Introduction to Genetics (General Biology 2)

1. Genetics
General Biology 2 Lesson 1

2. Genetics
•Branch of science which deals with the study
heredity and variations.
•Heredity – transmission of characteristics
•Variation – differences among the traits
that occur in member of the same species.

3. Terminologies

4. Pedigree
This makes use of diagrams
showing the ancestral
relationships and
transmission of genetic
traits over several
generations in a family.

5. Proband
• The first family member that seeks medical attention.
• The individual in the pedigree that led to the construction of
the pedigree.

6. Autosomal Trait
• A trait whose alleles that control it are found in the
autosomes (body chromosomes/non-sex
chromosomes).
• Autosomal refers to the 22 numbered chromosomes
as opposed to the sex chromosomes

7. Genotype
The gene pair an individual carries for a particular trait symbolized with a
pair of letters. By convention, uppercase letter (e.g., A) for a dominant allele
and lowercase letter (e.g., a) for the recessive allele. Any letter in the
alphabet may be used.
For a diploid organism with two alleles in a given gene pair, genotypes may
be written as:
• Homozygous dominant, i.e. with two dominant alleles (DD)
• Heterozygous, i.e. with a dominant and recessive allele (Dd). The
individual will show the dominant phenotype.
• Homozygous recessive, i.e. with two recessive alleles (dd)

8. Phenotype
The observable trait of an individual based on its genotype.
• Example: red flower, curly hair, blood types (i.e., the blood
type is the phenotype)
For a typical Mendelian trait, phenotypes may either be:
• Dominant. A trait that requires at least one dominant
allele for the trait to be expressed (e.g., Dd)
• Recessive. A trait that requires two recessive alleles for the
trait to be expressed (e.g., dd)

9. Phenocopy
A trait that is expressed due to specific environmental
conditions (i.e., having hair that is dyed of a different
color) and is not due to the genotype.

10. Twins
Identical twins. Also known as monozygotic twins, are derived
from a single fertilization event. After the first cleavage or cell
division of the zygote, the cells or blastomeres separate and
become independent blastocysts implanted in the mother’s
uterus.
Fraternal twins. Also known as dizygotic twins, are derived from
separate fertilization events (two eggs fertilized by two sperms)
within the fallopian tube, resulting in two separate zygotes.

11. Brief History

12. Gregor Johann Mendel
• He laid the foundation for the formal
discipline of genetics in 1866.
• When Mendel began his studies of
inheritance using the Pea plant (Pisum
sativum), there was no knowledge of
chromosomes or the role and
mechanism of meiosis.
• His work remained unnoticed until
about 1900, following the rediscovery of
his work, the concept of gene as a
hereditary unit was established.
• Even today, they serve as the
cornerstone of the study of Genetics.

13. Research
Method:
Crossing Pea
Plants

14. Advantages of pea plants
for genetic study
• There are many varieties with distinct
heritable features or characters (such as
flower color); character variants (such as
purple or white flowers); called traits
• Mating can be controlled
• Each flower has sperm-producing organs
(stamens) and an egg-producing organ (carpel)
• Cross-pollination (fertilization between
different plants) involves dusting one plant
with pollen from another

15. Mendel’s Experimental Approach
• Mendel chose to track only those characters that
occurred in two distinct alternative forms (such as
purple or white flower color).
• He made sure that he started his experiments with
varieties that were true-breeding (plants that produce
offspring of the same variety when they self-pollinate)

16. Mendel’s Experimental Approach
• In a typical experiment, Mendel mated two contrasting, true-
breeding varieties, a process called hybridization.
• The true-breeding parents are the P generation (parental
generation).
• The hybrid offspring of the P generation are called the F1
generation
• When F1 individuals self-pollinate or cross- pollinate with other
F1 hybrids, the F2 generation is produced

17. Laws of Inheritance

19. Law of Segregation
(First Mendelian Law)

20. Law of Segregation (First Mendelian Law)
• For every trait governed by a
pair of alleles, these alleles
segregate or separate during
gamete formation in meiosis.

21. Mendel’s Experiment
• Mendel derived the law of segregation by following a single
character.
• The F1 offspring produced in this cross were monohybrids,
individuals that are heterozygous for one character.
• A cross between such heterozygotes is called a monohybrid
cross.

22. Mendel’s Experiment
• When Mendel crossed contrasting, true-breeding white- and
purple-flowered pea plants, all of the F1 hybrids were purple.
• When Mendel crossed the F1 hybrids, many of the F2 plants had
purple flowers, but some had white.
• Mendel discovered a ratio of about three to one, purple to
white flowers, in the F2 generation.

23. P Generation
EXPERIMENT
(true-breeding
parents)
F1 Generation
(hybrids)
F2 Generation
Purple
flowers
White
flowers
All plants had purple flowers
Self- or cross-pollination
705 purple-
flowered
plants
224 white
flowered
plants

24. Mendel’s Experiment
• Mendel reasoned that only the purple flower factor was
affecting flower color in the F1 hybrids.
• Mendel called the purple flower color a dominant trait and
the white flower color a recessive trait.
• The factor for white flowers was not diluted or destroyed
because it reappeared in the F2 generation.

25. Mendel’s
Experiment
• Mendel observed
the same pattern
of inheritance in
six other pea
plant characters,
each represented
by two traits.
• What Mendel
called a
“heritable factor”
is what we now
call a gene.

26. Mendel’s Model
• Mendel developed a hypothesis to explain the 3:1 inheritance
pattern he observed in F2 offspring.
• Four related concepts make up this model
• These concepts can be related to what we now know about
genes and chromosomes

27. Concept 1: Alternative versions of genes account
for variations in inherited characters
• For example, the gene for flower color in pea plants exists in two
versions, one for purple flowers and the other for white flowers.
• These alternative versions of a gene are called alleles.
Allele for purple flowers
Locus for flower-color gene
Allele for white flowers
Pair of
homologous
chromosomes
Allele - a variant
of the sequence
of nucleotides at
a particular
location (locus)
on a DNA
molecule

28. Concept 2: For each character, an organism inherits
two alleles, one from each parent
• Mendel made this deduction without knowing about the role of
chromosomes.
• The two alleles at a particular locus may be identical, as in the true-
breeding plants of Mendel’s P generation.
• Alternatively, the two alleles at a locus may differ, as in the F1 hybrids.

29. Concept 3: If the two alleles at a locus differ, then
one (the dominant allele) determines the
organism’s appearance, and the other (the
recessive allele) has no noticeable effect on
appearance.
• In the flower-color example, the F1 plants had purple flowers
because the allele for that trait is dominant.

30. Concept 4 (now known as the LAW OF
SEGREGATION): The two alleles for a heritable
character separate (segregate) during gamete
formation and end up in different gametes.
• Thus, an egg or a sperm gets only one of the two alleles that are
present in the organism.
• This segregation of alleles corresponds to the distribution of
homologous chromosomes to different gametes in meiosis.

31. Useful Genetic Vocabulary
•An organism that has a pair of identical alleles for a gene
encoding a character is called a homozygote and is said to be
homozygous for that gene.
•In the parental generation, the purple-flowered pea plant is
homozygous for the dominant allele (PP), while the white plant
is homozygous for the recessive allele (pp).
•Homozygous plants “breed true” because all of their gametes
contain the same allele—either P or p in this example.
•If we cross dominant homozygotes with recessive homozygotes,
every offspring will have two different alleles—Pp in the case of
the F1 hybrids of our flower color experiment.

32. Phenotype
Purple
Purple
Purple
White
3
1
1
1
2
Ratio 3:1 Ratio 1:2:1
Genotype
PP
(homozygous)
Pp
(heterozygous)
Pp
(heterozygous)
pp
(homozygous)

33. •Because of the different effects of dominant and recessive
alleles, an organism’s traits do not always reveal its genetic
composition.
•Therefore, we distinguish between an organism’s appearance or
observable traits, called its phenotype, and its genetic makeup,
its genotype.
•For the case of flower color in pea plants, PP and Pp plants have
the same phenotype (purple flowers) but different genotypes.
•Note that the term phenotype refers to physiological traits as
well as traits that relate directly to appearance.

34. The Testcross
• How can we tell the genotype of an individual with the
dominant phenotype?
• Such an individual could be either homozygous dominant or
heterozygous.
• The answer is to carry out a testcross: breeding the mystery
individual with a homozygous recessive individual.
• If any offspring display the recessive phenotype, the mystery
parent must be heterozygous.

35. Dominant phenotype,
unknown genotype:
PP or Pp?
Recessive phenotype,
known genotype:
pp
Predictions
If purple-flowered
parent is PP
If purple-flowered
parent is Pp
or
Sperm Sperm
Eggs Eggs
or
All offspring purple 1/2 offspring purple and
1/2 offspring white
Pp Pp
Pp Pp
Pp Pp
pp pp
p p p p
P
P
P
p
TECHNIQUE
RESULTS

36. Law of Independent Assortment
(Second Mendelian Law)

37. Law of Independent Assortment (Second Mendelian
Law)
• A pair of alleles for one trait
will segregate or separate
independently of another pair
of alleles for another trait
during meiosis.
• This law applies only to genes
on different, non-homologous
chromosomes or those far
apart on the same
chromosome.
• Genes located near each other
on the same chromosome tend
to be inherited together.

38. Mendel’s Experiment
• Mendel identified his second law of inheritance by following
two characters at the same time.
• Crossing two true-breeding parents differing in two characters
produces dihybrids in the F1 generation, heterozygous for both
characters.
• A dihybrid cross, a cross between F1 dihybrids, can determine
whether two characters are transmitted to offspring as a
package or independently.

39. Mendel’s Experiment
•Imagine crossing two true-breeding pea varieties that differ in both of
these characters—a cross between a plant with yellow-round seeds
(YYRR) and a plant with green-wrinkled seeds (yyrr).
•The F1 plants will be dihybrids, individuals heterozygous for the two
characters being followed in the cross (YyRr).
•But are these two characters transmitted from parents to offspring as a
package?
•That is, will the Y and R alleles always stay together, generation after
generation? Or are seed color and seed shape inherited independently?
•The next figure shows how a dihybrid cross, a cross between F1
dihybrids, can determine which of these two hypotheses is correct.

40. P Generation
F1 Generation
Predictions
Gametes
EXPERIMENT
RESULTS
YYRR yyrr
yr
YR
YyRr
Hypothesis of
dependent assortment
Hypothesis of
independent assortment
Predicted
offspring of
F2 generation
Sperm
Sperm
or
Eggs
Eggs
Phenotypic ratio 3:1
Phenotypic ratio 9:3:3:1
Phenotypic ratio approximately 9:3:3:1
315 108 101 32
1/2
1/2
1/2
1/2
1/4
1/4
1/4
1/4
1/4
1/4
1/4
1/4
9/16
3/16
3/16
1/16
YR
YR
YR
YR
yr
yr
yr
yr
1/4
3/4
Yr
Yr
yR
yR
YYRR YyRr
YyRr yyrr
YYRR YYRr YyRR YyRr
YYRr YYrr YyRr Yyrr
YyRR YyRr yyRR yyRr
YyRr Yyrr yyRr yyrr

41. Mendel’s Experiment
• The F1 plants, of genotype YyRr, exhibit both dominant phenotypes,
yellow seeds with round shapes, no matter which hypothesis is correct.
• The key step in the experiment is to see what happens when F1 plants
self-pollinate and produce F2 offspring.
• If the hybrids must transmit their alleles in the same combinations in which
the alleles were inherited from the P generation, then the F1 hybrids will
produce only two classes of gametes: YR and yr.
• As shown on the left side of Figure 14.8, this “dependent assortment”
hypothesis predicts that the phenotypic ratio of the F2 generation will be
3:1, just as in a monohybrid cross:

42. Mendel’s Experiment
• The alternative hypothesis is that the two pairs of alleles segregate independently of
each other.
• In other words, genes are packaged into gametes in all possible allelic combinations,
as long as each gamete has one allele for each gene.
• In our example, an F1 plant will produce four classes of gametes in equal quantities:
YR, Yr, yR, and yr.
• If sperm of the four classes fertilize eggs of the four classes, there will be 16 (4*4)
equally probable ways in which the alleles can combine in the F2 generation.
• These combinations result in four phenotypic categories with a ratio of 9:3:3:1 (nine
yellow round to three green round to three yellow wrinkled to one green wrinkled):

43. Mendelian Modes of Inheritance

44. Monohybrid Cross – one factor cross
• In pea plants, having axial position of flowers on stem
(T) is dominant over the terminal position (t). A
heterozygous axial flower position in a pea plant is
allowed to pollinate by itself.

45. 2. Dihybrid Cross – Two – factor cross
• In pea plants, let us use the same example as in
monohybrid cross (Flower position). Then, let us
combine the traits with yellow and green seed color.
Cross heterozygous axial and yellow with another of the
same kind. Find the phenotypic ratio of the offsprings.

46. The Laws of Probability Govern Mendelian Inheritance
• Mendel’s laws of segregation and independent
assortment reflect the rules of probability.
• When tossing a coin, the outcome of one toss has no
impact on the outcome of the next toss.
• In the same way, the alleles of one gene segregate into
gametes independently of another gene’s alleles.

47. • The multiplication rule states that the probability that two or more
independent events will occur together is the product of their
individual probabilities.
• Probability in an F1 monohybrid cross can be determined using the
multiplication rule.
• Segregation in a heterozygous plant is like flipping a coin: Each
gamete has a chance of carrying the dominant allele and a
chance of carrying the recessive allele.
Multiplication Rules Applied to Monohybrid
Crosses
1
2
1
2

48. We can apply the same reasoning to an F1 monohybrid
cross.
• With seed shape in pea plants as the heritable character, the genotype of F1 plants is Rr.
• Segregation in a heterozygous plant is like flipping a coin in terms of calculating the
probability of each outcome: Each egg produced has a 1⁄2 chance of carrying the dominant
allele (R) and a 1⁄2 chance of carrying the recessive allele (r).
• The same odds apply to each sperm cell produced. For a particular F2 plant to have
wrinkled seeds, the recessive trait, both the egg and the sperm that come together must
carry the r allele.
• The probability that an r allele will be present in both gametes at fertilization is found by
multiplying 1⁄2 (the probability that the egg will have an r) * 1⁄2 (the probability that the
sperm will have an r).
• Thus, the multiplication rule tells us that the probability of an F2 plant having wrinkled
seeds (rr) is 1⁄4. Likewise, the probability of an F2 plant carrying both dominant alleles for
seed shape (RR) is 1⁄4.

49. Segregation of
alleles into eggs
Segregation of
alleles into sperm
Sperm
Eggs
1/2
1/2
1/2
1/2
1/4
1/4
1/4
1/4
Rr Rr
R
R
R
R
R
R
r
r
r
r r

r

50. Addition Rule
• The addition rule states that the probability that
any one of two or more exclusive events will
occur is calculated by adding together their
individual probabilities.
• The rule of addition can be used to figure out
the probability that an F2 plant from a
monohybrid cross will be heterozygous rather
than homozygous.

51. Solving Complex Genetics Problems with the Rules of
Probability
•We can apply the multiplication and addition
rules to predict the outcome of crosses involving
multiple characters.
•A dihybrid or other multi-character cross is
equivalent to two or more independent
monohybrid crosses occurring simultaneously.
•In calculating the chances for various genotypes,
each character is considered separately, and
then the individual probabilities are multiplied.

52. Probability of YYRR
Probability of YyRR
1/4 (probability of YY)
1/2 (Yy)
1/4 (RR)
1/4 (RR)
1/16
1/8
 




To give two examples, the calculations for finding
the probabilities of two of the possible F2
genotypes (YYRR and YyRR) are shown below:

53. Seatwork
In tomatoes, two pairs of gene affect the color of the ripe fruit as
follows
R, red flesh; r, yellow flesh; Y, yellow skin; y colorless skin
Dominance is complete for red flesh and yellow skin. If the genes
are independently segregating, calculate the expected
phenotype and genotype ratios from the following crosses:
a. Rryy x rrYy
b. RrYy x rrYy
c. RrYY x Rryy
d. RrYy x RrYy

54. Solution for Rryy x rrYy
Ry Ry ry ry
rY RrYy RrYy rrYy rrYy
rY RrYy RrYy rrYy rrYy
ry Rryy Rryy rryy rryy
ry Rryy Rryy rryy rryy
Possible combinations for Rrryy = Ry and ry
Possible combinations for rrYy = rY and ry
Probability Probability Probability Probability
RrYy 4/16 = 1/4 Rryy 4/16 = 1/4 rrYy 4/16 = 1/4 rryy 4/16 = 1/4

55. Another Solution for Rryy x rrYy
R r
r Rr rr
r Rr rr
y y
Y Yy Yy
y yy yy
Probability
RR 0
Rr 2/4 = 1/2
rr 2/4 = 1/2
Probability
YY 0
Yy 2/4 = 1/2
yy 2/4 = 1/2
Combi Probability
RrYy 1/2 (Rr) × 1/2 (Yy) = 1/4
Rryy 1/2 (Rr) × 1/2 (yy) = 1/4
rrYy 1/2 (rr) × 1/2 (Yy) = 1/4
rryy 1/2 (rr) × 1/2 (yy) = 1/4

56. Phenotypic and Genotypic Ratios for Rryy x rrYy
Combi Prob Phenotype Genotype
RrYy 1/4 Red flesh, yellow skin heterozygous red flesh,
heterozygous yellow skin
Rryy 1/4 Red flesh, colorless skin heterozygous red flesh,
homozygous colorless skin
rrYy 1/4 Yellow flesh, yellow skin homozygous yellow flesh,
heterozygous yellow skin
rryy 1/4 Yellow flesh, colorless skin homozygous yellow flesh,
homozygous colorless skin
Phenotypic ratio = 1:1:1:1
Genotypic ratio = 1:1:1:1

57. Dominant and Recessive Traits in Humans
Dominant Recessive
Free earlobe Attached ear lobe
Cleft chin No cleft chin
Widow’s peak No widow’s peak
Ability to roll the tongue Inability to roll the tongue
Straight thumb Hitchhiker’s thumb
Arm folding right on top Arm folding left on top
With dimples Without dimples

58. Dominant and Recessive Traits in Humans
Dominant Recessive
Huntington Disease Alkaptonuria
Marfan Syndrome Color blindness
Congenital night blindness Cystic fibrosis
Neurofibromatosis Muscular dystrophy
Porphyria Hemophilia
Ehler-Danlos Syndrome Sickle-cell anemia
Hypercholesterolemia Tay-Sach’s disease
Achondroplasia Phenylketonuria