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
9. Mendel’s Monohybrid Crosses
Mendel crossed tall and dwarf pea plants to
investigate how height was inherited
He emasculated the male plant then applied
pollen the stigma of other variety
The hybrids were uniformly tall
The same result for the reciprocal crosses
The dwarf seemed to disappear in the progeny
Mendel allowed them to undergo selffertilization
among 1064 progeny: 787 tall and 277 dwarf
ratio of approximately 3:1
15. Mendel’s Monohybrid Crosses
The hybrids had the ability to produce dwarf
progeny
The latent (masked) factor: recessive; the
expressed factor: dominant
The cross was called a monohybrid cross
Each trait was controlled by a heritable factor
(genes)
The dominant and recessive forms are called
alleles
Each of the parental strains carried two identical
copies of a gene homozygous
the hybrid two different alleles heterozygous.
16. Mendel’s Monohybrid Crosses
Mendel used symbols to represent the
hereditary factors
The allele for dwarfness, (recessive): d
The allele for tallness (dominant): D
The tall and dwarf pea strains: DD and dd
The allelic constitution: genotype
the physical appearance (tall or dwarf):
phenotype
As the parental strains, the tall and dwarf pea
plants form the P generation of the
experiment.
17. Mendel’s Monohybrid Crosses
The hybrid progeny first filial generation (F1)
The genotype F1: Dd (heterozygous)
The F1 phenotype: tall because D is
dominant over d
During meiosis, these F1 produce two kinds
of gametes, D and d, in equal proportions
They separate, or segregate, from each other
during gamete formation
Upon self-fertilization, the two kinds of
gametes produced by heterozygotes can
unite in all possible ways.
18. Mendel’s Monohybrid Crosses
Thus, they produce four kinds of zygotes: DD,
Dd, dD, and dd
Because of dominance, three of these
genotypes have the same phenotype
Thus, in the next generation, called the F2,
the plants are either tall or dwarf, in a ratio
of 3:1.
19. 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.
20. Genetics terms you need to know:
Homozygous: having identical alleles (one
from each parent) for a particular (AA or aa)
characteristic
Heterozygous: having two different alleles
(Aa)
for a particular characteristic
Dominant: the allele of a gene that masks or
suppresses the expression of an alternate
allele; the trait appears in the heterozygous (A)
condition
Recessive: an allele that is masked by a
(a)
dominant allele; does not appear in the
heterozygous condition, only in homozygous.
21. Genetics terms you need to know:
Genotype: the genetic makeup (AA, Aa or aa)
of an organisms
Phenotype: the physical appearance
of an organism
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.
24. Mendel’s Monohybrid Crosses
Mendel’s two principles of heredity:
1. The Principle of Dominance:
In a heterozygote, one allele may conceal the
presence of another
2. The Principle of Segregation:
In a heterozygote, two different alleles segregate
from each other during the formation of gametes.
25. Example 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?
26. 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?
Let B: black coat color; and b: brown coat
P: homozygous for black X homozygous brown coat
BB
bb
Gamete:
B
b
F1 :
All Bb
Genotypic ratio: 100% Bb
Phenotypic ration: 100% Black coat
27. Example 2
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?
28. 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?
D: allele for polydactyly; d: alelle for normal finger
D is dominant over d
P: heterozygous polydactyly
Dd
Gamete: D, d
F1 :
d
D
d
X normal
dd
d
Dd (polydactyly)
dd (normal)
Genotypic ratio: ½ Dd : ½ dd
Phenotypic ratio: ½ polydactyly : ½ normal
29. Dihybrid Cross
Mendel investigated the inheritance of seed
shape (smooth vs wrinkled) and seed color
(green vs 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).
31. Dihybrid Cross
Seed color: 2 alleles g (green) and G (yellow),
Seed texture: w (wrinkled) and W (round)
The parental strains, doubly homozygous
yellow, round plants: GG WW
green, wrinkled plants: gg ww
Gametes from GG WW plants: GW
gametes from gg ww plants: gw
Cross-fertilization produces F1 hybrids
(doubly heterozygous) GgWw
Phenotype yellow, round indicates that the G
and W alleles are dominant.
32. Dihybrid Cross
P:
round, yellow x wrinkled, green
F1:
100% round, yellow seeds
F1 intercross: round, yellow x round, yellow
F2 offspring: Total 556 seeds 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
36. 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
37. 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%.
38. Dihybrid Cross
Phenotype
We could What Mendel
expect
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
39. Mendel’s Principles
3. Principle of Independent Assortment:
The alleles of different genes segregate or assort
independently of each other
Genes get shuffled –
these many combinations
are one of the advantages
of sexual reproduction
43. Forked-line Method for Trihybrid Crosses
Split the individual gene
Make crosses
Combine them back
Genotypes:
Aa x Aa ¼ AA : ½ Aa : ¼ aa
Bb x Bb ¼ BB : ½ Bb : ¼ bb
Cc x Cc ¼ CC : ½ Cc : ¼ cc
Phenotypes:
Aa x Aa ¾ A- : ¼ aa
Bb x Bb ¾ B- : ¼ bb
Cc x Cc ¾ C- : ¼ cc
44. Trihybrid Cross: AaBbCc X AaBbCc
¼ BB
¼ AA
½ Bb
Genotypes:
¼ bb
¼ BB
½ Aa
½ Bb
¼ bb
¼ BB
¼ aa
½ Bb
¼ bb
¼ CC
½ Cc
¼ cc
¼ CC
½ Cc
¼ cc
¼ CC
½ Cc
¼ cc
¼ CC
½ Cc
¼ cc
¼ CC
½ Cc
¼ cc
¼ CC
½ Cc
¼ cc
¼ CC
½ Cc
¼ cc
¼ CC
½ Cc
¼ cc
¼ CC
½ Cc
¼ cc
¼ x ¼ x ¼ = 1/64 AABBCC
¼ x ¼ x ½ = 2/64 AABBCc
¼ x ¼ x ¼ = 1/64 AABBcc
¼ x ½ x ¼ = 2/64 AABbCC
¼ x ½ x ½ = 4/64 AABbCc
¼ x ½ x ¼ = 2/64 AABbcc
¼ x ¼ x ¼ = 1/64 AAbbCC
¼ x ¼ x ½ = 2/64 AAbbCc
¼ x ¼ x ¼ = 1/64 AAbbcc
¼ x ¼ x ¼ = 1/64 AABBCC
¼ x ¼ x ½ = 2/64 AABBCc
¼ x ¼ x ¼ = 1/64 AABBcc
¼ x ½ x ¼ = 2/64 AABbCC
¼ x ½ x ½ = 4/64 AABbCc
¼ x ½ x ¼ = 2/64 AABbcc
¼ x ¼ x ¼ = 1/64 AAbbCC
¼ x ¼ x ½ = 2/64 AAbbCc
¼ x ¼ x ¼ = 1/64 AAbbcc
¼ x ¼ x ¼ = 1/64 aaBBCC
¼ x ¼ x ½ = 2/64 aaBBCc
¼ x ¼ x ¼ = 1/64 aaBBcc
¼ x ½ x ¼ = 2/64 aaBbCC
¼ x ½ x ½ = 4/64 aaBbCc
¼ x ½ x ¼ = 2/64 aaBbcc
¼ x ¼ x ¼ = 1/64 aabbCC
¼ x ¼ x ½ = 2/64 aabbCc
¼ x ¼ x ¼ = 1/64 aabbcc
45. Trihybrid Cross: AaBbCc X AaBbCc
Phenotype
¾ B¾ A¼ bb
¾ B¼ aa
¼ bb
(¾)(¾)(¾)A-B-C-
27/64 A-B-C-
¼ cc (¾)(¾)(¼)A-B-cc
9/64 A-B-cc
¾C-
(¾)(¼)(¾)A-bbC-
9/64 A-bbC-
¼ cc (¾)(¼)(¼) A-bbcc
¾C- (¼)(¾)(¾) aaB-C-
9/64 A-bbcc
¾C-
3/64 aaB-C3/64 aaB-cc
¾C-
(¼)(¾)(¼) aaB-cc
(¼)(¼)(¾) aabbC-
3/64 aabbC-
¼ cc
(¼)(¼)(¼) aabbcc
1/64 aabbcc
¼ cc
Phenotypic ratio: 27:9:9:9:3:3:3:1
46. 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.
47. Dihybrid test cross
In monohybrid crosses, to know if a
dominant trait is homozygous (DD) or
heterozygous (Dd) it is necessary to carry out
a test cross
This is done with a homozygous recessive
(dd) 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 (wwgg)
48. Dihybrid testcross
Testcross results of four smooth round individuals
WwGG
WWGG
WWGG
xsx
wwgg
wg
WG
WwGg
WwGG
x
wwgg
wg
WG WwGg
wG wwGg
WWGg
WWGg
x
wwgg
wg
WG WwGg
Wg Wwgg
WwGg
WwGg
x
wwgg
wg
WG WwGg
Wg Wwgg
wG wwGg
wg wwgg
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
Phenotypic ratio
¼ smooth yellow
¼ smooth green
¼ wrinkle yellow
¼ wrinkle green
Mixed
49. Monohybrid and Dihybrid Problems
1. White (W) hair in sheep is caused by the
2.
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?
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?
50. Monohybrid and Dihybrid Problems
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?
51. 5.
Monohybrid and Dihybrid Problems
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) What are the genotypes of the parents and their offspring?
b) If one of the rough F1 animals is mated to its rough parent, what progeny
would you expect?
6.
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) White, disk x yellow, sphere gives 1/2 white, disk and 1/2 white, sphere.
b) White, sphere x white, sphere gives 3/4 white, sphere and 1/4 yellow,
c)
sphere.
Yellow, disk x white, sphere gives all white, disk progeny.
52. Monohybrid and Dihybrid Problems
7.
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?
8.
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?
53. 9. 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?
