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Growth & Development

  Heredity & Environment
- From One Cell to Many -

Growth & Development

Ψ  From One Cell to Trillions

  •  The zygote grows through a process of duplication & division.
   •  At about the eight-cell stage differentiation is added, cells begin to specialize.

Ψ  Terms you should know:

Genotype - A person's entire genetic inheritance, including genes that are not expressed in the person. Existent at conception!

Phenotype - All the genetic traits, including physical characteristics and behavioral tendencies, that are expressed in a person.

Carrier - A person who has a gene in his or her genotype that is not evident as part of the phenotype. Carriers can pass such a gene to their offspring.

Polygenic Traits - Characteristics produced by the interaction of many genes ( rather than by a single gene ).

Multifactorial Traits - Characteristics produced by the interaction of genetic & environmental influences.

Gene-Gene Interaction

Ψ  Whether or not a genetic trait becomes expressed is determined by;

    •  Interaction among the proteins synthesized according to the specific genes that affect that trait.
    •  Ongoing interaction between the genotype & the environment.

Ψ  Regulator gene - In humans there are about 100 genes involved in controlling the expression of one or more other (structural) genes. These genes regulate genetic interaction, they produce the differences between humans an chimps, etc.

Ψ  Additive Gene - One of a number of genes affecting a specific trait, each of which makes an active contribution to that trait. Skin color & height are determined by additive genes.

Ψ  Dominant Gene - The stronger of an interacting pair of genes.

Ψ  Recessive Gene - The weaker of an interacting pair of genes.

Ψ  X-Linked Genes - Genes that are on the X chromosome of the 23rd pair. Some bad traits can be passed from mother to son via the X.

    If an X-linked gene is recessive - the fact that it is on the X chromosome is critical. Recall that males have only one X chromosome. thus, whatever recessive genes a male happens to inherit from Mom cannot be counterbalanced by his second X (Dad gave him a Y ); therefore, the X recessive (if present) passed from Mom to Son will be always be expressed!

It's a Mickey Mouse World , isn't it?

Ψ  Genetic Imprinting - The tendency of certain genes to be expressed differently when they are inherited from the mother than when they are inherited from the father.

Ψ  One list of "Epigenetic rules"

•  Large numbers of genes interact to produce a trait.
 •  Environmental factors are important, affecting trait expression & form.
  •  Environmental factors can change during the life span; however, a trait that was quite
      plastic early in life can become hard to change later.
    •  Despite the rule above, even adults retain developmental flexibility.
     •  Despite all the rules, the "butterfly effect" is operational.

Ψ  Twins

  • Monozygotic (identical) twins have identical genes because they were formed from one zygote that split into two identical organisms very early in development.
  • Dizygotic (fraternal) twins are formed when two separate ova are fertilized by two separate sperm at roughly the same time. Such twins share about half their genes as do all siblings.
  • Ψ  A clone originates from a live organism. Cloning is the process of making an identical copy of something. When we hear of cloning successes, we learn about only the few attempts that worked. What we don't see are the many, many cloning experiments that failed! And even in the successful clones, problems tend to arise later, during the animal's development to adulthood.

    Ψ  ART: Defined in the broadest sense, the term assisted reproductive technologies (ART) could be applied to any therapy used to improve the chances of conception. In the current vernacular ART is reserved to describe those interventions related to in vitro fertilization (IVF) & embryo transfer.

    Growth & Development
    Robert C. Gates