Mendel’s Work
The Science of Heredity
Genetics- science involving the study of
heredity; how traits are passed from one
generation to the next
Gregor Mendel
(1822-1884)
Father of Genetics
Claim to Fame
• Experimented with pea plants (34 varieties)
• Crossed plants with various traits
tall, short, blossom color, pod color, pea
color, pod texture.
• Discovered that the
parent plants produced
offspring with
particular characteristics
Mendel’s Experiments
• Offspring were able to produce
“recessive” traits that existed in one of
the parents that were
not expressed visibly in
the offspring themselves
but were expressed in
the next generation
• E.G.: Tall plants
(from tall x short parents)
were able to produce
short offspring
Results:
• Dominant trait always appeared (no
mixing/blending of traits)
• Recessive trait
sometimes appeared
in the next generation
Traits
• Dominant Allele = trait that is expressed
when 2 different genes for a trait are
present
– represented with capital letters Y, T
• Recessive Allele = trait which is not
expressed when a dominant gene for a
trait is present
– represented with lower case letters y, t
• P Generation = beginning Parental
Generation
• F1 Generation = First Filial Generation
• F2 Generation = Second Filial Generation
Purebred = organism with genes for a
particular trait that are alike
YY, yy, BB, bb
Hybrid = organism with genes for a
particular trait that are different
Yy, Bb
Punnett Squares
• Developed by English
geneticist Reginald C. Punnett
• What is a Punnett Square?
–Chart that shows
possible gene
combinations in a
cross between 2
organisms
Let’s try it!!!
Phenotype and Genotype
• Phenotype = physical
appearance of an organism
• Genotype = its actual genetic
makeup (represented with
capital and/or lower case
letters)
BB & Bb are alike
phenotypically (both are
black) but different
genotypically
(genetically)
• Homozygous- when 2 genes
for a particular trait are the
same (TT or tt)
• Heterozygous- having 2
different forms of a gene for
a particular trait (Tt)
Incomplete Dominance
Some genes are neither dominant
nor recessive, traits appear to be
blended together
Examples: 4 o’clock flower,
palomino (horse)
Cross between chestnut brown & a white horse = Palomino