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Mendel and Mendelian
Genetics
I.
Gregor Mendel
• Heredity – passing of
characteristics (traits)
from parents to offspring
• Genetics – biology that
studies heredity
II. Pea Plant Experiments
• Breeding pea plants
– Reproduce sexually
• Fertilization – gametes
form zygote (fertilized
cell)
• Pollination – pollen grains
from male to female
II. Pea Plant Experiments
• Cross pollination – breed
one plant with another
(did it manually)
II. Pea Plant Experiments
• Mendel observed seven characteristics.
• Each occurred in two contrasting traits.
II. Pea Plant Experiments
(Monohybrid Cross)
• Mendel crossed short
plant with tall plant
(P Generation).
• ALL offspring (hybrids)
were tall. (F1 Generation)
• 2nd Generation (F2) – tall plants from 1st
generation to self-pollinate
III. Mendel’s Conclusions
• Each organism has two
factors that control each
trait
• Alleles – genes in
alternative form
• Genes – parts of
chromosomes (DNA) that
control specific
characteristics
III. Mendel’s Conclusions
• Organisms ALWAYS
have two alleles
(one on each
chromosome)
• Alleles can be
either dominant or
recessive
III. Mendel’s Conclusions
• Dominant trait –
masks the presence
of other traits for
the same
characteristic
• Recessive trait – is
masked by dominant
trait for the same
characteristic
IV. Rule of Dominance
• If an organism had one dominant allele,
this trait would be expressed, hiding the
recessive trait
• Capital letters = Dominant allele
• Lowercase = Recessive
V. Law of Segregation
1) There are alternate versions of genes called
alleles.
2) For each characteristic, an organism inherits
two alleles.
3) Alleles interact in a number of ways.
4) The two alleles for a trait separate during
gamete production
Vocabulary
• Phenotype – observed
characteristics (example:
long or short neck)
• Genotype – genetic
makeup of an individual
(example: Nn or nn)
Vocabulary
• Homozygous – two of
the same alleles
• Homozygous dominant two dominant alleles
(TT)
• Homozygous recessive –
two recessive alleles
(tt)
Vocabulary
• Heterozygous – two
different alleles
(Tt)
• One is dominant and
one is recessive
Example
• Long necks in giraffes
are dominant to short
necks (N and n)
• Genes vs. displayed
trait
VII. Law of Independent
Assortment
• Inheritance of one
trait will not affect
the inheritance of
another.
• Only true for traits
not linked.
VIII.
Punnett Squares
• Reginald Punnet (1905) developed
Punnet squares.
Monohybrid Cross
VIII.
Punnett Squares
• Shows probability, yet not exact
outcomes (genetics follows rule of
chance)