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
− 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 characteristics.
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
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
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 unit.
− 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
− Dominant: Describing the more powerful of two alleles in a heterozygous
− Recessive: Describing the weaker of two alleles in a heterozygous
Sex Chromosomes: The 23 rd 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
− 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.
Disorder and its Cause Method of Current method of
Nature Diagnosis treatment and prevention.
Hemophilia: Blood Heredity: X- Blood tests Treated by transfusion of
disease linked clotting factors. Genetic
characterized by recessive counselling can help
poor clotting ability trait. determine whether a couple
risk bearing a child with this
Diabetes mellitus: Heredity: Blood and Can often control it by special
Body's inability to multi-gene urine tests diet alone, in other cases, oral
metabolize exaggerated medication and/or insulin
carbohydrates and by injections are required to
maintain proper environment maintain the body's
glucose levels. al factors. equilibrium.
Phenylketonuria Heredity: Blood tests Genetic counselling can
(PKU): Inability to recessive prenatally or indicate the risk that a couple
convert allele at birth will have a PKU child. A
phenylalanine to special diet can be instituted
tyrosine; untreated, that will prevent the disorder's
leads to mental toxic affects.
Sickle cell anemia: Heredity: Two Blood tests Blood transfusions have until
Blood disease recessive recently been the only
characterized by alleles in treatment. However, the
malformation of red combination recent in utero treatment of a
blood cells that are fetus for an autoimmune
low in oxygen. disorder had brought hope
that sickle cell anemia and
other similar diseases may be
treated successfully before
birth. Down syndrome Heredity: Amniocentesi Special physical training,
(trisomy 21): extra full or s, special education, including
Physically and partial alphafetoprot speech therapy, surgical
mentally retarded chromosome ein assay, corrections of problems with
development; 21 chorionic villithe heart and with hearing are
sometimes sampling, sometimes necessary.
cardiovascular and chromosome
Turner (XO) Chromosoma Blood tests Hormone therapy can
Syndrome: l promote development of
Underdeveloped abnormality: secondary sex characteristics.
secondary sex only one X Counselling; special education
characteristics; Chromosome to lessen deficits in spatial
infertility; short instead of understanding.
stature; social two
Triple X (XXX) Chromosome Blood tests Special education to improve
Syndrome: Some abnormality: cognitive skills.
physical extra X
Klinefelter's (XXY) Chromosoma Bloods tests Testosterone treatments can
Syndrome: Some l enhance development of male
female physical abnormality: secondary sex characteristics
characteristics; extra X as well as sexual interest and
sterility; mild to chromosome assertiveness. Special
severe cognitive education to improve
difficulties cognitive skills.
XYY Syndrome: Chromosoma Blood tests Special education.
Unusual height; l
some cognitive abnormality:
impairment; extra Y
attention deficit chromosome
Fragile X Syndrome: Heredity: Blood tests No known treatment.
Physical breaking of
abnormalities; an X
mental retardation chromosome that deepens with near its tip
and social problems.
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