Sexual Reproduction and Inheritance

Development and its Significance

  • Development is crucial for understanding:
    • Normal biological processes.
    • Abnormalities.
    • Positioning of adult structures.

Simultaneous Processes in Development

  • Multiple processes occur simultaneously:
    • Gastrulation.
    • Neurulation.
    • Organogenesis.
  • Tissues form concurrently, sometimes using the same genes.

Heart Hand Syndrome (Holt-Oram Syndrome)

  • Example of developmental biology importance:
    • Thumb looks like a finger.
    • Enlarged heart (atrial defect).
  • Indicates:
    • Thumb and heart form at the same time.
    • Involves the same gene (TBX5).
  • TBX5:
    • Involved in thumb formation.
    • Involved in heart development.
    • Its function varies based on the environment.
  • Clinical relevance: Abnormal thumb shape may indicate heart issues.

Impact of Developmental Biology on Society

  • In vitro fertilization (IVF):
    • Result of developmental biology research.
    • Used by a significant number of couples (mentioned 25%, but confirm).
  • Understanding foods to avoid during pregnancy to prevent birth defects.
  • Screening for genetic mutations.
  • Future possibilities:
    • Tissue regeneration.
    • Better disease models.
    • Organ harvesting and transplantation.
    • Embryonic stem cells.
    • Cloning (ethical considerations).

Fertilization

  • Activates the egg to begin development.
  • Restores the diploid number of chromosomes (46 in humans, 23 pairs).
  • Sperm nucleus migrates to the female pronucleus to form a zygote.
  • Timing varies: 12 minutes in sea urchins, hours in humans.
  • Initiates signaling and molecular distribution among blastomeres.

Cleavage

  • Zygote undergoes divisions (cleavages).
  • No overall growth of the embryo until gastrulation.

Types of Sexual Reproduction

  • Diplomatic (vertebrates, including humans):
    • Two haploid gametes fuse to form a diploid individual.
  • Other organisms (plants, fungi) can have diploid gametes and haploid individuals.
  • Vertebrate method considered successful for dominance.

Sex Determination in Humans

  • Determined at conception.
  • SRY region:
    • Specific region on the Y chromosome.
    • Without SRY, cannot be male.
  • Default vertebrate state: female.

SRY Gene and Sex Determination

  • Females typically have Wnt4, which activates DAX1 and other genes, leading to ovary development.
  • SRY gene inhibits Wnt4 expression.
  • SRY gene allows activation of SOX9, leading to testes development.
  • Mutations can lead to:
    • XX individuals with the SRY region (rare).
    • XY individuals missing the SRY gene, resulting in a non-male phenotype.

Sex Determination in Other Species

  • Birds (Z and W chromosomes):
    • Default state: male.
    • Femaleness requires overriding maleness.
  • Mice and fish: Similar to humans, default state is female.
  • Commonality: Region on one chromosome that triggers male development.

Environmental Influences on Sex Determination

  • Turtles, crocodiles, and alligators: Temperature-dependent sex determination.
  • Thermosensitive period: Influences sex determination.
  • Example: Turtles: Cold temperatures favor male development, warm temperatures favor female development.
  • Mechanism: Temperature affects SOX9 expression.

Why Sexual Reproduction?

  • Disadvantages:
    • Need to find a partner.
    • Decreased reproductive capacity with age.
  • Benefits: The most significant is Variability
    • Genetic crossover during meiosis leads to unique individuals.
    • The number of combinations of genotypes and crossover events is very large, approximately 223,0002^{23,000}
    • This crossover ensures no two people (except identical twins) look the same.

Meiosis and Genetic Variation

  • Male germ cell: Sperm.
  • Female germ cell: Secondary oocyte.
  • Meiosis: Reduces chromosome number to haploid (23).
  • Fertilization: Restores diploid number (46).
  • Sex is determined at conception.
  • Meiosis separates homologous chromosomes and allows genetic crossover.

Stages of Meiosis

  • Prophase: Chromosomes materialize and align.
  • Metaphase: Chromosomes line up for cell division.
  • Cell division: Chromosomes split into pairs, reducing chromosome number.
  • Separation of chromatids: Results in four cells, each with one set of chromosomes (haploid).

Crossover Event in Meiosis I

  • Occurs in the first metaphase.
  • Parts of chromosomes exchange (e.g., blue to red).
  • Happens every time, in all chromosomes.
  • Results in haploid cells with different chromosome combinations.
  • Contributes to massive variation when combined with gametes from another partner.
  • Ensures siblings share traits but are not identical.

Differences in Meiosis Between Males and Females

  • Males: Meiosis occurs continuously during sperm production.
  • Females: Meiosis I occurs in early development, meiosis II only completes upon fertilization.
  • Eggs remain arrested between meiosis I and II for a long time.

Crossing Over and Recombination

  • Crossing over: Exchange of genetic information, leading to recombinant chromatids.
  • Increases genetic variation of gametes.
  • Consequences:
    • Offspring are never identical.
    • Variability within the population.
    • Potential for malformations if crossing over is uneven.

Inheritance of Characteristics

  • Crossing over allows changes in heritability.
  • Genetic maps can be created based on recombination frequency.
  • Recombination frequency:
    • Tends to be higher at the ends of chromosomes.
    • Can be used to map genes based on observed characteristics.

Genetic Maps and Recombination Frequency

  • High recombination frequency: Genes are further apart on the chromosome.
  • Low recombination frequency: Genes are close together (linked).
  • Linked genes: Lower chance of recombination, reduces mutation risk.

Alfred Sturtevant

  • Proposed using recombination frequency to map genes on chromosomes.
  • Genes are located close together on the chromosome are considered linked and have a lower frequency.
  • Chromosomes are made of thousands and thousands of genes.

Sex-Linked Inheritance

  • Two types:
    • Recessive: Requires the genetic variation on both X chromosomes (females) or a single X chromosome (males) to manifest the condition.
    • Dominant: A mutation on a single X chromosome is enough to cause the condition.
  • Examples of X-linked conditions: Hemophilia and muscular dystrophy.

Inheritance Patterns

  • Father with X-linked recessive condition:
    • Daughters will be carriers.
    • Sons will be unaffected (Y chromosome is normal).
  • Mother with X-linked recessive condition:
    • 50% chance of passing it to sons.
    • 50% chance of passing carrier status to daughters.

X-Linked Inheritance Examples

  • Males are hemizygous for genes on the X chromosome because they can get it from their mom or their dad.
  • Dominant female with hemizygous recessive male: All offspring will be dominant because they'll have the X chromosome from from mom, which has got the mutation.
  • Homozygous recessive female with homozygous dominant male: Daughters have the dominant phenotype, and males are carriers.

Color Blindness as an X-Linked Recessive Trait

  • Mom is a carrier:
    • One son will be colorblind.
    • One son will be unaffected.
    • Daughters will be carriers.

Aneuploidy and Chromosome Abnormalities

  • Aneuploidy: Abnormal number of chromosomes.
  • Can result from incorrect recombination during meiosis.
  • Duplication of a chromosome (e.g., trisomy 13, Down syndrome).
  • Missing parts of chromosomes.
  • Leads to birth defects.

Extra Sex Chromosomes and Chromosome Duplications

  • Extra sex chromosomes (X and Y): Lead to disorders of sexual development.
  • Famous chromosome duplications: Down syndrome, Edwards syndrome, and Patau syndrome.
  • Caused by failure of chromosomes to properly split and conjoin.

Chromosome Anomalies and Pregnancy

  • Chromosome anomalies typically occur early in development.
  • Most result in early pregnancy loss (spontaneous abortion).
  • Conditions like Down syndrome and Turner syndrome are compatible with life, but most are not.
  • Significance: Fifty percent of all conceptions end in spontaneous abortion, and fifty percent of these are due to chromosome abnormalities, i.e., twenty five percent of all conceptuses have a chromosomal anomaly.
  • Chromosome abnormalities account for seven percent of major live birth differences.

Summary

  • Sex determination.
  • Meiosis (creating haploid gametes).
  • Exchange of genetic information.
  • Chromosome numbers and abnormalities.