Monohybrid Crosses

1

• Monohybrid Crosses
• 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

2
Fig. 14.5
3
Fig. 14.6
4

• Monohybrid Crosses
• Testcross
• Individual with known phenotype but unknown
  genotype
• Ex Pea with purple flowers homozygous or
  heterozygous?
• Cross with known homozygous recessive

5
Fig. 14.7
6

• Dihybrid Crosses
• Cross two individuals with different alleles at
  two loci
• If alleles at different loci on non-homologous
  chromosomes, traits should assort independently

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Fig. 14.8
Why not exactly 9331?
8

• 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

9

• Probability
• Product Rule
• Predicts combined probability of independent
  events
• Probability of multiple independent events all
  occurring is product of probabilities for each
• Sum Rule
• Predicts combined probability of mutually
  exclusive events
• Probability of multiple exclusive events all
  occurring is sum of probabilities for each

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Fig. 14.9
11

• Probability
• 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
• Probability of a child having blue eyes?
• Probability of child 4 having blue eyes?
• Probability of all four children having blue
  eyes?
• Probability of at least one child having blue
  eyes?

12

• Inheritance
• Relationship between genotype and phenotype may
  be simple or complex
• Single pair of alleles may regulate single trait
• Single pair of alleles may regulate multiple
  traits
• Multiple alleles collectively may regulate single
  trait
• Multiple alleles collectively may regulate
  multiple traits
• Phenotype also may be influenced by environment
• Complete Dominance
• Codominance
• Both alleles expressed independently
• Ex Reddish-coated stallion x white-coated mare ?
  Roan
• Incomplete Dominance
• Intermediate phenotype

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Fig. 14.10
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• Inheritance
• 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

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Fig. 14.11
16

• Inheritance
• 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
• 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

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Fig. 14.12