Chapter 14

The Human Genome

Human Chromosomes

Our Focus this Chapter: • Karyotype• Sex Determination• Genes and the Environment• Genes and Inheritance

– Polygenic– Dominant vs. Recessive– Pedigree Charts and Punnett Squares

• Human Genes– Inheritance Patterns

• Gene Inheritance: Polygenic, dominant, recessive, codominant, sex-linked, linked

• Chromosomal Disorders

Karyotype

• Autosomes: Body chromosomes (#1-22)

• Sex Chromosomes– Male or Female?

(hint: the one in your notes is different from this one)#23

• Fertilization Chart – Haploid (23)– Diploid (46)

Sex Determination

• Punnett Square

• Probability:

50% male

50% female

Human Genes• What is a gene? A segment of DNA

coding for a protein (phenotype)

Genes and the EnvironmentInfluences on Gene

Expression (phenotype)• Nature vs. Nurture

– Gene expression• Nutrition• Exercise• Stress

– epigenetics

Genes and Inheritance• Polygenic Traits:-Multiple genes on different

chromosomes affecting one trait

• Combination of dominant and recessive

Ex: Hair color, Eye Color, Height

• Multiple Allele– More than 2 possible

alleles, in the population,for 1 genes

– Each individual still only has 2 alleles

Ex. Blood TypeIA, IB, i

Genes and InheritanceDominant vs. RecessiveRecessive alleles are masked

in the presence of a dominant allele

Incomplete Dominance is a form of intermediate

inheritance in which one allele for a

specific trait is not completely

dominant over the other allele.

Codominant

-(blood type)

A circle represents a female.

A square represents a male.

A horizontal line connecting a male and female represents a marriage.

A vertical line and a bracket connect the parents to their children.

A half-shaded circle or square indicates that a person is a carrier of the trait.

A completely shaded circle or square indicates that a person expresses the trait.

A circle or square that is not shaded indicates that a person neither expresses the trait nor is a carrier of the trait.

Section 14-1

Figure 14-3 A PedigreePedigree Chart(traces family history)

Problem Solving: p343

Use a Punnett Square (determines probability) to illustrate the genetic probability of the first couple’s offspring.

Blood Type• Multiple Allele (A, B, O)• Codominant (A and B) • Dominant/Recessive (A & B are dominant over O)

Blood Type in the Human Population

•  

Human Genes

• Blood Type• Disorders

– Gene• Autosomal

– Recessive

– Dominant

– Codominant

• Sex-linked– Recessive

– Chromosomal

Autosomal Recessive

• Both copies of the gene must be the recessive allele– Represented by a

lower case letter• Albinism – • Phenylketonuria – • Tay Sachs –• Galactosemia – • Cystic Fibrosis –

Chart p345

Autosomal Recessive

AlbinismGenotype:

aaExpression:• Missing Melanin

(pigment)– Lack of color in skin, hair

and eyes

• Vision impairmentTreatment: • Glasses, eye surgery

Autosomal Recessive

Phenylketonuria (PKU)Genotype: (chromosome 12)

ppExpression:• Missing phenylalanine

hydroxylase– Inability to process

phenylalanine• Abnormal pigment• Mental retardation

Treatment• Specialized diet low in

phenylalanine

Autosomal Recessive

Tay SachsGenotype: (chromosome 15)

ttExpression:• Lack enzyme

hexosaminidase A– Lipid accumulation in brain,

blindness, mental deficiency, death

Treatment:• None• Genetic testing available

Autosomal Recessive

Cystic FibrosisGenotype: (chromosome 12)

ccExpression:• Deletion of 3 bases = loss of

phenylalanine in gene– Abnormal chloride channels

cannot be transported to membrane

– No chloride channels causes airways to become clogged with thick mucous

Treatment• Drug therapies and lung

therapies extend lives

Autosomal Dominant

• Only one copy of the genes is necessary to exhibit the trait

• Represented by a capital letter– Achondroplasia – Huntington’s Disease– Hypercholesterolemia

Autosomal DominantHuntington DiseaseGenotype: (chromosome 4)

HhExpression:• Extensive repetition of CAG on

DNA• Adult-onset• Neurological Symptoms

Include:– Disorientation or confusion,

Loss of judgment, Loss of memory, Speech changes

• Death (usually by 50)Treatment• Medications used to slow

symptoms

Singer/Songwriter Woody Guthrie

“This Land is Your Land”

Autosomal DominantAchondroplasia (dwarfism)Genotype: (chromosome 4)

AaExpression:• Problems with bone

ossification of long bones– Effect growth– Average head and torso with

shortened arms and legsFun Fact• 80% of Achondroplasia cases

are new mutations primarily from men >35

Codominant

• 2 alleles can be expressed at the same time

• Sickle Cell Anemia HA – normal alleleHS - sickle cell

HA HA , HA HS, HS HS

Sickle Cell Anemia

HA HA HA HS HS HS

Normal Sickle Cell Trait

Sickle Cell Disease

Normal Hemoglobin

Both normal and abnormal hemoglobin

Abnormal hemoglobin

Normal rbc shape

Normal and sickle shape

Sickle shaped rbc

Distribution of Sickle Cell Anemia vs. Malaria

Sickle Cell Malaria

10% of African Americans and 40% of populations in Africa and Asia carry the gene for Sickle Cell Anemia

DNA normal hemoglobin CAC GTG GAC TGA GGA CTC CTC

mRNA

Amino acid sequence (p303)

DNA Sickle Cell Anemia CAC GTG GAC TGA GGA CAC CTC

mRNAamino acid

GUG CAC CUG ACU CCU GAG GAG

Val – His – Leu – Thr – Pro – Glu – Glu… 1 2 3 4 5 6 7 …146

From gene to molecule

GUG CAC CUG ACU CCU GUG GAGVal – His – Leu – Thr – Pro–VAL – Glu…1 2 3 4 5 6 7 …146

Sex-Linked Genes• Genes that are found on the X

(or Y) chromosome and are linked to a person’s gender.

• Recessive type seems to disappear only to reappear later.

• Males have a higher chance of having sex-linked disorders. – They only need one copy of

the gene in order to express it.

• Examples: colorblindness, hemophilia, Duchenne Musclar Dystropy

Slide 16

Figure 21.12Page 394

Sex-linked Recessive

Red-Green Colorblindness– Normal Vision

• would be able to see all #s• XCXC, XCXc

• XCY

– A colorblind person • could see 25 and 56• XcXc

• XcY

Sex-linked Recessive

• Red-Green Colorblindness

Sex-linked RecessiveHemophiliaGenotype:

XhXh, XhY

carrier: XHXh

Expression:• Lack of clotting factors for blood• Bleeder’s disease• Can die if bleeding is left untreated

Treatment: • Clotting factor replacement• Gene therapy* (what is gene

therapy?)

Royal Family and Hemophilia

Slide 18

I

II

III

IV

V

VI

Albert Victoria

Figure 21.13Page 394

Sex-linked RecessiveDuchenne Muscular DystrophyGenotype:

XmXm, XmY

carrier: XMXm

Expression:• Defective protein in muscle (dystrophin)• Fatigue• Learning difficulties (IQ can be below

75)• Muscle weakness • Eventually, inability to walk and breathe• Death by 25

Treatment: • Steroid drugs and supplements• Working on gene therapy

X-inactivation

• Females have 2 copies of X – One is randomly

turned off early in development

– Different cells have different Xs active

Chromosomal Disorders

• Rather than 1 gene effecting one protein an entire chromosome is deleted or added.

• Cause:– Non-disjunction of chromosomes (autosomes or sex

chromosomes) during meiosis

• Examples: Down Syndrome, Turner’s Klinefelter’s

Homologous chromosomes fail to separate

Meiosis I:Nondisjunction

Meiosis II

Section 14-2 Nondisjunction

Go to Section:

Homologous chromosomes fail to separate

Meiosis I:Nondisjunction

Meiosis II

Section 14-2

Nondisjunction

Go to Section:

Homologous chromosomes fail to separate

Meiosis I:Nondisjunction

Meiosis II

Section 14-2

Go to Section:

Nondisjunction

Chromosomal DisorderDown Syndrome• Genotype: (autosomal)

47 XX +2147 XX +21

• Expression:– Non-disjunction of chromosome

21– Mental retardation, growth failure,

intestinal and heart problems, leukemia, early onset Alzheimer’s

• Treatment: – No specific treatment

Slide 24

inc

ide

nc

e p

er 1

,00

0 b

irth

s

20

15

10

5

020 25 30 35 40 45

mother's age

Fig. 21.20a,bPage 399

Incidence of Down’s Syndrome and mother’s age

Chromosomal Disorders

• Non disjunction of sex chromosomes leads to extra of loss of an X or Y chromosome

Slide 25

Figure 21.21Page 399

XX

XX

O

XX

O

O

XX

Y

Y

X

X

x

x

x

x

XXY

XXX

YO

XO

gamete

nondisjunction

gamete

or

or

meiosis and gamete formation

possibilities at fertilization

genotype (phenotype)

(Klinefelter syndrome)

(“metafemale”)

(not viable)

(Turner syndrome)

Chromosomal Disorders

Turner’s SyndromeGenotype: (sex chromosome)

45 XOExpression:• Female• Sterile

– Sex organs do not fully develop during puberty

Treatment• Growth hormone treatment,

estrogen treatment

Chromosomal Disorders

Klinefelter’s SyndromeGenotype: (sex chromosome)

48 XXXYExpression:• Male• Sterile • Breast growth• Language problems• Lower energy

Treatment• Testosterone replacement,

speech therapy, breast reduction, physical therapy