Download 2. - Dickinson ISD

The Work of Gregor
Mendel:
Monohybrid, Dihybrid, Incomplete,
Codominance, Multiple Alleles, Polygenic
Traits
Genetics: study of hereditary
 Every living being-
plant, animal, microbe
or human being, has a
set of characteristics
inherited from its parent
or parents.
 Genetics: study of
hereditary
Gregor Mendel’s Peas:
 Austrian Monk in charge of the garden,
Studied peas
What he knew:
 fertilization- during sexual reproduction a
male sperm (pollen) and female egg (ova)
joined which produced a new cell, which
began to develop into a tiny embryo encased
within a seed.
 When Mendel took control of the garden, he
had a garden full of self-breeding garden
peas which were true-breeding
True-breeding pure genes, if allowed
to self pollinate, these
plants would produce
identical copies of
themselves.
What he had to work with
 One stock of seeds:
produced Tall plants,
 One stock of seeds:
produced Short Plants,
 One line produced:
green Seeds,
 Another line produced:
yellow Seeds
cross pollination
 Basis of Mendel’s Experiments: Tall, Short,
green Seeds, yellow seeds
 He wanted to cross breed these plants called
cross pollination
Genes and Dominance:
 Mendel studied 7
1.
Form of ripe seed
Smooth
Wrinkled
Color of seed
albumen
Yellow
Green
Color of seed coat
Grey
White
4.
Form of ripe pods
Inflated
Constricted
5.
Color of unripe pods
Green
Yellow
6.
Position of flowers
Axial
Terminal
7.
Length of stem
Tall
Dwarf
different pea plant traits
 Trait: a specific
2.
characteristic, such as
seed color or height 3.
Mendel crossed these plants and
studied their offspring
 He called each of the original plants the P
(Parental) generation
F1 generation
 He called the offspring, F1 or “first filial”
generation
 Filius and filia are the Latin words for “son” and “daughter”
 Therefore the child in the picture below is the F1 generation of
those parents
Hybrids
 The offspring of crosses between parents
with different traits are called hybrids
 Ex: toyota prius
Hybrids in other areas
 Mythology: Centaurs
 Biology: Zeedonk, Liger
So, what were the results? Did they
have a mixture of all the traits?
 NO, all the hybrids had the characteristics of
only ONE of the parents.
 In each cross, the character of the other
parent seemed to disappear!
First Conclusion:
 biological inheritance is determined by factors
that are passed from one generation to the
next
 Today, we called these GENES
 Different forms of a gene are called Allele
Second conclusion: Principle of
Dominance
 P of D: states that some
alleles are dominant
and others are
recessive
 An organism with a
dominant form will
ALWAYS show the trait
Mendel’s Findings
 Tall plant was Dominant and Short plant was
recessive/
 Yellow seeds Dominant and green seeds are
recessive
Some common human dominant traits
Segregation:
 Mendel wanted to know what happened to
the recessive traits. Did they disappear?
 He took the F1 generation and crossed them
with one another and made the F2 generation
F2 generation:
 the recessive traits reappeared
 ¼ or 25% of all the plants had the recessive
traits
He concluded the
 F1 plants produced gametes (sex cells), the
2 alleles segregated from one another so that
each gamete carried a single copy of the
gene
 In the F1 generation, each gamete had 1
copy of the Tall gene and one copy of the
short gene.
Genetics and Probability:
 Probability: the likelihood that an event will
occur
 Coin toss: 2 possibilities: head or tails
 The probability or chances are equal, 1 in 2
chance
 That is ½ or 50% chance
 If you flip a coin 3 times in a row what are the
chances that you will get heads every time: ½
x ½ x ½ = 1/8 1 in 8 chance of flipping heads
3 times in a row!
So what?
The principles of
probability are used to
predict the outcomes of
genetic crosses
Punnett Squares
 the gene combinations that might result from
a genetic cross can be determined by
drawing a diagram known as a Punnett
Square
Very important terms to know!
 Homozygous: 2 identical alleles (TT or tt)
considered true-breeding
 Heterozygous: 2 different alleles (Tt)
considered Hybrids
 Phenotypes: physical characteristics, like tall
or short
 Genotypes: genetic characteristics like TT,
Tt, or tt
Tutorial
 The figure represents a monohybrid cross of
F1-hybrid plants.
 Both parent plants are heterozygous (Ss) for
an allele that determines seed shape.
 Presence of the dominant allele (S) in
homozygous (SS) or heterozygous (Ss)
plants results in spherical seeds.
 Homozygous recessive (ss) plants have
dented seeds.
Setting up a Punnett square
 1. Set up a 2 by 2
Punnett square.
2. Write the alleles for parent 1 on the left
side of the Punnett square.
 Each gamete will have
one of the two alleles of
the parent.
 In this particular cross,
half of the gametes will
have the dominant (S)
allele, and half will have
the recessive (s) allele.
3. Write the alleles from parent 2
above the Punnett square.
 For this heterozygous
parent (Ss), half of the
gametes will have the
dominant (S) allele,
and half will have the
recessive (s) allele.
4. Fill the squares for parent 1.
 Fill each square with
the allele from Parent 1
that lines up with the
row.
5. Fill the squares for parent 2.
 Fill each square with
the allele from Parent 2
that lines up with the
column.
Interpreting the results of a Punnett
square
 We now have the information for predicting
the outcome of the cross.
 The genotypes in the four boxes of the
Punnett square are each equally likely to
occur among the offspring of this cross.
 We may now tabulate the results.
Genotypes that resulted from this
monohybrid cross (Ss x Ss)

 25%
50%
 homozygous heterozygous
dominant
dominant
25%
homozygous
recessive
Phenotypes that resulted from this
monohybrid cross (Ss x Ss)
Independent Assortment: Dihybrid
Crosses
 2 factor Cross
 Mendel crossed true breeding plants that
produced only round yellow peas (RRYY)
with plants that produced wrinkled green
peas (rryy)
 All the F1: round yellow peas
Showed that the allele for yellow and
round peas are dominant
 Provided the hybrid plants for the F1 cross to
produce the F2 generation
 F2: RrYy
 Found a grand mix of traits proving that the
genes practiced independent assortment
meaning that the seed shape and color are
independent of one another
F2 results//
 9:3:3:1
 9 yellow and round, 3 green and round, 3
yellow and wrinkled, 1 green and wrinkled
Tutorial: Dihybrid Crosses
 Determine all possible combinations of alleles
in the gametes for each parent.
 Half of the gametes get a dominant S and a
dominant Y allele; the other half of the
gametes get a recessive s and a recessive
y allele.
 Both parents produce 25% each of SY, Sy,
sY, and sy.
1. Punnett square.
 Since each Parent
produces 4 different
combinations of alleles
in the gametes, draw a
4 square by 4 square
punnett square.
2. Place in Gametes from Parent 1
 List the gametes for
Parent 1 along one
edge of the punnett
square.
3. Place Gametes from Parent 2
 List the gametes for
Parent 2 along one
edge of the punnett
square.
4. Fill in Alleles from Parent 1
 Fill out the squares with
the alleles of Parent 1.
5. Fill in Alleles from Parent 2
 Fill out the squares with
the alleles from Parent
2.
 The result is the
prediction of all possible
combinations of
genotypes for the
offspring of the dihybrid
cross, SsYy x SsYy.
Predicting the phenotype of
offspring
 There are 9 genotypes
for spherical, yellow
seeded plants. They
are:
 SSYY (1/16)
SSYy (2/16)
SsYY (2/16)
SsYy (4/16)
Predicting the phenotype of
offspring
 Two recessive alleles
result in green seeded
plants.
 There are 2 genotypes
for spherical, green
seeded plants. They
are:
 SSyy (1/16)
Ssyy (2/16)
Predicting the phenotype of
offspring
 Two recessive s alleles
result in dented seeded
plants.
 There are 2 genotypes
for dented, yellow
seeded plants. They
are:
 ssYY (1/16)
ssYy (2/16)
Predicting the phenotype of
offspring
 A ssyy plant would be
recessive for both
traits.
 There is only 1
genotypes for dented,
green seeded plants. It
is:
 ssyy (1/16)
A phenotypic ratio of 9:3:3:1
 A phenotypic ratio of 9:3:3:1 is predicted for
the offspring of a SsYy x SsYy dihybrid cross.
Summary of Mendel’s Principles:
 Inheritance of biological characteristics is
determined by genes passed from parents to
offspring
 In cases where 2 alleles exist: 1 is dominant,
1 is recessive
 Each adult has 2 copies of each gene, one
from each parent
 Genes for different things usually segregate
independently
Beyond Dominant and Recessive
alleles:
 some alleles are neither dominant nor
recessive
 many traits are controlled by multiple alleles
or genes
 If Mendel were given a mommy black mouse & a
daddy white mouse & asked what their offspring
would look like, he would've said that a certain
percent would be black & the others would be
white. He would never have even considered
that a white mouse & a black mouse could
produce a GREY mouse! For Mendel, the
phenotype of the offspring from parents with
different phenotypes always resembled the
phenotype of at least one of the parents.
 In other words, Mendel was unaware of the
phenomenon of INCOMPLETE DOMINANCE.
1. Incomplete Dominance:
 F1 Generation: red flower (RR) and white
flower (rr)
 F2 Generation: pink (Rr)
 With incomplete dominance, a cross
between organisms with two different
phenotypes produces offspring with a third
phenotype that is a blending of the parental
traits.
2. Codominance:
 Both alleles contribute to the phenotype

ex: chicken feathers
 In COdominance, the "recessive" &
"dominant" traits appear together in the
phenotype of hybrid organisms.
 I remember codominance in the form of an
example like so:
 red x black ---> red & black spotted
 In horses, gray horses (GG) are codominant
to white horses (WW). The heterozygous
horses(GW) is an appaloosa horse (a white
horse with gray spots on the rump and loins).
Cross a white horse with an appaloosa horse.
 Genotype:
GW (2); WW (2)
 Phenotype:
appaloosa (2)
white (2)
W
W
G
GW
GW
W
WW
WW
Polygenic traits
 interactions of several genes
 Ex: eye color, skin color
3. Multiple Alleles:
 more than two alleles that code for the trait
 Ex: coat color of rabbits
Blood Types
 A and B are codominant
 AA
= Type A
 BB = Type B
 AB = Type AB
 A and B are dominant over O
 AO
= type A
 BO = type B
 OO = type O
#1
 Show the cross between a mother who has type O
blood and a father who has type AB blood.
O
GENOTYPES
O
- AO (2) BO (2)
- ratio 1:1
A
AO
AO
PHENOTYPES:
- type A (2); type B (2)
- ratio 1:1
B
BO
BO
#2
 Show the cross between a mother who is
heterozygous for type B blood and a father who is
heterozygous for type A blood
 Genotype:
 Phenotype:
GENOTYPES:
-AB (1); BO (1);
AO (1); OO (1)
- ratio 1:1:1:1
PHENOTYPES:
-type AB (1); type B (1)
type A (1); type O (1)
- ratio 1:1:1:1
A
O
B
AB
BO
O
AO
OO
#3
 Show the cross between a mother who is
homozygous for type B blood and a father who is
heterozygous for type A blood
 Genotype:
 Phenotype:
Show the cross between a mother who is homozygous for type B
blood and a father who is heterozygous for type A blood
 Genotype:
AB (2), BO (2)
ratio - 1:1
B
B
A
AB
AB
O
BO
BO
 Phenotype:
Type AB (2);
Type B (2)
ratio – 1:1