Human Genetics
Chapter 14 in the
Textbook
Sex-Linked Inheritance
The gender of an individual is
determined by the combination
of sex chromosomes inherited.
• Females normally have two X
chromosomes.
• Males normally have one X and
one Y chromosome.
How are the X and Y
chromosomes different in size
and number of chromosomes.
Sex-Linked Inheritance
• Make a Punnett square to show the probability of
producing a male or female offspring.
– Females can only pass on a X chromosome.
– Every female egg cell contains 1 X chromosome.
– Males can pass on a X
or Y chromosome.
– Half of the sperm cell
contain an X
chromosome and half
contain a Y
chromosome.
Sex-Linked Inheritance
Sex-linked Genes: genes located on
a sex chromosome
• All genes found on the Y
chromosome are passed directly
from father to son.
• Males have only 1 X chromosome
– All alleles on the X chromosome
are expressed in males, even if
they are recessive
– Males are more likely to inherit
a trait that is found on X
chromosome.
– Many genetic disorder are
linked to the X chromosome.
Sex-Linked Inheritance
An example of a sex-linked disorder is colorblindness.
• Genes associated with colorblindness are found on the
X chromosome
• 1/12 males are colorblind, 1/200 females are colorblind
• Draw a Punnett square to show the probability that a
heterozygous normal vision female and a normal vision
male would have a colorblind child.
Karyotypes
Different organisms have different numbers of chromosomes
• Autosomes: Any
chromosome that is not
a sex chromosome (X or
Y chromosome)
– Humans have 22 pairs
autosomes
• Karyotype: a
photograph of the
complete diploid set of
chromosomes grouped
in homologous pairs and
arranged in order of
decreasing size.
Karyotypes
How to make a karyotype:
1. Photograph a cell going through
mitosis.
– Why couldn’t you photograph a
cell in interphase to make a
karyotype.
2. Match up the chromosomes in
homologous pairs.
– What do a pair of homologous
chromosomes have in common?
– How are the chromosomes in a
homologous pair different?
3. Arrange the chromosomes from
largest to smallest
Karyotypes
• What can be learned from making a karyotype?
– The number of
chromosomes in the
organism
– The gender
– The presence of
chromosomal disorders
• What does the karyotype
to the right tell you about
the organism?
Chromosomal Disorders
• Chromosomal mutation: mutation
that changes the number or
structure of chromosomes.
– Deletion: The loss of all or part
of a chromosome
– Duplication: A segment of the
chromosome is repeated
– Inversion: part of the
chromosome is reverse from its
usual direction.
– Translocation: one chromosome
breaks off an attaches to another
chromosome.
Chromosomal Disorders
Many chromosomal disorders are
caused by a mistake in meiosis
called nondisjunction.
• Nondisjunction: homologous
chromosomes fail to separate
during meiosis
– Abnormal number of
chromosomes find their way
into gametes
– Causes chromosomal
disorders.
– Which of the sperm cells have
an extra chromosome, which
are missing a chromosome?
Chromosomal Disorders
Disorders caused by
nondisjunction.
Down Syndrome
• Caused by an extra 21st
chromosome.
• Also known as trisomy 21
• Trisomy: having three
copies of a chromosome.
Chromosomal Disorders
Turner’s Syndrome
• Females are born with only one
X chromosome
• Monosomy X: having only one
X chromosome.
Klinefelter’s Syndrome
• Males born with two X
chromosomes and a Y
chromosome.
• Y chromosome determines if
the individual is male or female.
Pedigrees
Pedigree Chart: shows the absence or presence of a trait
according to the relationships between family members.
Pedigrees
Determine if the trait is dominant or recessive trait?
Determine the genotype of every member in the family.
Pedigrees
What can you learn by making a pedigree:
• How genetic disorders can be inherited from one
generation to the next. (sex-linked, autosomal
dominant, autosomal recessive)
• If you have a genetic disorder in your family, then
pedigree can determine how likely you are to passed
the disorder to future generation.
Genetic Disorders
Cystic Fibrosis
• Caused by a recessive allele
on chromosome 7
• It is inherited in a autosomal
recessive pattern.
• People with cystic fibrosis
lack one amino acid in the
CFTR gene, causing a thick
mucus that clogs the lungs.
Genetic Disorders
Ideogram: A diagram of a chromosome
• Ideograms show can show the
position of specific genes:
• The CFTR gene is located at 7q31.2
– 7th chromosome
– q = long arm of the chromosome
(p = short arm of the chromosome)
– 31.2 represents the position on the
arm (band number)
Genetic Disorders
Huntington’s Disease
• Caused by a dominant allele
• The DNA sequence – C A G – is
repeated over several times.
• This causes the Huntingin protein
to become deformed which
damages the nervous system.
• Most genetic disorders are
inherited as a recessive trait.
Why are dominant genetic
disorders more rare than
recessive disorder?
Genetic Disorders
Why does Huntington’s disorder
get passed from generation to
generation.
• Causes gradual damage to the
nervous system resulting in
loss of muscle control and
mental function.
• People who have the disease
show no symptoms until their
thirties or forties (after they
passed the allele to their
children).
Genetic Disorders
Sickle Cell Disease
• Characterize by “sickle” shaped red
blood cells
• Caused by a mutation of the
hemoglobin gene on chromosome 11.
• People who are heterozygous for the
sickle cell trait have both normal and
sickle shaped red blood cells.
– What is the pattern of inheritance
Genetic Disorders
• Sickle Cell allele is carried by 1 in 12 of African ancestry
• Most genetic disorders are extremely rare. Why is
sickle cell so common?
– Those who are heterozygous for Sickle cell are
resistant to malaria.
– The benefits of being heterozygous outweighed the
risks the of passing on both parents passing the
allele for the disease.