Mendelian Genetics, Pedigree Analysis,
and the Bioethics of Eugenics
ETHC 210: Ethics in Biotechnology / Bioethics — Quiz 3
Liberty University
Department of Biology & Health Sciences
June 10, 2026
Abstract
This paper provides a comprehensive review of Mendelian genetics principles and their
application to pedigree analysis, virtual laboratory simulation using Drosophila
melanogaster, and the critical bioethical history of the eugenics movement. The
foundational principles of Mendel's Law of Segregation and Law of Independent
Assortment are examined alongside the analytical skills required for interpreting pedigree
charts and predicting phenotypic ratios from monohybrid and dihybrid crosses. The paper
addresses the dark history of eugenics—from Francis Galton's pseudoscientific framework
through the devastating Buck v. Bell Supreme Court ruling—and evaluates how early
geneticists misappropriated Mendelian principles to justify coercive sterilization. Christian
worldview integration provides the ethical foundation for rejecting eugenics through the
doctrines of the imago Dei, divine sovereignty, and intrinsic human dignity.
Keywords: Mendelian genetics, pedigree analysis, eugenics, Buck v. Bell, Drosophila, imago Dei, Christian
bioethics
, ETHC 210 — Mendelian Genetics & Eugenics
1. Core Principles of Mendelian Genetics
Mendelian genetics, established through Gregor Mendel's pioneering experiments with Pisum
sativum (garden peas) in the 1850s and 1860s, provides the foundational framework for
understanding inheritance patterns in sexually reproducing organisms. Two fundamental concepts
underpin this framework. The genotype refers to an organism's specific allelic composition at a
given locus (e.g., AA, Aa, or aa), while the phenotype describes the observable physical or
biochemical characteristic resulting from that genotype (e.g., brown eyes, tall stature, or white
flowers). The distinction between genotype and phenotype is essential for understanding how
genetic information is transmitted and expressed across generations (Hartl & Jones, 2020).
1.1 Law of Segregation and Law of Independent Assortment
Mendel's Law of Segregation states that during gamete formation (meiosis), the two alleles
inherited from each parent for a given trait separate so that each gamete carries exactly one allele.
Upon fertilization, the offspring receives one allele from each parent, restoring the diploid pair.
This law explains why heterozygous parents can produce offspring with different genotypes and
why recessive traits can reappear after skipping a generation. Mendel's Law of Independent
Assortment extends this principle to multiple traits: alleles for different genes are distributed to
gametes independently of one another, provided the genes are located on different chromosomes or
are sufficiently far apart on the same chromosome to avoid linkage effects.
1.2 Test Crosses and Predicted Phenotypic Ratios
The test cross is a diagnostic breeding strategy used to determine the genotype of an
individual expressing a dominant phenotype (unknown: AA or Aa). By crossing this individual
with a homozygous recessive partner (aa) and analyzing the offspring, the unknown genotype can
be inferred: if any offspring display the recessive phenotype, the dominant parent must be
heterozygous (Aa). This method remains a staple of virtual genetics laboratories, including those
using Drosophila melanogaster as the model organism. For a monohybrid cross between two
heterozygotes (Aa x Aa), the expected phenotypic ratio is 3:1 (three dominant : one recessive). For
a dihybrid cross between double heterozygotes (AaBb x AaBb), the expected ratio is 9:3:3:1,
reflecting all combinations of dominant and recessive phenotypes across both traits.
Cross Type Parental Genotypes Expected Ratio Application
Monohybrid Aa x Aa 3:1 phenotypic Determining single-trait
dominance patterns
Dihybrid AaBb x AaBb 9:3:3:1 phenotypic Testing independent
assortment across two traits
Test Cross A_ x aa 1:1 if Aa; all dom. if AA Identifying unknown
dominant genotype
Back Cross Aa x AA or Aa x aa Varies Introgressing specific alleles
into breeding lines
Table 1. Common Genetic Cross Types, Expected Ratios, and Applications
Page 2
and the Bioethics of Eugenics
ETHC 210: Ethics in Biotechnology / Bioethics — Quiz 3
Liberty University
Department of Biology & Health Sciences
June 10, 2026
Abstract
This paper provides a comprehensive review of Mendelian genetics principles and their
application to pedigree analysis, virtual laboratory simulation using Drosophila
melanogaster, and the critical bioethical history of the eugenics movement. The
foundational principles of Mendel's Law of Segregation and Law of Independent
Assortment are examined alongside the analytical skills required for interpreting pedigree
charts and predicting phenotypic ratios from monohybrid and dihybrid crosses. The paper
addresses the dark history of eugenics—from Francis Galton's pseudoscientific framework
through the devastating Buck v. Bell Supreme Court ruling—and evaluates how early
geneticists misappropriated Mendelian principles to justify coercive sterilization. Christian
worldview integration provides the ethical foundation for rejecting eugenics through the
doctrines of the imago Dei, divine sovereignty, and intrinsic human dignity.
Keywords: Mendelian genetics, pedigree analysis, eugenics, Buck v. Bell, Drosophila, imago Dei, Christian
bioethics
, ETHC 210 — Mendelian Genetics & Eugenics
1. Core Principles of Mendelian Genetics
Mendelian genetics, established through Gregor Mendel's pioneering experiments with Pisum
sativum (garden peas) in the 1850s and 1860s, provides the foundational framework for
understanding inheritance patterns in sexually reproducing organisms. Two fundamental concepts
underpin this framework. The genotype refers to an organism's specific allelic composition at a
given locus (e.g., AA, Aa, or aa), while the phenotype describes the observable physical or
biochemical characteristic resulting from that genotype (e.g., brown eyes, tall stature, or white
flowers). The distinction between genotype and phenotype is essential for understanding how
genetic information is transmitted and expressed across generations (Hartl & Jones, 2020).
1.1 Law of Segregation and Law of Independent Assortment
Mendel's Law of Segregation states that during gamete formation (meiosis), the two alleles
inherited from each parent for a given trait separate so that each gamete carries exactly one allele.
Upon fertilization, the offspring receives one allele from each parent, restoring the diploid pair.
This law explains why heterozygous parents can produce offspring with different genotypes and
why recessive traits can reappear after skipping a generation. Mendel's Law of Independent
Assortment extends this principle to multiple traits: alleles for different genes are distributed to
gametes independently of one another, provided the genes are located on different chromosomes or
are sufficiently far apart on the same chromosome to avoid linkage effects.
1.2 Test Crosses and Predicted Phenotypic Ratios
The test cross is a diagnostic breeding strategy used to determine the genotype of an
individual expressing a dominant phenotype (unknown: AA or Aa). By crossing this individual
with a homozygous recessive partner (aa) and analyzing the offspring, the unknown genotype can
be inferred: if any offspring display the recessive phenotype, the dominant parent must be
heterozygous (Aa). This method remains a staple of virtual genetics laboratories, including those
using Drosophila melanogaster as the model organism. For a monohybrid cross between two
heterozygotes (Aa x Aa), the expected phenotypic ratio is 3:1 (three dominant : one recessive). For
a dihybrid cross between double heterozygotes (AaBb x AaBb), the expected ratio is 9:3:3:1,
reflecting all combinations of dominant and recessive phenotypes across both traits.
Cross Type Parental Genotypes Expected Ratio Application
Monohybrid Aa x Aa 3:1 phenotypic Determining single-trait
dominance patterns
Dihybrid AaBb x AaBb 9:3:3:1 phenotypic Testing independent
assortment across two traits
Test Cross A_ x aa 1:1 if Aa; all dom. if AA Identifying unknown
dominant genotype
Back Cross Aa x AA or Aa x aa Varies Introgressing specific alleles
into breeding lines
Table 1. Common Genetic Cross Types, Expected Ratios, and Applications
Page 2
