Heredity & Environment
Ψ The Beginnings of life
• The name for a human reproductive cell is gamete, they are either sperm or ovum.
• When a sperm & ovum fuse a living cell called a zygote is formed.
Ψ The genetic code
• The genetic code is a sequence of chemical compounds (called bases) that is held within DNA molecules & directs
development, behavior & form.
• The basic unit of genetic instruction is the gene, which is a discrete segment of a chromosome (gene carrier).
• Genes can have several variations (alleles).
• A karyotype portrays a person's 46 chromosomes (23 pairs).
• Sex is determined by the 23rd chromosome pair; XY is male, XX is female.
• The human genome is like a blueprint for creating humans. Your genome contains all the genetic (gene) information
needed to create you, & to determine how your cells function throughout your whole life. Every one of your body cells,
contains a copy of your genome. The human genome contains about 25,000 genes.
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
Ψ Dominant Gene - The stronger of an interacting pair of genes.Ψ
Ψ Recessive Gene - The weaker of an interacting pair of genes.Ψ
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!
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.
- From Genotype to Phenotype -
Ψ It is quite clear that genes, the prenatal & postnatal biochemical environment, & the more distant social environment
are all powerful influences on human development.
We must not ignore the genetic component of any given trait!
We must not forget that the environment affects every trait!
Inherited Abnormalities
Ψ Chromosomal Miscount - usually results in spontaneous abortion (miscarriage). An estimated half of all zygotes have
an odd number of chromosomes.Ψ Most zygotes with chromosomal abnormalities are aborted; but, one in 200 are born
& in every case have a recognizable syndrome, such as:
• Down syndrome: Trisomy - 21, most common extra-chromosome syndrome
• Klinefelter syndrome (23 location), impairs sexual maturation
• Fragile X syndrome - most common form of inherited mental retardation.
Ψ The variable that most often correlates with chromosomal abnormalities is maternal age.
Ψ Some methods of genetic testing follow:
• PKU or Phenylketonuria (every newborn in the US is tested)
• Alpha-Fetoprotein Assay
• Chorionic villi Sampling (CVS)
• Amniocentesis (reveals sex of the fetus)
• Ultrasound (sonogram)
Harmful Genes Dominant disorders:
• Huntington's Chorea - central nervous system disease
• Tourette's Syndrome - tics & twitches, sometimes with uncontrollable speech
Recessive disorders:
• Cystic Fibrosis
• Thalassemia
• Sickle-cell Anemia
Ψ Genes that predispose for schizophrenia occur on chromosomes 1,5,6,10,13,15, & 22.
Ψ Genetic counseling is the process of helping people understand & adapt to the medical,
psychological & familial implications of genetic contributions to disease. This process integrates the following:
• Interpretation of family & medical histories to assess the chance of disease occurrence
or recurrence.
• Education about inheritance, testing, management, prevention, resources & research.
• Counseling to promote informed choices and adaptation to the risk or condition.
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Growth & Development
Robert C. Gates