PSYC 251 Lecture Notes - Lecture 2: Down Syndrome, Eyelid, Black Canadians
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Chapter 2: Genetic Bases of Child Development
• 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
•1978, Louise Brown first test tube baby
•Assisted reproductive technology is no longer experimental, 130,000 times annually in
America, 8000 times in Canada producing 50,000 and 2,100 babies respectively
• In vitro fertilization involves mixing sperm and egg together in a dish and if eggs are
fertilized, then several eggs are placed in the mother’s uterus 24 hrs later for
• The fertilized egg may be placed in the uterus of a surrogate mother who carries the
baby to term
• Social and emotional development is perfectly normal
• Disadvantages: Often requires more than one attempt (1/3 succeed), higher chance of
multiple births i.e. twins or triplets, low birth weight and birth defects, expensiveand
usually not covered by health care (only half of provinces and not in USA)
• Autosomes are the first 22 pairs of chromosomes, each pair similar in size. The 23rd
pair of sex chromosomes determines the sex of the child, with X much larger than Y.
The egg contributes an X, and the sperm’s contribution (X or Y) determines the sex, with
the Y chromosome initiating a male pattern of development
• Each chromosome is one molecule of deoxyribonucleic acid (DNA). The rungs of the
helix carries the genetic code -> consists of pairs of nucleotides A-T, C-G. The order of
the pairs is the code that causes the cell to create specific amino acids, proteins etc.
•Each group of bases that provide a specific set of instructions is called a gene.
• The complete set of 25,000 genes make up a person’s heredity is known as the
genotype. Chromosome 1 has the most with 3,000 genes, Y the least with 200. Fewer
than 1% of genes causes differences between people.
• 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 dominant.
• 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 i.e. excessive exercise or high altitudes.
• 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.
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• 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 ancestry. This explains why the sickle-cell
disease primarily affects African Canadian children as Africans with it are less likely to
die of malaria and pass on their genes -> since Canada has no malaria, sickle-cell
should be less common in successive generations which research is finding.
Genetic Disorders: Some disorders are inherited while sometimes eggs or sperm have
more or less chromosome than the usual 23.
• Homozygous recessive disorders include: albinism, cystic fibrosis, phenylketonuria,
Tay-Sachs disease and obviously sickle cell 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 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.hence by the time the symptoms occur they have
• Disorders can be sex-linked, where the gene responsible is carried on a sex
Hemophilia, a disorder in which the blood does not clot easily so sufferers bleed
severely even with minor injuries, has its gene on the X chromosome – risk greater for
males since they have no normal X to protect against the recessive allele where as
females are normally carriers.
• most inherited disorders are rare -> PKU is 1/10000 births while huntington’s is even
• A genetic counsellor can construct a detailed family history to determine likelihood of
Abnormal Number of Chromosomes -> more common than inherited disorders.
• 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 (extra chromosome) 21,
resulting in cognitive deficits and developmental delay.
•People with downs have almond shaped eyes and a fold over the eyelid,
head/neck/nose is smaller, normal as an infant but soon mental and behavioural
development starts to lag, by childhood felay is apparent.
•Need special programs to prepare them for school, life expectancy is 25-60
•The extra chromosome is usually provided by the egg; the risk increases
markedly with maternal age due aging – perhaps due to deterioration in eggs 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
• Sex chromosomes can also be disrupted -> note X is always present -> necessary for
o XXX: Female normal appearance, menstrual irregularities, learning disorders, delayed
motor and language developement
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