I. Monohybrid Crosses

A. Simple cross• Cross two individuals with different alleles at a

particular locus• Individual may have one or two copies of allele• Both copies identical = homozygous• Different copies = heterozygous

• One allele may be dominant, the other recessive

Fig. 14.5

Fig. 14.6

I. Monohybrid Crosses

B. Testcross• Individual with known phenotype but unknown

genotype• Ex: Pea with purple flowers – homozygous or

heterozygous?

• Cross with known homozygous recessive

Fig. 14.7

II. Dihybrid Crosses

• Cross two individuals with different alleles at two loci

• If alleles at different loci on non-homologous chromosomes, traits should assort independently

Fig. 14.8Why not exactly 9:3:3:1?

II. Dihybrid Crosses

• Law of Independent Assortment• Pairs of alleles segregate independently of other

pairs of alleles during gamete formation

• Understanding principles of inheritance permits prediction of cross outcomes

III. Probability

A. Product Rule• Predicts combined probability of independent

events• Probability of multiple independent events all

occurring is product of probabilities for each

B. Sum Rule• Predicts combined probability of mutually

exclusive events• Probability of multiple exclusive events all

occurring is sum of probabilities for each

Fig. 14.9

III. Probability

C. Examples• Eye color in humans determined by one gene

with two alleles – B (Brown) & b (blue)• Two parents with brown eyes are heterozygous

for eye color (Bb)• Four children

1. Probability of a child having blue eyes?

2. Probability of child #4 having blue eyes?

3. Probability of all four children having blue eyes?

4. Probability of at least one child having blue eyes?

II. Inheritance

• Relationship between genotype and phenotype may be simple or complex

1) Single pair of alleles may regulate single trait

2) Single pair of alleles may regulate multiple traits

3) Multiple alleles collectively may regulate single trait

4) Multiple alleles collectively may regulate multiple traits• Phenotype also may be influenced by environment

A. Complete Dominance

B. Codominance• Both alleles expressed independently• Ex: Reddish-coated stallion x white-coated mare Roan

C. Incomplete Dominance• Intermediate phenotype

Fig. 14.10

IV. Inheritance

D. Multiple Alleles• Single individual has two alleles for each locus• Population may contain more than two alleles at

a locus• Three+ alleles at a locus = multiple alleles• Ex: Blood type (three alleles) – IA, IB, i

Fig. 14.11

IV. Inheritance

E. Pleiotropy• Single gene may affect multiple traits• Single gene products may affect many cells or cell types

in different ways• Ex: Cystic fibrosis, sickle cell disease

F. Epistasis• Presence of certain alleles at one locus can alter

expression of alleles at different locus• Ex: Coat color in dogs

• Color regulated by one allele pair (B = Black, b = brown)• Second allele pair (E = active, e = inactive) regulates

deposition of color in hair• EE and Ee dogs are pigmented, ee dogs are yellow• Gene for pigment deposition is epistatic to gene that codes

for Black or brown pigment

Fig. 14.12