antigens are cellular

nametags.◦ Your body makes antibodies

to fight off anything without your particular antigen.

Blood Types in Humans

Your special antigens are made by your DNA.

The antigens found on the red blood cells determine your blood type.◦ Type A◦ Type B◦ Type AB◦ Type O

Blood Types in Humans

Genotype IAIA or IAi

Antigens A antigens

Antibodies

Anti-B antibodies

Type A Blood

Genotype IBIB or IBi

Antigens B antigens

Antibodies Anti-A antibodies

Type B Blood

Genotype IAIB

Antigens A antigens and B

antigensAntibodies No

Antibodies

Type AB Blood

Genotype ii

Antigens No antigens

Antibodies Anti-A and Anti-B

antibodies

Type O Blood

Another antigen on the surface of the RBC, is the Rhesus Factor.◦ Named after it was

discovered in Rhesus Monkeys

People who have the Rh factor are positive.

People without the Rh factor are negative.

Rh Factor

Commonly, a person’s blood type combines their ABO type and Rh factor.◦ Type A neg.◦ Type O pos.

Blood Donations

rh

rh

rh

rhrh

rh

rhrh

Anti-A

Anti-B

Anti-Rh

Anti-A

Anti-B

Anti-Rh

Anti-A

Anti-B

Anti-Rh

Type A+

Type AB-

A person cannot receive any blood that contains an antigen that they posses the antibody for.◦ Type A individuals

produce anti-B antibodies.

◦ Giving that person type B blood can have dangerous effects.

◦ The anti-B antibodies will cause the Type B blood to stick together. This is called clumping.

Blood Donations

The Universal Recipient◦ Produces no antibodies◦ Can receive all types

of blood◦ Type AB+

The Universal Donor◦ RBC’s have no

antigens.◦ Can give blood to

anyone◦ Type O-

Blood Donations

Determining Blood Types

IAIAx IBIBType A Type B

IA

IA

IB IB

IAIB

IAIB

IAIB

IAIB

G:

P:

100% IAIB

100% AB

Determining Blood Types

Type AB x Type OIAIB ii

IA

IB

i i

IAi

IBi

IAi

IBi

G:

P:

50% IAi50% IBi

50% Type A50% Type B

Determining Blood TypesHospital Mix-UpOn a busy night at the Plainsboro Hospital, three families delivered three healthy baby boys. Unfortunately, the babies became mixed up during a rush and no one knows which baby belongs to which family. The nurses were able to take blood samples from each parent and each baby. Use your knowledge of blood type genetics to figure out which baby belongs to each family.

Parent Blood Type

Mr. Robinson AB

Mrs. Robinson B

Mr. Jones O

Ms. Jones AB

Mr. Jackson B

Ms. Jackson B

Baby Blood Type

Baby #1 O

Baby #2 AB

Baby #3 A

Baby #1 belongs to the JacksonsBaby #2 belongs to the RobinsonsBaby #3 belongs to the Jones

Codominance◦ Both A and B are

dominant.◦ In a hybrid (AB) both

types of antigen will be present.

Multiple Alleles◦ The ABO blood group has

three alleles that produce four phenotypes.

Polygenic◦ Two genes control blood

type: ABO and Rh factor

Characteristics of Blood System

How do scientists follow genetic disorders in our

genome?When Genetics Goes Wrong

Inheritance◦ Autosomal Recessive◦ Chromosome #7

Symptoms◦ Abnormally thick

mucous clogs pores in lungs, liver, pancreas.

Cystic Fibrosis

Cystic FibrosisGene Therapy is a hopeful avenue of treatment for those with cystic fibrosis.

Here’s how it works:1. Insert a working copy of

the gene into a virus.2. Load the virus into an

inhaler.3. Have the patient breath in

the virus with the working copy.

4. The virus then injects its DNA and the working CF gene into the patient’s cells What would be one potential

downside to using gene therapy?

Inheritance◦ Autosomal Recessive◦ Chromosome # 12

Symptoms◦ Victims are unable

to metabolize phenylalanine properly.

◦ Leads to mental retardation.

Phenylketoneuria (PKU)

Phenylketoneuria (PKU)All babies are tested for PKU in the first few days.

How would being diagnosed with PKU affect your life?

Inheritance◦ Autosomal

Codominant◦ Chromosome # 11

Symptoms◦ Defective hemoglobin

becomes sickle-shaped

◦ Pain crises result when, cells clog blood vessels.

Sickle Cell

http://www.ehponline.org/docs/2004/112-6/bloodcells.jpg

Sickle CellAA

Normal RBC/ Susceptible to Malaria

ASNormal RBC and Sickle

Cells/ Resistant to Malaria

SSSickle Cells/

Resistant to Malaria

If sickle-cell is so bad, why has the

gene not been taken out of the genome?

Inheritance◦ Autosomal Dominant◦ Chromosome # 4

Symptoms◦ Progressive

neurological disorder.◦ Nerve cells begin to

deteriorate resulting in a loss of coordination.

◦ Begins in late 40’s or early 50’s.

Huntington’s Disease

Huntington’s Disease

Hh Hh

hh hhH

h

h h

Huntington’s Disease is autosomal dominant.If one of your parents have it, you have a 50% chance of inheriting this progressive disorder.Question #1Genetic testing can determine if you have inherited the dominant allele or not. If one of your parent’s had it, would you want to know if you have the disease or not?

Question #2Should the results of genetic testing be given to employers by insurance companies? Is having the trait cause for firing somebody, even if they have not shown any symptoms?

Inheritance◦ Sex-linked Recessive◦ X Chromosome

Symptoms◦ Unable to distinguish

between red and green colors.

Color Blindness

Color Blindness

Normal Vision

ProtanopiaDeuteranopiaTritanopia

How would colorblindness affect the way you see the world?

Inheritance◦ Sex-linked Recessive◦ X Chromosome

Symptoms◦ Unable to clot

properly because of missing Factor VII.

◦ Seen in the Royal family of Britain

Hemophilia

Hemophilia

Inheritance◦ Chromosomal

Nondisjunction◦ Trisomy 21

Symptoms◦ Mild mental

retardation◦ Simian Crease,

Epicanthal fold, shorter limbs, poor muscle tone, protruding tongue

Down Syndrome

Down SyndromeWomen over the age of 35

increase the chances of a DS child up to 1 in 378 (over 45

is 1 in 30).

An amniocentesis extracts some fetal cells to prepare a

karyotype.

If you found out that your child would be born with Trisomy 21 (Down Syndrome), would you support terminating the pregnancy?

Polygenic Traits◦ Eye Color- 2 genes◦ Skin Color- 3 genes

Multifactorial◦ Height◦ Cholesterol◦ Behavioral Traits

Other Traits in Humans