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Punnett square – 1 gene. Independent Assortment. Punnett Square- 2 genes ( dihybrid ). Product rule. What are the chances that the offspring will be homozygous recessive for both traits? Multiply two probabilities together. Modes of inheritance.

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product rule
Product rule
  • What are the chances that the offspring will be homozygous recessive for both traits?
  • Multiply two probabilities together
modes of inheritance
Modes of inheritance
  • Autosomal Dominant – impacts both sexes equally, it does not ‘skip generations’
    • Carrier of gene always has trait
  • Autosomal Recessive – impacts both sexes equally, can ‘skip generations’
    • Carriers with one allele copy do not show trait
  • X-linked Dominant – affected males always pass trait on to daughters.
  • X-linked Recessive – affected females always pass trait on to sons.
  • Y-linked – passed through male line. Always like a dominant trait because only one copy.
  • Show relationships of family members and depict which relatives have phenotypes and sometimes genotypes
  • Pedigrees can ‘show’ mode of inheritance of a trait
mendelian trait
Mendelian trait
  • What happens if:
    • One allele combination results in death?
    • There are more than two alleles for a gene?
    • Dominance is not complete?
    • Other genes impact phenotype?
    • So on….
lethal allele combinations
Lethal allele combinations
  • If an allele combination is lethal, it is not seen in the offspring genotypes
multiple alleles
Multiple alleles
  • Each person only has two alleles for each gene, but many more alleles can exist in a population
  • Severity of a phenotype can depend of which two alleles the person has
    • Examples: Phenylketonuria (PKU) and Cystic Fibrosis
    • Symptoms and severity can vary depending on which alleles are present
dominance relationships
Dominance Relationships
  • Complete dominance: one allele is expressed over another
  • Incomplete dominance: heterozygote has an intermediate or different phenotype
    • Often the case at the molecular level
  • Codominance: both alleles are expressed (ABO blood types)
snapdragon flower color
Snapdragon flower color
  • Make a punnett square showing a cross of a pink snapdragon to a white one.
abo bloodtypes
ABO Bloodtypes
  • Make a punnett square showing the cross of A blood type (IAi) to AB blood type.
  • My blood type is A and my mother is type O. What are the possible blood types of my father?
  • Occurs when one gene (modifier) masks or affects another gene

Labrador retrievers coat color:

The Eumelanin gene (B gene) is responsible for pigment color.

Black (B) is dominate to brown (b)

The receptor gene (E gene) tells the cell to produce Eumelanin.

Expression (E) is dominate to no expression (e).

(B/_, E/_) (?/?, e/e) (bb, E/_)


A brown lab is crossed with a black lab. Two of the puppies are yellow, one is brown and one is black. What are the genotypes of the parents?

(B/_, E/_) (?/?, e/e) (bb, E/_)

penetrance and expressivity
Penetrance and Expressivity
  • Due to interactions among genes and environmental effects, the same genotype can result in variation of phenotype.
  • Penetrance: percentage of people who have the genotype and express the phenotype (all or nothing)
    • Complete or 100% penetrance is rare
  • Expressivity: severity or extent of expression
    • Many genotypes have variation in the extent in which they are expressed
  • A single gene that controls several functions or has more than one effect – hard to trace because can show different symptoms in different people
  • Example:

porphyria variegata

genetic heterogeneity
Genetic Heterogeneity
  • Different alleles/genes cause the same phenotype
  • Allelic Heterogeneity: multiple alleles at the same gene (Cystic Fibrosis has 1000 known alleles at the causative gene)
  • Locus Heterogeneity: separate genes involved-Often multiple genes involved in the same pathway or function
    • Retinitis pigmentosa has both locus and allelic heterogeneity