Lesson 4 section 5.1-5.2
Inheritance and Genetic crosses
A Brief History
In the past, people did not understand how
traits were inherited, but there were many
guesses based on things that could be
observed.
Two theories emerged….
Blending Theory - offspring are a straight
mix
Particulate Theory
traits are inherited as "particles", offspring
receive a "piece"
Golden Doodle
Who was Gregor Mendel?
He was known as the “FATHER OF GENETICS”
He discovered how traits were inherited
GENETICS – study of
heredity
HEREDITY – the passing
of traits from parents to
offspring
Mendel’s Peas
Mendel did his study on pea plants
which have many traits
tall/short
purple /white flowers
round/wrinkled seed
Traits in Pea Plants
Pea plants can be self-fertilized or cross-fertilized
True-Breeding Plants -always create plants that look
like themselves
Hybrids – offspring of true-breeding plants
HYBRIDS
Some traits are dominant over others.
Tall x Short = all tall offspring (hybrids)
*Tall is the dominant trait
T
* Short is recessive
t
Mendel discovered that each trait is
controlled by two factors (alleles)
Genes – factors that
determine your traits
Genes are located on
chromosomes
Quick Check - What do we know so far?
1. The “Father of Genetics” is ____________
2. Genetics is the study of _____________, which is how
traits are passed from _________ to ____________
3. Mendel studied what organism? ____________
4. If one trait covers up another one, we say that it is
______________,
the one that is covered up is ______
5. A “true-breeding” plant is one that can only produce plants
like itself a) true
b) false
Mendel’s Experiments
Draw the crosses
GENOTYPE - what
genes, letters, the
organism has (TT, Tt, tt)
TT x tt
PHENOTYPE - what it
looks like (tall or short)
Tt x Tt
TT Tt
Tt
tt
Check for understanding
1. The passing of traits from parents to offspring is known as _____
2. Who was the father of genetics? _________
3. Genes are located on _______________
4. Every gene is made of two
a. genotypes
b. alleles
c. cells
5. The organism’s outward appearance, such as wrinkled seeds are
referred to as the
a) phenotype
b) genotype
6. The letters (ex. RR) that represent the traits are referred to as the
a) phenotype
b) genotype
7. An organism that has two different alleles, or letters, such as Rr is:
a) homozygous
b) heterozygous
8. 7. An organism that has two of the same alleles, or letters, such as RR is:
a) homozygous
b) heterozygous
9. Which of the following sets would represent Mendel’s Parent (P) generation?
a) RR x RR
b) Rr x Rr
c) RR x rr
10. When two different alleles occur together, such as R r, the one that is expressed
is a) dominant b) recessive
11. What is the diagram shown below called?
What does
this letter
actually
represent?
Check for understanding
1. A one-eyed purple people eater is crossed with a two
eyed purple people eater. All of their offspring have two
eyes. Which trait is dominant?
2. If you use the letter E for this
gene. What is the genotype of
the offspring if the parents were
EE x ee
EE = two eyes
Ee = two eyes
ee = one eye
3. If you crossed the offspring
with each other? How many of the new
offspring would you expect to have two
eyes?
Mendel’s Principles
Dominance and Recessiveness
Some alleles can cover up or hide the other. The
one that is expressed is DOMINANT.
PRINCIPLE OF SEGREGATION
When a parent makes sperm or eggs, their genes
separate. The GAMETES (egg or sperm) contain
only one of the pair.
PRINCIPLE OF INDEPENDENT ASSORTMENT
Each trait is inherited independent of other traits.
Each gamete can inherit
any combination of genes
Possible:
GB
Gb
gB
gb
Mendel crossed plants with two separate traits to see the outcome.
Consider this cross:
Yellow, round seeded plant
YYRR x yyrr
What is the phenotype
of the offspring?
What is the genotype?
x Green, wrinkled seeded plant
What if the offspring RrYy self fertilize?
YyRr x
YyRr
Use
“FOIL” to
determine the
gametes created by
each parent:
What if the offspring RrYy self fertilize?
YyRr x YyRr
YR
Set up a 4x4 Punnett
Square.
YR
Fill out the offspring by
combining letters
(keep the R’s and Y’s
together)
Yr
yR
yr
Yr
yR
yr
What if the offspring RrYy self fertilize?
YyRr x YyRr
YR
Set up a 4x4 Punnett
Square.
YR
Fill out the offspring by
combining letters
(keep the R’s and Y’s
together)
Yr
yR
yr
Yr
yR
yr
YyRr x YyRr
What if the offspring RrYy self fertilize?
YR
Count the number for each
phenotype
Yellow, round? ______
Yellow, wrinkled? _____
YR
Yr
yR
Green, round? ____
Green, wrinkled? _____
yr
Yr
yR
yr
DIHYBRID CROSSES - involve 2 traits, heterozygotes
The ratio is
always:
9
3
3
1
Try this cross: A
yellow, round seeded
dihybrid (YyRr)
crossed with a green
seeded, wrinkled (yyrr)
YR
yr
yr
How many?
Yellow, round? ______
Yellow, wrinkled? _____
Green, round? ____
Green, wrinkled? ____
yr
yr
Yr
yR
yr
Beyond Mendel - experiments after Mendel showed there
were other types of INHERITANCE PATTERNS
Some genes did seem to “blend” together...
Codominance → both traits are
expressed in the heterozygotes
Show the cross of a red cow and a roan cow:
RR = Red Cow
RW = Roan Cow (heterozygote)
WW = White cow
Show the cross of a roan
cow (RW) and a white cow
(WW)
A white flower is crossed with a red flower.
All of the offspring are pink.
RR = red
RW = pink
WW = white
This is called
INCOMPLETE DOMINANCE
Show the cross of two
pink flowers:
Multiple Allele Traits - more than two alleles control the trait
Example: Blood Type
Blood Type
Genotype
A
AA, AO
B
BB, BO
AB
AB (codominant)
O
OO (recessive)
Show the cross between:
AB x O
AO x
BO
O x O
POLYGENIC TRAITS - when many genes control one trait,
usually resulting a wide RANGE of phenotypes
Examples: Skin color, height, eye color
(human)
Sometimes a set of alleles
can cover up another set.
Check out the coat color in
labradors. What’s going
on here?