AP Biology Crosby High School Chromosomal Basis AP Biology Crosby High School

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

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

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

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

Body Color and Wing Type

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

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

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

Sex-Linked Punnet Square

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

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

Tortoiseshell Cats

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

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

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

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

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

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

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

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