Download The Chromosome Theory of Inheritance

The Chromosome Theory of
Inheritance
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4-1
Outline of Chromosome Theory of
Inheritance
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Observations and experiments that placed the
hereditary material in the nucleus on the chromosomes
Mitosis ensures that every cell in an organism carries
same set of chromosomes.
Meiosis distributes one member of each chromosome
pair to gamete cells.
Gametogenesis, the process by which germ cells
differentiate into gametes
Validation of the chromosome theory of inheritance
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Evidence that Genes Reside in the
Nucleus
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1667 – Anton van Leeuwenhoek
Microscopist
 Semen contains spermatozoa (sperm animals).
 Hypothesized that sperm enter egg to achieve
fertilization
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1854-1874 – confirmation of fertilization through
union of eggs and sperm
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Recorded frog and sea urchin fertilization using
microscopy and time-lapse drawings and micrographs
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4-3
Evidence that Genes Reside in
Chromosomes
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1880s – innovations in microscopy and staining
techniques identified thread-like structures
Provided a means to follow movement of
chromosomes during cell division
Mitosis – two daughter cells contained same
number of chromosomes as parent cell (somatic
cells)
Meiosis – daughter cells contained half the number
of chromosomes as the parents (sperm and eggs)
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4-4
One Chromosome Pair Determines
an Individual’s Sex.
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Walter Sutton – Studied great lubber
grasshopper
Parent cells contained 22 chromosomes plus an
X and a Y chromosome.
Daughter cells contained 11 chromosomes and
X or Y in equal numbers.
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4-5
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After fertilization
Cells with XX
were females.
 Cells with XY
were males.
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Great lubber grasshopper
(Brachystola magna)
Fig. 4.5
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4-6
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Sex chromosome
Provide basis for sex
determination
 One sex has matching
pair.
 Other sex has one of
each type of
chromosome.
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Photomicrograph of human
X and Y chromosome
Fig. 4.6a
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4-7
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Sex determination
in humans
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Fig. 4.6b
Children receive
only an X
chromosome from
mother but X or Y
from father.
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4-8
At Fertilization, Haploid Gametes
Produce Diploid Zygotes.
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Gamete contains one-half the number of
chromosomes as the zygote.
Haploid – cells that carry only a single chromosome
set
 Diploid – cells that carry two matching chromosome
sets
 n – the number of chromosomes in a haploid cell
 2n – the number of chromosomes in a diploid cell
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4-9
diploid vs haploid
cell in
Drosophila
melanogaster
Fig. 4.2
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4-10
The number and shape of chromosomes
vary from species to species.
n
2n
Drosophila melanogaster
Drosophila obscura
4
5
8
10
Drosophila virilus
Peas
Macaroni wheat
Giant sequoia trees
6
7
14
11
12
14
28
22
Goldfish
Dogs
Humans
47
39
23
94
78
46
Organism
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4-11
Anatomy of a chromosome
Metaphase chromosomes are classified by the position of the centromere
Fig. 4.3
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4-12
Homologous chromosomes match in
size, shape, and banding patterns.
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Homologous chromosomes (homologs) contain
the same set of genes.
Genes may carry different alleles.
Nonhomologous chromosomes carry
completely unrelated sets of genes.
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4-13
Karyotypes can be produced by cutting
micrograph images of stained chromosomes
and arranging them in matched pairs
Human male karyotype
Fig 4.4
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4-14
Autosomes – pairs of nonsex chromosomes
Sex chromosomes and autosomes are arranged in homologous pairs
Note 22 pairs of autosomes and 1 pair of sex chromosomes
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4-15
There is variation between species in
how chromosomes determine an
individual’s sex.
__________________________________________________
Chromosome Females
Males
Organism
__________________________________________________
XX-XY
XX
XY
Mammals, Drosophila
XX-XO
XX
XO
Grasshoppers
ZZ-ZW
ZW
ZZ
Fish, Birds, Moths
__________________________________________________
Table 4.1
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4-16
Complement of sex chromosomes
Humans – presence of Y determines sex
Drosophila – ratio of autosomes to X chromosomes determines sex
Drosophila
Humans
XXX
XX
XXY
XO
XY
XYY
OY
Dies
Normal
female
Normal
female
Sterile
male
Normal
male
Normal
male
Dies
Klinefelter
male
(sterile);
tall, thin
Turner
female
(sterile);
webbed
neck
Nearly Normal
normal female
♀
Normal Normal or
male
nearly
normal
male
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Dies
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Mitosis ensures that every cell in an
organism carries the same chromosomes.
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Cell cycle – repeating pattern of cell growth and
division
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Alternates between interphase and mitosis
Interphase – period of cell cycle between
divisions/cells grow and replicate chromosomes
G1 – gap phase – birth of cell to onset of
chromosome replication/cell growth
 S – synthesis phase – duplication of DNA
 G2 – gap phase – end of chromosome replication to
onset of mitosis
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4-18
The cell cycle
Fig. 4.7a
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4-19
Chromosome replication during S
phase of cell cycle
Synthesis of
chromosomes
Note the
formation of
sister chromatids
Fig. 4.7 b
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4-20
Interphase
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Within nucleus
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G1, S, and G2 phase – cell growth, protein synthesis,
chromosome replication
Outside of nucleus
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Formation of microtubules radiating out into
cytoplasm crucial for interphase processes
Centrosome – organizing center for microtubules located
near nuclear envelope
 Centrioles – pair of small darkly stained bodies at center
of centrosome in animals (not found in plants)
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4-21
Mitosis – Sister chromatids separate
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Prophase – chromosomes condense
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Inside nucleus
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Fig. 4.8 a
Chromosomes condense into structures suitable for replication.
Nucleoli begin to break down and disappear.
Outside nucleus
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Centrosomes which replicated during interphase move apart and migrate to
opposite ends of the nucleus.
Interphase microtubules disappear and are replaced by microtubules that
rapidly grow from and contract back to centrosomal organizing centers.
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4-22
Mitosis - continued
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Prometaphase
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Fig. 4.8b
Nuclear envelope breaks down
Microtubules invade nucleus
Chromosomes attach to microtubules through kinetochore
Mitotic spindle – composed of three types of microtubules
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Kinetochore microtubules – centrosome to kinetochore
Polar microtubules – centrosome to middle of cell
Astral microtubules – centrosome to cell’s periphery
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Mitosis - continued
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Metaphase – middle stage
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Chromosomes move towards imaginary equator called
metaphase plate
Fig. 4.8 c
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4-24
Mitosis - continued
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Anaphase
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Separation of sister chromatids allows each chromatid to
be pulled towards spindle pole connected to by
kinetochore microtubule.
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Fig.to reproduce
4.8 d or display
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Mitosis – continued
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Telophase
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Fig. 4.8 e
Spindle fibers disperse
Nuclear envelope forms around group of chromosomes at each pole
One or more nucleoli reappear
Chromosomes decondense
Mitosis complete
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Mitosis - continued
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Cytokinesis - cytoplasm divides
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Fig. 4.8 f
Starts during anaphase and ends in telophase
Animal cells – contractile ring pinches cells into two halves
Plant cells – cell plate forms dividing cell into two halves
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4-27
Checkpoints
help regulate
cell cycle
Fig. 4.11
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4-28
Meiosis produces haploid germ cells.
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Somatic cells – divide mitotically and make up
vast majority of organism’s tissues
Germ cells – specialized role in the production
of gametes
Arise during embryonic development in animals and
floral development in plants
 Undergo meiosis to produce haploid gametes
 Gametes unite with gamete from opposite sex to
produce diploid offspring.
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Meiosis
Chromosomes replicate once.
Nuclei divide twice.
Fig. 4.12
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4-30
Meiosis – Prophase I
Feature Figure 4.13
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4-31
Meiosis – Prophase I continued
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Crossing over during prophase
produces recombined chromosomes.
Fig. 4.14 a-c
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4-33
Fig. 4.14 d, e
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How crossing over produces
recombined gametes
Fig. 4.15
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Meiosis I – Metaphase and Anaphase
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Meiosis – Telophase I and
Interkinesis
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Meiosis – Prophase II and
Metaphase II
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Meiosis – Anaphase II and
Telophase II
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Meiosis - Cytokenesis
Fig. 4.13
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Meiosis contributes to genetic
diversity in two ways.
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Independent assortment of nonhomologous
chromosomes creates different combinations of
alleles among chromosomes.
Crossing-over between homologous
chromosomes creates different combinations of
alleles within each chromosome.
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4-41
Fig. 4.17
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4-42
Gametogenesis involved mitosis and
meiosis.
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Oogenesis – egg formation in humans
Diploid germ cells called oogonia multiply by mitosis
to produce primary oocytes.
 Primary oocytes undergo meiosis I to produce one
secondary oocyte and one small polar body (which
arrests development).
 Secondary oocyte undergoes meiosis II to produce
one ovum and one small polar body.
 Polar bodies disintegrate leaving one large functional
gamete
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4-43
Oogenesis in humans
Fig 4.18
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Gametogenesis
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Spermatogenesis in humans
Symmetrical meiotic divisions produce four
functional sperm.
 Begins in male testis in germ cells called
spermatogonia
 Mitosis produces diploid primary spermatocytes.
 Meiosis I produces two secondary spermatocytes per
cell.
 Meiosis II produces four equivalent spermatids.
 Spematids mature into functional sperm.
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4-45
Spermatogenesis in humans
Fig. 4.19
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4-46
The chromosome theory correlates Mendel’s
laws with chromosome behavior during meiosis.
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Chromosome Behavior
Each cell contains two copies of each
chromosome
Chromosome complements appear
unchanged during transmission from
parent to offspring.
Homologous chromosomes pair and
then separate to different gametes.
Maternal and paternal copies of
chromosome pairs separate without
regard to the assortment of other
homologous chromosome pairs.
At fertilization an egg’s set of
chromosomes unite with randomly
encountered sperm’s chromosomes.
In all cells derived from a fertilized
egg, one half of chromosomes are of
maternal origin, and half are paternal.
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Behavior of genes
Each cell contains two copies of
each gene.
Genes appear unchanged during
transmission from parent to
offspring.
Alternative alleles segregate to
different gametes.
Alternative alleles of unrelated
genes assort independently.
Alleles obtained from one parent
unite at random with those from
another parent.
In all cells derived from a fertilized
gamete, one half of genes are of
maternal origin, and half are
paternal.
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4-47
Specific traits are transmitted with
specific chromosomes.
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A test of the chromosome theory
If genes are on specific chromosomes, then traits
determined by the gene should be transmitted with
the chromosome.
 T.H. Morgan’s experiments demonstrating sexlinked inheritance of a gene determining eye-color
demonstrate the transmission of traits with
chromosomes.
 1910 – T.H. Morgan discovered a white – eyed male,
Drosophila melanogaster, among his stocks.
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4-48
Nomenclature for Drosophila
genetics
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Wild-type allele - allele that is found in high
frequency in a population
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Mutant allele - allele found in low frequency
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Denoted with a “+”
Denoted with no symbol
Recessive mutation - gene symbol is in
lower case
Dominant mutation - gene symbol is in
upper case
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4-49
Examples of notations for
Drosophila
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Gene symbol is chosen arbitrarily
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e.g., Cy is curly winged, v is vermilion eyed, etc.
Cy, Sb, D are dominant (upper case letter).
vg, y, e, are recessive (lower case letter).
vg+ - wild-type recessive allele for vestigial
gene locus
Cy+ - wild-type dominant allele for curly
gene locus
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4-50
Criss-cross
inheritance of the
white gene
demonstrates
X-linkage.
Fig. 4.20
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4-51
Rare events of
nondisjunction in
XX female
produce XX and
O eggs.
Segregation in an XX female
Fig. 4.21 a
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4-52
Segregation in an
XXY female
Fig. 4.21 b
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4-53
X and Y linked traits in humans are
identified by pedigree analysis.
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X-linked traits exhibit five characteristics seen in
pedigrees.
Trait appears in more males than females.
 Mutation and trait never pass from father to son.
 Affected male does pass X-linked mutation to all
daughters, who are heterozygous carriers.
 Trait often skips a generation.
 Trait only appears in successive generations if sister
of an affected male is a carrier. If so, one half of her
sons will show trait.
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4-54
Example of sex-linked recessive trait
in human pedigree – hemophilia
Fig. 4.23 a
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4-55
Example of sex-linked dominant trait in
human pedigree – hypophosphatemia
Fig. 4.23 b
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4-56