Genetics and
Heredity
Watch this video
before completing your
foldable or taking
notes.
http://ed.ted.com/lessons/how-mendel-s-pea-plantshelped-us-understand-genetics-hortensia-jimenez-diaz
Use slides 4 – 18 to aide you in
completing your Genetics Foldable
Heredity
• The study of how traits are passed from parents
to offspring.
Genes and Alleles
• A gene is a segment
of DNA that codes
for a specific trait.
• Ex. Gene for hair
color
• An allele is an
alternative form of a
gene
• Ex. Allele for brown
hair
Dominant
• Trait that is always
observed; masks the
recessive form of a trait.
• Represented by CAPITAL
LETTERS
• W : Widow’s Peak
• H : Hitchhikers thumb
• E : Unattached earlobes
Recessive
• Only observed when TWO recessive alleles are
present.
• Represented by a lower case letter.
• w : Straight hair line
• h : Absent hitchhikers thumb
• e : Attached earlobes
Homozygous
• Same alleles joined together
• Examples:
• Homozygous Dominant
• RR, HH, TT, LL
• Homozygous Recessive
• rr, hh, tt, ll
Heterozygous
• Different alleles joined together
• Examples:
• Ww, Hh, Ee
Genotype
• Combination of alleles in an organism
Phenotype
• The physical appearance
• The outward expression of a gene
• What an organism looks like
Interpreting a Genotype
• G: dominant allele for green pea
• g: recessive allele for yellow pea
• GG
Gg
gg
Theory of Heredity
Inherited allele for
purple P
HOMOZYGOUS
purple flower PP
Inherited allele
for purple P
Theory of Heredity
Inherited allele for
purple flower (P)
HETEROZYGOUS
purple flower (Pp)
Inherited allele for
pink flower (p)
Theory of Heredity
Inherited for pink
flower (p)
HOMOZYGOUS pink
flower (pp)
Inherited for pink
flower (p)
G = Gray fur, g= White fur
Genotype for Fur
Color
Phenotype for Fur
Color
Subject A
Subject B
Subject C
GG
Gg
gg
same
What does homo- mean? ______________
different
What does hetero- mean? ______________
joined together
What does –zygous mean? ________________________
Define homozygous:
Homozygous means the same alleles have joined together
_______________________________________________
Define heterozygous:
Heterozygous means different alleles have joined together
_______________________________________________
Circle the following genotype(s) that are homozygous:
GG
Gg
gg
Circle the following genotype(s) that are heterozygous:
GG
Gg
gg
Now, read through slides 20 – 27.
On the back of your “Monohybrid
Punnett Squares” Notes summarize
Mendel’s conclusions and define:
- Law of Segregation
- Law of Independent Assortment
Gregor Mendel
 Studied patterns of
heredity (passing on of
characteristics from
parent to offspring)
 Used the common
garden pea in
experiments
Why did Mendel use peas?
 Sexually reproducing:
able to isolate both
male and female
gametes
 Easy to identify traits
(characteristics that are
inherited)
 Short life cycle: able to
be grown quickly
Mendel’s Experiment
PARENT GENERATION (P1)
Tall true breed x short true breed
FILIAL GENERATION (F1)
All tall hybrids
FILIAL GENERATION (F2)
75% tall hybrids, 25% short hybrids
What did Mendel observe?
 When a true-breeding tall plant is crossed
with a true-breeding short plant in the P
generation, the F1 height trait is always
predictable. 100% are tall plants.
P generation
F1
F2
What happens when the F1 tall plants
are crossed together?
 Mendel observed that the F2 generation, the
offspring of F1 plants, are always in a fixed
ratio of 3:1 tall:short.
 Why?
P generation
F1
F2
Mendel’s Conclusions
 There must be two variations for every trait,
where each variation is called an allele.
 Each offspring inherits only one allele from
each parent.
 The alleles are either dominant or
recessive.
 To show the recessive trait, two recessive
alleles must be inherited.
Mendel’s Conclusions - Law of
Segregation
 Mendel concluded only one allele is passed
from parent to offspring for each trait.
 F1 plants must be heterozygous because the
P generation only passed on one tall allele
and one short allele.
 The F1 plant will then pass on to its offspring
either a tall or a short allele, never both.
Mendel’s Conclusions - Law of
Independent Assortment
 Because organisms are made up of more
than one trait, Mendel concluded that the
inheritance of one trait does not influence the
inheritance of a second trait.
 Example: Height of the pea plant does not
influence the color of the peas

Height is independently assorted from color.