Transcript Foundations of Biology - Geoscience Research Institute

Chromosomes:
Vessels For The Genes
Timothy G. Standish, Ph. D.
©1999 Timothy G. Standish
Reasons Mendel’s
Work Was Ignored:
 There
was no physical element in which
Mendel’s inherited particles could be
identified.
 By the turn of the century, chromosomes
had been discovered (physical particles)
and biologists were better at math.
©1999 Timothy G. Standish
Chromosomes:
The Physical Basis of Inheritance
 1866
Mendel published his work
 1875 Mitosis was first described
 1890s Meiosis was described
 1900 Mendel's work was rediscovered
 1902 Walter Sutton, Theodore Boveri
and others noted parallels between
behavior of chromosomes and alleles.
©1999 Timothy G. Standish
Chromosomal Theory
of Inheritance
 Genes
have specific loci on
chromosomes.
 Chromosomes undergo segregation
(meiosis) and independent assortment,
 Thus alleles of genes are independently
assorted.
©1999 Timothy G. Standish
Chromosomal Theory
of Inheritance Telophase I
E
Prophase I
Crossing Over
e
Replication
E
n
E e
n N
E
e
n
N
e
n
N
e
E
e
n
N
N
n
e
N
E
N
E
e
E
n
Telophase II
N
n
©1999 Timothy G. Standish
Independent Assortment
Eggs
As long as genes are on
different chromosomes,
they will assort
independently
Sperm
EN En
eN
en
EN
EENN
EENn
EeNN
EeNn
En
EENn
EEnn
EeNn
Eenn
eN
EeNN
EeNn
eeNN
eeNn
en
EeNn
Eenn
eeNn
eenn
©1999 Timothy G. Standish
Two Genes On One
Chromosome Telophase I
Prophase I
Replication
E
E
e
e
e
E
e
A
A
E
a
A
a
a
A
E e
A A
a a
Telophase II
a
As long as genes on the same
chromosome are located a long
distance apart, they will assort
independently due to crossing
over during Prophase I of
meiosis
E e
E
e
E
e
A
A
a
a
©1999 Timothy G. Standish
Thomas Hunt Morgan
First to associate a trait (gene) with a
chromosome.
 Worked with fruit flies (Drosophila
melanogaster)
 Why fruit flies?

–
–
–
–
Short generation time (≈ 2 weeks)
Survives and breeds well in the lab
Very large chromosomes in some cells
Many aspects of phenotype are genetically
controlled.
©1999 Timothy G. Standish
Drosophila Mutations
©1999 Timothy G. Standish
More Drosophila Mutations
Wild Type ++
ebony body ee
white eyes ww
©1999 Timothy G. Standish
X Chromosome Human and
Drosophila Genes Are Easy To Find
 In
humans and Drosophila, males are XY
 Thus males are haploid for the X
chromosome
 Because of this, recessive genes on the X
chromosome show up far more commonly
in male than female phenotypes
©1999 Timothy G. Standish
Morgan’s Discovery Of An XLinked Drosophila Gene
X+ X+
A white-eyed
male was
discovered
P
X
1/4
Xw X+ Xw X+
Y
X+Y X+Y
F1
X
1/4
Xw
1/2
X+ Xw
X+
X+ X+ Xw X+
Y
X+Y XwY
F2
©1999 Timothy G. Standish
The Key To Morgan’s Discovery
The key to Morgan’s discovery was the
observation that all the white-eyed individuals in
the F2 generation were males
 Without this vital data on the association of white
eyes with being male, the gene for white eyes
could have been seen as a simple recessive trait on
an autosome
 This illustrates the importance of recording all the
data possible and being alert to the possibility of
interesting things being present in the data
 “Fate favors the prepared mind” (Louis Pasteur)

©1999 Timothy G. Standish
Human X-linked Recessive
Genes
 Brown
enamel - Tooth enamel appears
brown rather than white
 Hemophilia - Two types:
– A - Classic hemophilia, deficiency of
blood clotting factor VIII
– B - Christmas disease, deficiency of
blood clotting factor IX
©1999 Timothy G. Standish
X-linked Recessive Genes
Related to sight
Coloboma iridis - A fissure in the eye’s iris
 Color Blindness - Two types:

– Deutan - Decreased sensitivity to green light
– Protan - Decreased sensitivity to red light
Congenital night blindness - Not due to a
deficiency of vitimin A
 Mocrophthalmia - Eyes fail to develop
 Optic atrophy - Degeneration of the optic
nerves

©1999 Timothy G. Standish
Royal Pedigree
Edward Duke of Kent (1767-1820)
Victoria Princess of Saxe-Coburg (1786-1861)
Albert of Saxe-Coburg (1819-1861)
Victoria Queen of England (1819-1910)
Victoria
(1840-1901)
Leopold Duke
of Albany
(1853-1884)
Alice
(1843-1878)
Alix (Alexandra)
(1872-1918)
King Edward VII
of England (1841-1910)
Tsar Nicholas II
of Russia (1868-1918)
Olga
Marie
(1895-1918) (1899-1918)
Tatiana
(1897-1918)
Emperor Frederick III of
Germany (1831-1888)
Beatrice
King Alfonso XIII
(1857-1944) of Spain (1841-1910)
Irene
(1866-1953)
Victoria
(1866-1953)
Alexis
(1904-1918)
Anastasia
(1901-1918)
©1999 Timothy G. Standish
Variation In Chromosome
Number - Polyploidy


Polyploid individuals have more than two sets of
chromosomes
Many important commercial plants are polyploid:
– Roses
– Navel oranges
– Seedless watermelons

Polyploid individuals usually result from some sort of
interruption during meiosis
1n Gamete
+
Interruption
of meiosis
Pro or Metaphase I Metaphase II
2n
Gametes
3n Zygote
©1999 Timothy G. Standish
Variation In Chromosome
Number - Aneuploidy



Polyploid humans are unknown, but individuals with extra
individual chromosomes are known.
Having extra chromosomes or lacking some chromosomes
is called aneuploidy
Aneuploid individuals result from nondisjunction during
meiosis
+
Zygote
Metaphase I
Anaphase I
+
Zygote
©1999 Timothy G. Standish
Aneuploidy In Humans








Most human aneuploids spontaneously abort
The most viable variations in chromosome number are
those that deal with the sex chromosomes:
XO - Turner’s Syndrome - Phenotypically females
XXX…- “Super” females
XYY… - “Super” Males - On average tend to be larger and
less intelligent
XXY - Klinefelter’s Syndrome - Phenotypically male
Of the non-sex chromosome aneuploidys, Down’s
Syndrome, extra chromosome 21, tends to be the most
viable
Down’s Syndrome is more common in children of mothers
who gave birth after age 40
©1999 Timothy G. Standish
Gene Dosage
 There
seem to be elegant mechanisms for
maintaining the correct dosage of genetic
material in each cell
 When aneuploidy causes a change in the
relative dose of one chromosome,
problems result
©1999 Timothy G. Standish
The Lyon Hypothesis
Having extra chromosomes causes problems (i.e.,
Downs Syndrome)
 Men have only one X chromosome and they are
normal (at least they think so)
 Women have two X chomosomes and they are
normal
 Mary Lyon proposed that the extra dosage of X
chromosome that women have is compensated for
by turning off one of the X chromosomes.
 This turned off chromosome can be observed as a
“Barr Body” in metaphase female nuclei

©1999 Timothy G. Standish
Consequences of X Chromosome
Dosage Compensation
 Early
during development, X chromosomes
are randomly turned off in female cells
 All daughter cells have the same X
chromosome inactivated as their parental
cell.
 Thus, females are a mosaic of patches of
cells some patches expressing the genes on
the paternal X chromosome, other patches
expressing the maternal X chromosome
©1999 Timothy G. Standish
Consequences of X Chromosome
Dosage Compensation
XX
Zygote
XX
XX
At some pont (probably later than the 4 cell
stage) half the X chromosomes are turned off
Daughter cells inherit the mother cell’s
XX XX
combination off and on X chromosomes
XX
Cell division
Because of dosage
compensation, females are
thought to be a mosaic of
patches of cells with each
patch expressing the same
X chromosome, but none
expressing both
chromosomes
XX
Different patches of cells
inherit different act X
chromosomes
©1999 Timothy G. Standish
Why Calico Cats
Are Usually Female
 Orange
coat color is a sex-linked trait in cats
(it is on the X chromosome)
 A female cat heterozygous for orange, has
skin patches expressing the orange X with
the other X chromosome turned off. In other
patches the opposite occurs.
©1999 Timothy G. Standish
©1999 Timothy G. Standish