Patterns of Inheritance: Mendelian Genetics and Beyond

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Patterns of Inheritance

Introduction to Genetics

Genetics is the scientific study of heredity, the process by which traits are passed from one generation to the next. Modern genetic testing has made it possible for individuals to learn about their genetic makeup, reflecting the deep interest in understanding inheritance.

Heredity: Transmission of traits from parents to offspring.
Genetics: The study of heredity and variation in organisms.
Character: A heritable feature that varies among individuals (e.g., flower color).
Trait: A variant of a character (e.g., purple or white flowers).

Ancient Theories of Inheritance

Early theories, such as Hippocrates's "pangenes" and the blending hypothesis, were rejected because they could not explain the reappearance of traits or the mechanism of inheritance.

Pangenes: Incorrect idea that particles from all parts of the body are passed to offspring.
Blending Hypothesis: Suggested offspring are a blend of parental traits; rejected due to the reappearance of traits in later generations.

Mendel’s Experiments and Laws

Gregor Mendel, through experiments with pea plants, established foundational principles of inheritance. He proposed that genes exist in alternative forms called alleles and that inheritance follows specific laws.

True-breeding: Organisms that produce offspring identical to themselves.
Hybrid: Offspring of two different true-breeding parents.
P Generation: Parental generation.
F1 Generation: First filial generation, offspring of P generation.
F2 Generation: Second filial generation, offspring of F1.

Mendel’s Law of Segregation

This law describes the inheritance of a single character and explains how alleles separate during gamete formation.

Alleles: Alternative versions of a gene.
Homozygous: Two identical alleles for a gene.
Heterozygous: Two different alleles for a gene.
Dominant allele: Determines phenotype when present.
Recessive allele: Has no effect unless both alleles are recessive.
Law of Segregation: Allele pairs separate during gamete formation, so each gamete carries only one allele for each gene.

Punnett Square: A diagram used to predict the outcome of a genetic cross.

Example: In a cross between two heterozygous pea plants (Pp), the F2 generation shows a 3:1 ratio of dominant to recessive phenotypes.

Homologous Chromosomes and Alleles

Homologous chromosomes carry alleles for the same genes at corresponding loci. The physical basis for allele segregation is the separation of homologous chromosomes during meiosis.

Homologous chromosomes: Chromosome pairs with the same genes at the same loci.
Meiosis: The process that ensures segregation of alleles.

Mendel’s Law of Independent Assortment

This law states that alleles of different genes assort independently during gamete formation, revealed by dihybrid crosses.

Monohybrid cross: Cross between individuals heterozygous for one character.
Dihybrid cross: Cross between individuals heterozygous for two characters.
Law of Independent Assortment: Alleles of different genes segregate independently.

Testcrosses

A testcross is used to determine the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual.

Testcross: Reveals unknown genotype based on offspring phenotypes.

Probability in Genetics

Genetic outcomes can be predicted using probability rules.

Rule of Multiplication: Probability of two independent events both occurring is the product of their probabilities.
Rule of Addition: Probability of an event occurring in alternative ways is the sum of their probabilities.

Example: Probability of offspring with genotype AA BB CC from a cross between AA Bb CC and Aa Bb Cc:

For each gene, calculate probability separately and multiply:

Pedigrees and Human Genetics

Family pedigrees are used to track inheritance patterns in humans and determine genotypes.

Pedigree: Diagram showing family relationships and inheritance of traits.

Single-Gene Disorders

Many human traits and disorders are controlled by single genes, inherited as dominant or recessive traits.

Recessive disorders: Require two recessive alleles (e.g., cystic fibrosis).
Dominant disorders: Require only one dominant allele (e.g., Huntington’s disease).
Carrier: Heterozygous individual for a recessive disorder, phenotypically normal.

Disorder	Inheritance	Example
Cystic Fibrosis	Recessive	CFTR gene mutation
Huntington's Disease	Dominant	HTT gene mutation
Sickle-cell Disease	Recessive	HBB gene mutation
Additional info: Table 9.9 lists more autosomal disorders.

Genetic Testing and Screening

Technologies such as carrier screening, fetal testing (amniocentesis, chorionic villus sampling), fetal imaging, and newborn screening provide information for reproductive decisions but raise ethical concerns.

Amniocentesis: Sampling amniotic fluid for genetic testing.
Chorionic Villus Sampling (CVS): Sampling placental tissue for genetic testing.
Ultrasound Imaging: Non-invasive imaging of fetus.

Variations on Mendel’s Laws

Incomplete Dominance

In incomplete dominance, the phenotype of heterozygotes is intermediate between the two parental phenotypes.

Complete dominance: One allele completely masks the other.
Incomplete dominance: Heterozygote shows intermediate phenotype (e.g., red and white snapdragons produce pink flowers).

Multiple Alleles and Codominance

Some genes have more than two alleles, and in codominance, both alleles are fully expressed in heterozygotes.

Multiple alleles: More than two possible alleles for a gene (e.g., ABO blood group).
Codominance: Both alleles are expressed (e.g., IAIB genotype results in AB blood type).

Blood Type	Genotype
A	IAIA or IAi
B	IBIB or IBi
AB	IAIB
O	ii

Pleiotropy

Pleiotropy occurs when one gene affects multiple phenotypic characters. Sickle-cell disease is an example, affecting hemoglobin, red blood cell shape, and resistance to malaria.

Pleiotropy: One gene influences multiple traits.
Sickle-cell disease: Codominant at molecular level; carriers have resistance to malaria.

Polygenic Inheritance

Polygenic inheritance involves multiple genes contributing to a single phenotypic character, such as human height or skin color.

Polygenic inheritance: Additive effect of two or more genes on a single trait.
Example: Human height is influenced by several genes.

Environmental Influence

Many traits are influenced by both genetic and environmental factors. Mendel was able to ignore environmental effects in pea plants due to controlled conditions.

Environmental factors: Can affect expression of genetic traits.

The Chromosomal Basis of Inheritance

Chromosome Theory of Inheritance

Genes are located on chromosomes, which undergo segregation and independent assortment during meiosis, providing the physical basis for Mendel’s laws.

Chromosome theory: Genes occupy specific loci on chromosomes.
Segregation: Separation of homologous chromosomes in meiosis I.
Independent assortment: Random orientation of chromosome pairs in metaphase I.

Linked Genes and Crossing Over

Genes located close together on the same chromosome (linked genes) tend to be inherited together. Crossing over during meiosis can separate linked genes, producing recombinant gametes.

Linked genes: Located near each other on the same chromosome.
Crossing over: Exchange of genetic material between homologous chromosomes.
Recombination frequency: Percentage of recombinant offspring among total.

Genetic map: Ordered list of loci based on recombination frequencies (linkage map).

Sex Chromosomes and Sex-Linked Genes

Sex Determination

In mammals, sex is determined by the presence of X and Y chromosomes. The SRY gene on the Y chromosome triggers male development. Other species may use different systems or environmental factors.

XX: Female
XY: Male
SRY gene: Initiates development of testes.
Environmental sex determination: In some reptiles, temperature during incubation determines sex.

Sex-Linked Genes and Disorders

Genes located on sex chromosomes exhibit unique inheritance patterns. Most X-linked disorders are recessive and affect males more frequently.

Sex-linked gene: Located on a sex chromosome (X or Y).
X-linked recessive: Males affected if they inherit one recessive allele; females must inherit two.
Examples: Hemophilia, color blindness.

Y Chromosome and Human Evolution

The Y chromosome is passed from father to son and can be used to trace paternal ancestry and human evolutionary history.

Y chromosome: Used in studies of human male lineage.
Example: Genetic similarity among central Asian males traced to Genghis Khan.

Summary Table: Key Terms and Concepts

Term	Definition
Allele	Alternative form of a gene
Genotype	Genetic makeup of an organism
Phenotype	Observable traits of an organism
Homozygous	Two identical alleles for a gene
Heterozygous	Two different alleles for a gene
Dominant	Allele that determines phenotype
Recessive	Allele masked by dominant allele
Pleiotropy	One gene affects multiple traits
Polygenic	Trait controlled by multiple genes
Linked genes	Genes located close together on a chromosome
Sex-linked	Gene located on a sex chromosome

Additional info:

Checkpoint questions throughout the chapter encourage critical thinking and application of concepts.
Figures referenced in the notes illustrate key concepts such as Punnett squares, pedigrees, and chromosomal behavior.
Table 9.9 (inferred) lists autosomal disorders and their inheritance patterns.