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Chapter 2

Chapter 2 Notes.docx

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Department
Psychology
Course Code
PSYC 251
Professor
Elizabeth Kelley

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Page 44-63, 20 pages Page 1 of 4
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
development.
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 phenotypethe 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.
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
ancestry.
Genetic Disorders
Inherited Disorders
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

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Page 44-63, 20 pages Page 1 of4 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 rd • 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 development. • 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 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. • 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 ancestry. Genetic Disorders Inherited Disorders • 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 males. • A genetic counsellor can construct a detailed family history to determine likelihood of inherited disorders. 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  hormone therapy 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 or labour. Behavioural Genetics • Behavioural Genetics deals with the inheritance of behavioural and psychological trait
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