Chromosomes • Humans have 23 pairs of chromosomes. • The autosomes refer to pairs 1 - 22. 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: 1856-1865 • 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