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Conceiving new life
Conceiving new life
Dr. Ali D. Abbas
Instructor, Fundamentals of Nursing Department, College of Nursing, University of Baghdad
[email protected]
LEARNING OBJECTIVES
After mastering the contents of this lecture, the student should be able to:
1. Explain the fertilization.
2. Identify the mechanisms of heredity.
3. Identify the patterns of genetic transmission.
4. Describe influences of heredity and environment.
5. Identify some characteristics influenced by heredity and environment.
TERMINOLOGIES
Animalculists
Heritability
Fertilization
Heterozygous
Genetic
Homozygous
Genotypes
Phenotypes
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Conceiving new life
CONTENTS
1. Introduction
2
2. How fertilization takes place 2
3. Mechanisms of heredity 4
Genetic code 4
What determines sex? 6
Patterns of genetic transmission 7
4. Influences of heredity and environment 8
Studying heredity and environment 8
Some characteristics influenced by heredity
and environment 9
5. References 10
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Conceiving new life
Introduction
The animalculists (so named because the male sperm were
then called animalcules) claimed that fully formed "little people"
were contained in the heads of sperm, ready to grow when
deposited in the nurturing environment of the womb. The ovists,
inspired by the influential work of the English physician William
Harvey, held an opposite but equally incorrect view: that a female's
ovaries contained tiny, already formed humans whose growth was
activated by the male's sperm.
Finally, in the late eighteenth century, the German-born
anatomist Kaspar Friedrich Wolff demonstrated that embryos are
not preformed in either parent and that both contribute equally to
the formation of a new being.
Although scientists have now found a way to clone (make a
genetic copy of) a human embryo, and this has value for
therapeutic research purposes, ethical and religious concerns
about the dignity of individual human life make it unlikely—at least
in the near future—that cloning will become a common means of
reproduction.
How fertilization takes place
Fertilization, or conception, is the process by which sperm and
ovum the male and female gametes, or sex cells combine to create
a single cell called a zygote, which then duplicates itself again and
again by cell division to become a baby. At birth, a girl has all the
ova (plural of ovum) she will ever have about 400,000. These
immature ova are in her two ovaries (see Fig.1), each ovum in its
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Conceiving new life
own small sac, or follicle. In a sexually mature woman, ovulation
rupture of a mature follicle in either ovary and expulsion of its
ovum occurs about once every 28 days until menopause. The ovum
is swept along through the fallopian tube by tiny hair cells, called
cilia, toward the uterus, or womb. Fertilization normally occurs
during the brief time the ovum is passing through the fallopian
tube.
Fig. (1) Female reproductive system
Sperm are produced in the testicles (testes), or reproductive
glands, of a mature male (refer to Fig.2) at a rate of several
hundred million a day and are ejaculated in the semen at sexual
climax. They enter the vagina and try to swim through the cervix
(the opening of the uterus) and into the fallopian tubes, but only a
tiny fraction make it that far.
Fig. (2) Male reproductive system
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Conceiving new life
Fertilization is most likely if intercourse occurs on the day of
ovulation or during the five days before.
If fertilization does not occur, the ovum and any sperm cells in
the woman's body die. The sperm are absorbed by the woman's
white blood cells, and the ovum passes through the uterus and
exits through the vagina.
Mechanisms of heredity
The science of genetics is the study of heredity, the inborn
factors,
inherited
from
the
biological
parents
that
affect
development. When ovum and sperm unite, they endow the baby
to be with a genetic makeup that influences a wide range of
characteristics from color of eyes and hair to health, intellect, and
personality.
Genetic code
The basis of heredity is a chemical called deoxyribonucleic
acid (DNA), which contains all the inherited material passed from
biological parents to children. DNA carries the biochemical
instructions that direct the formation of each cell in the body and
tell the cells how to make the proteins that enable them to carry
out specific body functions.
The structure of DNA resembles a long, spiraling ladder made
of four chemical units called bases (see Fig. 3).
The complete sequence of genes in the human body
constitutes the human genome. The genome specifies the order in
which genes are expressed, or activated.
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Fig. (3) DNA
The genetic code
Every cell in the normal human body except the sex cells has
23 pairs of chromosomes 46 in all. Through a type of cell division
called meiosis, each sex cell, or gamete (sperm or ovum), ends up
with only 23 chromosomes one from each pair.
Thus, when sperm and ovum fuse at conception, they produce
a zygote with 46 chromosomes, 23 from the father and 23 from the
mother.
At conception, then, the single celled zygote has all the
biological information needed to guide its development into a
human baby. This happens through mitosis, a process by which the
cells divide in half over and over again. When a cell divides, the
DNA spirals replicate themselves, so that each newly formed cell
has the same DNA structure as all the others.
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Conceiving new life
What determines sex?
At the moment of conception, the 23 chromosomes from the
sperm and the 23 from the mother's ovum form 23 pairs. Twentytwo pairs are autosomes, chromosomes that are not related to
sexual expression. The twenty-third pair is sex chromosomes one
from the father and one from the mother that govern the baby's
sex.
Sex
chromosomes
are
either
X
chromosomes
or
Y
chromosomes. The sex chromosome of every ovum is an X
chromosome, but the sperm may contain either an X or a Y
chromosome. The Y chromosome contains the gene for maleness,
called the SRY gene. When an ovum (X) is fertilized by an Xcarrying sperm, the zygote formed is XX, a female. When an ovum
(X) is fertilized by a Y-carrying sperm, the resulting zygote is XY, a
male (see Fig.4).
Fig. (4)
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Conceiving new life
Patterns of genetic transmission
Dominant and Recessive Inheritance: Can you curl your
tongue? If so, you inherited this ability through dominant
inheritance. If your parents can curl their tongues but you cannot,
recessive inheritance occurred. How do these two types of
inheritance work?
Genes that can produce alternative expressions of a
characteristic (such as ability or inability to curl the tongue) are
called alleles. Every person receives a pair of alleles for a given
characteristic, one from each parent. When both alleles are the
same, the person is homozygous for the characteristic; when they
are different, the person is heterozygous. In dominant inheritance,
when a person is heterozygous for a particular trait, the dominant
allele governs. In other words, when an offspring receives two
contradictory alleles for a trait, only one of them, the dominant
one, will be expressed. Recessive inheritance, the expression of a
recessive trait, occurs only when a person receives two recessive
alleles, one from each parent.
Multifactorial transmission, a combination of genetic
and environmental factors, plays a role in the expression of most
traits.
Genotypes
and
Phenotypes:
Multifactorial
Transmission If you can curl your tongue, that ability is part of
your phenotype, the observable characteristics through which your
genotype, or underlying genetic makeup, is expressed. Except for
monozygotic twins, no two people have the same genotype. The
phenotype is the product of the genotype and any relevant
environmental influences. The difference between genotype and
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phenotype helps explain why a clone can never be an exact
duplicate of another human being.
Influences of heredity and environment
While certain rare physical disorders are virtually 100 %
inherited, phenotypes for most complex normal traits, such as
those having to do with health, intelligence, and personality are
subject to a complex array of hereditary and environmental forces.
Studying heredity and environment
One approach to the study of heredity and environment is
quantitative: it seeks to measure how much heredity and
environment influence particular traits. This is the traditional goal
of the science of behavioral genetics.
Measuring Heritability: Heritability is a statistical
estimate of how great contribution heredity makes toward
individual differences in a specific observed trait at a certain time
within a given population. Heritability does not refer to the relative
influence of heredity and environment in a particular individual;
those influences may be virtually impossible to separate. Nor does
heritability tell us how traits develop. It merely indicates the
statistical extent to which genes contribute to a trait.
Effects of the Prenatal Environment: One largely
neglected environmental influence is what happens in the womb.
Two newer types of twin studies co-twin control and chorion
control studies allow researchers to look at the nature and timing
of nongenetic influences in utero.
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Some characteristics influenced by heredity and environment
1. Physical and physiological traits.
Not only do monozygotic twins generally look alike, they are
also more concordant than dizygotic twins in their risk for such
medical disorders as hypertension (high blood pressure), heart
disease, stroke, rheumatoid arthritis, peptic ulcers, and epilepsy.
Life span, too, seems to be influenced by the genes. Obesity is a
multifactorial condition.
2. Intelligence and school achievement.
Heredity exerts a strong influence on general intelligence and
also on specific abilities. Still, experience counts, too; an enriched
or
impoverished
environment
can
substantially
affect
the
development and expression of innate ability.
3. Personality.
Certain aspects of personality appear to be inherited, at least
in part. Analyses of five major groupings of traits extraversion,
neuroticism (a group of traits involving anxiety),conscientiousness,
agreeableness, and openness to experience suggest a heritability of
about 40 percent. Setting aside variances attributable to measurement error brings heritability closer to 66 percent for these trait
groupings.
4. Psychopathology.
There is evidence for a strong hereditary influence on such
conditions as schizophrenia, autism, alcoholism, and depression.
All tend to run in families and to show greater concordance
between monozygotic twins than between dizygotic twins.
However, heredity alone does not produce such disorders; an
inherited tendency can be triggered by environmental factors. For
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example, researchers have linked a gene or genes on chromosome 1
to vulnerability to alcoholism or depression, or both, depending on
circumstances.
References
Diane, P.; Sally, O., and Ruth, F.: Human Development, 9th ed., McGraw
Hill Company, United States, 2004, P.P. 54-99.
Papalia, D.; Olds, S., and Feldman, R.: Human Development, 9th ed.,
McGraw Hill Company, United States, 2008, P.P. 63-72.
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