5 Ch. Opening: In 1910, T. H. Morgan discovered a white-eyed mutation with unusual "sex-linked" inheritance
T.H. Morgan, Drosophila, and Sexlinkage
T.H. Morgan started genetic studies with fruit flies (Drosophila melanogaster), 19091930s. Made many discoveries, but the first was discovery of sex-linkage. Sex linkage of white-eyed mutant (w) was important proof that genes were on chromosomes.
Fig. 5.2 Human Sex Chromosomes
Differ in size and morphology
Terminal regions similar, but few genes in common. Y has few genes, X many genes X and Y pair and disjoin in meiosis as if they were homologous chromosomes
More on sex chromosomes
Many organisms have sex chromosomes like X and Y. Chromosomes other than sex chr.= autosomes. XY males are said to be hemizygous, have only one copy of X chr. genes. XY males always show phenotype of recessive alleles of X chr. genes.
Morgan's observation:
Crossed red-eyed female to white-eyed male. F1 all red eyes.
F2 all females red eyes, males 1/2 red and 1/2 white.
Fig. 5.3 Morgan's hypothesis diagramed
w is on X chr.
There is no w+ allele on Y
Fig. 5.4a Tests of predictions of Morgan's Hypothesis
Draw Punnett squares of all crosses in Fig. 6.4 for practice.
Fig. 5.4b Tests of predictions of Morgan's Hypothesis
A male gets his X chromosome from his mother, not from his father!
These observations by Morgan were first direct evidence placing a gene on a specific chromosome.
Fig. 5.6
Bridges explained these exceptions as examples of nondisjunction in aberrant meioses.
Review Fig. 5.7 and 5.8: Chromosomal basis of Segregation and Independent Assortment
Sex linkage in human pedigrees:
Carriers always female. Affected individuals are usually (but not always) male.
Remember that males get their X from mother, and females get an X from each parent.
Fig. 5.9 X-linked hemophilia in European Royalty
New mutation
Fig. 5.10 Analysis of an X-linked pedigree for color-blindness
Fig. 5.11 Human sex determination
SRY gene on Y chr. encodes Testis Determining Factor (TDF)
Drosophila sex determination depends on X:A ratio (not in book)
Male: X:A ratio=0.5 Female: X:A ratio=1.0
Problem: Females have two copies of each X chr. gene, but males have only one copy. Many X chr. genes needed equally by both sexes.
Drosophia and mammals solve this problem different in ways.
Dosage compensation in Drosophila
When Sxl is off (in males), X chr. genes are transcribed twice as much.
Fig. 5.16 X chr. inactivation in mammals. Mammals solve problem of X dosage by inactivating one X in female embryos at 1000 cell stage.
X-inactivation and the calico cat
Consider a cat heterozygous for alleles of X-linked gene o. Genotype = Oo O = black coat, o = orange coat (recessive).
When X-inactivation occurs, either O or o allele is on inactivated X --> patches of orange or black fur!
Fig. 5.17 Color mosaics in female mammals
Tortoiseshell cat
Calico cat White markings come from another gene
All tortoiseshell cats are female.
What about Oo males?
OY males are solid black.
There are no Oo males (X-linked)!
Fig. 5.18 Barr bodies are only in female nuclei XY male Nucleoli
B
B
XX female
B
Ch. 6 chromosome number and structure variation
Cytogenetics and karyotype. Variation in chr. number.
Polyploidy and aneuploidy
Variation in chr. structure.
Deletions, duplications, inversions, and translocations.
Skip pp. 159 (middle)-end of chapter
Fig. 6.8 Origin of a plant species by hybridization and chr. doubling
Initial hybrid sterile (poor meiotic pairing)
Chr. doubling creates fertile teraploid.
Fig. 6.9 Origin of hexaploid wheat
Two rounds of hybridization and doubling
Aneuploidy: A change in number of part of chr. set, gain or loss of a single chr. Examples: Down syndrome: Chr. 21 trisomy Klinfelter syndrome: XXY sterile male Turner syndrome: XO female (monosomy for X)
Fig. 6.13 Down syndrome karyotype
Chr. 21 trisomy
Changes in chromosome structure:
Normal chr.- ABCDEF Deletion (Deficiency)- ABEF Cri-du-chat syndrome= deletion chr. 5 Duplication- ABCDCDEF
Fig. 6.16 Cri-du-chat syndrome
Deletion
Changes in chromosome structure (continued):
Inversion- ABDCEF
Translocation- Exchange between two different chr.
Fig. 6.19 Structure of an inversion
Fig. 6.20 Pericentric and paracentric inversions
Fig. 6.21 Pairing in inversion heterozygotes
Chromosome pairing of normal and inverted chromosomes forms a loop.
Paired chr. in inversion heterozygote
Fig. 6.22 Structure and pairing of translocations
Reciprocal translocatons involve exchange between two chr.
Altered pairing of translocation homologs in meiosis can cause mis-segregation of chr.