2. 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
4. 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
5. 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
6. 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.
8. 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
10. 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
11. 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
12. 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
13. 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%
15. 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
16. 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
18. 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
19. 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)
21. 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
23. 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
24. 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
26. 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
