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CHAPTER 11
Mendelian Genetics
A. GREGOR MENDEL
A. Gregor Mendel: was an Austrians monk who
discovered the basic principles of heredity through
experiments in his pea plant garden in 1865.
Through these experiments he made two major
discoveries.
A. GREGOR MENDEL
1.  Law of Segregation: Two forms of a gene
(alleles) present in an individual can separate,
with half the progeny inheriting one allele and
half of the progeny inheriting the other allele
during gamete formation.
1. LAW OF SEGREGATION
A. GREGOR MENDEL
2.  Law of Independent Assortment: Allele
pairs separate independently during the
formation of gametes. This means traits are
transmitted to offspring independently of one
another.
2. LAW OF INDEPENDENT
ASSORTMENT
B. REGINALD PUNNETT
B.  Reginald Punnett: was an English scientist who
developed the Punnett Square by connecting
Mendelian with statistics.
B. REGINALD PUNNET
1.  Punnett Square: a type of grid used to
show the gametes of each parent and their
possible offspring.
1. PUNNETT SQUARE
Example –
P = Purple
p = White
Parental (P) Generation
Pp
Pp
Pp
Pp
Genotypic Percentage: 100% Pp
Phenotypic Percentage:
100% Purple
1. PUNNETT SQUARE
Example –
P = Purple
p = White
Offspring (F1) Generation
P p
P
P
Pp
Genotypic Ratio (PP:Pp:pp):
1:2:1
Phenotypic Ratio (Purple:White):
3:1 . VOCABULARY
v Allele: one member of a pair of genes occupying
a specific spot on a chromosome that controls the
same trait.
•  Example: Humans have one gene that controls whether they
will have attached or detached earlobes. That one gene has
two alleles. One allele that codes for attached earlobes, and
one allele that codes for detached earlobes.
VOCABULARY
v Dominant Allele: An allele that is expressed in the
organism’s phenotype. The dominant allele will always
mask the recessive allele in simple Mendelian genetics
and is represented by an uppercase letter.
•  Example: The dominant allele in humans is detached earlobes,
designated by an uppercase “E”. The recessive allele in humans
is attached earlobes, designated by a lowercase “e”. If an
individual had a genotype of “Ee”, their phenotype would be
detached earlobes, because that is the dominant trait.
VOCABULARY
v Recessive Allele: An allele that is expressed in the
organism’s phenotype when no dominant allele is
present.
•  Example: The dominant allele in humans is detached earlobes,
designated by an uppercase “E”. The recessive allele in
humans is attached earlobes, designated by a lowercase “e”.
To express the recessive trait in the phenotype, the genotype
must be “ee”.
VOCABULARY
v Genotype: the genetic makeup of a particular
organism.
•  Example: The potential genotypes for the earlobe trait
in humans could be “EE”, “Ee”, or “ee”.
VOCABULARY
v Phenotype: the observable characteristics of an
organism resulting from its genotype.
•  Example: The potential phenotypes for the earlobe trait
in humans could be attached or detached.
VOCABULARY
v Homozygous: having two identical alleles
for a particular gene.
•  Examples – PP, RR, pp, or rr
VOCABULARY
v Heterozygous: having two different alleles
for a particular gene.
•  Examples – Pp, Aa, Ff, or Qq
VOCABULARY
v Gene: Sequence of DNA that codes for a protein
and thus determines a trait (e.g. factor that is passed
from parent to offspring)
VOCABULARY
v Genetics: Scientific study of heredity
VOCABULARY
v Gamete: sex cell
v Somatic Cell: Body cell (e.g. any cell besides a sex
cell)