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CHROMOSOMES
Chapter 11
A. What Is a Chromosome?
A long, continuous strand of DNA, plus
several types of associated proteins,
and RNA.
Chromosome pairs are distinguished by:
size, banding pattern & centromere
position.
B. Linked Genes
Genes carried on the same chromosome.
Linked genes violate Mendel’s law of
independent assortment because
they may not separate during
crossing-over of meiosis I.
What types of gametes are
expected from this
individual?
¼ PL ¼ pl ¼ Pl ¼ pL
What types of gametes are
expected from this
individual?
½ PL ½ pl
The further apart two linked genes are,
the more likely they will separate
during gamete formation.
Parental gametes retain
the gene combinations
from the parents.
Recombinant gametes
result from the mixing
of maternal & paternal
alleles during crossingover.
Closely linked genes yield few recombinant
chromosomes - will NOT obtain expected
9:3:3:1 phenotypic ratio.
Knowing allele arrangement is important
in predicting trait transmission.
Ex. Two allele combinations are possible
for a pea plant with genotype PpLl.
Alleles in coupling tend to be transmitted
together.
 Alleles in repulsion separate with each
generation.

C. Sex Determination
Mechanism by which an individual
develops as a male or a female.
1. Total chromosome number
 is diploid
(develops from a fertilized ovum)
 is haploid
(develops from an unfertilized ovum)
Ex. bees
2. X-O System (number of X
chromosomes determines sex)
 is XX
 is XO
Ex. grasshoppers, crickets & roaches
3. X-Y System (presence of Y
chromosome determines sex)
 is XX
 is XY
Ex. all mammals
SRY gene
In X-Y system, 
determines sex
of offspring.
D. Inheritance of Sex-Linked Traits
Most sex-linked traits are carried on
the X chromosome (X-linked) & are
recessive.
Ex. colorblindness, hemophilia
 more common in 
  cannot be a carrier ( is hemizygous)
  inherits condition from his mother,
NOT his father
Hemophilia: recessive X-linked trait
Genotype
XHXH
XHXh
XhXh
XHY
XhY
Phenotype
non-carrier 
carrier 
 with hemophilia
normal 
 with hemophilia
What is the probability that a carrier 
and a normal  will have a son with
hemophilia?
¼ or 25%
What is the probability that a noncarrier  and a hemophiliac  will
have a son with hemophilia?
ZERO
E. X Inactivation
Female mammals have 2 alleles for
every gene on the X chromosome,
while males have only 1.
This inequality is balanced by “turning
off” one X chromosome in each cell
of a 3 week old  embryo.
 some
cells turn off paternal X
 some cells turn off maternal X
Inactivated X
appears as a darkstaining structure
called a Barr body.
How many Barr bodies would cells of a
male possess?
X inactivation is responsible for the
appearance of calico cats.
The earlier X inactivation occurs, the
larger the patches.
F. Chromosome Abnormalities
1. Polyploidy - extra full sets of
chromosomes.
 animal polyploids spontaneously abort
or die shortly after birth
 plant polyploids are relatively
common (wheat, lilies)
2. Aneuploidy - an extra (trisomy) or
missing (monosomy) chromosome.
Aneuploidy is usually due to a meiotic
error called nondisjunction.
 Autosomal
aneuploids
 trisomy 13
 trisomy 18
 trisomy 21 (Down
syndrome)
 Sex
chromosome aneuploids
 Turner syndrome
XO 
 Triplo-X
XXX 
 Klinefelter syndrome
XXY 
 Jacobs syndrome
XYY 
3. Deletion - part of
a chromosome is
missing.
4. Duplication part of a
chromosome is
present twice.
5. Inversion - part
of a chromosome is
reversed.
6. Translocation - nonhomologous
chromosomes exchange parts (reciprocal
translocation) or combine (Robertsonian
translocation).