Transcript Heredity-Gene/Sex Linkage PPT Lecture

Chapter 15 (Part 5)
INTRODUCTION TO LINKED
GENES AND SEX LINKAGE
AP Biology/ Ms. Gaynor
Locating Genes on
Chromosomes
• Genes
– Are located on chromosomes
– Code for protein that
Figure 15.1
Chromosome Theory of Inheritance
• The chromosome theory of
inheritance states that
–Genes have specific loci on
chromosomes
–Chromosomes undergo
segregation and independent
assortment during meiosis
Gene Linkage
• Linked genes
– Usually inherited together because
located near each other on the SAME
chromosome
• Genes closer together on the same chromosome are more
often inherited together
• Each chromosome
– Has 100’s or 1000’s of genes
• Sex-linked genes exhibit unique
patterns of inheritance; genes linked on
the X or Y chromosome
Morgan’s Experimental Evidence
• Thomas Hunt
Morgan
–Provided convincing
evidence that
chromosomes are the
location of Mendel’s
heritable factors
(alleles)
Sex Linkage Explained
http://nobelprize.org/nobel_prizes/medicine/articles/lewis/index.html
• Thomas Hunt Morgan
(Columbia University 1910)
• Fruit Flies (Drosophila)
melanogaster)
© 2007 Paul Billiet ODWS
Morgan’s Choice of
Experimental Organism
• Morgan worked with fruit
flies
–Lots of offspring
–A new generation can be
bred every two weeks
–They have only 5 pairs of
chromosomes
Morgan and Fruit Flies
• Morgan first observed and noted
– Wild type (most common) phenotypes
that were common in the fly populations
• Traits alternative to the wild type are
called mutant phenotypes
w+
WILDTYPE
w
MUTANT
The case of the whiteeyed mutant
Character
Eye color
Traits
Red eye (wild type)
White eye (mutant)
P Phenotypes
Wild type (red-eyed) female x White-eyed male
F1 Phenotypes
All red-eyed
Red eye is dominant to white eye
Hypothesis
A cross between the F1 flies should
give us: 3 red eye : 1 white eye
F2
Phenotypes
Numbers
So far so good…
Red eye
White eye
3470
82%
782
18%
An interesting observation
F2
Phenotypes
Redeyed
males
Redeyed
females
Whiteeyed
males
White-eyed
females
Numbers
1011
2459
782
0
24%
58%
18%
0%
© 2007 Paul Billiet ODWS
Correlating Behavior of a Gene’s
Alleles with Behavior of a
Chromosome Pair
• Morgan mated male flies with white
eyes (mutant) with female flies with
red eyes (wild type)
–The F1 generation all had red eyes
–The F2 generation showed the 3:1
red: white eye ratio (as expected),
but only males had white eyes
Morgan determined that whiteeye mutant allele must be
located on the X chromosome
EXPERIMENT
Morgan mated a wild-type (red-eyed) female
with a mutant white-eyed male. The F1 offspring all had red eyes.
P
Generation
X
F1
Generation
Morgan then bred an F1 red-eyed female to an F1 red-eyed male to
produce the F2 generation.
RESULTS
The F2 generation showed a typical Mendelian
3:1 ratio of red eyes to white eyes. However, no females displayed the
white-eye trait; they all had red eyes. Half the males had white eyes,
and half had red eyes.
F2
Generation
Figure 15.4
Morgan Also Tried A
Reciprocal Cross …
Morgan crossed white-eyed female x
red-eyed male
Result
All red-eyed females and all white-eyed
males
This confirmed what Morgan suspected
The gene for eye color is linked to the X
chromosome
Morgan’s Discovery: Sex Linked
Traits
• Eye color is linked on X
Chromosome
• Females carry 2 copies of gene;
males have only 1 copy
• If mutant allele is recessive, white
eyed female has the trait on both X’s
• White eyed male can not hide the
trait since he has only one X.
Gene are LINKED to
chromosomes
• Morgan’s discovery
–transmission of X chromosome in
fruit flies correlates with
inheritance of the eye-color trait
– 1st solid evidence that a specific
gene is associated with a specific
chromosome
Another X-Linked Trait = RedGreen Colorblindness
Normal vision
Color blind simulation
http://www.onset.unsw.edu.au/issue1/colourblindness/colourblindness_print.htm
X-Linked Trait =
Male Pattern Baldness
Baldness
Another X-Linked Trait =
Hemophilia
• About 85% of hemophiliacs
suffer from classic
hemophilia
– 1 male in 10 000
– They cannot produce
clotting factors (proteins)
• The genes for both forms
of hemophilia are sex
linked
• Hemophiliacs have trouble
clotting their blood
Another X-Linked Trait = Duchenne
Muscular Dystrophy
• Recessive disorder
– 1 male in 3,600
– Defective muscle protein called dystrophin
•
•
•
•
•
Fatigue
Muscle weakness (legs, pelvis, arms, neck)
Difficulty with motor skills
Frequent falls
Progressive disease  trouble walking
leads to wheelchair bound patients
Another X-Linked Trait = Duchenne
Muscular Dystrophy
The Chromosomal Basis of Sex
• An organism’s sex is an inherited
phenotype determined by the
presence or absence of certain
chromosomes
– XX = girl (homozygous)
– XY = boy (hemizygous)
Inheritance of Sex-Linked
Genes
• The sex chromosomes
– Have genes for many characters
unrelated to sex (especially the X
chromosome)
• A gene located on either sex
chromosome
– Is called a sex-linked gene
(Usually on X chromosome
called x-linked)
What genes are on the
X chromosome?
• carries a couple thousand
genes but few, if any, of
these have anything to do
directly with sex
determination
• Larger and more active than
Y chromosome
What genes are on the
Y chromosome?
• Gene called SRY triggers
testis development, which
determines male sex
characteristics
• This gene is turned “on” ~6
weeks into the development
of a male embryo
• Y-Chromosome-linked
diseases are rare
Sex-linked genes follow specific patterns
of inheritance
• Fathers  pass sex-linked alleles to
ALL their daughters but NONE to their
sons
– XY (Father)  XX (daughter)
– XY (Father)  XY (son)
• Mothers  can pass sex-linked alleles
to BOTH sons and daughters
– XX (Mother)  XX (daughter)
– XX (Mother)  XY (son)
Sex Linkage
• If sex-linked recessive on Xn
– Females have to be Xn Xn to show sexlinked trait
– Xn X Females do NOT show sex-linked
trait
– Males have to be Xn Y to show sex-linked
trait
**Most sex-linked disorders affect
males; sometimes females
Sex Linkage
• If sex-linked dominant on XN
–Females have to be XN Xn or XN XN
to show sex-linked trait
–Males have to be XN Y to show
sex-linked trait
X inactivation in Female
Mammals
• In mammalian females
– One of the two X chromosomes in each
BODY cell is usually randomly
inactivated in EACH cell during
embryonic development
– Inactive X condenses into a Barr body
Inactivation of X is Random
• The inactivation is random
and approximately ½ the
X’s from Mom and ½ the
X’s from Dad will inactivate
• Females usually have a mosaic of cell types
for characteristics on X chromosome
– Ex: Tortoiseshell and Calico cats
Two cell populations
in adult cat:
Active X
Early embryo:
Orange
fur
X chromosomes
re 15.11
Cell division
Inactive X
and X
chromosome Inactive X
inactivation
Black
fur
Allele for
black fur
Active X