AP Biology Notes: Cell Development and Inheritance

Five Things to Know about Cell Development: 

  1. Fertilization, the first step in embryonic development, is the fusion of one sperm with one egg (gametes) to produce a zygote that becomes a fully formed organism.
  2. In gene regulation, negative control occurs when a repressor switches genes off by inhibiting transcription, while positive control occurs when an activator switches genes on by promoting transcription. Inducers are molecules that bind to repressors and/or activators to increase gene expression.
  3. Operons generally occur in bacterial genomes and are sets of genes that perform a biological function like metabolizing lactose. When the substrate is absent, a repressor binds to the operator to inhibit transcription of the operon, and when present, the substrate binds to the repressor and induces transcription.
  4. Though each contains the same set of genetic material, cells in multicellular organisms have specific and different functions. The regulation of gene expression causes cells to undergo differentiation and specialization.
  5. Intercellular and intracellular signals regulate gene expression. For example, in embryos of multicellular organisms, the concentration of morphogens determines spatial organization of tissues and organs.

Five Things to Know about Cellular Inheritance: 

  1. Chromosomes are the basic units of inheritance. How they reorganize and combine directly influences the genetic material present in an offspring.
  2. Different versions of a gene that code for the same trait are called alleles. In classical (Mendelian) genetics, an individual receives one allele from each parent. Individuals with matching alleles are homozygous for that trait while those with different alleles are heterozygous. Usually, one version of the allele is dominant (e.g., brown eye color) and the other is recessive (e.g., blue eye color). Heterozygotes are “ruled” by the dominant allele.
  3. Geneticists perform test crosses to determine the genetic makeup (genotype) of organisms displaying the dominant phenotype. A Punnett square is used to illustrate a test cross. Mendel’s Law of Segregation states that an individual’s alleles separate during meiosis, and either may be passed on to the offspring. Mendel’s Law of Independent Assortment states that inheritance of a particular allele for one trait does not affect inheritance of other traits.
  4. Sex-linked genes in humans occur on the X or Y chromosome. Males who inherit recessive X-linked genes from their mothers always express the trait in question, since men have only one X chromosome.
  5. Pedigrees are family trees that enable us to study the inheritance of a particular trait across many related generations. They can also help us determine the type of transmission through generations: autosomal recessive/dominant, X-linked recessive/dominant, and so on.

Key Topics–Cell Development and Inheritance

Remember that the AP Biology exam tests you on the depth of your knowledge, not just your ability to recall facts. While we have provided brief definitions here, you will need to know these terms in even more depth for the AP Biology exam.

Cell Development: Fertilization and Early Development

  • Fertilization: The fusion of the sperm and egg to produce a zygote
  • Gametogenesis: The formation of gametes
  • Egg (ovum): The female gamete; it is nonmotile, large in comparison to male gametes, and stores nutrients
  • Sperm (spermatozoon): A male gamete
  • Zygote: A cell resulting from the fusion of gametes
  • Cleavage: The division in animal cell cytoplasm caused by the pinching in of the cell membrane
  • Pollination: The transfer of pollen to the micropyle or to a receptive surface that is associated with an ovule (such as a stigma)

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