Transcript The Human Genome

The Human
Genome
Chapter 14
Human Chromosomes

Karyotype—picture of paired homologous
chromosomes

Humans have 46 chromosomes
2
of those are sex chromosomes (X or Y)
 Females have XX; Males have XY
 The remaining 44 are called autosomes.
Gametes

All egg cells contain 22 autosomes and 1 X.

50% of sperm contain 22 autosomes and 1
X; the other 50% contain 22 autosomes and
1Y

The sperm determines the sex of the
offspring!
Human Traits

Pedigree Chart—shows relationships within a
family; can be used to determine how a trait is
passed from one generation to the next

Many traits are polygenic

Many traits are influenced strongly by
environmental factors
 Environmental
effects on gene expression are not
inherited
 The gene may be expressed in a later generation
under the right conditions
Human Genes

The human genome contains tens of thousands
of genes.
 Example:
Chromosome 22 is one of the smallest
human autosomes.
 22
contains approximately 43 million DNA bases and
contains as many as 545 different genes.

Many human traits are controlled by more than
one gene (polygenic) or have incomplete or codominant alleles.
Blood Typing

ABO blood groups
 Controlled
by a gene that codes for a hemoglobin
protein, either A, B, or none
 The protein is what makes different blood types
incompatible
 A & B are co-dominant
 O is recessive

Rh factor—controlled by an additional gene, Rh+
is dominant; Rh- is recessive
Sex-Linked Genes

Genes that are located on either the X or Y
chromosome are called sex-linked.

The X chromosome is much larger and contains
hundreds more genes than the Y.

Males have just one X chromosome. All X-linked
alleles are expressed in males, even if they are
recessive.

Examples: Colorblindness, Hemophilia, and
Muscular Dystrophy
X-chromosome Inactivation

In females, one X chromosome is randomly
switched off. It is called a Barr body.

Different cells can switch off different X
chromosomes.
 Example: All
calico cats are female. Their patches of
different colors are controlled by different X
chromosomes. Male cats can have only one color of
spots because they have only one X chromosome.
Chromosomal Disorders

The most common error in meiosis occurs when
homologous chromosomes fail to separate. This
is called nondisjunction.

If nondisjunction occurs, abnormal numbers of
chromosomes may find their way into gametes
and a chromosomal disorder may result.

Example: Down Syndrome (trisomy 21), Turner’s
syndrome, and Klinefelter’s syndrome
Human Genetics

DNA fingerprinting—a tool that analyzes sections of
DNA that vary widely from individual to individual

The Human Genome Project—started in 1990,
scientists from around the world successfully
mapped and sequenced all human DNA by the end
of 2000.

Gene Therapy—process of changing the gene that
causes a genetic disorder. An absent or faulty gene
is replaced by a normal, working gene.