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GENES AND HEREDITY
Mendelian Genetics Part 1
Introduction
• The passing of traits from parents to offspring is
called heredity.
• Your traits are a result of the interactions of the
genes from both your parents (50% from mom,
50% from dad)
• There are over 8 x106 combinations from the 23
chromosomes inherited from each parent.
• Your makeup is one of over 6.4 x 1013 (64
trillion) combinations of your parents gametes.
Early Beliefs and Mendel
• Early beliefs on heredity were flawed in
many ways.
• Gregor Mendel is known as
the father of genetics because
his experiments laid the
foundation for the science of genetics.
• Mendel was an Austrian monk (18221884) who worked with garden peas.
Mendel cont’d
• Pea plants were ideal because they: 1) had
many contrasting traits, 2) reproduced quickly
and in large numbers, 3) self-fertilized as well as
cross-fertilized.
• Mendel crossed pure-breeding plants that
always produced the same type of offspring with
other pure-breeding plants.
• He crossed traits like: tall plants x short plant,
round seeds x wrinkled seeds, green seeds x
yellow seeds and looked for a pattern in the
offspring.
Mendel’s Experiments
• Prior to Mendel it was believed that the crossing
of traits would create a blend.
• Mendel showed that this was not the case for
when he crossed pure tall plants with pure short
plants, all the offspring were tall.
• Mendel showed that one trait always dominated
over another.
• He reasoned that things called “factors” control
the traits of a plant.
Mendel’s Experiment cont’d
• These factors were later called genes.
• He also realized that there were alternate
forms of genes called alleles.
• Smooth (S) and wrinkled (s) are alleles for
seed shape and the smooth is dominant
while wrinkled is recessive.
• Notice the upper case letter denotes the
dominant allele.
Monohybrid Crosses
• When pure smooth
plants (SS) are crossed
with pure wrinkled (ss)
plants, all the first
generation plants (F1)
were smooth (Ss).
• The next year Mendel
crossed the F1 plants
together to see if the
wrinkled trait would
reappear in the second
generation (F2)
The 3:1 Ratio
• Mendel recovered 7324
seeds, 5474 were
smooth while 1850 were
wrinkled.
• The ratio 5474:1850 can
be simplified to 3:1.
• The chart to the right is
called a punnett
square and explains
how these alleles could
form offspring in a 3:1
ratio.
Summary of Mendel’s Work
• Traits are controlled by genes which occur in
pairs. Different forms of a gene are called
alleles.
• One allele can mask the expression of another
which is called the principle of dominance (eg.
S-smooth is dominant over s-wrinkled)
• Therefore SS is smooth, Ss is also smooth,
while only ss is wrinkled.
• Pairs of alleles separate during sex cell
formation so that each sex cell (sperm or egg)
only has one member of each pair -- law of
segregation.
Meiosis Supports Mendel
• 25 years after Mendel’s
work, the study of meiosis
in cells explains how
genes segregate into sex
cells or gametes.
• A hybrid parent Ss will
produce 50% S gametes
and 50% s gametes.
• Now mathematics can be
applied in biology to solve
heredity problems
---- Mendelian genetics!
Review
Why did Mendel choose pea plants?
• Pea plants have many contrasting traits,
reproduce quickly with lots of offspring,
and can be cross-fertilized.
What are alleles? Give some examples.
• Alleles are different versions of genes.
E.g. Tall & short plants, brown & blue eyes
Review
At what stage of meiosis do pairs of alleles
separate? (Mendel’s law of segregation)
• When homologous chromosomes
separate during anaphase 1.
If you are hybrid for freckles (Ff), what % of
your gametes will have the dominant
freckles allele (F) ?
• 50%
Review
What are the chances of a couple who are hybrid brown
eyed (Bb x Bb) of producing a child with blue eyes (bb)?
Bb x Bb
B
b
B
BB
Bb
b
Bb
bb
there is a ¼ chance of producing blue-eyed child, or 25%