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Genetics
Gregor Mendel (1822-1884)
Mendel’s
Experiments
• Crossed tall pea plants with
short (dwarf) ones
• Crossed yellow seeded
plants with green seeded
ones
• Crossed inflated pods
with constricted pods
• Crossed terminal flowers
with axial flowers
From these
experiments…
• Mendel discovered
dominant and recessive
genes
Mendel also…
• Crossed the offspring from
the original experiments
• For example, he crossed the
two tall offspring (they had
one tall parent and one short
parent)
• The third generation had 787
tall offspring and 277 short
ones
• That’s a ratio of 3 tall:1 short
• He also crossed 2 of the
offspring of the yellow X
green cross
• The third generation had
6,022 yellow offspring to
2,001 green offspring; that’s
a ratio of 3:1
Mendel discovered…
• The Principle of
Segregation – genes
occur in pairs and the
members of the pair
segregate during gamete
formation
Mendel Also...
• Studied crosses between
pea plants that differed in
two characteristics
• For example, one parent had
round yellow peas and one
parent had wrinkled green
peas
Do round and yellow always
have to be inherited together?
• Mendel obtain yellow round
ones, wrinkled green ones,
yellow wrinkled ones, and
round green ones.
Mendel Discovered...
• Principle of Independent
Assortment – traits are not
inherited together
Today…
• We use the terms
homozygous and
heterozygous
Homozygous
• “homo” means the same
• Genes in a pair are
homozygous if they’re the
same
• For example, two genes for
tall or two genes for short
Heterozygous
• “hetero” means different
• Genes are heterozygous if
the genes in the pair are
different
• For example, one purple
flowered gene and one white
flowered gene
Mendel used…
• Letters to represent genes
and we still do today
• For example, we can use
the letter “P” for purple
flowers or “T” for tall
plants
So…
• Two tall genes can be
written “TT” – that’s
homozygous
• One tall gene and one
short gene can be written
“Tt” – that’s heterozygous
Questions
• What would this gene pair
be (homozygous or
heterozygous)?
•RR
• What would this gene pair
be?
•Ss
Two More Genetics Terms to
Know
• Phenotype – physical trait (purple
flowers, tall plants, yellow peas,
etc.)
• Genotype – kinds of genes in the
pair (PP, Pp, or pp) also
(homozygous dominant,
heterozygous, homozygous
recessive)
In Review…
• Genes can be dominant or
recessive – dominant
genes can hide recessive
genes
• If a gene pair is RR, the
“R” trait shows up
• If a gene pair is Rr, the
“R” trait shows up
• If a gene pair is rr, the “r”
trait shows up
A human example…
• Free or
attached ear
lobes
•Free ear lobes are
dominant
•Attached ear lobes
are recessive
• A person with two
dominant genes (EE) or
one dominant and one
recessive (Ee) will have
free ear lobes
• A person with attached
ear lobes has two genes
for attached lobes (ee)
Another human
example
• Hitchhiker’s
Thumb
• Having a hitchhiker’s
thumb is recessive
• Not having a hitchhiker’s
thumb is dominant