Chromosomes §Humans have 23 pairs of chromosomes. §The autosomes refer to pairs They are identical in both males and females. §Pair #23 is the sex chromosomes. §A female has XX. §A male has XY. §Thus, it is the father that determines the sex of the offspring.

Chromosome Disorders §Nondisjunction: homologous chromosomes fail to segregate or move to opposite poles during meiosis. §Trisomy: condition of having 3 chromosomes instead of a normal pair. Examples include: Down’s Syndrome-extra #21. Klinefelters- XXY

§Monosomy: condition of having 1 chromosome instead of the normal pair. Example: Turner’s Syndrome- The 23rd chromosome has no pair. Only one X is present.

Genetics: Study of Heredity §Gregor Mendel: “Father” of Genetics §Mendel’s Conclusions: §1) Inherited characteristics are controlled by genes (factors) that occur in pairs. §2) Principle of Dominance and Recessiveness: One gene in a pair may mask the other, preventing the other from having an effect.

§3) Law of segregation: A pair of genes is segregated or separated during the formation of gametes. (Anaphase I of meiosis) §4) Law of independent Assortment: genes separate and distribute to gametes in a way that is independent of other gene pairs.

§Symbols used in genetics: Letters are used to represent the genes. § A capital letter is use to indicate a dominate trait. §A lower case letter is used to indicate a recessive trait. §The letter is chosen by the dominant trait.

§Genotype: indicates the actual genes, represented by letters. §For example: BB or Bb or bb. §Phenotype: indicates what you can actually “see”. For example Brown hair. §Homozygous: paired genes that are the same. For example: BB or bb.

§Heterozygous: paired genes that are not the same. For example Bb. §Alleles: Alternative forms of a gene. For example: B and b. §Multiple alleles: traits with more than two alleles. For example Blood type, A, B and O.

§Punnett Square: used to determine various combinations of genes that can result from a particular cross. §Monohybrid cross: when only one pair of contrasting traits is considered in a cross. For example, TT x tt §Dihybrid cross: two pairs of traits are considered. For example: RRYY x rryy.

§Traits that Mendel observed in peas: §seed texture: round - R wrinkled - r §color: yellow- Y green - y §height: tall - T short - t

§Incomplete Dominance: neither gene is dominant or recessive, they blend. For example: If a red flower is crossed with a white flower, the result would be pink. §Capital letters are used for both genes. §Co-Dominance: both genes dominate, or show. For example: Blood type AB