2. Introduction
۞
Genetics is the branch of biology that
studies heredity
۞
Genetics is the branch of biology that
studies the storage, duplication, and
transfer of information
۞
Organisms inherit characteristics from
their parents
۞
The information for these
characteristics is contained in an
organism’s DNA.
3. The History of Modern Genetics
۞ Modern genetics began early in
the 20th century with the
pioneering work of Gregor Mendel
۞ Mendel worked on the garden pea
(Pisum sativum), and he
established that certain traits
could be passed from generation
to generation
Gregor Mendel
۞ Prior to Mendel, heredity was regarded
as a "blending" process and the
offspring were essentially a "dilution"of
the different parental characteristics.
5. Gregor Mendel’s Peas
۞ Mendel studied a number of
characteristics in pea plants:
Plant height - short or tall
Seed color - green or yellow
Seed shape - wrinkled or round
Flower color - white or purple
Pod shape - constricted or smooth
Pod color - yellow or green
Flower position - terminal or axial
8. Gregor Mendel’s Peas
Cross Pollinating Peas
Instead of self pollination, the pollen
from one plant is used to pollinate the
egg cells of another plant
۞ Cut away anthers
۞ Hand pollinated using pollen from a
chosen plant.
۞
10. Genetics terms you need to know:
Gene – a unit of heredity;
a section of DNA sequence
encoding a single protein
۞ Genome – the entire set
of genes in an organism
۞ Alleles – two genes that occupy the
same position on homologous
chromosomes and that cover the same
trait (Different forms of a specific
gene) (A or a)
۞ Locus – a fixed location on a strand of
DNA where a gene or one of its alleles
is located.
۞
11. Genetics terms you need to know:
Homozygous – having identical alleles
(one from each parent) for a particular
characteristic (AA or aa)
۞ Heterozygous – having two different
alleles for a particular characteristic (Aa)
۞ Dominant – the allele of a gene that
masks or suppresses the expression of
an alternate allele; the trait appears in
the heterozygous condition (A)
۞ Recessive – an allele that is masked by
a dominant allele; does not appear in
the heterozygous condition, only in
homozygous. (a)
۞
12. Genetics terms you need to know:
۞
۞
۞
۞
۞
۞
Genotype – the genetic makeup of an
organisms (AA, Aa or aa)
Phenotype – the physical appearance
of an organism (Genotype +
environment)
Monohybrid cross: a genetic cross
involving a single pair of genes (one
trait); parents differ by a single trait
P = Parental generation
F1 = First filial generation; offspring
from a genetic cross
F2 = Second filial generation of a
genetic cross.
13. Mendel's data: Monohybrid Cross
X
P:
round
wrinkled
Mendel's insight
F1:
All round
selfing
F2:
wrinkled
round
5474 : 1850
2.96 :
1
"round" F1 is actually
heterozygous: Aa
A = round - dominant
a = wrinkled - recessive
Parents are AA and aa
14. Mendel's data: Monohybrid Cross
X
P
Mendel's insight
“purple" F1 is actually
heterozygous: Bb
F1
selfing
F2
705
:
224
3.15
:
1
B = purple - dominant
b = white - recessive
Parents are BB and bb
Law of segregation:
alleles segregate from
each other in gametes,
1/2 get one, 1/2 get the
other.
17. Use of Punnet Square for F2
Determining the outcome of a single-trait cross
♂
♀
p
PP
Pp
Pp
P
P
pp
p
Genotypic ratio:
1 PP : 2 Pp : 1pp
Phenotypic ratio:
3 purple:1white
21. Monohybrid Crosses Yielded Consistent Results
Therefore, the Principle of Segregation indeed is a general
principle of genetics.
22. Consistency is Good
No matter what the
character, Mendel
observed a 3:1 ratio of
characters in the F2.
23. Dihybrid Cross
۞
۞
۞
Mendel investigated the inheritance of
seed shape (smooth v wrinkled) and seed
color (green v yellow) at the same time
From his monohybrid crosses he knew
that smooth seeds were dominant to
wrinkled seeds and yellow seeds were
dominant to green seeds
He chose to cross plants that were pure
breeding for both dominant features
(smooth and yellow seeds) with plants
that were pure breeding for both recessive
features (wrinkled and green seeds).
24. Dihybrid Cross
Parents
plants with round x plants with wrinkled
and yellow seeds
and green seeds
F1 offspring 100% plants with round and yellow seeds
F1 intercross plants with round x plants with round
and yellow seeds and yellow seeds
F2 offspring Total 556 seeds collected from F2 plants
Ratio
315
108
101
32
9
3
3
1
round and yellow seeds
round and green seeds
wrinkled and yellow seeds
wrinkled and green seeds
However, the ratio of dominant to recessive features is still 3:1
25. Dihybrid Cross
Law of independent assortment:
Alleles of separate genes assort
independently into the gametes
SY
Sy
sY
sy
Genotypic ratio =
1SSYY : 2SSYy : 1SSyy :
2SsYY : 4SsYy : 2Ssyy :
1ssYY : 2ssYy : 1ssyy
Phenoypic ratio =
9
:3
:3
:1
26. Dihybrid Cross
۞ A dihybrid cross can be treated as two
separate monohybrid crosses
۞ The expected probability of each type of seed
can be calculated:
Probability of an F2 seed being round = 75% or ¾
Probability of an F2 seed being wrinkled =25% or ¼
Probability of an F2 seed being yellow = 75% or ¾
Probability of an F2 seed being green = 25% or ¼
27. Dihybrid Cross
Therefore:
Probability of an F2 seed being round and yellow
= ¾ x ¾ = 9/16 = 56.25%
Probability of an F2 seed being round and green
= ¾ x ¼ = 3/16 = 18.75%
Probability of an F2 seed being wrinkled and yellow
= ¼ x ¾ = 3/16 = 18.75%
Probability of an F2 seed being wrinkled and green
= ¼ x ¼ = 1/16 = 6.25%
28. Dihybrid Cross
We could
expect
What Mendel
observed
556 x 9/16 round yellow
313
315
556 x 3/16 round green
104
108
556 x 3/16 wrinkled yellow
104
101
556 x 1/16 wrinkled green
35
32
Phenotype
31. Trihybrid Cross - Phenotypes
Forked-line Method
-
bb
-
aa
bb
-
A-B-C-
A-B-C-
cc
A-B-cc
A-B-cc
-
A-bbC-
A-bbC-
cc
A-bbcc
A-bbcc
-
aaB-C-
aaB-C-
cc
aaB-cc
aaB-cc
-
aabbC-
aabbC-
cc
aabbcc
aabbcc
Phenotypic ratio: 27:9:9:9:3:3:3:1
32. Mendel’s Principles
۞ Principle of Dominance:
One allele masked another, one allele
was dominant over the other in the F1
generation
۞ Principle of Segregation:
When gametes are formed, the pairs of
hereditary factors (genes) become
separated, so that each sex cell
(egg/sperm) receives only one kind of
gene (allele).
33. Mendel’s Principles
۞ Principle of Independent Assortment:
“Members of one gene
pair segregate
independently from
other gene pairs during
gamete formation”
Genes get shuffled –
these many combinations
are one of the advantages
of sexual reproduction
34. Exercises
1.
For the following genotypes, tell how
many gametes will be produced
a) BB
b) Ss
c) DDRr
Solution
# gametes = 2n
Use this formula:
where n is number of heterozygous genes.
a) BB, n=0, 20=1 gamete: B
b) Ss, n=1, 21=2 gametes: S, s
c) DDRr, n=1, 21=2 gametes: DR, Dr
35. Exercises
2.
List all possible gametes produced by
a)
b)
c)
d)
aabbcc
kkLLMm
PpQQRr
RrSsTt
Solution:
a) aabbcc:
1 gametes: abc
b) kkLLMm: 2 gametes: kLM and kLm
c) PpQQRr: 4 gametes: PQR, PQr, pQR, pQr
d) RrSsTt:
8 gametes: RST, RSt, RsT, Rst,
rST, rSt, rsT, rst
36. Monohybrid Test Cross
How can you determine genotype from individual
expressing dominant phenotype? - DD or Dd?
Cross individual with dominant phenotype to a
homozygous recessive individual.
37. Dihybrid test cross
۞ In monohybrid crosses, to know if a
dominant trait is homozygous (SS)
or heterozygous (Ss) it is necessary
to carry out a test cross
۞ This is done with a homozygous
recessive (ss) individual
۞ The same is true for a dihybrid cross
where the test cross is made with an
individual which is homozygous
recessive for both characters (ssyy)
38. Dihybrid testcross
Testcross results of four smooth round individuals
SSYY
SSYY
SsYY
SsYY
SSYy
SSYy
SsYy
SsYy
SSYYs x ssyy
x
SsYY x ssyy
SSYy x ssyy
SsYy x ssyy
sy
Sy SsYy
sy
SY SSYy
sY ssYy
sy
SY SsYy
Sy Ssyy
sy
SY SsYy
Sy Ssyy
sY ssYy
sy
Phenotypic ratio
All smooth yellow
All smooth yellow
Phenotypic ratio
½ smooth yellow
½ wrinkle yellow
All yellow
Phenotypic ratio
½ smooth yellow
½ smooth green
All smooth
ssyy
Phenotypic ratio
¼ smooth yellow
¼ smooth green
¼ wrinkle yellow
¼ wrinkle green
Mixed
39. Monohybrid Problems
1.
In rabbits the allele for black coat color (B) is dominant
over the allele for brown coat color (b). What is the
genotypic ratio and phenotypic ratio be for a cross
between an animal homozygous for black coat color and
one homozygous for brown coat color?
2.
White (W) hair in sheep is caused by the dominant gene
while black (w) hair is recessive. A heterozygous white
male and a black female are parents of a black
lamb. What is the probability that their next lamb will
be white? What are the genotypic and phenotypic
ratios?
3.
Albinism is recessive in humans. An albino man marries
a woman who is not albino, but had an albino
father. What is the probability of this couple having a
child that is not an albino? What are the genotypic and
phenotypic ratios?
40. Monohybrid Problems
4.
5.
6.
Todd and Melissa are college students who are planning to
get married. They are currently taking a genetics course and
decided to determine the eye color of any possible children
they might have. Blue eyes are recessive to brown
eyes. Todd has brown eyes, like his three brothers. His
mother and grandmother have blue eyes, but his father and
all other grandparents have brown eyes. Brown eyed Melissa
has one blue eyed sister and one brown eyed sister and a
mother with blue eyes. Her father and all of her
grandparents have brown eyes. Construct an accurate
punnett square to determine the possible eye colors of their
yet to be born children. What are the genotypic and
phenotypic ratios?
In Teenage Mutant Ninja Turtles, green shells are dominant
over brown shells. Leonardo, who is heterozygous for a
green shell, marries the lovely Mona Lisa, who has a brown
shell. What are the genotypic and phenotypic ratios?
In humans, polydactyly (an extra finger on each hand or toe
on each foot) is due to a dominant gene. When one parent is
polydactylous, but heterozygous, and the other parent is
normal, what are the genotypic and phenotypic ratios of their
children?
41. Monohybrid Problems
5.
In guinea pigs, rough coat (R) is dominant over smooth
coat (r). A rough coated guinea pig is bred to a smooth
one, giving eight rough and seven smooth progeny in the
F1.
a)
b)
6.
What are the genotypes of the parents and their offspring?
If one of the rough F1 animals is mated to its rough parent, what
progeny would you expect?
In summer squash, white fruit (W) is dominant over
yellow (w), and disk-shaped fruit (D) is dominant over
sphere-shaped fruit (d). The following problems give the
phenotype of the parents and their offspring. Determine
the genotypes of the parents in each case:
a)
b)
c)
White, disk x yellow, sphere gives 1/2 white, disk and 1/2 white,
sphere.
White, sphere x white, sphere gives 3/4 white, sphere and
1/4 yellow, sphere.
Yellow, disk x white, sphere gives all white, disk progeny.
42. Dihybrid Problems
1.
In human, aniridia, (a type of blindness resulting from
absence of an iris) is due to a dominant gene. Migraine (a
sickening headache) is due to a different dominant gene. A
man with aniridia, whose mother was not blind, marries a
woman who suffers from migraine. The woman’s father did
not suffer from migraine. In what proportion of their
children would both aniridia and migraine be expected to
occur?
2.
In watermelons, solid green color (G) is dominant over
striped pattern (g), and short shape (S) is dominant over
long shape (s). What is the probability of each phenotype
of possible offspring if a heterozygous solid, long
watermelon cross pollinates with a heterozygous solid,
heterozygous short watermelon?
3.
Having two eyebrows is dominant (E) over having one
large eyebrow (e). Also having six fingers (F) is dominant
over having five fingers (f). What is the probability of each
phenotype if a man that has one eyebrow and twelve
fingers total (heterozygous), has children with a woman
that is heterozygous for both traits?
