The Chromosome Theory of Inheritance Evidence that chromosomes contain the genetic material: That genes reside in the nucleus was proved by using microscopes and observing the equal contribution of the sperm nucleus and the egg during fertilization. That genes are part of the chromosome was shown by (a) one chromosome pair determines sex e.g. XY, XX (b) diploid zygotes 2n are formed by fusion of haploid (n) gametes (c) number and shape of chromosomes vary from species to species(metacentric, acrocentric) (d) karyotypes show homologous chromosomes (autosomes and sex chromosomes)

Sex Determination: Humans, XY (males), XX (females) and Y determines maleness Drosophila, X/A = 1 (females), X/A = 0.5 (males), X/A = 0.5 - 1 (intersexes) and Y determines fertility Moths, XX (females), XO (males) Birds & butterflies, ZZ (males), WZ (females)

Mitosis It is a somatic division. It ensures that number and content of chromosomes are unchanged in every cell of the organism. Cell cycle involves G1, S, G2 and finally Mitosis followed by Cytokinesis Interphase is the longest and is divided to G1 (gap 1) where cells synthesize material needed for growth and cellular functions, S (synthesis) where DNA duplicates and each chromosomes becomes two sister chromatids attached at the centromere and G2 (gap 2) where cells grow but growth is less than in G1.

Meiosis contributes to genetic diversity in three ways: Independent assortment of different pairs of homologous chromosomes Crossing-over and shuffling of maternal and paternal genes Sexual reproduction and the random fusion of a sperm and an egg during fertilization

1) Egg formation (Oogenesis) in ovaries Oogonia (2n) in ovaries divide by mitosis to produce primary oocytes which divide by meiosis I to produce two assymetric haploid (n) cells, first polar body and a secondary oocyte. The latter divides by meiosis II to produce two assymetric haploid (n) cells, a second polar body and a mature ovum (n). The latter becomes very rich in nutrients to sustain the early embryo after fertilization Gametogenesis in Humans

2) Sperm formation (spermatogenesis) in humans: In the male testis the spermatogonia (2n) divide by mitosis to produce primary spermatocytes which undergo meiosis I to produce two symmetrical cells each haploid (n) and known as seconadry spermatocytes. Each secondary spermatocyte then undergoes meiosis II to produce two spermatids each haploid (n). Each of the spermatids then develop into a mature sperm by forming a whip-like tail and concentrate its chromosomes in the head.

Sex-linked traits: the gene is part of chromosome X. So such traits are hemizygous in males of Drosophila and humans. Traits display crisscross inheritance i.e. the males inherit their phenotype from their mother and the females inherit their phenotype from their father.