 |
| | | | |
Child Development - A Thematic Approach
, Fifth Edition
Danuta Bukatko - College of the Holy Cross
Marvin W. Daehler - University of Massachusetts, Amherst
| | |
| | Chapter Outline
Chapter 3:
Genetics and Heredity
-
Principles of hereditary transmission.
The genetic potential that a person inherits is called the genotype. A person's observable characteristics and behavior are called the phenotype. The genotype is formed when human reproductive cells called gametes (egg and sperm) combine during fertilization to provide the newly created offspring with basic units of heredity called genes. The genes comprise segments of larger units called chromosomes. Alleles are different forms of a gene. For instance, we have alleles resulting in
blue eyes, and alleles resulting in brown eyes. The entire set of genes, the human genome, is being mapped.-
The building blocks of heredity.
The genes and chromosomes are made up of deoxyribonucleic acid, or DNA. DNA is constructed of pairs of nucleotides. Human beings have forty-six chromosomes organized into twenty-three homologous
pairs. Twenty-two of the paired chromosomes are similar in males and females
and are called autosomes. In the female, the twenty-third pair consists of two X chromosomes. In the male, the twenty-third pair consists of one X chromosome and one Y chromosome. A karyotype is a pictorial representation of an individual's chromosomes. -
Cell division and chromosome duplication.
The fertilized egg, or zygote, begins as one cell containing forty-six chromosomes, and the number of cells
increases by mitosis, a process of cell division that produces new cells containing duplicates
of all forty-six chromosomes. The gametes are formed by a process of cell division called meiosis. In meiosis, genetic material is exchanged during crossing over and cell division proceeds so that one chromosome from each pair is randomly
selected for each human egg and sperm cell. -
Gene expression.
Each characteristic, or trait, is coded by at least one pair of genes on
an autosome, one inherited from the mother and the other from the father.
If the two variants, or alleles of a gene are identical, the trait for which they code is said to be homozygous. If two alleles are different, the trait that they determine is said to be heterozygous. The phenotype is influenced by whether dominant, recessive, or codominant allelic forms are inherited. Many traits are polygenic, or determined by more than one gene. -
Gene functioning and regulation of development.
Structural genes code for the production of enzymes and other proteins.
Regulator genes control structural genes. Proteins are produced in the cytoplasm
of a cell through a process initiated by mRNA. Phenylketonuria, or PKU, is a condition in which the amino acid phenylalanine cannot be metabolized.
As a consequence, normal development is disrupted. Some phenotypic expressions
are different depending on whether a gene fails to be inherited from the mother or the father, a phenomenon known
as genomic imprinting.
-
Gene and chromosomal abnormalities.
Mutations, or sudden changes in genetic material, occur relatively often and provide genetic diversity among individuals. They can also lead to severe
disruption in physical and behavioral development.-
Gene variations.
Some gene disorders, such as Williams Syndrome, are caused by the inheritance of a single dominant allele. Others, such as sickle cell anemia and PKU, are caused by the inheritance of two recessive alleles. Many disorders,
including fragile X syndrome, are sex linked because they are associated with genes on the twenty-third pair of chromosomes. -
Chromosome variations.
Down syndrome, or trisomy 21, is caused by an extra twenty-first chromosome. It is one of the most common
genetic causes of mental retardation. Structural aberrations of chromosomes often lead to mental retardation
and severe physical deformities.
A varying number of sex chromosomes may be inherited, leading to anomalies
that show a wide variety of phenotypic expressions. Studies suggest that children with sex chromosome abnormalities may be more vulnerable to environmental
stressors than children with the normal number of sex chromosomes.
-
Genetic counseling
Prospective parents at risk for bearing children with a genetic disease as a result of family history or other factors,
can receive genetic counseling. Genetic screening can indicate whether couples planning to conceive a child are heterozygous
or homozygous for a genetic disorder. Genetic screening can also be conducted on the fetus to determine if it possesses a genetic
disorder.-
Prenatal diagnosis.
A number of procedures are now available to detect potential genetic and
other disorders in the fetus. In fetal blood sampling, typically performed after about eighteen weeks gestational age, blood withdrawn
from the umbilical cord is analyzed. In amniocentesis, typically performed about fifteen to eighteen weeks gestational age, a small amount of amniotic fluid is extracted via a needle inserted through the
mother's abdomen to examine the chromosomes in the fetal cells present in the fluid.
In chorionic villus sampling, typically performed about ten to twelve weeks gestational age, a sample of the membrane surrounding the embryo (the chorion) is cultured to
examine the chromosomal makeup of the cells. Maternal blood screening is a less invasive procedure that can provide evidence of increased risk
for various disorders. Ultrasonography is a nonintrusive procedure that produces a picture of the embryo or fetus
from sound waves that pass through the mother's abdominal wall and reflect off the fetus. Some risk may be associated with
each of these procedures, though ultrasound is now routinely used in many countries. -
Ethical and social issues.
The use of prenatal diagnostic tests often occurs relatively late in pregnancy.
In addition, obtaining results may take from days to weeks, sometimes contributing to considerable anxiety on the part of prospective parents. Prospective
parents can decide whether or not to undergo such tests. Members of various
ethnic groups differ in the likelihood of having such tests carried out.
Among the concerns of many is the potential inappropriate use of genetic information gained from the
tests.
Controversy: Should Sex Preselection Be Permitted?
Prospective parents sometimes prefer to have a boy and sometimes a girl.
Recent advances may now permit couples to more effectively influence the likelihood of delivering a son
or a daughter. Could such an option lead to disproportionate numbers of males
and females in a society? Many people appear to support the opportunity to
make choices about such matters. However, if sex preselection becomes popular, others fear that it could
be the precursor to efforts to have "designer children," offspring more likely to have certain specific traits or abilities. |
-
Developmental and behavioral genetics
-
The methods of behavioral geneticists.
Behavior geneticists often conduct selective breeding experiments in animals to learn more about the influence of genetics and the environment
on behavior. The study of twins provides a means of determining the role
of nature and nurture in humans. When identical (monozygotic) twins are observed to be more similar in a trait than fraternal (dizygotic) twins, one possible interpretation is that the trait is determined by the common
genotype. Because most twins also share common environments, researchers
study the similarities between twins who have been adopted or otherwise raised in different environments. The degree of resemblance among monozygotic
and dizygotic twins, siblings, and adopted children is estimated using concordance rates and heritability of complex traits and behaviors. -
Conceptualizing the interaction between genotype and environment.
The nature of the contributions made by the genotype and the environment
is complex because they often interact. The concept of a range of reaction suggests that genotypes establish limits on the phenotypes that can be shaped by environment. With some attributes,
a canalization process takes place so that the genotypic influence on the phenotype dominates,
and only extreme variations in the environment can alter the phenotypic expression. -
Conceptualizing the correlation between genotype and environment.
The correlations that occur between genotype and environment make it difficult
to estimate to what degree each contributes to development. Accurate estimates of their contributions are complicated by passive,
reactive (or evocative), and active (or niche-picking) correlations between the genotype and the environment. Because of these
influences, siblings sometimes become more similar as they grow older and identical twins sometimes become similar even when reared
apart. -
Hereditary and environmental influences on behavior.
Heritability is the proportion of variability in the phenotype accounted for by genetic influences within current known environmental variation.
Indications that heredity contributes to intelligence come from studies showing
that correlations in performance on intelligence tests increase as genetic
relationships increase. Similar correlations are seen between genetic relationships and personality characteristics,
temperament, mental illness, and other characteristics. That the environment
also contributes to human behavior is revealed by research showing greater
similarities among genetically related individuals reared together than among genetically
related individuals reared apart. The environment individuals share may be
less effective in producing similarities than psychologists originally believed.
However, the experiences unique to individuals, that is, their shared environment, may make a powerful
contribution to individual differences within families.
Research Applied to Parenting: Treating Siblings Fairly
Parents should expect siblings to differ from one another, and children in the same family will often seek out ways to
be unique. Siblings, by virtue of their variations in age and background,
experience family events in dissimilar ways and need to be treated differently.
Nevertheless, to the extent that it is age appropriate, impartial treatment of siblings contributes to
less conflict and more effective interactions. "Favoritism" has negative consequences for sibling interactions. |
| |
| |
|
|
|
