1. INTRODUCTION TO LINKED GENES AND SEX LINKAGE AP Biology/ Ms. Gaynor
Chapter 15 (Part 5)
INTRODUCTION TO LINKED GENES AND SEX LINKAGE
AP Biology/ Ms. Gaynor

2. Locating Genes on Chromosomes
Are located on chromosomes
Code for protein that
Figure 15.1

3. 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

4. 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

5. Morgan’s Experimental Evidence
Thomas Hunt Morgan
Provided convincing evidence that chromosomes are the location of Mendel’s heritable factors (alleles)

6. Sex Linkage Explained Thomas Hunt Morgan (Columbia University 1910)
Thomas Hunt Morgan (Columbia University 1910)
Fruit Flies (Drosophila) melanogaster)
© 2007 Paul Billiet ODWS

7. 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

8. 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
WILDTYPE w+ w
MUTANT

9. The case of the white-eyed mutant
Character Traits
Eye color 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

10. Hypothesis
A cross between the F1 flies should give us: 3 red eye : 1 white eye F2
Phenotypes
Red eye
White eye
Numbers
3470
82%
782
18%
So far so good…

11. An interesting observationF2
Phenotypes
Red-eyed males
Red-eyed females
White-eyed males
White-eyed females
Numbers
1011
2459
782
24%
58%
18% 0%
© 2007 Paul Billiet ODWS

12. 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

13. Morgan determined that white-eye mutant allele must be located on the X chromosome
Figure 15.4
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.
Morgan then bred an F1 red-eyed female to an F1 red-eyed male to
produce the F2 generation.
RESULTS P
Generation F1 X F2
Morgan mated a wild-type (red-eyed) female
with a mutant white-eyed male. The F1 offspring all had red eyes.
EXPERIMENT

14. 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

15. 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.

16. 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

17. Another X-Linked Trait = Red-Green Colorblindness
Normal vision
Color blind simulation

18. Another X-Linked Trait = Hemophilia
About 85% of hemophiliacs suffer from classic hemophilia
1 male in
They cannot produce factor VIII
The rest show Christmas disease where they cannot make factor IX
The genes for both forms of hemophilia are sex linked
Hemophiliacs have trouble clotting their blood

19. 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

20. The Chromosomal Basis of Sex
An organism’s sex is an inherited phenotype determined by the presence or absence of certain chromosomes
XX = girl
XY = boy

21. 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)

22. 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

23. 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

24. 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)

25. Sex Linkage
If sex-linked recessive on Xn
Females have to be Xn Xn to show sex-linked 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

27. 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

28. 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

30. 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
Orange
fur
Inactive X
Early embryo:
X chromosomes
Allele for
black fur
Cell division
and X
chromosome
inactivation
Black
Figure 15.11