Patterns of Inheritance: Codominance & Multiple Alleles

Other Patterns of Inheritance

Incomplete Dominance

• Some alleles are neither completely dominant nor recessive.
• One allele is not completely dominant over another.
• Red + white = pink
• Offspring are all pink.

Codominance

• Codominance is a situation in which the phenotypes produced by both alleles are clearly expressed.
• Chicken feather color, human protein controlling blood cholesterol levels, are examples of codominance.
• black + white = black & white
• A blood + B blood = AB blood type
• Heterozygous chickens have a color described as "erminette", speckled black & white feathers.
• Unlike the 4 o'clock flowers, which turned pink when white & red flowers crossed.

Multiple Alleles

• Many genes exist in several different forms and are therefore said to have multiple alleles.
• An individual organism usually only has 2 copies of each gene, but many different alleles are found in the organisms population.
• Rabbit coat color is determined by a single gene with 4 different alleles.
• Human blood has 3 multiple alleles: A, B, O

Polygenic Traits

• Polygenic traits are controlled by 2 or more genes.
• Human eye color is controlled by an estimated 100 genes.

Genes & the Environment

• Environmental conditions can affect gene expression & influence genetically determined traits.
• An individual's actual phenotype is influenced by its environment as well as its genes

Genetics and Probability

• Probability refers to the fraction or % of likelihood of an event taking place.
• Punnett Square - grid or chart that shows all possible gene combinations for a cross.
• Genotype- pair of alleles. (RR), (Rr) (rr)
• Phenotype - allele being expressed
• The principle of independent assortment states that a gene for different traits can segregate independently during the formation of gametes.
• It accounts for the many different genetic variations in organisms.
• When a plant with a genotype (RrYy) produces gametes, the alleles R & r and Y & y will separate and can combine 4 different ways.

Mendel Genetics

Gregor Mendel Genetics

• Genetics - study of heredity
• Gregor Mendel discovered the principles of genetics.
• Mendel conducted an experiment with pea plants to determine how traits were passed to their offspring.

The Role of Fertilization

• Mendel knew that the pollen came from the male reproductive system & the female portion produced the egg.
• When these combine we call it fertilization
• Trait- a specific characteristic
• Hybrid - a cross between 2 different organisms

Genes & Alleles

• All individual's characteristics are determined by factors that are passed from one parent generation to the next.
• Genes
• Alleles - different forms of a gene
• Principle of Dominance
• Principle states that 1 that gene in a pair may prevent the other gene in the pair from being expressed.
• Dominant gene - gene that is being expressed
  • Always written with a capital letter
• Recessive gene - gene that is present but hidden
  • Lower case letter
• Ways genes can be expressed: Homozygous dominant (RR), Homozygous recessive (rr), Heterozygous(Rr)

Segregation

• Segregation - The separation of alleles during gamete formation.
• Gametes - reproductive cells of an animal or plants, female - ovum oregg, male - sperm
• During gamete formation, the alleles for each gene segregate from each other, so that each gamete carries only 1 allele for each gene.

The Formation of Gametes

• During gamete formation, the alleles for each gene segregate from each other, so that each gamete carries only one allele for each gene.
• Each F1 plants produces 2 kinds of gametes, green & yellow pod, even though all the pods in F1 were green.

Explore inheritance patterns beyond simple dominance. Learn about incomplete dominance where alleles blend (e.g., pink flowers from red and white parents) and codominance, where both alleles are fully expressed (e.g., black and white speckled chickens). Also covered are multiple alleles.