Mendelian genetics

1. Mendelian Genetics 101

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. 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.”

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

5. Gregor Mendel
Father of 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 Artificially Cross-Pollinated Pea Plants

9. 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
-He focused on 7

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

11. Mendel’s Experimental Design

12. Mendel’s Experimental Design

13. Mendel’s Experimental Design

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

15. 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.

16. 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

17. Mendel’s Experimental Design: What we know now

18. Mendel’s Experimental Design: What we know now

19. Mendel’s Experimental Design: What we know now

20. 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

21. Production of gametes: Meiosis
• The number of chromosomes is reduced (by
half).
• Alleles for genes are separated into the
gametes.
• Gametes then combine during sexual
reproduction to form zygotes with complete
sets of chromosomes.
• Mendel’s flowers examples
• Dog examples handout

22. Gametes for Single Characteristic

23. Two characteristics
Heterozygous for both
characteristics
_____
_____
__________

24. Mendel’s Laws of Inheritance
• “LAW OF DOMINANCE” – One factor in a pair
may prevent the other from having an effect;
that factor dominates or masks the other.
in modern terms:
• “LAW OF DOMINANCE” – recessive alleles will
always be masked by dominant alleles

25. Mendel’s Laws of Inheritance
• “LAW OF SEGREGATION” – paired factors
separate during the formation of reproductive
cells
in modern terms:
• "LAW OF SEGREGATION" - alleles separate
from one another during meiosis in the
formation of gametes.

26. Mendel’s Laws of Inheritance
• “LAW OF INDEPENDENT ASSORTMENT” – factors
for individual characteristics are not paired
together.
in modern terms:
• "LAW OF INDEPENDENT ASSORTMENT“ - genes at
different locations are separated independently
of one another during gamete formation.
– Far away from each other on the same chromosome
– On separate chromosomes.