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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 23rd 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