Chapter 14Human Genetics

Human Genes and Chromosomes

• 31 thousand genes located over 46 chromosomes

• Autosomes – 22 pairs (44 total)

• Sex Chromosomes – 1 pair (2 total) – XX = female – XY = male– Some genes are located on the sex

chromosomes….they are called “sex-linked”

Sex Determination:

-will a child be male or female?

X

X Y

X

XX

XY

XX

XY

X X

Y

X

x

x

eggs sperm

female(XX)

male(XY)

Men show sex-linked traits more often…

Why?

Sex-linked genes…aka: X-linked genes

A. HemophiliaB. ColorblindnessC. Duchenne Muscular Dystrophy

**Must take into consideration the X and the Y chromosomes when doing these problems

**We refer to X-linked genes with superscriptsex: XHXh=female carrier of hemophilia

XhXh=female hemophiliac Xh Y =male hemophiliac

Why are most of these genes located on the Xchromosome?

Completing sex-linked Punnett Squares:

Hemophilia

– The X chromosome carries genes that help control blood clotting.

– In hemophilia, a protein necessary for normal blood clotting is missing.

– Hemophiliacs can bleed to death from cuts and may suffer internal bleeding if bruised.

Practice Problem:

What are the chances for a mother that is a carrier for hemophilia and a hemophiliac father to have hemophiliac children?

Father(normal vision)

ColorblindNormal vision

Mother (carrier)

Daughter(normal vision)

Son(normal vision)

Daughter(carrier)

Son(colorblind)

Male

Female

Go to Section:

Color blindness:•Three human genes associated with color vision are located on the X chromosome. •In males, a defective version of any one of these genes produces colorblindness.

Duchenne muscular dystrophy

– Duchenne muscular dystrophy is a sex-linked disorder that results in the weakening and loss of skeletal muscle.

– It is caused by a defective version of the gene that codes for a muscle protein.

Pedigrees and sex-linked traits:

Terms to know:•Normal= unaffected with disease unshaded in pedigrees

•Affected= has diseasefully shaded in pedigrees

•Square= male•Circle= female

Practice Problem:This pedigree shows the inheritance of hemophilia:

Find the phenotypes for #1, #2, and #4

If #5 marries a woman who is heterozygous for the hemophiliac gene, XHXh, what are their chances for having children with hemophilia?

Genetic Disorders: AutosomalCaused by Recessive Alleles

Recessive disorders only appear if two recessive alleles are present.

• Albinism • Cystic fibrosis • Galactosemia

Cystic FibrosisSufferers of cystic fibrosis produce a thick, heavy mucus that

clogs their lungs and breathing passageways.

• The most common allele that causes cystic fibrosis is missing 3 DNA bases.

• As a result, the amino acid phenylalanine is missing from the CFTR protein.

• Normal CFTR is a chloride ion channel in cell membranes.• Abnormal CFTR cannot be transported to the cell membrane.• The cells in the person’s airways are unable to transport

chloride ions. • As a result, the airways become clogged with a thick mucus.

Genetic Disorders: Autosomal

Caused by Dominant Alleles :

Dominant disorders are caused by dominant alleles, so only one allele is needed for the disease to show.

• Achondroplasia• Huntington’s Disease • Hypercholesterolemia

Genetic Disorders: ChromosomalCaused by Nondisjunction– Down Syndrome—

Trisomy Chromosome 21– Down syndrome produces

mild to severe mental retardation.

– It is characterized by:• increased susceptibility to

many diseases • higher frequency of some

birth defects

Karyotype: picture of human chromosomes

Changes in Chromosome Number usually the result of Nondisjunction

n + 1

n + 1

n - 1

n - 1chromosome alignments at metaphase I

nondisjunction at anaphase I

alignments at metaphase II anaphase II

Nondisjunction in

Sex

Chromosomes

• Turner’s Syndrome—Monosomy Sex Chromosome (X0)

• A female with Turner’s syndrome usually inherits only one X chromosome (karyotype 45,X).

• Women with Turner’s syndrome are sterile.

• Klinefelter’s Syndrome—Trisomy Sex Chromosome (XXY)

• The extra X chromosome interferes with meiosis and usually prevents these individuals from reproducing.

Pedigree Analysis• Technique to study patterns of human

inheritance• Visual representation of family history• Several possible modes of inheritance are able

to be identified:– Autosomal dominant traits– Autosomal recessive traits– Sex-linked traits

What information do we find in a pedigree?

Male with “normal” traitFemale with “normal” trait

Female affected

Male affected

Horizontal lines between a male and a female = marriage Vertical lines coming down from a marriage = offspring

Autosomal Dominant• Appears in both

sexes with equal frequency

• Can be passed on to the next generation by both males and females

• Does not skip a generation

• Ex: Huntington’s Disease

Autosomal Recessive• Appears at equal

frequency in both sexes.

• Only appears when the affected individual has received one allele from each parent

• Skips generations• Ex: Cystic Fibrosis

Sex-linked• more frequent in males• trait tends to skip generations

MCAS practice questionPeople who are tune deaf are unable to follow a rhythm. Scientists have evidence that tune deafness

can be genetic. The pedigree below traces the inheritance of tune deafness in a family. Individuals in the pedigree are numbered. Scientists have analyzed the inheritance patterns for tune deafness and have concluded that tune deafness is caused by an autosomal dominant allele, T

a. Provide evidence from the pedigree that shows that the tune deafness allele is autosomal dominant, not autosomal recessive. Explain.

a. Identify the genotypes of individuals 5 and 6, and then draw the Punnett square for the cross of these two individuals.

CASE STUDY: ABO Blood Typing

What inheritance patterns does human blood type demonstrate?

ABO GeneDetermined by a single gene with 3 alleles, 2 of which are codominant •Type A Blood (type A marker)

– Makes antibody for B

•Type B Blood (type B marker)– Makes antibody for A

•Type AB Blood (both A & B markers)– Makes no antibodies

•Type O Blood (neither marker)– Makes antibodies for A and B

Risks of Blood Transfusions

• Donor RBCs may not have the same kind of recognition molecules (“markers”) on their surfaces as Recipient RBCs

• Potential Result: – Agglutination, a defense response. CLUMPING

• Antibodies act against foreign cells and cause them to clump together

• Antibodies are produced against antigens NOT present on RBCs

When does agglutination occur?

• Type A – antibodies ignore A marker, attack B markers

• Type B– antibodies ignore B marker, attack A markers

• Type AB– antibodies ignore both forms

• Type O– antibodies attack both A and B markers

Phenotype(Blood Type) Genotype

Antigen on Red Blood Cell

Safe Transfusions

To From

Go to Section:

Blood Transfusions and Agglutination

Blood Typing in Forensics• Can be used to determine the blood type of a

potential suspect in a crime• Test uses two solutions each containing antibodies

to type A and type B antigens. – Solution 1: Anti-A; when mixed with type A blood will

cause it to form clumps.– Solution 2: Anti-B; when mixed with type B blood will

cause it to form clumps. • If blood clumps under contact with both Anti-A and Anti-B,

then it is type AB• O blood does not clump

Rh Gene

• Rh marker determined by a single gene with 2 alleles

– Rh+ allele=Dominant=marker present– Rh- allele=recessive=marker not present