Download Genetics

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.
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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