1. AP Biology Crosby High School
Chromosomal Basis
AP Biology
Crosby High School

2. Thomas Hunt Morgan Morgan’s choice of organism Discovered Mutant type
Drosophila Melanogaster
Short life span (new generation every 2 weeks)
Only 4 chromosome pairs
Discovered Mutant type
Wild type = occurs most often in nature (w+)
Mutant type = alternative traits to wild type (w)
Discovery of Sex Linkage

3. Morgan Discovered Sex-linkage
Sex-linked genes
White-eyed male and red-eyed female give red eyed F1
F2 gives 3:1 but all white eyes are male
Females receive XX while males receive only One X

4. Linked Genes Results differ from Independent Assortment
Body color and Wing Size
Mixture observed due to Crossing Over

5. Independent Assortment and Crossing Over
Creates genetic recombinants
Parental Type: same as one of the parents
Recombinants: matches neither parent
50% recombinants observed for 2 genes on different chromosome
Expected Results
Unlinked – 1:1:1:1
Linked – 1:1:0:0

6. Body Color and Wing Type

7. Alfred H. Sturtevant Recombinant frequencies relfect gene separation
The farther apart two genes the more likely they are to separate due to crossing over
Created Linkage maps
Body color (b) wing size (vg) and cinnabar (cn)
Cn and b = 9%
Cn and vg = 9.5%
B and vg = 17%
One map unit equals 1% chance recombination
Often called a centimorgan

9. Linkage Maps Maximum 50 map units
Multiple crossovers can cancel each other
Found 4 clusters of linked genes in D. melanogaster
4 chromosome pairs
Cytological maps use chromosome features

10. Sex Chromosomes Basis of sex varies with organisms
SRY (Sex-determining Region of Y)
SRY triggers other proteins to mature gonads into testis
Sex-linked genes have unique inheritance
Passed along X chromosome
Carrier mother
Sons: ½ receive trait
Daughters: ½ are carriers for trait
XaY x XAXa
Daughters: ½ receive trait

11. Sex-Linked Punnet Square

12. Sex-Linked Disorders Duchenne Muscular Dystrophy (20 years) Hemophilia
Progressive weakening of muscles
Loss of coordination
Lack of Dystrophin
Hemophilia
Lack of blood clotting proteins
Given intravenous injections

13. X – Inactivation in Females
One X becomes inactive
Barr body: inactive X lies on inside of Neu. Env.
Mosaic Females
Some cells father derived X
Some cells mother derived X
Tortoiseshell cats and female sweat glands
Determined XIST
X-inactive specific transcript
X inactivated by methylation

14. Tortoiseshell Cats

15. Alteration of Chromosome Numbers
Aneuploidy: abnormal number of chromosomes
Trisomic: 3 homologous chromosomes
Monsomic: 1 chromosome
Polyploidy: Extra complete sets of chromosomes
Triploidy: 3 complete sets
Tetraploidy: 4 complete sets

16. Chromosome Alterations
Deletion: portion of chromosome is lost
Duplication: portion of chromosome is repeated
Inversion: Detaches and reattaches in the opposite order
Translocation: Reattaches to non-homologous chromosome

18. Down Syndrome 1/700 Trisomy 21 Symptoms Facial features Short
Heart defects
Prone to respiratory infection
Mental retardation

19. Klinefelter Syndrome (XXY)
1/2000
Male sex organs
Smaller than average testicles
Breast enlargement

20. Turner Syndrome (XO)
1/5000
Immature sex organs
Sterile

21. Cri du chat “Cry of the cat” Caused by a deletion Mental retardation
Small head
Sometimes a cleft lip

22. Imprinting Prader-Willi Angelman
Deletion on chromosome 15 inherited from father
Mental retardation, short, obese, small hands and feet
Angelman
Deletion on chromosome 15 inherited from mother
Spontaneous laughter, jerky movements, motor and mental symptoms

23. Extracellular Genes Present in Chloroplasts and Mitochondria
Passed down through mother
Exist in the cytoplasm of the ovum
Sperm contains little more than DNA