Genetic Principles: Inheritance, Chromosome Changes, Sex Determination

Posted on Sep 5, 2025 in Biology

Mendelian Genetics: Independent Assortment & Probability

This section delves into Mendel’s fundamental principles and the statistical tools essential for genetic analysis.

Independent Assortment: Genetic Diversity

Mendel’s Law of Independent Assortment explains how different genes on non-homologous chromosomes segregate randomly during meiosis, leading to significant genetic diversity. This process can generate 2n possible gametes, where ‘n’ is the number of heterozygous gene pairs.

Laws of Probability in Genetics

Genetic ratios can be predicted using two key probability rules:
  • Product Law: Calculates the probability of two or more independent events occurring together (e.g., the chance of inheriting specific alleles from different genes).
  • Sum Law: Determines the probability of mutually exclusive outcomes occurring (e.g., the chance of an offspring having one of several possible genotypes).

Chi-Square Analysis: Testing Genetic Hypotheses

Chi-square analysis is a statistical test used to evaluate whether observed genetic results significantly differ from expected results. A p value less than 0.05 (p < 0.05) typically leads to the rejection of the null hypothesis, suggesting that the observed deviation is not due to random chance.

Pedigree Analysis: Mapping Inheritance Patterns

Pedigrees are visual charts that map the inheritance patterns of traits or diseases across generations within a family. Standardized symbols are used: circles represent females (♀) and squares represent males (♂). Examples include tracking recessive conditions like albinism or dominant conditions like Huntington’s disease.


Modifications of Mendelian Inheritance

This section expands on inheritance patterns that deviate from classic Mendelian ratios, showcasing the complexity of genetic expression.

Non-Mendelian Allele Expression

  • Incomplete Dominance: Neither allele is completely dominant, resulting in a blended phenotype in heterozygotes (e.g., pink snapdragons from red and white parents).
  • Codominance: Both alleles are expressed simultaneously and distinctly in heterozygotes (e.g., MN blood groups in humans, where both M and N antigens are present).
  • Multiple Alleles: More than two alleles exist for a single gene within a population, leading to a wider range of phenotypes (e.g., ABO blood types in humans, determined by IA, IB, and i alleles).
  • Lethal Alleles: Certain alleles can cause the death of an organism, often before birth, altering expected Mendelian ratios (e.g., the yellow coat color allele in mice, which is dominant for coat color but recessive lethal).

Gene Interactions & Epistasis

  • Epistasis: One gene’s alleles mask or modify the expression of alleles at a different gene locus (e.g., the Bombay phenotype, where a gene masks the expression of ABO blood group antigens).
  • Gene Interactions: Two or more genes interact to produce a single phenotype, often resulting in modified Mendelian ratios (e.g., the 9:7 ratio observed in sweet pea flower color).

Complex Genetic Phenomena

  • Pleiotropy

    One gene affects multiple distinct phenotypic traits (e.g., the SRY gene triggers testes development and also influences secondary male sexual characteristics).

  • Heterogeneous Traits

    The same phenotype can result from mutations in different genes (e.g., hereditary deafness, which can be caused by mutations in over 100 different genes).

Complementation Analysis

This technique tests whether two mutations causing similar phenotypes are located in the same gene or in different genes.
  • If offspring from a cross between two individuals with similar mutant phenotypes show a wild-type phenotype, the mutations are in different genes (complementation occurred).
  • If no complementation occurs (offspring still show the mutant phenotype), the mutations are alleles of the same gene.


Chromosome Number Variations

This section examines significant variations in chromosome number, which can have profound effects on an organism’s development and viability.

Aneuploidy: Abnormal Chromosome Counts

Aneuploidy refers to an abnormal number of chromosomes, typically resulting from nondisjunction—the failure of homologous chromosomes or sister chromatids to separate properly during meiosis. This can lead to:
  • Monosomy: The absence of one chromosome from a pair (e.g., Turner syndrome, 45,X, characterized by a single X chromosome).
  • Trisomy: The presence of an extra chromosome (e.g., Down syndrome, 47,XX or XY,+21; Patau syndrome, 47,XX or XY,+13; Edwards syndrome, 47,XX or XY,+18). These conditions often cause severe developmental issues.

Polyploidy: Extra Chromosome Sets

Polyploidy involves the presence of extra complete sets of chromosomes. While rare and often lethal in animals, it is common and often beneficial in plants.
  • Autopolyploidy: Extra chromosome sets originate from the same species (e.g., tetraploid potatoes).
  • Allopolyploidy: Extra chromosome sets result from hybridization between two different species (e.g., Triticale, a hybrid of wheat and rye).
Plants with polyploidy often exhibit desirable traits like larger fruits (e.g., seedless watermelon) and increased hardiness.

Mechanisms & Significance of Chromosome Number Changes

Mechanisms contributing to these variations include nondisjunction (which is linked to increased maternal age in cases like Down syndrome) and induced chromosomal doubling (e.g., using colchicine). Polyploids often propagate asexually, as odd-numbered sets (e.g., triploids) are typically sterile due to difficulties in meiosis, whereas even-numbered sets (e.g., tetraploids) can form viable gametes. These mutations underscore the role of genetic diversity in evolution and agriculture.


Structural Chromosome Mutations

This section focuses on changes in the composition and arrangement of chromosomes, known as structural chromosomal mutations, and their impact on gene function and phenotype.

Types of Structural Mutations

These mutations arise from breaks or errors during recombination or meiosis, disrupting gene function:

  • Deletions

    Loss of a segment of a chromosome (e.g., Cri du Chat syndrome, caused by a deletion on the short arm of chromosome 5 (5p−), leading to a characteristic cat-like cry and developmental delays).

  • Duplications

    Repetition of a segment of a chromosome.

  • Inversions

    A segment of a chromosome is reversed end-to-end.

  • Translocations

    A segment of one chromosome moves to a different chromosome.

Fragile Sites & Associated Disorders

Fragile sites are specific regions on chromosomes that are prone to breakage under certain cellular stresses (e.g., folate deficiency). These sites are linked to various genetic disorders:

  • Fragile X Syndrome

    Caused by an expansion of CGG trinucleotide repeats (more than 230 repeats) in the FMR1 gene on the X chromosome. This leads to methylation and silencing of the gene, resulting in intellectual disability and macroorchidism.
  • Other fragile regions (e.g., FRA16D in the WWOX gene) are correlated with cancer development through the inactivation of tumor suppressor genes.

Mechanisms & Clinical Impact of Structural Changes

Mechanisms such as trinucleotide expansions, translocations (e.g., familial Down syndrome), and epigenetic silencing highlight how structural changes can profoundly alter gene expression. These alterations often lead to phenotypic abnormalities or diseases, emphasizing the critical interplay between chromosomal integrity and genetic disorders.


Sex Determination & Sex Chromosomes

This section examines the diverse mechanisms by which sex is determined across different species, focusing on both genetic and environmental factors.

Genetic Sex Determination Systems

  • Human XX/XY System & SRY Gene

    In humans, the XX/XY system dictates sex. The presence of the SRY gene (Sex-determining Region Y) on the Y chromosome is crucial; it initiates male development.

  • Other Genetic Systems

    Other species utilize different genetic systems, such as the ZZ/ZW system in birds and some reptiles (where ZW is female, ZZ is male) or the X:A ratio in Drosophila (fruit flies), where the ratio of X chromosomes to autosomes determines sex, with the Y chromosome primarily ensuring male fertility.

Environmental Sex Determination (TSD)

Some species, like turtles and crocodiles, exhibit temperature-dependent sex determination (TSD), where the incubation temperature of eggs determines the sex of the offspring.

Sex Chromosome Abnormalities

Chromosomal abnormalities arising from nondisjunction of sex chromosomes can lead to specific syndromes:

  • Klinefelter Syndrome (XXY)

    Individuals have an extra X chromosome, often resulting in infertility and developmental issues.

  • Turner Syndrome (45,X)

    Individuals have only one X chromosome, leading to infertility and various developmental challenges.

Broader Implications of Sex Determination

The chapter also explores pseudoautosomal regions (regions on X and Y chromosomes that pair during meiosis), hormonal regulation of sex development, and the evolutionary significance of diverse sex-determination strategies, highlighting their medical and biological implications.