Human Genetics
Chapter 14 in the
Textbook
Human Chromosomes
What makes us different from other
animals such as a chimpanzee or
earthworm?
If the genome were typed into books,
you'd need 200 volumes, each 1,000
pages long, to write it all out. And yet,
knowing the sequence does not mean
you know where the genes are, how
they are regulated, or what they do.
Human Chromosomes
The size of a genome does not
necessarily have anything to do with
the complexity of the organism.
The lowly onion has a genome that is 20
times the size of the human genome.
Human genome is thought to be
composed of 30 thousand genes spread
out over ____ chromosomes
Human Chromosomes
However the study of any
genome begins with
chromosomes
What is a chromosome?
Threadlike structure within the
nucleus that contains genetic
information that is passed from
one generation to the next.
Sex-Linked Inheritance
The gender of an individual is
determined by the combination
of inherited sex chromosomes.
• Females normally have two X
chromosomes.
• Males normally have one X and
one Y chromosome.
How are the X and Y
chromosomes different?
• 22,700,000 bases or 78 genes make up the Y
chromosome, the smallest in our genome.
• 155 million basses or 900 genes make up the X
chromosome.
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 cells
contain an X
chromosome and half
contain a Y
chromosome.
Sex-Linked Inheritance
Sex-linked Genes: genes located on
a sex chromosome (X or Y)
• 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 recessive trait that is found on
X chromosome.
– More than 100 genetic
disorders are linked to the X
chromosome.
Sex-Linked Inheritance
An example of a sex-linked disorder is colorblindness.
• Genes associated with colorblindness are found only 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.
Example: Color Blindness Test
Various tests for color blindness.
Sex-Linked Traits
• It is possible for a
female to be a carrier of
an X (sex)-linked trait,
but not express it.
Using the picture and your
prior knowledge explain
why the statement
above
X-linked recessive, carrier mother
Unaffected
father
Unaffected
Affected
Carrier
Unaffected
son
U.S. National Library of Medicine
Carrier
mother
Unaffected
daughter
Carrier
daughter
Affected
son