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Chapter 2: Genetic Bases of Child Development
MECHANISMS OF HEREDITY
The Biology of Heredity
• There are 200-500 million sperm in the 5ml of semen ejaculate; of these, a few hundred reach the
fallopian tubes. Only one will penetrate the cellular wall of the egg, after which chemical changes block
out other sperm.
• Most cells in the body have 46 chromosomes; the egg and sperm gametes each have 23 – when
combined at fertilization, the new individual will have the full set of 23 pairs
• In vitro fertilization involves mixing sperm and egg together in a dish and then placing several
fertilized eggs in the mother’s uterus for implantation; this assisted reproductive technology produces
50,000 babies each year.
• The fertilized egg may be placed in the uterus of a surrogate mother who carries the baby to term.
• Disadvantages: Often requires more than one attempt (1/3 succeed), higher chance of multiple births,
low birth weight and birth defects, expensive
• Autosomes are the first 22 pairs of chromosomes, each pair similar in size. The 23 pair of sex
chromosomes determine the sex of the child, with X much larger than Y. The egg contributes an X, and
the sperm’s contribution (X or Y) determine the sex, with the Y chromosome initiating a male pattern of
• Each chromosome is one molecule of deoxyribonucleic acid (DNA). A group of nucleotide bases
provide a set of biochemical instructions as a gene
• The complete set of 25,000 genes make up a person’s heredity, or genotype. These genetic
instructions, in conjunction with environmental influences, produce a phenotype – the observable
expression of genotype in an individual’s physical, behavioural, and psychological features.
• Only identical twins are genetically identical. Even then, the expression of their genes differ; only 10%
provide active instructions at any time, with some genes turned on for a few hours only – these are
controlled by regulator genes that code for hormones.
Single Gene Inheritance
• Genes come in different forms, or alleles, which as a pair may either be homozygous or heterozygous.
In a heterozygous situation, often one allele is dominant and the other recessive; the chemical
instructions of the dominant allele will be followed – for sickle cell alleles on chromosome 11, normal is
• Incomplete Dominance occurs when one allele does not dominate completely, with resulting
phenotype falling between phenotype of either allele. E.g. Heterozygous individuals may have sickle-cell
trait: temporary, mild form of disease when they are in situations with deficient oxygen.
• Only 1/3 of human genes show Mendelian genetics of having alleles and simple dominance; typically
many genes contribute to a trait in polygenic inheritance
• Impact of heredity depends on environment, where an allele may have survival value in one
environment but not others. The sickle-cell allele is protective against malaria, common in parts of Africa
and other malaria-prone regions; however, it is virtually non-existent in children of European or Asian
• Homozygous recessive disorders include: albinism, cystic fibrosis, phenylketonuria, Tay-Sachs disease
• Very few serious disorders are caused by dominant alleles as individuals typically do not live long
enough to reproduce. An exception is Huntington’s Disease, a fatal disease characterized by Page 44-63, 20 pages Page 2 of4
progressive nervous system degeneration due to a mutation on chromosome 4. Individuals with this
disorder are normal until middle age, when they start developing muscle spasms, etc.
• Disorders can be sex-linked, where the gene responsible is carried on a sex chromosome.
Hemophilia, where blood clotting is disordered, has its gene on the X chromosome – risk greater for
• A genetic counsellor can construct a detailed family history to determine likelihood of inherited
Abnormal Number of Chromosomes
• Individuals who have extra, missing, or damaged chromosomes from the normal complement of 46 will
have disturbed development
• Down Syndrome is a genetic disorder caused by trisomy 21, resulting in cognitive deficits and
developmental delay. The extra chromosome is usually provided by the egg; the risk increases markedly
with maternal age due aging and the accumulation of exposure to hazardous materials over time.
• Most damage, extra, or missing autosomes lead to spontaneous abortion since each autosome
contains huge amounts of important genetic material and are crucial to normal development.
• Sex chromosomes can also be disrupted:
o XXX: Female normal appearance, menstrual irregularities, learning disorders
o XYY: Male, tall stature, tend to have low verbal IQ; not more likely to be violent or criminal
o XXY (Klinefelter): Sterile male with under-developed secondary characteristics, learning
disorders hormone therapy
o XO (Turner): Sterile female, physical abnormalities such as very short stature and webbed
neck, poor spatial abilities, underdeveloped sex organs and secondary sex characteristics
o Fragile X: Minor to severe mental retardation, delayed speech and motor development
• No disorders consist solely of Y chromosomes; X chromosome seems necessary for life.
HEREDITY, ENVIRONMENT & DEVELOPMENT
• Congenital defects refer to any defects present at birth: can be caused by either genes (chromosomal
or genetic abnormalities) or by environmental influences such as difficulties during prenatal development
• Behavioural Genetics deals with the inheritance of behavioural and psychological trait