Gene: a specific sequence of nucleotides (or heriditary
unit) that codes for a particular polypeptide
Allele: an alternate form of a gene
Homozygous: having two copies of the same allele of a
Heterozygous: having two different alleles of the gene in
Phenotype: the physical appearance or manifestation
coded for by a genotype.
Genotype: the genetic make-up of an organism.
1) Somatic mutations: occur in somatic (body) cells and
can be passed on to daughter cells (those that result
from mitotic divisions), but are not passed on to
2) Germ line mutations: mutations that occur in the cells
that give rise to gametes (usually during the process of
meiosis). These are often passed on to sexually-
1) Aneuploidy: failure of chromosomes to separate during
anaphase I or II of meiosis leading to gametes with one
extra or one fewer of a particular chromosome than a
2) Polyploidy: individuals with cells having one or more
complete extra sets of chromosomes (triploids, tetraploids,
etc.). This occurs either via autopolyploidy (malfunction
during meiosis) or allopolyploidy (hybridization involving
diploid gametes from different species).
Central Dogma, revisited…
The Universal Genetic Code
codons Amino acids coded for
by specific codons
Point mutations: change in a single nucleotide.
Silent mutations: nucleotide change does not affect
amino acid sequence.
Redundancy in Genetic Code: different codons code for same
Missence mutations: point mutations that change codons and amino
acid sequence and, in turn, the protein produced.
Nonsense mutations: point mutations that create a Stop codon in
place of an “active” codon and halts translation.
Frame-shift mutation: the insertion of a nucleotide into a normal
sequence; this alters all the following codons beyond the insertion.
Types of chromosomal mutations:
Deletion: removal of chromosomal segment
Duplication and deletion occur when homologs break at different
places and unequally swap parts during crossing-over.
Inversions: the flipping of chromosomal segments.
Reciprocal translocations: the exchange of segments
between non-homologous chromosomes
Gregor Mendel’s experiments with peas
Cross two parental
plants with different
Gather mature seeds
and grow them.
offspring from those
Analyze the seeds from
the above crosses for
Both parents are
homozygous; one dominant
(SS, smooth seeds) and the
other recessive (ss; wrinkled
F1 generation are all
heterozygous (Ss), but
appear smooth seeded.
F2 generation are ¼
& ¼ homozygous recessive.
Punnett Square: parental cross
s Ss Ss
s Ss Ss
Homozygous dominant = SS; heterozygous = Ss; homozygous
recessive = ss; Phenotypes: both homozygous dominant and
heterozygous individuals have smooth seeds; homozygous
recessives have wrinkled seeds.
Punnett Square: F1 generation
S SS Ss
s Ss ss
Genotypes: ¼ homozygous dominant, ½ heterozygous, ¼
homozygous recessives. Phenotypes: ¾ smooth, ¼ wrinkled.
Dihybrid cross: a cross involving two loci (2 genes) each
with two alleles.
where one allele is not
expressed preferentially to
another when they are
combined in the
Incomplete Dominance: when heterozygotes have a
phenotype that is intermediate between the two homozygous
Codominance: when both alleles are expressed in the
Example: blood types.
Pleiotrophy: when a single allele causes more than one
distinguishable phenotypic effect.
Epistasis: when the phenotypic effects of one gene are
influenced by another gene.
Many characteristics (height, skin color, etc) are the product
of the interaction of many genes. These interactions usually
produce a wide variation in trait expression (quantitative
Many genes have multiple alleles (not just one or two).
Gregor Mendel (July 20, 1822 - January 6, 1884). Moravian monk (ordained as
priest, August 6, 1847). Discoverer of the fundamental principles of "mendelian"
Exercise: Work in partners or small groups (tables, perhaps).
Beads represent alleles (i.e., black = dominant allele, white = recessive). Two
blacks will equal the homozygous dominants, a black and white the
heterozygotes, two white beads the homozygous recessive condition).
With beads combined in equal numbers in a jar, choose beads and assign
characters to a hypothetical plant.
Black beads White beads
purple flwrs white flwrs
Axial flowers terminal flwrs
yellow seeds green seeds
smooth seed wrinkled seeds
Tall stems short stems
pod inflated pod constricted
Construct ten plants based on all the above characteristics.
Calculate and record the ratio of each set of characteristics (i.e., purple
flwrs vs white flwrs; yellow seeds vs green seeds, etc).
Now do the same with 50 plants.