Basic Mendelian Genetics

1. Genetics and Heredity

2. Inheritance
• Parents and offspring often
share observable traits.
• Grandparents and
grandchildren may share
traits not seen in parents.
• Why do traits disappear in
one generation and
reappear in another?

3. Background
Organisms usually
resemble their parents
because they inherit
certain characteristics
from them.
These characteristics,
called traits, are
determined by genetic
information on
chromosomes.
Genetic information =
segments of DNA =
genes

4. Definitions
Genetics = the branch of biology that
studies heredity
Heredity = the passing on of
characteristics from parents to
offspring
***from the Latin word hered-,
meaning “heir.”

5. Gregor Mendel
Father of modern genetics
Gregor Mendel, an
Austrian monk,
carried out the first
important studies
on heredity
(1800s).

6. History
Mendel was the first person to
succeed in predicting how traits would
be transferred from one generation to
the next.
Earlier observers looked at many
traits at once-- Mendel focused on
one at a time

7. Mendel Combined:
• Plant breeding
• Statistics
• Careful record keeping
Mendel’s findings of transmission of traits are
now considered the Laws of Inheritance.
Mendel’s Experiments

8. Mendel studied the pea plant Pisum
sativum
- easy to cultivate and a short life cycle
- easy to control pollination
- keep unwanted pollen out
- cross-fertilize artificially
-had discontinuous characteristics
-Ex: flower color, seed texture
-knew of at least 34 such traits

9. Mendel Artificially Cross-Pollinated
Pea Plants

10. Mendel studied pea traits, each
with two distinct phenotypes
trait

11. TERMINOLOGY
PHENOTYPE - appearance
("pheno-"=visible, as in
"phenomenon")
GENOTYPE - genetic make-up, not
always visible, but detectable by
performing crosses
ALLELES - variants of a gene.

12. Gene Seed shape Seed color Flower
color
Flower
position
Pod shape Pod
color
Plant
height
Dominant
allele
Round (R) Yellow (Y) Purple
(P)
Axial (A) Inflated
(I)
Green
(G)
Tall
(T)
Recessive
allele
Wrinkled
(r)
Green (y) White
(p)
Terminal
(a)
Constricted
(i)
Yellow
(g)
Short
(t)
Mendel‘s Experiments

13. Three important written conventions
for writing genotypes:
The same letter is used for different
alleles of the same gene.
Uppercase letters are used for
dominant alleles and lowercase
letters are used for recessive
alleles.
The letter for the dominant allele is
always written first.

14. TERMINOLOGY
HOMOZYGOUS - having two alleles
that are alike
HETEROZYGOUS - having two
unlike alleles
DOMINANT - showing a phenotypic
effect in heterozygous form
RECESSIVE - showing a phenotypic
effect only when homozygous

15. Mendel’s Laws of Inheritance
"LAW OF SEGREGATION" - dominant and
recessive alleles of heterozygote separate
from one another during meiosis
"LAW OF INDEPENDENT ASSORTMENT"
for 2 genes at a time: genes at different
locations are chosen (sampled)
independently of one another during gamete
formation.
“LAW OF DOMINANCE” – recessive alleles
will always be masked by dominant alleles