Download View PDF

14-2: HUMAN
CHROMOSOMES
HUMAN GENES AND
CHROMOSOMES
•  Using chromosome 21 & 22 as examples
• Both contain long stretches of non-coding
DNA where rearrangement occurs.
•  Genes on the same chromosome are linked
because they were inherited together
(linkage groups).
•  Linked genes may be separated by
recombination (crossing over in meiosis).
SEX-LINKED GENES
•  Sex-Linked Genes: genes
located on the X or Y
chromosome of your 23rd
pair.
•  100+ sex-linked genetic
disorders mapped to the X
chromosome.
THINK-PAIR-SHARE*
Based on this picture, why do you think
more sex-linked disorders are mapped
to the X chromosome and not the Y
chromosome?
•  Colorblindness: 3 human genes associated
with color-vision are located on the X
chromosome.
•  Red-green
colorblindness:
•  in 1 in 10 males but
only in 1 in 100
females in the U.S.
•  Males have only 1 X
chromosome.
Therefore, all X-linked
alleles are expressed in
males, even if they are
recessive.
A COLORBLIND LIFE…
https://www.youtube.com/
watch?v=FKSOe5NK_qQ
•  Hemophilia: disorder
where a protein necessary
for normal blood clotting
is missing.
•  2 genes carried on the X
chromosome help control blood
clotting.
•  Affects 1 in 10,000 males.
•  Treatable with protein clotting
injections
POST-IT-NOTE CHECK
Name 2 sex-linked disorders
and
Describe each of them using words,
pictures or 2-word phrases.
DID YOU KNOW…
Abraham Lincoln may have
had Hemophilia.
Bill Clinton is Colorblind.
X-CHROMOSOME INACTIVATION
•  British geneticist, Mary Lyon, found that in
female cells, one X chromosome is switched
off, forming a Barr body.
•  Influences fur color spots on cats depending on which X
chromosome is inactivated.
CHROMOSOMAL DISORDERS
•  Nondisjunction: error in meiosis where
homologous chromosomes don’t
separate.
• If nondisjunction occurs, abnormal numbers of
chromosomes find their way into gametes and a
mistake in chromosome numbers result.
https://www.youtube.com/watch?
v=EA0qxhR2oOk
Down Syndrome (Trisomy 21):
• Trisomy is when 2 copies of an
autosomal chromosome fail to
separate during meiosis,
resulting in an individual with 3
copies of a chromosome.
• Affects 1 in 800 babies in U.S.
Sex Chromosome Disorders:
•  Turner’s Syndrome: female inherits only one X
chromosome (genotype: XO) resulting in sex organs
that do not develop = infertility.
•  Klinefelter’s Syndrome: (genotype: XXY) prevents
reproduction in males.
•  No reports of baby without an X chromosome.
•  Y-chromosome contains a sex-determining region that
produces males sexual development (no matter how
many X chromosomes) and in its absence, a female
develops.
SUMMARY
Write 5-7 sentences addressing
the various concepts you
learned throughout this lesson.