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Chapter 15 notes
The Chromosomal Basis of
Inheritance
Concept 15.1
In 1902 Sutton noted the parallels
between the behavior of chromosomes
and the behavior of Mendel’s factors.
Chromosomal theory of inheritance:
genes have specific loci on
chromosomes, and the chromosomes
undergo segregation and independent
assortment
Concept 15.1
Concept 15.1
Morgan traced a gene to a specific
chromosome
- Morgan used Drosophila “fruit flies”
for his experiments
- only 4 pairs of chromosomes
- wild type flies have red eyes
- white eyes is a mutant phenotype
Concept 15.1
- Morgan mated the white-eyed male to
a red-eyed female
- all F1 offspring were red-eyed
- The F2 offspring were not 3:1; instead
all females were red-eyed while half of
the males had red and half had white
eyes
- eye color was linked to the fly’s sex
Concept 15.1
Genes located on sex chromosomes are
called sex-linked genes
- Morgan’s evidence that a specific gene
is carried on the X chromosome helped
confirm the chromosomal theory of
inheritance.
Concept 15.1
Concept 15.2
linked genes: genes that are located on
the same chromosomes and tend to be
inherited together
- linked genes deviate from expected
Mendelian ratios
-ex. in flies body color and wing shape
are inherited together
Concept 15.2
Concept 15.2
Genetic recombination: the production
of new combinations of traits inherited
from two parents
yellow-round x green-wrinkled
YyRr
x
yyrr
Parental types: when the offspring
phenotypes are identical to the parents
- ex ¼ YyRr, ¼ yyrr
Concept 15.2
Recombinants: when the offspring
phenotypes are new combinations
ex ¼ Yyrr, ¼ yyRr
When 50% of all offspring are
recombinants, we say there is a 50%
frequency of recombination
- 50% frequency is observed for genes
located on different chromosomes
Concept 15.2
Geneticists can use recombination data to
map a chromosomes genetic loci
Genetic map: an ordered list of the
genetic loci along a particular
chromosome
Linkage map: a genetic map based on
recombination frequencies
Concept 15.2
Map of body-color (b), wing-size (vg) and
cinnabar (cn)
- cn and b is 9%
- cn and vg is 9.5%
- b and vg is 17%
Concept 15.2
Concept 15.3
Sex-linked genes have unique patterns of
inheritance
- fathers pass sex-linked alleles to
daughters, but not sons
- mothers pass sex-linked alleles to
both sons and daughters
Concept 15.3
Because males only have one locus, they
cannot be heterozygous
Sex-linked disorders in humans
- muscular dystrophy: 1/3500 males
in the US
- hemophilia: absence of proteins for
blood clotting
Concept 15.3
Concept 15.4
Nondisjunction: the members of a pair
of homologous chromosomes do not
move apart properly during meiosis I or
II.
- gametes are (n+1) or (n-1) if they
have too many or too few
chromosomes
Concept 15.4
Aneuploidy: having an abnormal
chromosome number
Trisomic: a chromosome in triplicate
(2n+1); ex. trisomy 21 (Down’s
syndrome)
Monosomic: if a chromosome is missing
(2n-1)
Concept 15.4
Concept 15.4
Polyploidy: organisms that have more
than two complete chromosome sets
- triploidy (3n)
- tetraploidy (4n)
Polyploids are more normal in appearance
than aneuploids
Concept 15.4
Concept 15.4
Breakage of a chromosome can lead to
four types of changes in chromosome
structure
Deletion: occurs when a chromosomal
fragment lacking a centromere is lost
during cell division
Concept 15.4
Duplication: a fragment becomes
attached as an extra segment to a
sister chromatid
Inversion: a chromosomal fragment can
reattach to the original chromosome in
reverse order
Translocation: the fragment joins a
nonhomologous chromosome
Concept 15.4
Concept 15.4
Alterations in chromosome number and
structure are associated with several
human disorders
- Down syndrome: 1/700 children
born; extra 21st chromosome
- Klinefelter syndrome: XXY male
- Turner syndrome: XO female
Concept 15.4
Concept 15.4
Concept 15.4