1. 5.2 Extending Mendel’s Laws
Big Ideas
Review Mendelian Model of Inheritance (page 129)
Not all inheritance patterns can be explained by dominant and recessive alleles
Incomplete Dominance, Codominance
A single trait can be influenced by many genes, each with two or more alleles
The environment usually influences an organisms phenotype

2. Incomplete Dominance
Complete Dominance: one allele is expressed and the other is not
Incomplete Dominance: an inheritance pattern in which the phenotype of a heterozygous individual falls between the two parental phenotypes
There is not a blending of alleles, but rather the dominant allele is only partially expressed
Proof: both alleles segregate during gamete formation
Ex: Snapdragon Punnett Square
P RR x rr F1 4 Rr, Rr x Rr F2 RR (red), 2 Rr (pink), rr (white)

3. Dominance and recessiveness explain some simple forms of inheritance, but for most traits inheritance patterns are more complex
Ex: In the Andalusion breed of chickens, a cross between a black bird and a white bird produces hybrid offspring which have grey-blue feathers, called “blues”

4. Codominance
Similar to incomplete dominance in which neither allele is dominant to the other
Codominance: both alleles are completely expressed at the same time and so a heterozygous individual has a mixture of the two parental phenotypes
Ex: A cross between a true breeding red bull and a white cow produces roan calves
Alleles for both hair colours are expressed

5. Genes With Multiple Alleles
For most genes, more than two alleles exist in a population
The existence of multiple alleles means that there may be many possible genotypes for a particular gene and many possible phenotypes
Ex: ABO blood type in humans involves three alleles
A person can have one of four blood types (phenotypes): A, B, AB, O

6. A person’s red blood cells may be coated with:
The letter of the blood type refers to two types of carbohydrates (A and B) that are found on the surface of red blood cells (page 141)
A person’s red blood cells may be coated with:
carbohydrate A (blood type A)
carbohydrate B (blood type B)
both carbohydrate A and B (blood type AB)
neither carbohydrate A or B (blood type O)
The alleles IA and IB are codominant, meaning that an IA IB heterozygous individual expresses both the IA and the IB traits resulting in the blood type AB
Alleles are IA, IB and i, do a cross after A: IAIA and IAi B: IBIA and Ibi AB: IAIB O: ii When transfusing blood, it is important to match the recipients blood type with a compatible donor, if incompatible a fatal immune response can occur due to the detection of foreign cells, antibodies will attach to the foreign carbohydrate and will cause the blood cells to clump together, causing blockages

7. Traits Influenced By Multiple Genes
Pea plant height, snapdragon colour and ABO blood type are each under the control of a single gene
More commonly, a trait is influenced by two or more genes
Polygenic Trait: has a range of phenotypes due to the additive effects of multiple genes
Ex: In humans, eye colour, skin colour (~60), height exhibit polygenic inheritance
Dozens to hundreds of genes are thought to influence polygenic traits such as susceptibility to certain diseases

8. The Environment Influence
Different allele combinations account for a great variety of phenotypes
However, for most traits the product of a genotype is not a single, rigidly defined phenotype, but a range of possibilities arising from the combination of heredity (genetics) and the environment
Environment: refers to the conditions in which a cell or organism lives

9. Ex: although a tree’s genotype does not change throughout its lifetime, the shape, size and colour of its leaves greatly depends on the tree’s exposure to sunlight and humidity
Ex: Hydrangea flower colour can be influenced by the nutrients and pH of the soil

10. Ex: Siamese cats have white fur except on their face, ears, feet, tail
This pattern is due to a mutation in a gene for pigment production
As a result of the mutation, the enzyme responsible for black fur is sensitive to heat
The enzyme is active only in the cooler temperatures found in the cat’s extremities, creating this characteristic appearance
Other breeds of cats do not have the mutant allele that produces the Siamese phenotype
Homework: page 145 #2-9,12