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Genetics
Core Concepts
• Mendel’s Law of Segregation states that there are two alleles for
every gene determining a specific characteristic, and these alleles are
segregated into separate gametes during reproduction.
• When the 2 different alleles occur together in one individual
(heterozygote), the dominant allele will be the one that is expressed in
the phenotype of the individual.
• The Law of Addition is used to combine probabilities, when there are
2 or more ways to arrive at a given outcome.
• The Law of Multiplication is used to combine probabilities of 2 or
more different events that need to occur in combination.
• Mendel’s Law of Independent Assortment says that 2 or more
different genes, if found on separate chromosomes, are determined
independently of each other.
• Many characteristics have been found to follow patterns of inheritance
that are modifications of Mendel’s rules.
Keywords
•
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•
•
•
gene
allele
dominant
recessive
character
trait
phenotype
genotype
•
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pure breeding
hybrid
homozygous
heterozygous
P (parental
generation)
• F1 (first filial
generation)
• F2 (second filial
generation)
• Punnett Square
• forked line
• polygenes
• contributory
allele
• noncontributory
allele
• continuous
variation
• discontinuous
variation
Mendel’s
Discoveries
• Pre-Mendel beliefs in
genetics: blending
theory
• Augustinian monk and
science teacher
• Why Pisum sativum?
– Pea plants available in
many varieties
• character (heritable feature)
• trait (character variant)
– Perfect flowers
• cross-pollination and selfpollination
– Short generation time
– Many offspring
CHARACTERS
TRAITS
MONOHYBRID CROSS
inheritance of a single trait
P Generation
(true-breeding
parents)
x
Purple
flowers
White
flowers
F1 Generation
(hybrids)
All plants had
purple flowers
self-pollination
F2 Generation
705 plants
224 plants
Allele for purple flowers
Locus for flower-color gene
Homologous
pair of
chromosomes
Allele for white flowers
• Alternative versions
of genes exist (alleles)
– Dominant
– Recessive
• Organism inherites 2
alleles
– Dominant is fully
expressed
– Recessive has no
visible effect
• Law of segregation
– 2 alleles for one
character separate
and go to different
gametes
True-breeding plants have identical
alleles.
Gametes each contain only one allele
for the flower-color gene. Every gamete
produced by one parent has the same allele.
P Generation
x
Purple flowers
PP
White flowers
pp
P
p
Appearance:
Genetic makeup:
Gametes:
F1 hybrids have a Pp combination.
Purple-flower allele is dominant,
all hybrids have purple flowers.
Hybrid plants produce gametes,
two alleles segregate: half the
gametes receiving the P allele and
the other half the p allele.
Punnett square: shows all possible
combinations of alleles in offspring
from an F1 x F1 (Pp x Pp) cross.
F1 Generation
Purple flowers
Pp
Appearance:
Genetic makeup:
1/
2
Gametes:
1/
2
P
F1 sperm
P
p
F2 Generation
P
Each square represents an equally probable
product of fertilization.
Pp
PP
F1 eggs
p
Random combination of the gametes
results in the 3:1 ratio that Mendel
observed in the F2 generation.
p
pp
Pp
3
:1
Other terms in
genetics
• Homozygous –
identical alleles,
true-breeding
• Heterozygous –
different alleles
• Phenotype –
physical
appearance
• Genotype – genetic
make-up
The test-cross
APPLICATION
X
An organism displaying the dominant phenotype can either
be homozygous or heterozygous for the trait. A test-cross will
help determine which.
Dominant phenotype,
unknown genotype:
PP or Pp?
TECHNIQUE
The individual with the unknown genotype is crossed with
a homozygous individual expressing the recessive trait
(white flowers in this example). By observing the
phenotypes of the offspring resulting from this
cross, we can deduce the genotype of the purple-flowered
parent.
Recessive phenotype,
known genotype:
pp
If PP,
then all offspring
purple:
p
If Pp,
then 2 offspring purple
and 1⁄2 offspring white:
1⁄
p
p
p
Pp
Pp
pp
pp
RESULTS
P
P
Pp
Pp
P
p
Pp
Pp
Dihybrid cross
• Illustrates the inheritance
of two characters
• Produces four
phenotypes in the F2
generation
• Law of Addition combines probabilities
for mutually exclusive
events
• The Law of
Multiplication combines probabilities of
2 or more independent
events that need to occur
together
Law of independent
assortment
- allele pairs separate independently
during the formation of gametes
Forked Line Method
AaBbCc x AaBbCc
Monohybrid cross problems
1.
2.
3.
4.
In pea plants, spherical seeds (S) are dominant to dented seeds (s). In a genetic cross of
two plants that are heterozygous for the seed shape trait, what fraction of the offspring
should have spherical seeds?
In pea plants, yellow seed color is dominant to green seed color. If a heterozygous pea
plant is crossed with a plant that is homozygous recessive for seed color, what is the
probability that the offspring will have green seeds?
A. If all of the offspring of a particular cross have the genotype Yy, what must the genotype
of the parents be?
B. To identify the genotype of yellow-seeded pea plants as either homozygous dominant
(YY) or heterozygous (Yy), you could do a test cross with plants of genotype _______.
In rabbits, black fur (B) is dominant to white fur (b). Perform the following crosses. For each
cross, give the phenotype and genotype of all offspring.
A. BB x bb
B. Bb x Bb
C. BB x Bb
In summer squash, white fruit color (W) is dominant over yellow fruit color (w). If a squash
plant homozygous for white is crossed with a plant homozygous for yellow, what will the
phenotypic and genotypic ratios be for:
A. the F1 generation? B. the F2 generation?
C. What will the phenotypic and genotypic ratios of the offspring be if you perform a
testcross with the F1 generation?
Dihybrid and trihybrid cross problems
1.
The ability to roll one’s tongue is dominant over non-rolling. The ability to taste
certain substances is also genetically controlled. For example, there is a
substance called phenylthiocarbamate (PTC for short), which some people can
taste (the dominant trait), while others cannot (the recessive trait).
A woman who is both a homozygous tongue-roller and a non-PTC-taster marries
a man who is a heterozygous tongue-roller and is a PTC taster, and they have
three children: a homozygous tongue-roller who is also a PTC taster, a
heterozygous tongue-roller who is also a taster, and a heterozygous tongue-roller
who is a non-taster.
A. If these parents have more children so that they had 12 in all, how many of
those 12 would you expect to be non-tasters who are homozygous for tonguerolling? If the first child (the homozygous tongue-roller who is also a PTC taster)
marries someone who is heterozygous for both traits, draw the Punnett square
that predicts what their children will be.
2. Using the forked-line method, determine the genotypic and
phenotypic ratios of these trihybrid crosses:
A. AaBbCc x AaBBCC
B. AaBBCc x aaBBCc
Non-mendelian inheritance
1.
Codominance
–
–
–
2.
Incomplete dominance
–
–
3.
two dominant alleles affect
phenotype in separate ways
both alleles manifest
e.g. roan coloring in horses –
both red and white
phenotype of offspring is
between phenotypes of two
parents
e.g. red and white parents
give rise to pink offspring
Multiple alleles
–
–
genes with more than two
alleles that control the
phenotype
e.g. ABO blood type system
has 3 alleles--A,B,i. A and B
are codominant, i is recessive
to both
Sample problems
1. A cross between a blue blahblah bird & a white blahblah bird produces
silver blahblah birds.
a) What are the genotypes of the parent blahblah birds in the original cross?
b) What is the genotype of the silver offspring?
c) What would be the phenotypic ratios of offspring produced by two silver
blahblah birds?
2. The color of fruit for plant "X" is determined by two alleles. When two
plants with orange fruits are crossed the following phenotypic ratios are
present in the offspring: 25% red fruit, 50% orange fruit, 25% yellow fruit.
What are the genotypes of the parent orange-fruited plants?
3. Predict the phenotypic ratios of offspring when a homozygous white cow is
crossed with a red roan bull.
4. What should the genotypes & phenotypes for parent cattle be if a farmer
wanted only cattle with red fur?
Sample problems
5. A cross between a black cat & a tan cat produces a tabby pattern (black &
tan fur together).
a) What pattern of inheritance does this illustrate?
b) What percent of kittens would have tan fur if a tabby cat is crossed with a
black cat?
6. A woman with Type O blood and a man who is Type AB are expecting a
child. What are the possible blood types of the kid?
7. What are the possible blood types of a child whose parents are both
heterozygous for "B" blood type?
8. What are the chances of a woman with Type AB and a man with Type A
having a child with Type O?
9. Determine the possible genotypes & phenotypes with respect to blood
type of the offspring of a couple whose blood types are A and B. Both sets
of grandparents have the AB blood type.