Sexual Reproduction and Genetics

The Benefits of Sex

Patients cured by blood stem cells gene therapy could still pass the recessive mutant allele on to their future children.

  • Gene therapy for ADA-SCID (Severe Combined Immune Deficiency caused by ADA mutations) target somatic cells, specifically blood stem cells

  • These cells are not a part of the germline, which includes sperm and eggs

  • The therapy does not alter the DNA in germ cells, so the underlying mutation in the patient’s genome is still present and can inherited by their offspring

Somatic Cells

  • All body cells except sperm and eggs

  • Perform day-to-day bodily functions (e.g., muscle contraction, nerve signaling, skin protection)

  • Typically diploid (2 copies of each chromosome)

  • Divide by mitosis

  • Not passed on to offspring; mutations or changes affect only the individual

  • Examples: skin cells, muscle cells, neurons, liver cells

Germ-line cells

  • Cells that give rise to gametes (sperm and egg cells)

  • Carry and pass genetic material to the next generation

  • Haploid once matured (1 copy of each chromosome), but diploid in earlier precursor stages

  • Undergo meiosis to produce gametes

  • Passed on to offspring; mutations in germ-line DNA can be inherited

  • Examples: Sperm, eggs, and their precursors in tests or ovaries


Meiosis and Fertilization

Meiosis - Produce haploid gametes (sperm or eggs) from diploid germ cells

  • reduces the chromosome number by half to create haploid gametes

  • Starts with one diploid cell

  • Induces one round of DNA replication and two rounds of cell division

  • Crossing over and independent assortment occur, leading to genetic variation

  • End result: 4 genetically unique haploid cells (n)


  1. Interphase (before Meiosis begins)

    • DNA is replicated - each chronometer now has two sister chromatids

  2. Meiosis I - separates homologous chromosomes

    • Prophase I

      • Chromosomes condense

      • Homologous chromosomes pair up and exchange segments (crossing over)

      • Spindle forms

    • Metaphase I

      • Homologous pairs line up at the metaphase plate

    • Anaphase I

      • Homologous chromosomes separate (sister chromatids stay together)

    • Telophase I & Cytokinesis

      • Tow haploid cells form, each with duplicated chromosomes (sister chromatids)

  3. Meiosis II - separates sister chromatids (No new DNA replication)

    • Prophase II

      • New spindle forms in each haploid cell

    • Metaphase II

      • Chromosomes (sister chromatids) line up at the metaphase plate

    • Anaphase II

      • Sister chromatids separate and move to opposite ends of

    • Telophase II & Cytokinesis

      • Four haploid gametes form, each genetically unique


Fertilization

  • joins a haploid sperm and haploid egg to form a diploid zygote

  • Each parent contributes half the chromosomes (23 in humans)

  • The zygote has 46 chromosomes - a full set - and begins dividing by mitosis to form a multicellular organism

  1. Gamete fusion: one haploid sperm and one haploid egg unite

  2. Zygote formation: forms a diploid zygote (2n) with one set of chromosomes from each parakeet

  3. Zygote divides by mitosis: the zygote undergoes mitotic cell divisions to grow into a multicellular organism


Mitosis - general cell division

  • Produces identical diploid cells for growth, repair, and asexual reproduction

  • Involves one round of DNA replication and one round of cell division

  • No crossing over or genetic shuffling

  • End result: 2 genetically identical diploid cells (2n)


How they work together:

  • meiosis creates gametes

  • Fertilization restores the diploid chromosome number by uniting two haploid gametes

  • The resulting zygote divided by mitosis, building the body of a new organism


Mendel and the Laws of Inheritance

  1. Law of Segregation

    • each individual has two alleles for each gene (one from each parent)

    • These alleles separate (or segregate) during the formation of gametes

    • Each gamete receives only one allele for each gene

    • This law reflects what happens during anaphase I of meiosis, when homologous chromosomes (each carrying an allele) are separated into different gametes

  2. Law of Independent Assortment

    • genes for different traits are inherited independently of one another (as long as they are on different chromosomes)

    • The allele a gamete receives for one gene does not affect the allele it receives from another gene

    • This law is seen during metaphase I, when homologous chromosome pairs align randomly at the metaphase plate

    • This random assortment creates many possible combinations of chromosomes in gametes — a source of genetic variation

  • Mendel discovered that inheritance is based on discrete units (genes) passed from parent to offspring

  • His laws of segregation and independent assortment are physically realized through meiosis, the process by which gametes are formed

  • Without meiosis, the patterns Mendel observed would not occur — and sexual reproduction wouldn’t generate genetic variation

Mendel’s Pea Plant Experiment

  1. Started with purebred plants

  2. Crossed two different varieties

  3. Observed how traits were passed down through multiple generations

  • In the first generation, once one trait appeared — this was the dominant trait

  • In the second generation, the recessive trait reappeared in a 3:1 ration

  • He deduced that…

    • Traits are controlled by factors (genes)

    • Each parent contributes one allele per gene

    • Alleles can be dominant or recessive

  • Let’s say we are looking at a human genetic trait such as earlobe attachment:

    • Free earlobes (F) are dominant

    • Attached earlobes (f) are recessive

    • In the F1 generation, all are Ff (free earlobes), in F2, there is a 3:1 ratio — FF, Ff, fF, ff

  • Meiosis makes this possible

    • When humans made egg or sperm cells, the two alleles for each gene are segregated during meiosis — just as Mendel predicted

    • Independent assortment and crossing over during meiosis also create new combination, increasing genetic variation among siblings


Genetics as an Environmental Tool

Genetics provides powerful tools for exploring how traits are inherited and how genes function. By observing mutation and analyzing inheritance patterns, scientists can uncover relationships between genes and phenotypes and map genetic pathways

  1. Classical Genetic Approach

    • traditional method to study gene function by observing mutations and their effects on phenotypes

    • Often uses model organisms like fruit flies, mice, yeast, or plants

    • Tracks how traits are passed across generation

    • Foundational for identifying genes involved in specific biological processes

  2. Genetic Screening

    • Identify mutants that show a desired phenotypes (e.g., abnormal development or behavior)

    • Mutations induced randomly (via chemicals or radiation)

    • Helps pinpoint genes that contribute to specific cellular or developmental function

    • Screening for mutants with faulty wing development in flies to find gens involved in wing formation

  3. Genes and Phenotypes

    • Phenotype: observable trait or characteristic (e.g., eye color, enzyme function)

    • Gene: controls the expression of a trait

    • Mutations in genes lead to altered phenotypes, allowing inference about gene function

    • Studying phenotype changes helps us figure out what the gene does

  4. Meiosis and Genetic Mapping

    • During meiosis, recombination (crossing over) occurs between homologous chromosomes

    • By analyzing how frequently traits are inherited together, scientists can create a genetic map

    • Genes that are closer together on a chromosome are less likely to be separated by recombination

    • Determines the relative position of genes on chromosomes

  5. Complementation Test

    • Determine whether two mutations affect the same gene or different genes

    • Cross two mutants with similar phenotypes:

      • If offspring shows normal phenotype, the mutations complement = different genes

      • If offspring still have mutant phenotype = same gene

    • Confirms whether multiple mutations affect one gene or multiple genes in a pathway


Human Genetics: Understanding Disease and Evolution Through DNA

  1. Linked Blocks are polymorphisms & our ancestry

    • Polymorphisms: natural variations in DNA; most common type is the SNP (Single Nucleotide Polymorphism)

    • SNPs that are at or near each other on a chromosome tend to be inherited together in blocks called haplotype blocks

    • Recently arisen alleles are found in larger haplotype blocks because they haven’t yet been broken up by recombination

    • Blocks are passed down from our ancestors and tracking them helps scientists trace lineage and population history

  2. Polymorphisms are clues to evolutionary history

    • Different populations carry distinct SNP patterns that reveal:

      • Migration patterns

      • Interbreeding

      • Adaptations to different environments

    • SNPs provide molecule fossil record of human evolution

  3. Genetics studies and human disease

    • Genetics may help uncover causes of both rare and common diseases

    • Many severe, rare diseases (e.g., cystic fibrosis or sickle cell anemia) are caused by mutations in single genes

    • In contrast, common diseases (e.g., diabetes, heart disease, or depression) are often influenced by:

      • Multiple genetic mutations

      • Environmental factors (diet, stress, pollutants)

      • Gene-environment interaction

  4. SNPs and disease tracking

    • SNPs can flag areas of the genome where disease-related genes are located

    • If a certain SNP is common among people with a disease, it may lie near a disease-causing gene

    • Scientists use this to narrow down the search for disease-associated mutations

  5. Genome-wide associations studies (GWAS)

    • GWAS scan genome for SNPs associated with diseases in large populations

    • By comparing DNA from:

      • People with the disease

      • People without the disease

    • GWAS can identify statistically significant SNPs linked to the disease

    • Has helped find genes involved in Alzheimer’s, obesity, asthma, and type 2 diabetes

In Summary…

  • Haplotype blocks → inherited patterns from ancestors

  • SNPs → common markers used to study variation

  • New alleles → found in larger haplotype blocks

  • Polymorphisms → offer insight into human evolution

  • Single-gene mutations → cause rare, severe diseases

  • Multifactorial traits → involve many genes + environment

  • GWAS & SNPs → tools to link DNA changes to diseases