D’YOUVILLE COLLEGE
BIOLOGY 102 - INTRODUCTORY BIOLOGY II
LECTURE # 24
CHROMOSOMAL BASIS OF INHERITANCE
1.
Chromosomal Basis of Inheritance:
• discovery of meiosis & parallel behavior of meiotic chromosomes with
Mendel’s laws for heritable factors (gene alleles) (fig. 15 – 2 & ppt. 1); conclusion:
genes are on chromosomes (= chromosomal theory of inheritance)
• Morgan’s discovery of sex linkage (X linkage): white-eyed mutants in
Drosophila (fig. 15 – 3): white-eyed male x red-eyed female produces F2 with 3:1 ratio
but only males with white eyes (fig. 15 – 4 & ppt. 2); concluded gene alleles were
carried on X chromosome (further analysis below)
2.
Chromosomal Basis of Sex Inheritance:
• sex chromosomes & autosomes: humans have 1 pair of sex chromosomes +
22 pairs of autosomes (44A, XX for females or 44A, XY for males) (fig. 15 – 5)
• X-Y system: males heterogametic; e.g., Drosophila & mammals; Y does not
carry genes that match those on X, but does carry gene for maleness (SRY = sexdetermining region of Y) in humans (codes for DNA binding protein that activates
autosomal genes for maleness during development)
• Z-W system: females heterogametic; e.g. birds, butterflies, moths, some fish
• X-O system: males have single X (XO), females are diploid for X (XX); e.g.
grasshoppers, many insects
Bio 102, spr. 13
lec. 24 - p. 2
• haplo-diploid system: males haploid, females diploid, e.g., honeybees; drones =
males; workers = sterile females; queen = fertile female (fig. 15 – 6 & ppts. 3 &. 4)
Bio 102, spr. 13
3.
lec. 24 - p. 3
Sex-linked (usually X-linked) traits:
• different pattern from autosomal inheritance (fig. 15 – 7 & ppt. 5)
• males (hemizygous for X-linked genes) show all recessive traits (no
masking by heterozygous condition), e.g., color blindness, hemophilia & Duchenne
muscular dystrophy
• sex chromatin (Barr body): dense-staining chromatin in interphase nuclei
- one X chromosome inactivated in all cells: one-half of cells X1, one-half of
cells X2, e.g., tortoise-shell pattern in cats (fig. 15 – 8 & ppt. 6)