2. INTRODUCTION TO GENETICS
GENETICS – branch of biology that deals with heredity and
variation of organisms.
Chromosomes carry the hereditary information (genes)
• Arrangement of nucleotides in DNA
• DNA RNA Proteins
3. HUMAN GENOME
Most human cells
contain 46 chromosomes:
2 sex chromosomes (X,Y):
XY – in males.
XX – in females.
22 pairs of chromosomes
named autosomes.
4. 4
Gregor Mendel
(1822-1884)
• Father of Genetics
• Mendel was the first biologist to
use Mathematics – to explain
his results quantitatively.
• Responsible for the Laws
governing Inheritance of Traits
5. HUMAN CASE: CYSTIC FIBROSIS
• Mendel‟s Principles of Heredity apply universally to all
organisms.
• Cystic Fibrosis: a lethal genetic disease affecting
Caucasians.
• Caused by mutant recessive gene carried by 1 in 20
people of European descent (12M)
• One in 400 Caucasian couples will be both carriers of
CF – 1 in 4 children will have it.
• CF disease affects transport
in tissues – mucus is accumulated
in lungs, causing infections.
6. Jason’s Story
Today you will use Mendel‟s
principles of inheritance to learn
more about how a boy name
Jason inherited cystic fibrosis.
6
13. 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
• Locus
• Allele
14. CHROMOSOME LOGICAL STRUCTURE
Locus – a fixed location on a strand of
DNA where a gene or one of its alleles is
located.
Allele – two genes that occupy the same
position on homologous chromosomes
and that cover the same trait (like
„flavors‟ of a trait); one variant form of a
gene/marker at a particular locus.
Locus1
Possible Alleles: A1,A2
Locus2
Possible Alleles: B1,B2,B3
15. Homozygous – having identical genes (one from each
parent) for a particular characteristic. (DD or dd)
Heterozygous – having two different genes for a
particular characteristic. (Dd)
Dominant – the allele of a gene that masks or suppresses
the expression of an alternate allele; the trait appears in
the heterozygous condition.
Recessive – an allele that is masked by a dominant allele;
does not appear in the heterozygous condition, only in
homozygous.
Genetics terms you need to know:
16. Genotype – the genetic makeup of an organisms (TT, Tt, tt)
Phenotype – the physical appearance (tall, purple, wrinkled)
of an organism (Genotype + environment)
Monohybrid cross: a genetic cross involving a single pair of
genes (one trait); parents differ by a single trait.
Dihybrid cross: a genetic cross involving two pair of genes
(two traits); parents differ by two traits
P = Parental generation
F1 = First filial generation; offspring from a genetic cross.
F2 = Second filial generation of a genetic cross
17. GENOTYPES DETERMINES PHENOTYPES
At each locus (except for sex chromosomes)
there are 2 genes. These constitute the
individual‟s genotype at the locus.
The expression of a genotype is termed a
phenotype. For example, hair color, weight,
or the presence or absence of a disease.
19. Jason’s Parent
Both of Jason‟s parents are carriers for the recessive CF
gene, which means their genotype is heterozygous for CF
19
Inheritance Patterns of CF:
CC = normal
Cc = carrier, no symptoms
cc = has cystic fibrosis
20. Chromosomes (and genes) occur in pairs
Homologous Chromosomes
New combinations of genes occur in sexual reproduction
• Fertilization from two parents
21. Genotype & Phenotype in Flowers
21
Genotype of alleles:
R = red flower
r = yellow flower
All genes occur in pairs, so 2 alleles
affect a characteristic
Possible combinations are:
Genotypes RR Rr rr
Phenotypes RED RED YELLOW
24. MONOHYBRID CROSS
Parents differ by a single trait.
Crossing two pea plants that differ in stem size, one tall one short
T = allele for Tall
t = allele for dwarf
TT = homozygous tall plant
t t = homozygous dwarf plant
T T t t
25. MONOHYBRID CROSS FOR STEM
LENGTH:
T T t t
(tall) (dwarf)
T t
(all tall plants)
Parental (P)
Generation
Pure breeds-
homozygous
First Filial (F1) Generation
Hybrid- Heterozygous
26. PUNNETT SQUARE
A useful tool to do genetic crosses
For a monohybrid cross, you need a square divided by
four….
Looks like
a window
pane…
We use the
Punnett square
to predict the
genotypes and phenotypes of
the offspring.
28. USING A PUNNETT SQUARE
STEPS:
1. determine the genotypes of the parent organisms
2. write down your "cross" (mating)
3. draw a p-square
Parent genotypes:
PP and pp
Cross
PP pp
29. PUNNETT SQUARE
4. "split" the letters of the genotype for each parent
& put them "outside" the p-square
5. determine the possible genotypes of the offspring
by filling in the p-square
6. summarize results (genotypes & phenotypes of
offspring)
Pp Pp
Pp Pp
P P
p
p
Genotypes:
100% Pp
Phenotypes:
100% Purple flowers
PP pp
30. MONOHYBRID CROSS: F2
GENERATION
If you let the F1 generation self-fertilize, the
next monohybrid cross would be:
Pp Pp
(purple) (purple)
PP Pp
Pp pp
P p
P
p
Genotypes:
1 PP= Purple flowers
2 Pp = Purple flowers
1 pp = white flowers
Genotypic ratio= 1:2:1
Phenotype:
3 Purple flowers
1 white flowers
Phenotypic ratio= 3:1
31. SECRET OF THE PUNNETT
SQUARE
Key to the Punnett Square:
Determine the gametes of each parent…
How? By “splitting” the genotypes of each parent:
If this is your cross
P P p p
P P p p
The gametes are:
32. Once you have the gametes…
P P p p
Pp Pp
Pp Pp
P
P
p p
33. Shortcut for Punnett Square…
You only need one box!
P P p p
P
p Genotypes:
100% Pp
Phenotypes:
100% Purple flower plan
• If both parents are HOMOZYGOUS
Pp
35. If you have another
cross…
A heterozygous with a homozygous
T t t t
T
t
t
T t
t t
Genotypes:
50% T t
50 % t t
Phenotypes:
50% Tall plants
50% Dwarf plants
You can
still use the
shortcut!
37. P1 Monohybrid Cross
Trait: Seed Shape
Alleles: R – Round r – Wrinkled
Cross: Round seeds x Wrinkled seeds
RR x rr
37
R
R
rr
Rr
RrRr
Rr
Genotype: Rr
Phenotype: Round
Genotypic
Ratio: All alike
Phenotypic
Ratio: All alike
38. P1 Monohybrid Cross Review
Homozygous dominant x
Homozygous recessive
Offspring all Heterozygous (hybrids)
Offspring called F1 generation
Genotypic & Phenotypic ratio is ALL
ALIKE
38
39. Know do the F1 Monohybrid Cross
Trait: Seed Shape
Alleles: R – Round r – Wrinkled
Cross: Round seeds x Round seeds
Rr x Rr
39
R
r
rR
RR
rrRr
Rr
Genotype: RR, Rr, rr
Phenotype: Round &
wrinkled
G.Ratio: 1:2:1
P.Ratio: 3:1
40. F1 Monohybrid Cross Review
Heterozygous x heterozygous
Offspring:
25% Homozygous dominant RR
50% Heterozygous Rr
25% Homozygous Recessive rr
Offspring called F2 generation
Genotypic ratio is 1:2:1
Phenotypic Ratio is 3:1
40
41. Practice…
Work the P1, F1, and both F2
Crosses for each of the other
Seven Pea Plant Traits
41
43. Principle of Dominance
43
1. One allele masked another, one allele was
dominant over the other in the F1 generation.
44. Mendel’s Principles
2. 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.
46. Use what you have learned so far to
determine the genotypes for Jason’s
parents
46
47. Did you say both parents have the
genotype…
47
Cc
If so…you are correct!
48. Now perform a monohybrid cross
using the genotypes of Jason’s
parents to determine the probability
that their children would not have CF,
and the probability that Jason would
have CF.
48
49. INHERITANCE PATTERN OF CF
Since both parents carry the recessive gene of Cystic Fibrosis (c), that
is, they are heterozygous (Cc), one in four of their children is expected
to be homozygous for cf and have the disease:
C C C c
C c c c
C c
C
c
CC = 25% chance of being normal
Cc = 50% chance carrier, no symptoms
cc = 25% chance that Jason would have cystic fibrosis
50. Probabilities…
Of course, the 1 in 4 probability of getting the disease is just an
expectation, and in reality, any two carriers may have normal
children.
However, the greatest probability is for 1 in 4 children to be
affected.
Important factor when prospective parents are concerned about
their chances of having affected children.
Now, 1 in 29 Americans is a symptom-less carrier (Cf cf) of the
gene.
51. LAW OF INDEPENDENT
ASSORTMENT
Alleles for different traits are
distributed to sex cells (&
offspring) independently of one
another.
This law can be illustrated using
dihybrid crosses.
51
52. Dihybrid Cross
A breeding experiment that tracks the
inheritance of two traits.
Mendel‟s “Law of Independent
Assortment”
a. Each pair of alleles segregates
independently during gamete formation
b. Formula: 2n (n = # of heterozygotes)
52
53. Dihybrid Cross
Traits: Seed shape & Seed color
Alleles: R round
r wrinkled
Y yellow
y green
53
RrYy x RrYy
RY Ry rY ry RY Ry rY ry
All possible gamete combinations
57. DIHYBRID CROSS: FLOWER
COLOR AND STEM LENGTH
TT PP tt pp
(tall, purple) (short, white)
Possible Gametes for
parents
T P and t p
F1 Generation: All
tall, purple flowers
(Tt Pp)
TtPp TtPp TtPp TtPp
TtPp TtPp TtPp TtPp
TtPp TtPp TtPp TtPp
TtPp TtPp TtPp TtPp
tp tp tp tp
TP
TP
TP
TP
58. DIHYBRID CROSS: FLOWER COLOR
AND STEM LENGTH (SHORTCUT)
TT PP tt pp
(tall, purple) (short, white)
Possible Gametes for
parents
F1 Generation: All
tall, purple flowers
(Tt Pp)
T t P pTP
t p
T P
t p
59. DIHYBRID CROSS F2
If F1 generation is allowed to self pollinate, Mendel observed 4
phenotypes:
Tt Pp Tt Pp
(tall, purple) (tall, purple)
Possible gametes:
TP Tp tP tp
Four phenotypes
observed
Tall, purple (9); Tall, white
(3); Short, purple (3);
Short white (1)
TTPP TTPp TtPP TtPp
TTPp TTpp TtPp Ttpp
TtPP TtPp ttPP ttPp
TtPp Ttpp ttPp ttpp
TP Tp tP tp
TP
Tp
tP
tp
60. DIHYBRID CROSS
9 Tall purple
3 Tall white
3 Short purple
1 Short white
TTPP TTPp TtPP TtPp
TTPp TTpp TtPp Ttpp
TtPP TtPp ttPP ttPp
TtPp Ttpp ttPp ttpp
TP Tp tP tp
TP
Tp
tP
tp
Phenotype Ratio = 9:3:3:1
62. PRINCIPLE OF INDEPENDENT
ASSORTMENT
Based on these results, Mendel postulated the
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
63. Relation of gene segregation to
meiosis…
There‟s a correlation between the movement of
chromosomes in meiosis and the segregation of alleles that
occurs in meiosis
64. TEST CROSS
When you have an individual with an unknown genotype, you do a
test cross.
Test cross: Cross with a homozygous recessive individual.
For example, a plant with purple flowers can either be PP or Pp…
therefore, you cross the plant with a pp (white flowers,
homozygous recessive)
P ? pp
65. TEST CROSS
If you get all 100% purple flowers, then the unknown parent was
PP…
P p P p
P p P p
P P
p
p
P p p p
P p p p
P p
p
p
•If you get 50% white,
50% purple flowers,
then the unknown
parent was Pp…
66. Summary of Mendel‟s laws
66
LAW
PARENT
CROSS
OFFSPRING
DOMINANCE TT x tt
tall x short
100% Tt
tall
SEGREGATION
Tt x Tt
tall x tall
75% tall
25% short
INDEPENDENT
ASSORTMENT
RrGg x RrGg
round & green
x
round & green
9/16 round seeds & green
pods
3/16 round seeds & yellow
pods
3/16 wrinkled seeds & green
pods
1/16 wrinkled seeds & yellow
pods
72. SUMMARY OF GENETICS
Chromosomes carry hereditary info (genes)
Chromosomes (and genes) occur in pairs
New combinations of genes occur in sexual
reproduction
Monohybrid vs. Dihybrid crosses
Mendel‟s Principles:
• Dominance: one allele masks another
• Segregation: genes become separated in gamete
formation
• Independent Assortment: Members of one gene pair
segregate independently from other gene pairs during
gamete formation
