Chapter 3: Forming a New Life ~ Conception, Heredity, and Environment
Becoming Parents: How Conception Occurs:
Bronfenbrenner’s bioecological approach includes factors in the
Microsystem: birth planned/accident, wanted/unwanted, parents
Macrosystem: if culture encourages large/small families, values one sex over the
How does conception normally occur, and how have beliefs about conception changed?
Early beliefs about conception reflected incorrect beliefs about nature and male
and female anatomy
o Aristotle thought birth of male was natural order, female was when
development was disturbed (woman only receptacle, child formed of
o Hippocrates thought fetus results from joining male/female seeds.
o 18 century, finally, Kaspar Friedrich Wolff said embryos are forms with
equal contributions from each parent.
o Union of sperm and ovum (gametes) fuse to produce a zygote; also called
o One-celled organism resulting from fertilization
Duplicates itself by cell division to form an infant.
o Inability to conceive after 12 months of trying
7% of Canadians
Common causes of infertility in men and woman chart (p. 57)
What causes infertility, and what are alternative ways of becoming parents?
Most common cause in men: low sperm count
o As low as 60/200 million makes conception unlikely
o Some just can’t swim well or ejaculatory duct is blocked
o Some genetic basis in this
Most common cause in women: blockage of Fallopian tubes (many cases is from
scar tissue from STI’s.
o Mucus in cervix may prevent sperm from penetrating it
o Disease of uterine lining may prevent implantation of fertilized ovum
Women fertility begins to decline late 20’s, especially 30’s, and men; age does
not affect it as much, but declines in late 30’s.
Sometimes hormone treatment, drug therapy, or surgery may correct it; but fertility
dugs may increase likelihood of high-risk births. Men in fertility treatment have increased
risk of producing sperm with chromosomal abnormalities. A daily supplement of
coenzyme Q10 (antioxidant) increases sperm motility. Unless this is a known cause for failure to conceive, chances of success after 18
months – 2 years are high, but after a year or so of trying, pregnancies must be monitored
Assisted reproduction involve ethical and practical issues
o About 76% are successful, much fewer end in live births (24% in Canada).
o Artificial insemination: injection of sperm into a woman’s vagina, cervix
or uterus (man has low sperm count). Stimulating her ovaries to produce
excess ova can increase success.
o In vitro fertilization (most common)
Multiple births can occur by fertilization of more than one ovum
(or one ovum that has split) or by the splitting of one fertilized
Intracytoplasmic sperm injection (ICSI): sperm injected into ovum
o In Vitro maturation (IVM) is earlier in monthly cycle, when 30-50 egg
follicles are developing. You harvest many follicles before ovulation is
complete, and allow them to mature in the lab can diminish likelihood of
o Blastocyst transfer: fertilized ovum is kept in the culture until it grows into
blastocyst stage (linked to increase in identical twins).
o Surrogate Motherhood: fertile woman impregnated by prospective father
(usually artificial insemination), carries the baby and gives the baby to the
father and his mate.
o 2004, Canadian gov’t passed Canadian Assisted Human Reproductive
Act: strict conditions on the practice, including ban on payment to
Mechanisms of Heredity
What genetic mechanisms determine sex, physical appearance, and other characteristics?
Monozygotic (one-egg) twins:
o Twins resulting from the division of a single zygote after fertilization; also
called identical twins
Same genetic make up but may differ in some ways
About 4 per 1,000 births
Dizygotic (two-egg) twins:
o Twins conceived by the union of two different ova (or a single ovum that has
split) with two difference sperm cells; also called fraternal twins.
Different genetic make up
Deoxyribonucleic acid (DNA):
o Chemical that carries inherited instructions for the formation and function of
Carries biochemical instructions, or genetic code, that governs bodily
functions and determines inherited characteristics.
Double helix made of pairs of chemical units called bases. Genetic Code:
o Sequence of base pairs within DNA, which determine inherited
Adenine (A), Thymine (T), Cytosine (C), Guanine (G).
o Coils of DNA that carry the genes
o Small segments of DNA located in specific positions on particular
o Basic functional units of heredity.
o Each seems to be located by function in a definite position on a
o Sequence of bases in a gene tells the cell how to make proteins that
enable it to do specific functions.
o Human Genome:
Complete sequence or mapping of genes in the human body and
Meiosis: cell division which the sex cells undergo when they are developing.
At conception, each human being receives 23 chromosomes from mother, and 23 from
father. They form 23 pairs of chromosomes (22 pairs of autosomes, 1 pair of sex
3/4 of genes every child receives are identical to other children: monomorphic genes
1/4 are polymorphic genes, which define each person as an individual.
Mitosis: process in which the cells divide in half over and over again. When a cell
divides, DNA spirals replicate themselves, so each structure has t he same DNA as othe
Genetic action that triggers growth of body and brain is often regulated by hormonal
levels—both in the mother and the baby, which are effected by environmental conditions
like nutrition and stress.
o The 22 pairs of chromosomes not related to sexual expression
o Pair of chromosomes that determines sex: XX in the normal female, XY in the
o The Y chromosome contains the gene for maleness, called the SRY
Initially, the reproduction system is identical in males in females until 6-8 weeks after
conception, male embryos begin producing testosterone. Hormones must first signal the
SRY gene, which triggers cell differentiation.
o The development of female reproductive system depends on a signally
molecule called Wnt-4. The extra X chromosome may explain why women
are generally healthier and more long-lived.
1860’s—Gregor Mendel laid foundation for understanding patterns of inheritance. He
crossbread pea plants with green seeds with plants with yellow seeds—found a dominant
and recessive. He found hat colour and shape were independent of each other. He showed
that hereditary traits are transmitted separately. Alleles:
o Two or more alternative forms of a gene that can occupy the same position on
paired chromosomes and affect the same trait
Genes can produce alternative expressions of a characteristic (roll tongue).
o Possessing two identical alleles for a trait
o Possessing differing alleles for a trait
o Pattern of inheritance in which, when a child received contradictory alleles,
only the dominant one is expressed
Heterozygous for a particular trait: When offspring receives contradictory
alleles for a trait, only one of them, the dominant one, will be expressed.
o Pattern of inheritance in which a child receives identical recessive alleles,
resulting in expression of a nondominant trait
Need a recessive allele from each parent.
o Pattern of inheritance in which multiple genes at different sites on
chromosomes affect a complex trait
o Interaction of several genes.
Most normal humans are this or multifactorial transmission. Except
monozygotic twins (each child has unique genotype—but phenotype may
not express underlying genotype).
o Permanent alterations in genes or chromosomes that may produce harmful
o Combination of genetic and environmental factors to produce certain complex
Ex. children with ADHD more likely to show early anti-social behaviour if
they were of low birth weight.
o Observable characteristics of a person
Is the product of a genotype. The difference between genotype and
phenotype helps explain why a clone can never be an exact duplicate.
Ex. curling your tongue.
o Genetic makeup of a person, containing both expressed and unexpressed
Experience modifies the expression of the genotype.
o To make a genetic copy of an individual Genetic and Chromosomal Abnormalities:
How are birth defects and disorders transmitted?
Birth disorders are fairly rare (3%), and most are usually cleft lip or palate, followed by
Down syndrome. Serious malformations involve the eye or circulatory, orofacial,
gastronomical, or musculoskeltal systems. Birth disorders are 27% of infant deaths in
Canada. Serious ones involves circulatory or central nervous system.
Birth defects chart on p. 64
In genetic defect we see the dominant and recessive transmission in humans and the
sex-linked inheritance. Some are mutations.
Birth defects and diseases may result from simple dominant, recessive, or sex-linked
inheritance, from mutations; genome imprint