Download 1.Trait

The introduction to modern
genetics:
 Mendel
discovered the basic
principles of heredity by breeding
garden peas in carefully planned
experiments
© 2011 Pearson Education, Inc.
I. The Origins of Genetics:
A. Terms associated with genetics:
1.Trait:
A characteristic determined by a genes
2. INHERITANCEThe passing of traits by heredity
3. HEREDITYThe transmission of traits from
parents to offspring
B. The History of Genetics
Gregor Mendel:
 The “father of modern
genetics” – Developed
Mendelian Genetics
 An
Austrian monk in the
mid-1800’s
 Studied
math and science
Mendel’s Garden
Mendel’s Studies:
 Mendel made
observations within the
monastery's garden.
 Noticed that in pea
plants, seven specific
traits could be seen.
 Mendel was interested in
testing the predictability
of the traits
C. Mendel’s Breeding Experiments
 Crossed
pea plants with purple
flowers with plants with white
flowers
 Mendel counted all offspring and
analyzed the data
Traits Expressed as Simple Ratios
D. Monohybrid Crosses
 Cross
involving only one pair of
contrasting traits.
STEP 1:
 Produce TRUE BREEDING
strains
 Ex: Purple plants only produce
purple offspring
 These plants make up the “P
Generation” (parental)
Step 2:
 Cross-pollinate
two P generation
plants with contrasting traits
 F1- First Filial Generation:
 The offspring of the P generation
 Record the number of F1 plants
with each trait
STEP 3:
 F1 plants self-pollinate
 The offspring are called F2
Second Filial
 Each plant was counted
E. Mendel’s Results
 F1 showed only one form of the
trait
 Missing trait reappeared in some of
the F2 generation
 3:1 ratio in the F2 generation
II. Mendel’s Theory
A. Mendel’s Findings in
Modern Times
 Dominant-
First letter of the trait in
capital (P- purple)
 RecessiveFirst letter of the
dominant trait in lower
case (p- white)
B. Mendel’s 4 Hypotheses:
1. For each trait an individual has 2
copies of the gene (one from
each parent)
2. There are alternative versions of
genes ALLELES
Mendel’s Hypotheses Continued
3.
Dominance and Recessiveness:

One factor is Dominant, and
hides, the other factor
Recessive
4.
Segregation:
The egg and sperm carry only
one of the possible alleles

C. The Laws of Heredity
1. Law of Segregation:
 The 2 alleles for a trait segregate
when gametes are formed
2. Law of Independent Assortment:
 Dominant factors don’t always go
with other dominant factors
 Recessive factors don’t always
go with other recessive factors
9-2: Genetic Crosses
1. Genotype:
 The genetic makeup of
an organism
2. Phenotype:
 External appearance of
an organism
Homozygous vs. Heterozygous
3. Homozygous: 2 alleles are the
same
- Homozygous Dominant (PP)
- Homozygous Recessive (pp)
4. Heterozygous: alleles are
different (Pp)
- Display the characteristics of the
dominant trait
B. Probability:
 Likelihood that a specific event will occur
Probability = # of one kind of outcome
total # of all possible outcomes
The laws of probability govern
Mendelian inheritance
Mendel’s laws of segregation and
independent assortment reflect the rules
of probability
 The alleles of one gene segregate into
gametes independently of another gene’s
alleles
EX: Outcome of one coin toss is irrelevant in
next coin toss

© 2011 Pearson Education, Inc.
The Multiplication and Addition Rules
Applied to Monohybrid Crosses
The multiplication rule:
 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 12 chance
of carrying the dominant allele and a 12
chance of carrying the recessive allele
© 2011 Pearson Education, Inc.
Figure 14.9
Rr
Segregation of
alleles into eggs

Rr
Segregation of
alleles into sperm
Sperm
1/
R
2
2
Eggs
4
r
2
r
R
R
1/
1/
r
2
R
R
1/
1/
1/
4
r
r
R
r
1/
4
1/
4
The addition rule:
 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
© 2011 Pearson Education, Inc.
C. Studying Heredity
1. Punnet Squares
 Diagram that predicts the
outcome of a monohybrid cross
 Four Boxes in a square
2. Test Crosses
 Testing
procedure used to determine
the genotype of an individual
 A procedure in which an individual of
unknown genotype is crossed with a
homozygous recessive individual
B. Incomplete Dominance
 Individual
displays a phenotype
intermediate between the two parents
Ex: Red Snapdragon + White
Snapdragon = Pink Snapdragon
Ex: Straight hair + Curly hair = Wavy hair
4. Codominance:
 Two
dominant alleles
are expressed at the
same time
 The phenotype is
unique (not
intermediate between
2 original)
 Ex: IA + IB = AB blood
type
Codominance Example: Roan Cattle
(White and Red)
Dihybrid Crosses
Monohybrid
Crosses