Human Chromosomes• The 46 human chromosomes contain 6

billion base pairs (bp) of DNA.

• Genes make up only 2 percent of the DNA.

• Average human gene is about 3,000 bp. The largest gene is more than 2 million bp.

Genetic Disorders• Recessive alleles – phenotype expressed in

homozygous recessive individuals only.

• Dominate alleles – phenotype expressed if the dominate allele is present (homozygous and heterozygous dominant).

• Codominant alleles – both alleles are dominant.

Human Genes and Chromosomes• Chromosome 22 – approx. 43 million bp;

545 different genes. Genetic disorders include a form of leukemia and neurofibromatosis. Also contains long stretches of repetitive DNA that are unstable sites where rearrangements can occur (breakage).

• Chromosome 21 – approx. 32 million bp; about 225 genes. Genetic disorders include amyotrophic lateral sclerosis (ALS). Also contains many regions with no genes at all.

• Many genes that are located close together on the same chromosome are linked, meaning that they tend to be inherited together. May be separated by crossing-over during meiosis.

Chromosomal Disorders• Errors in chromosomal separation can occur

during meiosis.

• Nondisjunction – homologous chromosomes fail to separate. If this occurs, abnormal numbers of chromosomes may find their way into gametes, and a disorder of chromosome numbers may result.

Homologous chromosomes fail to separate

Meiosis I:Nondisjunction

Meiosis II

Section 14-2

Nondisjunction

Homologous chromosomes fail to separate

Meiosis I:Nondisjunction

Meiosis II

Section 14-2

Nondisjunction

Homologous chromosomes fail to separate

Meiosis I:Nondisjunction

Meiosis II

Section 14-2

Nondisjunction

Down Syndrome• Trisomy 21 – an individual is born with three

copies of autosomal chromosome 21.

• In U.S., occurs in about 1 in 800 babies born.

• Produces mild to severe mental retardation and an increased susceptibility to many diseases and a higher frequency of birth defects.

Sex Chromosome Disorders• Turner’s syndrome (karyotype 45,X) – female that

inherits only one X chromosome. The women are sterile and their sex organs do not develop at puberty.

• Klinefelter’s syndrome (karyotype 47,XXY) – Males with an extra chromosome X. (Y chromosome has sex-determining region.) Cannot reproduce.

• XXXY and XXXXY have been reported.

Karyotyping • To analyze chromosomes, cells are

photographed during mitosis when chromosomes are fully condensed and easy to see.

• Chromosomes are cut out of photographs and lined up with homologous chromosome and in order of largest to smallest.

The Human Genome

• Sex chromosome = one of two chromosomes that determine an individual’s sex; females have two X chromosomes; males have one X chromosome and one Y chromosome.

• Autosomes = chromosome that is not a sex chromosome.

• 46,XX = female

• 46,XY = male

Polygenic traits• Trait controlled by more than one gene.

• Examples: height, skin color, and eye color.

• None of the genes for a polygenic trait are dominant. Each gene has an active allele and an inactive allele.

• Active alleles have an additive effect on the phenotype.

• Inactive alleles do not affect the phenotype.

• Because of the additive effects, a continuous range of phenotypes is possible.

Sex-Linked Genes• Genes located on the X or Y chromosomes.

• Males have one X chromosomes. All X-linked alleles are expressed in males, even if they are recessive.

• Because males pass their X chromosomes along to their daughters, X-linked genes move from fathers to daughters and then to sons of the daughters.

Colorblindness• 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.

• The most common form, red-green colorblindness, is found in about 1 in 10 males and 1 in 100 females.

Father(normal vision)

ColorblindNormal vision

Mother (carrier)

Daughter(normal vision)

Son(normal vision)

Daughter(carrier)

Son(colorblind)

Section 14-2

Male

Female

Figure 14-13 Colorblindness

Father(normal vision)

ColorblindNormal vision

Mother (carrier)

Daughter(normal vision)

Son(normal vision)

Daughter(carrier)

Son(colorblind)

Section 14-2

Male

Female

Figure 14-13 Colorblindness

Hemophilia• X-linked disorder. Two genes that help

control blood clotting are found on the X chromosome.

• Recessive allele of either gene causes hemophilia. A protein necessary for normal blood clotting is missing.

• Occurs in about 1 in 10,000 males.

• Treated by injections of clotting proteins.

Duchenne Muscular Dystrophy• Sex-linked disorder that results in the

progressive weakening and loss of skeletal muscles.

• In the U.S., 1 in 3,000 males affected.

• Caused by a defective version of the gene that codes for a muscle protein.

caused by

Section 14-1

includeincludeinclude

Concept Map

AutosomolDisorders

Recessive alleles

Dominant allelesCodominant

alleles

Albinism Galactosemia Tay-Sachs disease

Huntington’s disease

Sickle cell disease

Cystic fibrosis

Phenylketonuria AchondroplasiaHypercholes-

terolemia

Blood Types: A, B, AB, ORh factor: +, -

Phenotype(Blood Type Genotype

Antigen on Red Blood Cell

Safe Transfusions

To From

Section 14-1 Blood TypesMultiple alleles & Codominance

Symptoms: feverchillsrashflank pain or back painbloody urinefainting or dizziness

Treatment:transfusion is stoppedcorticosteroidsantihistaminesacetominophen (Tylenol)

Transfusion Reaction

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 Pedigree

A pedigree chart shows the relationships within a family.

From Gene to Molecule• Cystic fibrosis – caused by the deletion of

three bases in the DNA of the gene resulting in an amino acid (phenylalanine) missing from the protein. The protein produced by the gene is a chloride ion pump. The deletion of phenylalanine prevents the protein from being transported to the cell membrane.

Chromosome # 7

CFTR gene

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.

Section 14-1

Figure 14-8 The Cause of Cystic Fibrosis

• Sickle cell disease – characterized by sickle-shaped red blood cells that are more rigid than normal cells and tend to get stuck in capillaries leading to damage to cells, tissues, and organs.

• The normal allele differs from the sickle cell allele by one DNA base leading to a substitution of valine for glutamic acid.

Sickle cell disease and malaria:• In parts of the world

where malaria is a major health threat, the presence of the sickle cell allele in heterozygous individuals confers resistance to malaria.

X-Chromosome Inactivation• In female cells, one X chromosome is

randomly switched off. The turned-off chromosome forms a dense region in the nucleus called a Barr body.

• Also occurs in cats and other mammals (ex, white cat with orange and black spots).

Links from the authors on DNA samples

Career links on geneticists

More information on the Human Genome Project

Interactive test

For links on pedigrees, go to www.SciLinks.org and enter the Web Code as follows: cbn-4141.

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