1. The Story of How DNA Works
Genetics!!
The Story of How DNA Works

2. Heredity

3. Gregor Mendel (1822 - 1884) Monk / Austro-Hungarian Empire
Lots of time....worked in monastery
garden growing PEAS!
Investigated inheritance
of pea plant traits
He kept detailed journals that were
neglected until the early 20th century
Only GOD can control traits
not science!
Charles Darwin never knew about
Mendel’s discoveries

4. .......controlled by a single gene with how many alleles?
So why the pea? 7 Pea Plant Traits A trait is a specific characteristic such as....
controlled by a single gene with how many alleles?

5. Remember...1 gene = 2 alleles (for now!)
Gene - portion of the DNA that controls a trait (earlobe placement), specific location on a chromosome (locus)
Allele - variations of a gene (attached or free)

6. Pea Plant Reproduction
Female part where the pollen sticks
True-breeding purple and white plants
are cross-pollinated
Male parts
(sperm)
Female part where the eggs are
Female part that is
the egg that turns into
a seed
All of the flowers were purple.
What happened to
the white ones?
True breeding - organism that always shows the same traits when crossed with itself or with the same type of organism
(Pugs always produce Pug pups)

7. Why only purple flowers?
Principle of Dominance
One allele, the dominant masks
the effect of the other.
When the dominant allele is present
only that allele will be expressed.
The other trait is recessive, it is only observed
when both alleles are recessive.
So...the purple allele is dominant and is expressed making all of the flowers purple.

8. Recessive DOMINANT Always expressed as a capital letter P = purple
Always expressed as a lower case letter
p = white
To represent a trait the two alleles are written together
Pp, pp or PP

9. What does each represent?
PP =
Purple flowers
pp =
White flowers
Pp =
Purple flowers

10. Homozygous / Heterozygous
Homo - means the same
Hetero - means different
Homozygous - both alleles are the same, PP
(also referred to as pure)
Homozygous dominant - PP
Homozygous recessive - pp
Heterozygous - alleles are different, Pp
(Also referred to as a hybrid)

11. Mendel’s Law of Segregation
Because of meiosis each parent randomly contributes
one allele (single gene inheritance) to its offspring,
producing a pair of alleles at fertilization
Somatic cell from mom
egg allele options p Pp P Pp
Meiosis
Fertilization
from dad p Pp
zygote
sperm allele options P
Alleles for one gene on one set of homologous
chromosomes

12. How can we predict the results of a genetic cross?
Punnett square - simple way to predict
what traits an organism will have
Y = yellow
y = green

13. 100% heterozygous for yellow seeds
Let’s cross a green seeded pea plant with a homozygous yellow seeded pea plant
Green = yy
Yellow = YY
Why? y y Y Y y Y y
Yellow is dominant Y Y y Y y
Results:
100% heterozygous for yellow seeds

14. Phenotype
The physical characteristics of an organism (what you can observe):
tall, blue eyes, green seeds
Genotype
The genetic make-up of the gene (what the alleles are): Pp, YY or Gg
Genotype for the pea plant cross: 100% Yy
Phenotype for the cross:
100% yellow seeds

15. Laws of Probability are used to predict results
Lets practice!!