• Sequence of DNA that codes for a particular trait. (Ex: tongue rolling) Gene

• Alternative versions of a gene. (Ex: can roll tongue vs. cannot roll tongue) Allele

• The combination of alleles present in an individual’s DNA. (Ex: RR, Rr, rr) Genotype

• The physical characteristics resulting from the genotype. (Ex: can or cannot roll tongue) Phenotype

• Genotype with two copies of the same allele for a gene. (Ex: RR or rr) Homozygous

• Genotype with two different alleles for a gene. (Ex: Rr) Heterozygous

Dominant – The allele that is

expressed when the combination of alleles is heterozygous

– Only one copy of the allele is needed for the phenotype to be expressed

– Genotype vs. Phenotype

• Homozygous dominant or heterozygous: dominant gene is expressed

• Heterozygous (“carrier”): gene is expressed

• Homozygous recessive: dominant gene is not expressed

Recessive – The allele that is not

expressed when the combination of alleles is heterozygous

– Two copies of the allele are required for the phenotype to be expressed

– Genotype vs. Phenotype

• Homozygous dominant or heterozygous: recessive gene is not expressed

• Homozygous recessive: recessive gene is expressed

• “Carrier” state does not exist

When Mendel Cross pollinated Pea plant with different traits, only one trait seemed to be “expressed” in that first generation of offspring. This trait “dominated” the other.

Yet, when that first generation of plants were self pollinated, on average, three of every four plants had the dominant trait, and one of every three had the weaker or recessive trait.

Mendel’s Laws

When Mendel Cross pollinated Pea plant with different traits, only one trait seemed to be “expressed” in that first generation of offspring. This trait “dominated” the other.

Yet, when that first generation of plants were self pollinated, on average, three of every four plants had the dominant trait, and one of every three had the weaker or recessive trait.

Mendel’s Laws

Mendel’s Laws

Law of Dominance

In a cross of parents that are true-breeding for

contrasting traits, only one form of the trait will

appear in the next generation.

All offspring from that cross will be heterozygous

and express only the dominant trait

RR x rr yields

all Rr (round

seeds)

Mendel’s Laws

Law of Segregation

The two alleles for a trait must separate from each

other when gametes are formed.

A parent randomly passes only one allele for each

trait to each offspring.

Law of Independent Assortment

Alleles for different traits are inherited

independently of each other.

Mendel’s Laws

Law of Segregation

Before sexual

reproduction during

meiosis, the two

alleles from the parent

become separated so

each sex cell has only

one allele

Mendel’s Laws

Law of Independent Assortment

Inheritance of

one allele does

not affect which

other allele will

be inherited

(unless the

alleles are on the

same

chromosome)

An Example of How Traits Pass from Parents to First Offspring Generation

Probability

Probability

How likely it is that something will happen

Note: It does not tell you what will definitely

happen, but only the chances that something will.

How do we determine Probability?

Determining Probability

Determine the number of times a specific event can

occur in relation to the total number of events that can

occur:

Example: What is the probability of flipping a

coin on heads?

Number of specific events (heads) = 1

Total number of events that can occur (heads

or tails) = 2

Answer: The probability is ½ or 50% chance of

landing on heads (1:1 heads to tails ratio)

13 copyright cmassengale

Types of Genetic Crosses

Monohybrid Cross

Cross involving a single trait

Ex: Flower color

Dihybrid Cross

Cross involving two traits

Ex: Flower color and plant height

Parental P1 Generation

The parental generation in a breeding experiment.

F1 Generation

The first-generation offspring in a breeding experiment (1st filial generation)

From breeding individuals from the P1 generation

F2 Generation

The second-generation offspring in a breeding experiment (2nd filial generation)

From breeding individuals from the F1 generation

Generation “Gap”

The gene combinations that might result from a genetic cross can be determined by drawing a diagram known as a Punnett Square.

Punnett Squares can be used to compare the genetic variations that will result from a cross.

A capital letter represents the dominant allele for tall.

A lowercase letter represents the recessive allele for short.

In this example:

– T = tall

– t = short

Gametes produced by each F1 parent are shown along the top and left side.

Possible gene combinations for the F2 offspring appear in the four boxes.

Probability: Measure of the likelihood of an event happening.

Punnett Square: A chart designed to show the probability each possible

genotype in offspring based on parental genotypes.

Determine all the possible gametes that can

be formed from each parental genotype.

AB Ab aB

AB

Father (AaBb) Mother (AABb)

ab

Ab

DD Homozygous

Dominant

Dd Heterozygous

(Carrier)

dD Heterozygous

(Carrier)

dd Homozygous

Recessive

D d

Father (Dd)

Mother (Dd)

D d

d

D

d

D

Parental (P) Generation: Ex: Dd (mother) x Dd (father)

POSSIBLE GAMETES:

True-breeding Red Dragons Population consistently

produces only red offspring.

True-breeding Green Dragons Population consistently

produces only green offspring.

RG Red

RG Red

RG Red

RG Red

100% of offspring are

RED.

So RED is dominant

over GREEN.

True-breeding Green

(GG) Parent

True-breeding

Red (RR) Parent

G G

R

R

50% of offspring are RED, and

50% are GREEN.

Since there are GREEN

offspring, the RED dragon must

be heterozygous (Rr) & does

carry the green allele.

Green (rr) Parent

Rr Red

Red (R?) Parent

r r

R

?

Rr Red

?r Green

?r Green

Cc Carrier

c c

C

c

Cc Carrier

cc C.F.

cc C.F.

F1 Offspring: 50% Carriers

50% Cystic Fibrosis

P Generation:

Father (Cc)

Carrier X

Mother (cc)

Cystic Fibrosis

CC Normal

C c

C

c

Cc Carrier

Cc Carrier

cc C.F.

F1 Offspring: 25% Normal, 50% Carriers

25% Cystic Fibrosis

P Generation:

Father (Cc)

Carrier X

Mother (Cc)

Carrier

Cc Carrier

c c

C

C

Cc Carrier

Cc Carrier

Cc Carrier

F1 Offspring: 100% Carriers

P Generation:

Father (CC)

Normal X

Mother (cc)

Cystic Fibrosis

CC Normal

C C

C

c

CC Normal

Cc Carrier

Cc Carrier

F1 Offspring: 25% Normal & 50% Carriers

P Generation:

Father (Cc)

Carrier X

Mother (CC)

Normal

Cc Carrier

c c

C

C

Cc Carrier

Cc Carrier

Cc Carrier

F1 Offspring: 100% Carriers

P Generation:

Father (CC)

Normal X

Mother (cc)

Cystic Fibrosis

CC Normal

C c

C

c

Cc Carrier

Cc Carrier

cc C.F.

F1 Offspring: 25% Normal, 50% Carriers

25% Cystic Fibrosis

P Generation:

Father (Cc)

Carrier X

Mother (Cc)

Carrier

CC Normal

C C

C

c

CC Normal

Cc Carrier

Cc Carrier

F1 Offspring: 50% Normal, 50% Carriers

P Generation:

Father (Cc)

Carrier X

Mother (CC)

Normal

Cc Carrier

c c

C

c

Cc Carrier

cc C.F.

cc C.F.

F1 Offspring: 50% Carriers & 50% Cystic Fibrosis

P Generation:

Father (Cc)

Carrier X

Mother (cc)

Cystic Fibrosis