Lecture G4 (Extensions of Mendelian Genetics)

Do all inheritance patterns follow simple Mendelian rules?

No. Many traits show non-Mendelian inheritance, such as incomplete dominance, codominance, multiple alleles, and pleiotropy

What does “extensions of Mendelian genetics” mean?

Inheritance patterns that differ from Mendel’s simple 3:1 or 1:2:1 ratios due to more complex allele interactions

Why is genotype-phenotype relationship rarely straightforward?

Because alleles can have different dominance levels, more than two alleles can exist, and some genes affect multiple traits

What are the three degrees of dominance for a single gene?

1. Complete dominance

2. Incomplete dominance

3. Codominance

What is complete dominance?

The heterozygote shows the same phenotype as the dominant homozygote

• keep in mind, one generation is skipped in order to determine the phenotype

What are the F2 generation ratios in complete dominance?

• genotype ratio → 1:2:1

• Phenotype ratio → 3:1

Examples of complete dominance

• albinism

  • Recessive trait

  • A = melanin

  • a = decreases or absent melanin

  • Must have both “a”s in order for the disease to be completely expressed

• Achondroplasia

  • Dominant trait

  • D = affected

  • d = unaffected

What is incomplete dominance?

Neither allele is completely dominant, so heterozygotes show an intermediate phenotype

• dont have complete dominance, so dont use a big or little letter

What are the F2 generation ratios in incomplete dominance?

• genotype ratio → 1:2:1

• Phenotype ratio → 1:2:1

Example of incomplete dominance

Feather color in andalusian chickens

• CᴮCᴮ = black

• CᵂCᵂ = white

• CᴮCᵂ = gray (intermediate).

What is codominance?

Both alleles in a heterozygote are fully and separately expressed in the phenotype

• coworkers, work together

What are the F2 generation ratios in codominance?

• genotype ratio → 1:2:1

• Phenotype ratio → 1:2:1

Example of codominance in humans

ABO blood groups - IA and IB alleles are both expressed, producing type AB blood

Example of codominance in animals

Coat color in shorthorn cattle

• R₁R₁ = red

• R₂R₂ = white

• R₁R₂ = roan (red and white hairs intermixed).

What are multiple alleles?

The presence of more than two possible alleles for a gene in a population, even though each individual carries only two

Example of a gene within multiple alleles

The ABO blood group gene (IA, IB, and i alleles)

How do multiple alleles affect inheritance patterns?

They increase genetic diversity and can produce more than two possible phenotypes in a population

What is another example of multiple alleles?

The mouse agouti gene, which has one wild-type and many mutant alleles that affect coat color

What is pleiotropy?

When a single gene affects more than one distinct phenotypic trait

Give an example of pleiotropy in dogs

In australian shepherds

• MM = solid coat

• Mm = merle coat

• mm = white coat (can cause blindness or deafness)

  • This one gene influences color, vision, and hearing

What is a pleiotropic lethal allele?

An allele that causes both a visible phenotype and affects survival (often lethal in homozygotes)

Example of a pleiotropic lethal allele in horses

Overo Lethal White Syndrome (OLWS)

• Oo = overo coat pattern

• OO = lethal

How does sickle-cell disease show pleiotropy?

One gene mutation (HbS) affects hemoglobin structure, oxygen transport, cell shape, and causes multiple health effects — but also gives malaria resistance

What are the main types of dominance in genetics?

1. Complete dominance

2. Incomplete dominance

3. Codominance

What are the key exceptions to Mendelian inheritance?

1. Incomplete dominance

2. Codominance

3. Multiple alleles

4. Pleiotropy

What is the overall message of extensions to Mendel’s laws?

Inheritance is more complex than simple dominant/recessive patterns; genes can interact in multiple ways and influence multiple traits