14.1 Human Chromosomes
• What makes us human? What makes us
different from other animals such as a
chimpanzee?
• About 1% of our DNA differs from a chimp
• The number of chromosomes and the
genes found on those chromosomes.
• Karyotype –
photograph of
chromosomes taken
during mitosis, then
arranged in
decreasing size
• Where did these
chromosomes come
from?
23 from mom
23 from dad
5,000
14.1 Human Chromosomes
14.1 Human Chromosomes
• Chromosome pairs 1 - 22 = autosomes
• Chromosome pair 23 = sex chromosomes
(XX or XY)
• Females are written as 46XX; males are
written as 46XY.
Sex Chromosomes
• Males and females born in
50:50 ratio
• X chromosome carries about
1,200 genes. Y carries only
about 140.
Transmission of Human Traits - Recessive Alleles
Cystic Fibrosis
– Autosomal recessive disorder on chromosome 7
– People with cystic fibrosis lack one amino acid in
the 1,480 amino acid sequence that makes up the
CFTR protein
– CFTR protein is a transport protein located in the
cell membrane – absence causes a thick mucus
that clogs the lungs and makes patient susceptible
to infection.
Transmission of Human Traits - Dominant Alleles
• Huntington disease, achondroplastic
dwarfism, and polydactyly
• If a genetic disorder is dominant, it is
expressed even if the individual is
heterozygous.
Codominant Alleles
• Both alleles contribute to the phenotype of
the organism.
• Sickle Cell Disease
– Characterize by “sickle” shaped red blood
cells, cause blockage and severe pain in
limbs and organs
Sex-Linked Genes
• Sex-linked Genes – genes
located on the X
chromosome
– More than 100 sex-linked
genetic disorders have been
linked to the X chromosome
– Y chromosome is much
smaller and contains only a
few genes
• Males have only 1 X
chromosome
– All alleles on the X
chromosome are expressed in
males, even if they are
recessive
X Chromosome Disorder Colorblindness
• 3 genes associated with colorblindness are
found on the X chromosome
• 1/10 males are colorblind
• 1/100 females are colorblind
• Why is colorblindness more common in males?
Males need only one allele to be colorblind.
Females need two alleles to be colorblind.
Human Pedigrees
How does one trace an inherited trait
through generations? How does one
predict who in the next generation will
show that trait?
The Pedigree Chart – show the relationships
within a family
Human Pedigrees
Circle represent female
Square represents male
Horizontal line represents marriage
Vertical line represents children
Human Pedigrees
Unshaded circle or square
indicates that the person
does not express the trait
Half shaded circle or
square indicates the
person is a carrier for the
trait.
Completely shaded circle
or square indicates that
the person expresses the
trait.
How many
generations are
shown? 3
14-2 Human Genetic Disorders
• Genome – all the DNA contained in an
organism. 3.2 billion letters of coding in the human
genome.
• Genetic disorders result from:
– Changes in the DNA sequence that alter amino acids
and could alter phenotype (cystic fibrosis)
– Chromosomal disorders – errors during meiosis that
affect the number of chromosomes present in an
individual (Down Syndrome)
Chromosomal Disorders
• Nondisjunction – homologous
chromosomes fail to separate during
meiosis
– Abnormal number of chromosomes find their
way into gametes
Chromosomal Disorder
• Trisomy – having
three copies of a
chromosome
– Trisomy in
chromosome 21 –
Down Syndrome
– 1/800 births, causes
mild to severe mental
retardation
Chromosomal Disorder
Turner Syndrome
• Females are born with only
one X chromosome
• Females are sterile because
sex organs do not develop
at puberty.
Klinefelter’s Syndrome
• Males born with an extra X
chromosome
• Male is sterile
Autosomal Dominant Traits
• Appear with equal frequency in
both sexes
• Are not likely to skip generations
• Both sexes can transmit the trait to
the offspring
• Affected people must have
affected parents
Autosomal Recessive Traits
• Appear with equal frequency in
both sexes (unless penetrance
differs in males and females)
• Appear only when two recessive
alleles are inherited
• These traits seem to skip
generations
• If trait is uncommon most parents
carrying the allele are
heterozygous (carriers)
X-linked Recessive Traits
• X-linked traits are more readily
inherited by males
• Because males tend to inherit the
trait from unaffected females,
these traits tend to skip
generations
• Sons can only inherit the trait from
their mothers not their fathers
• All daughters will inherit an
affected allele from affected
fathers
X-linked Dominant Traits
• Appear in males and females (often more
females than males)
• Each child with an X-linked dominant trait must
have an affected parent
• Do not skip generations
• Affected men pass the trait to all of their
daughters but none of their sons
• Affected women pass the trait on to about ½ of
their daughters if heterozygous
• Red-green color blindness is the inability
to distinguish the colors red and green.
The gene for this trait is located on the Xchromosome. The allele for normal color
vision(XB) is dominant and the allele for
color blindness (Xb) is recessive. A color
blind woman and a man with normal vision
plan to have their first child.
Complete a punnet square for the couple.