Chapter 14 –

Mendelian Genetics

Blending vs. Particulate Theory of Inheritance

Traits of offspring were a “blend” of the parental traits.

Infer over time, populations begin to look uniform and look alike

Mendel observed that genes retain their separate identities

Over time, populations do not become uniform

Often traits that seemed to have “disappeared” would reappear in subsequent generations

His observations lead to what’s now accepted as the Particulate Theory of inheritance

Vocabulary Review

Trait: Characteristic of an organism

Ex. Plant Height

Allele: Different forms of a gene responsible for different traits

Ex. T = Tall t = short

Genotype = Genetic make-up of characteristics represented by 2 alleles in diploid organisms

Phenotype = expressed trait, based on the genotype

TT or Tt tt

Tall Short

Homozygous = When both alleles of the genotype are the same

Heterozygous = When both alleles of the genotype are different

(Hybrid)

TT tt

Tt

P = Parental Generation

F1 = 1st Filial Generation

F2 = 2nd Filial Generation

X T T

t Tt Tt

t Tt Tt

X T t

T TT Tt

t Tt tt

Genotypic Ratio of monohybrid cross

1:2:1

Phenoytypic Ratio of monohybrid cross:

3:1

Single-Factor Crosses

Memorize these ratios!

Two Factor Crosses (2 traits)

X

Y = yellow, y = green R = Round, r = wrinkled

RrYyX

RrYyDihybrid Cross

Memorize these ratios!

Test CrossesIf a plant has a dominant phenotype, (for example yellow seeds) and

we are unsure of its genotype (YY, or Yy), you can determine it’s genotype by crossing it with another with a recessive phenotype (green seeds) with the genotype yy.

YY or Yy?

X

yy

If F1 = 50% Yellow, 50% green

Then P = Yy (hybrid)

Yy

If F1 = 100% Yellow

Then P must be = YY

Yy yy

Pedigree

male femaleMating couple

Children/Siblings

Shaded = trait being followed

aa

A = tongue roller

a = can not roll tongue

??

?

? ?

aaaa Aa

Aa

AA

Can you figure out the rest of the genotypes on your own?

Other Pedigree Symbols

• Fraternal twins

• Identical twins

Examples of connected symbols:Examples of connected symbols:

Unique Degrees of Dominance(exceptions to the rule)

Incomplete Dominance: Dominant trait “blends” when combined with a recessive allele

Notice how the genotypes are written…

X

CRCR CWCWCRCW

Unique Degrees of DominanceCo-Dominance: When there are multiple alleles that are dominant and are of equal strength

- then both dominant alleles will be expressed when combined

- a dominant allele will always mask a recessive allele.

Notice how the genotypes are written… Example: Blood Type

I’m the dominant allele for Type A blood!

I’m the dominant allele for Type B blood !

Don’t count me out just ‘cuz I’m recesssive. I’m

type O allele!

What’s expressed

when we’re together?

IA IA or IA i = type A bloodIA IB = type AB bloodIB IB or IB i = type B bloodi i = type O blood

Unique Gene Interactions

Pleiotrophy: The ability of a gene to affect an organism in many phenoytypic ways

Ex. Sickle Cell Anemia

Blood Clumping

Physical Weakness or Brain Damage

Unique Gene InteractionsPolygenic Inheritance: when multiple genes have an added effect on a single phenotype (Opposite of Pleiotrophy) – ex. Skin color, heightNotice the range in genotypes…

aabbcc AABBCC

Unique Gene InteractionsEpistasis: when a gene at one locus alters the expression of a gene at another locus

Alleles for Fur Color:

B = Black Fur

b = brown fur

C = Color

c = albino

BBCC, BBCc, BbCC, BbCc

bbCC, bbCc

BBcc, Bbcc, bbcc

Since cc genotype is albino, the alleles for fur color (B or b) are not expressed

Genetics is cool! But

wait…sample probability

problems to come!

How do you calculate probability? (Alternatives to Punnett Squares)

Sample Problem

Brown eyes are dominant over blue eyes. Parent A has blue eyes, while Parent B is heterozygous for brown eyes. What is the probability that they will have a child with blue eyes?

Parent A (blue eyes) = bb

Parent B (brown eyes) = Bb

Process:

Blue-eyed Child has to be bb

Probability of parent A donating one “b” allele= 1

Probability of parent B donating the other “b” allele = ½

1 X ½ = ½ (50% probability)

X b b

B Bb Bb

b bb bb

Check your work!

50% probability for blue eyes

Rule of Multiplication

Rule of Multiplication and Addition

Sample Problem #2:In a cross between AaBbCc

x Aabbcc, what is the probability that at least two of the three recessive traits is present in the offspring?

Steps for solving:1. Write out the genotypic

possibilities2. Use rule of X (multiply

probabilities of each genotypic combination)

3. Use rule of +

AAbbcc

Aabbcc

aaBbcc

aabbCc

aabbcc

AA (1/2) x bb (1/2) x cc (1/2) = 1/8

Aa (1/2) x bb (1/2) x cc (1/2) = 1/8

Aa (1/4) x Bb (1/2) x cc (1/2) = 1/16

Aa (1/4) x bb (1/2) x Cc (1/2) = 1/16

Aa (1/4) x bb (1/2) x cc (1/2) = 1/16

Sum of the fractions: 6/16 = 3/8