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3.2Genetics and Heredity

Beyond Mendel — EST 2 Biology Revision Notes & Topical Questions

25 min read 42 parts Lesson 2 of 6 2 quizzes

📚 EST 2 Biology Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

SAT Biology beyond Mendel: recognise inheritance patterns that extend beyond complete dominance.

  • incomplete dominance
  • codominance
  • Polygenic traits
  • Pleiotropy

Part 1 of 5Incomplete dominance

Incomplete dominance
In incomplete dominance, the heterozygous phenotype is intermediate between the two homozygous phenotypes, such as red and white parents producing pink offspring.
A heterozygote cross gives a 1:2:1 genotype ratio and also a 1:2:1 phenotype ratio because each genotype has a distinct appearance.

Part 2 of 5Codominance and blood groups

Codominance and blood groups
Codominance
In codominance, both alleles are fully expressed in the heterozygote. Human Iᴬ and Iᴮ alleles are codominant, producing blood group AB.
The i allele is recessive to both Iᴬ and Iᴮ. Blood group A may be IᴬIᴬ or Iᴬi, group B may be IᴮIᴮ or Iᴮi, group AB is IᴬIᴮ, and group O is ii.

Part 3 of 5Multiple alleles and polygenic traits

A gene may have more than two alleles in a population even though one diploid individual carries only two. The ABO system is an example with three alleles.
Polygenic traits
Polygenic traits are influenced by several genes and often show continuous variation, as in height or skin pigmentation. Environmental effects can broaden this variation.

Part 4 of 5Epistasis and pleiotropy

Epistasis
Epistasis occurs when one gene changes or masks the expression of another gene. Coat colour pathways often show this interaction because pigment production and deposition are controlled separately.
Pleiotropy
Pleiotropy occurs when one gene affects several traits. A single altered protein can influence multiple organs and therefore produce a set of related symptoms.

Part 5 of 5Sex-linked inheritance

Sex-linked inheritance
X-linked recessive alleles are more often expressed in males because a male has only one X chromosome. A female generally needs two recessive copies to be affected.
A carrier female can pass the allele to sons or daughters. Fathers transmit their X chromosome to all daughters and their Y chromosome to all sons, so an X-linked allele never passes directly from father to son.

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