Unit 3: Sex Determination and Extranuclear Inheritance

1. Mechanisms of Sex Determination

Sex determination is the biological system that analyzes developmental traits and determines whether an organism will develop into a male, female, or hermaphroditic individual. Organisms utilize chromosomal, environmental, or hormonal signals to control this process.

Chromosomal Mechanisms of Sex Determination

Chromosomal sex determination relies on the presence, combination, or ratio of specific sex chromosomes (allosomes) compared to autosomes.

Definition: Heterogametic sex produces two different types of gametes with respect to sex chromosomes, whereas Homogametic sex produces only one uniform type of gamete.

• XX-XY System:
  • Mechanism: Females are homogametic (XX), producing gametes with one X chromosome. Males are heterogametic (XY), producing 50% X-bearing and 50% Y-bearing sperm.
  • Examples: Humans, mammals, and Drosophila melanogaster.
• XX-XO System:
  • Mechanism: Females possess two X chromosomes (XX), while males possess only one X chromosome and no Y chromosome (XO).
  • Examples: Grasshoppers, cockroaches, and true bugs (Order Hemiptera).
• ZZ-ZW System:
  • Mechanism: Females are heterogametic (ZW), producing two types of eggs (Z and W). Males are homogametic (ZZ), producing uniform Z-bearing sperm.
  • Examples: Birds, reptiles, some fishes, and butterflies.
• ZZ-ZO System:
  • Mechanism: Females have a single Z chromosome (ZO), while males possess two Z chromosomes (ZZ).
  • Examples: Certain species of moths and butterflies.
• Genic Balance Theory (Calvin Bridges):

  Developed using Drosophila melanogaster, this theory states that sex is determined not simply by the Y chromosome, but by the ratio of the number of X chromosomes (X) to sets of autosomes (A).

  Formula / Ratio: Sex Index Ratio = X / A

  • X / A = 1.0: Normal Female (e.g., 2X : 2A or 3X : 3A)
  • X / A = 0.5: Normal Male (e.g., 1X : 2A)
  • X / A > 1.0: Metafemale / Superfemale (e.g., 3X : 2A, ratio = 1.5)
  • X / A < 0.5: Metamale / Supermale (e.g., 1X : 3A, ratio = 0.33)
  • 0.5 < X / A < 1.0: Intersex (e.g., 2X : 3A, ratio = 0.67)

  Observation: In Drosophila, the Y chromosome is required for male fertility, but it does not determine male sex.

• Haplodiploidy:

  Sex is determined by the number of chromosome sets an individual receives. Unfertilized haploid eggs develop into fertile males (drones) via parthenogenesis. Fertilized diploid eggs develop into females (queens or worker bees).

  Examples: Honeybees, ants, and wasps (Order Hymenoptera).

Environmental Mechanism of Sex Determination

In environmental sex determination (ESD), non-genetic cues encountered during embryonic or larval development establish the sex of the organism.

• Temperature-Dependent Sex Determination (TSD):

  Incubation temperature during a critical thermosensitive period (TSP) determines gonad differentiation.

  • Pattern I (FTM - Female High, Male Low): Warm temperatures produce females; cooler temperatures produce males (e.g., many freshwater turtles).
  • Pattern II (MTF - Male Intermediate, Female Extremes): Intermediate temperatures produce males; both high and low extreme temperatures produce females (e.g., crocodiles, alligators, snapping turtles).
• Location / Position-Dependent Sex Determination:

  In the marine worm Bonellia viridis, larvae that settle on the ocean floor develop into adult females. Larvae that land on or enter the proboscis of an existing adult female are exposed to chemicals and develop into tiny parasitic males living inside the female body.

Hormonal Mechanism of Sex Determination

Sex hormones circulating during embryonic or larval differentiation modify phenotypic sex expression, regardless of underlying genotype.

• Free-Martin Condition in Cattle:

  When fraternal (dizygotic) twin calves of opposite sexes share a single placenta, vascular anastomoses allow blood fusion between them.

  Anti-Müllerian Hormone (AMH) and testosterone produced early by the male twin enter the female twin's circulation. This suppresses female reproductive tract development, causing the female calf to be born as a sterile, masculinized individual called a freemartin.

• Hormonal Sex Reversal in Amphibians and Fishes:

  Administering exogenous estrogens to genetically male (XY or ZZ) larvae converts them into functional females. Conversely, administration of androgens to genetically female larvae causes them to develop into functional males.

2. Sex-Linked, Sex-Influenced, and Sex-Limited Inheritance

Traits associated with biological sex can be divided into three distinct inheritance modes depending on whether genes are located on sex chromosomes or autosomes, and how hormones alter their expression.

Sex-Linked Inheritance

Sex-linked traits are determined by genes located on the sex chromosomes (X or Y chromosomes in mammals).

Important Concept: Criss-cross inheritance refers to the transmission of a trait from a affected father to his phenotypic normal carrier daughter, who then passes it to 50% of her sons.

• X-Linked Recessive Inheritance:
  • Males are hemizygous for X-linked genes (they carry only one X chromosome). A single recessive mutant allele on the X chromosome results in the mutant phenotype in males.
  • Females express the trait only if they carry two copies of the mutant allele (homozygous recessive).
  • Examples: Hemophilia A, Red-Green Color Blindness, Duchenne Muscular Dystrophy.
  • Exam Note: Carrier females (X_N X_n) crossed with normal males (X_N Y) pass the mutant trait to half of their sons.
• X-Linked Dominant Inheritance:
  • Expressed in both heterozygous females and hemizygous males.
  • Affected males transmit the trait to 100% of their daughters, but 0% of their sons.
  • Example: Hypophosphatemic Rickets (Vitamin D-resistant rickets).
• Y-Linked (Holandric) Inheritance:
  • Genes present exclusively on the non-homologous region of the Y chromosome.
  • Transmitted strictly from father to son (100% male offspring affected).
  • Examples: Hypertrichosis of the ear pinna, SRY gene (Sex-determining Region Y).

Sex-Influenced Inheritance

Sex-influenced traits are controlled by autosomal genes, but the expression of alleles depends on the biological sex and sex hormone profile (testosterone/estrogen levels) of the individual.

• Mechanism: An allele behaves as dominant in one sex but as recessive in the other.
• Example: Pattern Baldness in Humans

  Let B be the baldness allele and b be the non-bald allele:

  • Genotype BB: Bald in both males and females.
  • Genotype Bb: Bald in males (high testosterone converts expression to dominant phenotype), non-bald in females.
  • Genotype bb: Non-bald in both males and females.
• Example: Horns in Sheep

  In Dorset/Rambouillet sheep crosses, the horned allele (H) acts as dominant in males, producing horns in Hh males, but acts as recessive in females, requiring HH genotype for females to develop horns.

Sex-Limited Inheritance

Sex-limited traits are controlled by autosomal genes that are expressed in only one sex due to anatomical or physiological limitations. Expression in the non-expressing sex is zero (0%).

• Mechanism: Both sexes inherit the autosomal genes equally, but the gene remains completely unexpressed in one sex.
• Examples:
  • Milk production and milk fat composition in dairy cows (bulls pass these autosomal genes to daughters but never produce milk themselves).
  • Egg production, shell quality, and clutch size in poultry.
  • Cock-feathering vs. Hen-feathering in fowl: Autosomal recessive genotype (h/h) causes long, pointed cock-feathering only in male chickens; females carrying (h/h) remain hen-feathered due to estrogen suppression.

3. Maternal Effects and Cytoplasmic Inheritance

Non-Mendelian inheritance patterns frequently arise when nuclear factors act prior to fertilization or when traits are encoded by cytoplasmic organelles.

Maternal Effects

A maternal effect occurs when the phenotype of the offspring is determined exclusively by the genotype of the mother, independent of the offspring's own genotype or paternal genotype.

Cause: Storage of maternal gene products (mRNA, proteins) in the egg cytoplasm prior to meiosis and fertilization. These pre-stored maternal products direct early embryonic development.

• Classic Example: Shell Coiling in Snail (Limnaea peregra)
  • Shell coiling direction is controlled by nuclear autosomal genes: Dextral (right-handed coiling, allele D) is dominant over Sinistral (left-handed coiling, allele d).
  • The orientation of the mitotic spindle during the second cleavage division is directed by maternal proteins in the egg cytoplasm.
  • Cross 1: DD (Female, Dextral) x dd (Male, Sinistral)
    • F1 Genotype: Dd
    • F1 Phenotype: Dextral (because mother was DD)
    • F2 Genotypes: 1 DD : 2 Dd : 1 dd
    • F2 Phenotypes: 100% Dextral (because maternal genotype was Dd)
    • F3 Phenotypes: 3 Dextral (from DD and Dd mothers) : 1 Sinistral (from dd mothers)
  • Cross 2 (Reciprocal): dd (Female, Sinistral) x DD (Male, Dextral)
    • F1 Genotype: Dd
    • F1 Phenotype: Sinistral (because mother was dd)
    • F2 Phenotypes: 100% Dextral (because maternal F1 genotype was Dd)

Cytoplasmic Inheritance

Cytoplasmic inheritance (extranuclear or organelle inheritance) refers to the transmission of genes located in cytoplasmic organelle DNA, specifically Mitochondria (mtDNA) and Chloroplasts (cpDNA).

Definition: Cytoplasmic inheritance is non-Mendelian inheritance where traits are transmitted uniparentally via the maternal line, as the female gamete supplies virtually all cytoplasm and organelle genomes to the zygote.

• Key Characteristics of Cytoplasmic Inheritance:
  1. Reciprocal crosses yield completely different phenotypic results.
  2. Progeny always exhibit the maternal phenotype (maternal inheritance).
  3. F2 generation does not show standard Mendelian ratios (3:1 or 9:3:3:1).
  4. Traits cannot be mapped to nuclear linkage groups.
• Example 1: Plastid Inheritance in Four O'Clock Plant (Mirabilis jalapa)

  Discovered by Carl Correns in 1909. Mirabilis jalapa has branches with green leaves, white leaves, or variegated (patchy) leaves.

  • Pollen from any branch crossed onto flowers of a Green branch yields 100% Green offspring.
  • Pollen from any branch crossed onto flowers of a White branch yields 100% White offspring (which die due to lack of chlorophyll).
  • Pollen from any branch crossed onto flowers of a Variegated branch yields Green, White, and Variegated offspring in variable, non-Mendelian proportions.
  • Explanation: Egg cells from variegated branches randomly receive normal chloroplasts, mutant chloroplasts, or a mixture of both during cytoplasmic division.
• Example 2: Poky Mutant in Neurospora crassa

  Slow-growing mutant strain ("poky") of bread mold caused by a mutation in mitochondrial ribosomal RNA, resulting in defective cytochrome synthesis. Maternal parent carrying poky transmits the slow-growth trait to 100% of ascospore progeny.

4. Mitochondrial Mutations

Mitochondria contain their own circular, double-stranded DNA genome (mtDNA) that replicates independently of nuclear DNA. Because mitochondrial genes code for essential electron transport chain proteins and tRNAs/rRNAs, mitochondrial mutations impair cellular respiration and ATP generation.

Characteristics of Mitochondrial DNA (mtDNA)

• Structure: Circular, double-stranded, compact DNA with no introns.
• Lack of Histones: Vulnerable to oxidative damage caused by reactive oxygen species (ROS) produced during oxidative phosphorylation.
• High Mutation Rate: Mitochondrial mutation rate is 10 to 20 times higher than that of nuclear DNA due to limited DNA repair mechanisms.

Types and Examples of Mitochondrial Mutations

• 1. Petite Mutants in Yeast (Saccharomyces cerevisiae):

  Petite mutants form unusually small colonies on glucose medium because respiratory deficiency forces them to rely solely on anaerobic fermentation.

  • Segregational Petites: Caused by mutations in nuclear genes encoding mitochondrial proteins. Follow standard Mendelian inheritance (1 petite : 1 wild-type ratio in tetrad analysis).
  • Neutral Petites: Caused by total loss or major deletion of mitochondrial DNA. When crossed with wild-type, the wild-type mitochondrion dominates, yielding 100% normal progeny.
  • Suppressive Petites: Caused by deletion mutations in mtDNA that replicate faster than wild-type mtDNA. When crossed with wild-type, progeny are predominantly petite.
• 2. Cytoplasmic Male Sterility (CMS) in Plants:

  CMS is caused by mitochondrial gene mutations that prevent functional pollen development without altering female fertility. CMS is widely utilized in commercial agriculture for cost-effective hybrid seed production (e.g., maize, rice).

  Restorer of Fertility (Rf): Nuclear genes (Rf genes) can suppress mitochondrial CMS and restore pollen production in hybrid plants.

• 3. Human Mitochondrial Disorders:

  Mitochondrial genetic diseases are inherited exclusively through the maternal line. They predominantly affect tissues with high energy demands (nervous system, skeletal muscle, heart).

  • Leber's Hereditary Optic Neuropathy (LHON): Caused by missense mutations in mtDNA genes encoding NADH dehydrogenase subunits (Complex I). Leads to sudden central vision loss in young adults.
  • Myoclonic Epilepsy with Ragged Red Fibers (MERRF): Caused by a point mutation in the mitochondrial tRNA-Lysine gene. Characterized by muscle spasms, ataxia, and irregular red muscle fibers visible under microscopic staining.
  • Kearns-Sayre Syndrome (KSS): Caused by large deletion mutations in mtDNA, leading to paralysis of eye muscles, pigmentary degeneration of the retina, and cardiac conduction blocks.

Key Principles of Mitochondrial Inheritance

Heteroplasmy vs. Homoplasmy: Heteroplasmy is the condition in which a cell or tissue contains a mixture of normal (wild-type) and mutant mitochondrial genomes. Homoplasmy refers to a uniform cell population containing exclusively normal or exclusively mutant mtDNA.

• Replicative Segregation:

  During cell division, mitochondria partition randomly into daughter cells. Over multiple cell divisions, the proportion of mutant mtDNA in daughter cells can drift randomly toward mostly normal or mostly mutant genotypes.

• Threshold Effect:

  A specific proportion of mutant mtDNA (typically 60% to 80%) must be exceeded before cellular respiratory failure occurs and clinical symptoms manifest in tissues.

Common Exam Mistake to Avoid: Do not confuse Maternal Effect with Maternal Inheritance (Cytoplasmic Inheritance). Maternal effect is driven by nuclear genes of the mother acting through egg cytoplasm, leading to delayed Mendelian segregation in the F3 generation. Cytoplasmic inheritance is driven by organelle DNA (mtDNA/cpDNA), passed directly from mother to offspring without involvement of nuclear chromosomes.