extension to Mendel: dominance

Intended Learning Outcomes

• Explain some of the ways genes may interact to affect the phenotype.

• Distinguish among:

  • Incomplete dominance

  • Codominance

  • Multiple alleles

  • Epistasis

  • Pleiotropy

  • Polygenic traits

• Be able to provide an example of each of the extensions to Mendelian inheritance discussed in lecture.

Extensions to Mendel

Dominance

• Mendel’s model of inheritance is based on certain assumptions:

  • Each trait is controlled by a single gene.

  • Each gene has only two alleles.

  • There is a clear dominant-recessive relationship between the alleles.

• However, most genes do not meet these criteria!

• Special cases in genetics include:

  • Incomplete dominance

  • Codominance

  • Polygenes

  • Epistasis

  • Pleiotropy

Complete Dominance – Mendelian Genetics

• Under complete dominance:

  • The phenotype of the heterozygote is identical to that of the homozygous dominant phenotype.

  • Example Genotypes:

  • AA and Aa result in the dominant phenotype.

  • aa results in the recessive phenotype.

Incomplete Dominance

• Definition:

  • The phenotype of the heterozygote is intermediate between the homozygous phenotypes.

  • Example Alleles:

  • CRCR = ‘Red’ phenotype

  • CRCW = intermediate phenotype (pink)

  • CWCW = ‘White’ phenotype.

Codominance

• Definition:

  • The phenotype of the heterozygote expresses both homozygous phenotypes for the alleles being studied.

  • Example Alleles:

  • RwRw = ‘White’ phenotype

  • RwRr = ‘Roan’ phenotype (displays aspects of both phenotypes simultaneously)

  • RrRr = ‘Red’ phenotype.

Multiple Alleles

• Definition:

  • A single gene can have more than two different forms, or alleles.

  • Note: Each individual can possess only two alleles at a time, due to genes being paired on homologous chromosomes (with exceptions).

  • Example: Rabbit coat color and ABO blood group.

Notes on Notation Styles

• Complete dominance examples:

  • Aa, Rr, +e, Vgvg, etc.

• Other dominance types examples:

  • BW, CACa, C+Cc, RWRr, etc.

• Multiple alleles examples:

  • Blood groups: AB (O), IAi, IAIA, CCch, etc.

Common Misconceptions

Misconception 1

• Dominance does not necessarily correlate with being:

  • Stronger

  • Better

  • Normal

  • More common in the population

  • Healthy

  • Original (pre-mutation).

• Example: Huntington's disease is caused by a dominant allele but is rare in the population.

Misconception 2

• A single gene may have alleles with varying dominance relationships.

  • For instance, two alleles can be codominant while a third allele may be completely recessive to the first two yet dominant to a fourth.

Overdominance (Heterozygote Advantage)

• Definition:

  • Heterozygotes exhibit a phenotype that lies outside the phenotypic ranges of the homozygote parents.

  • Example: Sickle cell anaemia.

  • Homozygous dominant = normal phenotype but susceptible to malaria

  • Homozygous recessive = abnormal phenotype, lower life expectancy

  • Heterozygote = demonstrates partial immunity to malaria due to altered blood proteins and red blood cell shape.

ABO Blood Groups

Codominance and Multiple Alleles

• ABO blood group system involves codominance and multiple alleles.

• Three alleles at the I gene where an individual can carry only two.

  • IA and IB are codominant and both dominate the i allele.

  • Alleles represent blood phenotypes A, B, and O.

Antigen Details

• Two types of antigens: A and B found on red blood cells.

• Antigens determine the blood type and the antibodies produced by the body.

• Possible blood phenotypes: A, B, AB, or O.

ABO Blood Type Details

ABO Blood Group Gene Interaction

• Gamete combinations involving blood types need to be assessed.

  • Avoid confusion: Individuals cannot carry more than two alleles due to the paired nature of chromosomal genes.

Rhesus Blood Groups

Rh Factor Details

• Rh+ indicates the presence of the Rh antigen, while Rh- indicates no antigen.

• If an Rh- person receives Rh+ blood, antibodies develop against the Rh antigen.

• Reexposure will initiate an immune response against Rh+ blood cells.

Understanding Blood Types

Blood Group Compatibility Chart

• A chart detailing which blood types can donate or receive from each other is crucial for ensuring safe transfusions.

Environmental Influence

Impact on Traits

• Phenotypic traits can be influenced by both genotypes and environmental factors (expressed as: P = G imes E ).

• Example: In Hydrangeas, flower color is genetically determined but also influenced by the pH level of the soil.

Specific Case Studies

• Coat color in Himalayan rabbits and Siamese cats relies on temperature—an allele produces enzymatic activity that allows pigment production only below 30°C.

Incomplete Penetrance

• Definition:

  • Not all dominant alleles are expressed consistently across individuals.

• Concept of penetrance is defined as the probability that an individual with a dominant allele will exhibit its associated phenotype (e.g., polydactyly trait evident only in 65% of carriers).

• Variable expression of a trait is described as ‘expressivity.’

Summary of Extensions to Mendelian Genetics

• Draw a table comparing:

  • Polygenic inheritance

  • Pleiotropy

  • Epistasis

  • Environmental influence

  • Incomplete penetrance

  • Multiple alleles

Monohybrid Practice Exercises

1. In tomatoes, the dominant red fruit color crosses with homozygous yellow to determine F1, F2 ratios, and analyze offspring results.

Dihybrid Practice Exercises

1. Analyze a double heterozygote cross with respect to gamete production and apply a Punnett square for predictions.

Review Questions

• Question regarding incomplete dominance in tomato color genetics: A pure breeding red flower crossed with a pure breeding white flower will yield F1 offspring with what ratios?

• Understanding multiple alleles and distinguishing genetic interactions, epistasis, and pleiotropy in genetic expressions.