Chapter 2 – Heredity and the Environment
Transactions among a vast array of hereditary and environmental factors begin before birth. Such
transactions between genes and the environment make each newborn unique, and they continue to
shape the individual's characteristics throughout his or her lifespan.
Genotype: The particular set of genes that a person inherits from her parents. With the
exception of identical twins, no two individuals have the same genotype.
Phenotype: Created by the interaction of a person's genotype, or genetic makeup, with the
environment; the visible expression of the person's particular physical and behavioural
The Process of Genetic Transmission
In a woman's oviduct, the sperm (the male germ cell) and ovum (the female germ cell, or egg, it is
90000 times as heavy as the sperm that penetrates it) unite to create a new living organism called a
zygote, that has the potential to develop into a human being. This beginning is the start of a 9-month
long period that normally ends with a full-term baby 7 or 8 pounds and roughly 50 cm long.
Chromosomes: Thread-like structures, located in the central portion, or nucleus, of a cell, that
carry genetic information to help direct development. When the egg and sperm unite 23
chromosomes from each of these cells unite together to create 23 chromosome pairs or 46
chromosomes. Half of the chromosomes come from the father and the other half from the
mother, this is possible because each chromosome is homologous (similar in shape and
function) to one another. These 23 homologous pairs of chromosomes are passed on to every
cell in the body except the reproductive cells, which contain only 23 single chromosomes, this
possible due to meiosis.
Meiosis: The process by which a germ cell divides to produce new germ cells with only half the
normal complement of chromosomes; thus, male and female germ cells( sperm and ovum)
each contain only 23 chromosomes so that when they unite, the new organism they form will
have 46 chromosomes, half from each parent.
Crossing over: The process by which equivalent sections of homologous switch places
randomly, shuffling the genetic information each carries during meiosis.
How does the union of the egg and sperm cell become a complex human being?
This happens by a process called mitosis (the process in which a body cell divides in two, first
duplicating its chromosomes so that the new, daughter cells contain the usual 46 chromosomes)
which occurs in all autosomes (chromosomes that contain matching pairs) and sex chromosomes.
Thus, the zygote divides and continues to divide, each time producing new cells that have the full
complement of 46 chromosomes, and gradually becomes a multi-cellular organism.
Genes, DNA, and Proteins
Deoxyribonucleic acid (DNA): A ladder-like molecule that stores genetic information in cells
and transmits it during reproduction, made up of nucleotides (a compound containing a
nitrogen base, a simple sugar, and a phosphate group) that are held together by two long
twisted parallel strands. Only bases that are compatible with each other will form a bond
(adenine and thymine, cytosine and guanine).
Gene: A portion of DNA that is located at a particular site on a chromosome and that codes for
the production of certain kinds of proteins. Genes never work in isolation, but always in combination with environmental influences.
Proteins: Fundamental components of all living cells, they are any of a group of complex
organic molecules containing carbon, hydrogen, oxygen, nitrogen, and usually sulphur, and that
are composed of on or more chains of amino acids.
The Transmission of Traits
Gregor Mendel (through his work with pea plants) worked out the mechanisms or laws of inheritance
of characteristics. These two principles are known as:
Principle of segregation: states that each inherited trait comes from one's parent as a separate
Principles of independent assortment: states that the inheritance of various traits occurs
independently of one another.
Allele: An alternative form of a gene; typically, a gene has two alleles, one inherited from the
individual's mother and one from the father.
Homozygous: Describing the state of an individual whose alleles for a particular trait from each parent
are the same.
Heterozygous: Describing the state of an individual whose alleles for a particular trait from each
parent are different.
Co-dominance: A genetic pattern in which heterozygous alleles express the variants of the trait
for which they code simultaneously and with equal force.
Dominant: Describing the more powerful of two alleles in a heterozygous combination.
Recessive: Describing the weaker of two alleles in a heterozygous combination.
Sex Chromosomes: The 23 pair of chromosomes, which determine the individual's gender and are
responsible for sex related characteristics; in females, this pair normally compromises two X
chromosomes, in males and X and Y chromosome.
X-linked genes: Genes that are carried on the X chromosome and that may have no analogous
genes on the Y chromosome in males. In females X-linked recessive genes are expressed much
less frequently compared to males, because they have two X chromosomes, and have a chance
of inheriting a dominant and counteracting allele on the other X chromosome.
Hemophilia: A disorder caused by an X-linked recessive gene, in which the blood fails to clot;
found more often in males than in females. Many other x-linked recessive disorder are more
common in men than in women, including colour blindness, certain form of night blindness,
atrophy of the optic nerve, and so on.
Interactions among genes
Some traits that are influenced by genes do not tend to run in the families, development of such traits
usually depends on a certain configuration of many genes, and that particular configuration is not
likely to be passed on from parent to child. Furthermore, a single pair of alleles may influence more
than one trait. Moreover they may do this not directly, but indirectly through their effect on the
expression of still other genes, these genes are termed modifier genes.
Genetic Disorders Disorder and its Nature Cause Method of Current method of treatment and
Hemophilia: Blood Heredity: X- Blood tests Treated by transfusion of clotting
disease characterized by linked recessive factors. Genetic counselling can help
poor clotting ability trait. determine whether a couple risk
bearing a child with this disorder.
Diabetes mellitus: Body's Heredity: multi- Blood and urine Can often control it by special diet
inability to metabolize gene tests alone, in other cases, oral medication
carbohydrates and exaggerated by and/or insulin injections are required
maintain proper glucose environmental to maintain the body's equilibrium.
Phenylketonuria (PKU): Heredity: Blood tests Genetic counselling can indicate the
Inability to convert recessive allele prenatally or at risk that a couple will have a PKU
phenylalanine to birth child. A special diet can be instituted
tyrosine; untreated, that will prevent the disorder's toxic
leads to mental affects.
Sickle cell anemia: Blood Heredity: Two Blood tests Blood transfusions have until recently
disease characterized by recessive alleles been the only treatment. However,
malformation of red in combination the recent in utero treatment of a
blood cells that are low fetus for an autoimmune disorder
in oxygen. had brought hope that sickle cell
anemia and other similar diseases
may be treated successfully before
Down syndrome (trisomy Heredity: extra Amniocentesis, Special physical training, special
21): Physically and full or partial alphafetoprotein education, including speech therapy,
mentally retarded chromosome 21 assay, chorionic surgical corrections of problems with
development; sometimes villi sampling, the heart and with hearing are
cardiovascular and chromosome sometimes necessary.
respiratory abnormalities analysis.
Turner (XO) Syndrome: Chromosomal Blood tests Hormone therapy can promote
Underdeveloped abnormality: development of secondary sex
secondary sex only one X characteristics. Counselling; special
characteristics; infertility; Chromosome education to lessen deficits in spatial
short stature; social instead of two understanding.
neck; cardiovascular and
Triple X (XXX) Syndrome: Chromosome Blood tests Special education to improve
Some physical abnormality: cognitive skills.
abnormalities, including extra X menstrual irregularities chromosome
limitations on cognitive
Klinefelter's (XXY) Chromosomal Bloods tests Testosterone treatments can enhance
Syndrome: Some female abnormality: development of male secondary sex
physical characteristics; extra X characteristics as well as sexual
sterility; mild to severe chromosome interest and assertiveness. Special
cognitive difficulties education to improve cognitive skills.
XYY Syndrome: Unusual Chromosomal Blood tests Special education.
height; some cognitive abnormality:
impairment; attention extra Y
Fragile X Syndrome: Heredity: Blood tests No known treatment.
Physical abnormalities; breaking of an X
mental retardation that chromosome
deepens with time; near its tip
psychological and social
A major reason why potentially harmful diseases survive is that they are not harmful in the
heterozygous state, these individuals that carry the recessive allele survive and reproduce. The
defective allele also survives from generation to generation, even though it is activated only 25% of
the time. Some potentially harmful alleles may survive because they are actually beneficial in
combination with a normal allele (sickle cell anemia).
Chromosomal Abnormalities – Developme