[2024]Digital Global Overview Report 2024 Meltwater.pdf
Genetics chapter 3 part 2
1. EXAM INFORMATION!
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3. 3.3 The Chromosome Theory of Heredity
Proposes That Genes Are Carried on
Chromosomes
• Sutton and Boveri proposed
that chromosome behavior in
meiosis mirrors hereditary
transmission of genes
• The Boveri-Sutton Chromosome
Theory
• Correctly explains the
mechanism underlying
Mendelian Genetics by
identifying chromosomes as
the paired factors required by
Mendel’s Laws. States that
chromosomes are linear
structures with genes located
at specific sites called loci
Walter
Sutton
Theodor
Boveri
4. SOMEONE WAS SKEPTICAL
OF THE „GENE
HYPOTHESIS‟…
Thomas Hunt Morgan was skeptical:
Pea plants are domesticated, what happens in the real world? With
a wild, natural species?
What species did he find?
5. 3.3 The Chromosome Theory of Heredity
Proposes That Genes Are Carried on
Chromosomes
• Morgan studied fruit flies, Drosophila melanogaster, to test Mendel’s rules
on a natural, rather than domesticated, species
• Hung buckets of rotting fruit in trees in the then rural Long Island to attract
fruit flies. Found them easy to work with, and have a life cycle of 12-14 days.
• The term “wild type” signifies the phenotype most common in a
population
http://www.youtube.com/watch?v=LOGeTdcnqFM
6. Thomas Hunt Morgan
• Although at first wild-type phenotypes
prevailed, over several years, Morgan’s
lab members found numerous phenotypic
variants and analyzed these in controlled
crosses
• He concluded from his results that genes
were carried on chromosomes
(independently proposed)
• Great mentor, encouraged the use of fruit
fly in students, several mentees went on
to win Nobel Prizes
• His graduate student, then wife and
mother of his four children, Lilian Mogand
made important contributions in the
laboratory (X-linked inheritance).
• Strongly against eugenics movement
• Unraveled many basic principles of
genetics and inheritance.
7.
8. Chromosomal Basis of Sex
• Nettie Stevens (working with Tenebrio
molitor beetles): sex-dependent
hereditary differences are due to the
presence of two X chromosomes in
females and an X and a smaller Y
chromosome in males
• Diploid cells of females contained 20 large
chromosomes; males contained 19 large
chromosomes and one small one
• Half of the sperm of males contained 10
large chromosomes and the other half had 9
large and one small chromosome
• Sex-linked inheritance refers to
transmission of traits on sex
chromosomes
Mealworm
9. The White-Eye Mutant
• The first mutant identified
in Morgan’s lab was a male
with white eyes instead of
the normal red color
• The mutant white-eyed
male was crossed to a
normal red-eyed female,
producing many F1, all
with red eyes
• Thus the white-eyed
mutant allele was what?
10. NOT a normal 3:1! Only the males have white eyes!
11. Continued crosses…
• When the original white-eyed male was crossed to one of the
F1 females, a 1:1 ratio of white eyes and red eyes was
observed in both male and female offspring
12. Reciprocal Cross
• A cross between a white-eyed female and red-eyed male produced
red-eyed female and white-eyed male F1; a cross between these
produced F1 with red and white eyes in equal frequencies among
both male and female progeny
13. The Gene for Eye Color Is on the X Chromosome
• The differences in phenotype
according to gender are not
anticipated according to
Mendel’s laws
• The transmission of the X
chromosome can account for the
results obtained by Morgan’s lab
• X-linked inheritance is the term
for traits carried on the X
chromosome; males have only
one X and so are called
hemizygous for X-linked traits
14. Figure 3.18
X & Y chromosome
segregation of cross A, B, C.
-Allele genotype is written as a
superscript to the X
chromosome (XW+, XW)
-Hemizygous Y chromomsomes
do not carry a genotype
-Use a Punnet square to cross
two sex chromosomes exactly
as one would for two alleles
Let’s draw one..
Heterozygous red female x white male
15. Exceptional phenotypes!
• Morgan’s work led him to propose
the chromosome theory of heredity
• Calvin Bridges studied “exceptional
phenotypes”; very rare cases of
unexpected eye color
• Observed exceptional
phenotypes: in a cross between a
white-eyed female and red-eyed
male, unexpected rare (1/2,000)
offspring were observed: females
with white eyes or males with red
eyes (only red females and white
males are predicted to occur)
• What was happening to
the X chromosome?
17. Nondisjuntion
• Bridges saw that the
exceptional females
had three X
chromosomes and one
Y; and the exceptional
males had just one X
chromosome
• Failed chromosome
separation is
nondisjunction
18. 3.4 Sex Determination Is Chromosome and
Genetic
• Chromosomal sex is
the presence of
chromosomes
characteristic of each
sex and is determined
at the moment of
fertilization
• Phenotypic sex is the
internal and external
morphology of each
sex, and results from
differences in gene
expression
-Females have two X chromosomes
and males have one X chromosome
-Thus in flies;
Males: X0, XYY, or XY (normal)
Females are XXY or XX (normal)
19. • Sex is determined by
the X : A ratio
• -X =number of X
chromosomes;
A = number of haploid
sets of autosomes
This ratio influences the
actions of three sexdeterming genes:
Deadpan, Sisterless and
Sex-lethal genes
(complicated!)
20. Sex Determination in Humans
• XX = female, XY= male.
• The X chromosome contains genetic
information essential for both sexes; at
least one copy of an X is required.
• Absence of X is lethal
• The male-determining gene is located on
the Y chromosome. A single Y, even in the
presence of several X’s, will still produces a
male phenotype.
• SRY gene on the Y chromosome
determines maleness
• The absence of Y results in a female
phenotype.
22. SRY Gene
(Sex-determing
Region Y)
• Expression of SRY
initiates testicular
development of the
undifferentiated
gonads
• The absence of
SRY expression
allows the default,
female state, to
develop
22
23. SRY is a transcription factor
• SRY is a transcription
factor needed for malespecific gene expression
• Issues with the sex genes
themselves can alter the
male phenotype
• Androgen-insensitivity
syndrome
• Caused by the defective androgen
receptor
• Can be mild to complete
androgen insensitivity
• XY ‘females’
24. • XX = female, XY= male.
• SRY gene on the Y chromosome determines maleness
• Turner syndrome: XO; 1/3000 female births
• Amenorrhea, sterility, web necked, horse shoe kidney
• Klinefelter syndrome: XXY, or XXXY, or XXXXY, or XXYY; 1/1000
male birth
• infertility, gynechomastia, hypogonadism
• Poly-X females: 1/1000 female births
• Tall stature, menstrual irregularity, possible reduction of fertility
26. Diversity of Sex Determination
• A different system, the
Z/W system, is used
by birds, some
reptiles, some fish,
butterflies, and moths
• In this system females
have two different sex
chromosomes (ZW)
and males have two
sex chromosomes that
are the same (ZZ)
27. Z-Linked Inheritance
• Barred feathers: Zlinked dominant allele
(ZB)
• Non-barred feathers is
recessive (Zb)
• In sex-linked traits,
sexes are NOT
affected equally (F2)
• How do we verify
that sex is affecting
outcome?
28. Z-Linked Inheritance
• Reciprocal Cross
(reverse sexes)
yields different
results from the
previous cross.
This indicates that
feather form is
sex-linked.
29. Heterogametic vs Homogametic Sex
• Heterogametic sex: the sex
chromosomes do not match, and
thus the gametes produced by
that sex do not match.
http://www.cals.ncsu.edu/course/ent425/
In humans, the males (XY) are the
heterogametic sex. Males produce
both X and Y gametes
• Homogametic sex: sex
chromosomes match and will
produce the same sex-ed
gametes
• In many species (birds, fish,
reptiles) females can be
heterogametic sex (ZW system)
Gametes
X&Y
Gametes
X&X
30. Haplodiploidy
• Sex can also be
determined by the
number of
chromosome sets
• In haplodiploidy,
males develop
from unfertilized
eggs and are
haploid (n),
females develop
from fertilized eggs
and are diploid
(2n).
31. Environmental Sex
Determination
• Sex is determined by
the temperature the
eggs are incubated at
• In the red-eared slider
turtle, eggs above 30⁰
C produce all females
• In the snapping turtle,
eggs below 22⁰ C or
above 28⁰ C will be
female. Between 2527⁰ C males
predominate
Temperature-dependent sex determination in three reptile
species: the American alligator (Alligator mississippiensis),
the red-eared slider turtle (Trachemys scripta elegans), and
the alligator snapping turtle (Macroclemys temminckii). (After
Crain and Guillette 1998.)
33. 3.5 Human Sex-Linked Transmission Follows
Distinct Patterns
• In X-linked recessive
inheritance, females
homozygoous for the
recessive allele or males
hemizygous for it display
the recessive phenotype
• In X-linked dominant
traits, heterozygous
females and males
hemizygous for the
dominant allele express
the dominant phenotype
• Hemizygous males
display any allele on their
single X whether the allele
is recessive or dominant in
females
34. Features of X-Linked Recessive Inheritance
1. Many more
males than
females have
the trait due to
hemizygosity
35. X-Linked Recessive
#2
#3
#4
1.
Many more males than females
have the trait due to hemizygosity
2.
A recessive male mated to a
homozygous dominant female
produces all offspring with the
dominant phenotype, and all
female offspring are carriers
3.
Matings of recessive males with
carrier females
give half dominant and half recessive
offspring of both sexes
4.
Matings of homozygous recessive
females with dominant males produce
all dominant (carrier) female offspring
and all recessive male offspring
36. Hemophilia A Is an X-Linked Recessive Trait
• Hemophilia A is caused by a
mutation in the factor VIII
gene on the X chromosome
• The mutant allele produces
a nonfunctional bloodclotting protein
• A de novo (newly occurring)
mutation is thought to have
been passed from Queen
Victoria of England to some
of her offspring
37. 1. Many more males than females have the trait due to hemizygosity
39. X-Linked Dominant Trait Transmission
•
The distinctive characteristics of X-linked
dominant traits are
1. Heterozygous females mated to
wild-type males transmit the
dominant allele to half their progeny
of each sex
2. Dominant males mated to
homozygous recessive females
pass the trait to all their daughters
and none of their sons
3. The trait appears equally frequently
in males and females
40. Congenital Hypertrichosis
• Congenital hypertrichosis
(CGH) is a rare X-linked
dominant disorder in
humans
• It leads to a large
increase in the number
of hair follicles on the
body, and males and
females have more body
hair than normal
41. Modified pedigree with CGH
#1?
#2
1. Heterozygous females mated to wild-type males transmit the
dominant allele to half their progeny of each sex
2. Dominant males mated to homozygous recessive females pass the
trait to all their daughters and none of their sons
3. The trait appears equally frequently in males and females
42.
43. Y-Linked Inheritance
• Y-linked traits are transmitted
in an exclusively male-to-male
pattern
• In mammals, there are fewer
than 50 genes on the Y
chromosome; many play roles
in male sex determination or
development
• Though males have only one Y
chromosome, they are not
hemizygous for it, as most of
the genes on the Y are present
in two copies
44. The Pseudoautosomal Region
• Two small regions of homology,
the pseudoautosomal regions
(PAR1 and PAR2), exist
between the X and Y
chromosomes
• These allow homologous
pairing between the X and Y at
meiosis
• There is evidence that crossing
over occurs within these
regions during meiosis
http://php.med.unsw.edu.au/embryology
45.
46. 3.6 Dosage Compensation Equalizes Dosage of
Sex-Linked Genes
• In organisms with sex chromosomes, there is a gender
imbalance between the copy number of genes on the sex
chromosomes
• Any mechanism that compensates for the difference in
number of copies of genes between males and females is
called dosage compensation
47.
48. Random X-Chromosome Inactivation in
Placental Mammals
• Early in mammalian
development, one of two X
chromosomes in each female
somatic cell is randomly
inactivated
• The random X inactivation
hypothesis is also called the
Lyon hypothesis, after Mary
Lyon, who first proposed it
(1962)
• The inactive X chromosome is
visible near the nuclear wall, as
a condensed Barr body, first
visualized by Murray Barr (1949)
49. Female Mammals Are Mosaics
• Once X inactivation has
occurred in a cell, it is
permanent in all the
descendants of that cell
• Female mammals are mosaics
of two populations of cells; one
expresses the maternal X and
the other the paternal X
• Alleles of both chromosomes
are expressed approximately
equally over the whole organism
50.
51. Calico and Tortoiseshell Cats Are Visibly Mosaic
• In cats, the X chromosome
carries a gene responsible for
coat color
• One allele specifies a black
color; the other a yellow color
• X inactivation in heterozygous
females leads to a pattern of
orange and black patches that
is unique to each individual
52. Mechanism of X Inactivation
• Random X inactivation
requires an X-linked gene
called Xist (X-inactivationspecific-transcript)
• The gene produces large
RNA molecules that spread
out and cover (or paint) the
chromosome to be
inactivated
• Xist can only act on the
chromosome from which it is
being transcribed and not
the homolog (i.e., it acts
in cis)
epigenie.com
