EDP-6505 HUMAN DEVELOPMENT: CHILD AND ADOLESCENCEChapter 3 (Shaffer) – Hereditary Influences on Development

Genotype is the genetic endowment that an individual inherits.

Genes are hereditary blueprints for development that are transmitted unchanged from generation to generation.

PRINCIPLES OF HEREDITARY TRANSIMISSION

Conception is the moment of fertilization, when a sperm penetrates an ovum, forming a zygote.

A.) The Genetic Code After conception, this new cell, called a zygote, is

only 1/20th of the size of the head of a pin. The new cell nucleus contains 46 elongated,

threadlike bodies called chromosomes, each of which consists of thousands of chemical segments, or genes – basic units of heredity.

Chromosomes come in matching pairs. Each member of a pair corresponds to the other in size, shape, and the heredity functions it serves. One member of each chromosome pair comes from the mother’s ovum and the other form the father’s sperm cell (23 chromosomes from each parent).

1

B.) Growth of the Zygote and Production of Body Cells

As the zygote moves through the fallopian tube toward its prenatal home in the uterus, it begins to reproduce itself through the process of mitosis.

At first, the zygote divides into two cells, but the two soon become four, four become eight, eight become sixteen, and so on.

Just before each division, the cell duplicates its 46 chromosomes, and these duplicate sets move in opposite directions.

2

C.) The Germ (or Sex) Cells Production of Gametes through Meiosis

1.) In addition to body cells, human beings have germ cells that serve one special hereditary function – to produce gametes (sperm in males and ova in females).

2.) When male germ cells in the testes and female germ cells in the ovaries produce sperm and ova, they do so through a process called meiosis.

3.) The germ cell first duplicates its 46 chromosomes.

4.)Then an event called crossing-over often takes place: Adjacent chromosomes cross and break at one or more points along their length, exchanging segments of genetic material. This transfer of genes during crossing-over creates new and unique hereditary combinations.

3

Hereditary Uniqueness1.) Because each chromosome pair segregates independently of all other pairs according to the principle of independent assortment, there are many different combinations of chromosomes that could result from the meiosis of a single germ cell.

D.) Multiple Births There is one circumstance under which two people

share a genotype. Occasionally, a zygote that has begun to duplicate splits into separate but identical cells, which then become two individuals. These are called monozygotic (or identical) twins because they have developed from a single zygote and have identical genes.

More common are dizygotic (or fraternal) twins – pairs that result when a mother releases two ova at the same time and each is fertilized by a different sperm.

4

E.) Male or Female? Karyotypes (chromosomal portraits) reveal that 22

of the 23 pairs of human chromosomes (called autosomes) are similar in males and females. Sex is determined by the 23rd pair.

In males, the 23rd pair consists of one elongated body known as an X chromosome and a short, stubby companion called a Y chromosome.

In females, both these sex chromosomes are Xs.

F.) What Do Genes Do? At the most basic, biochemical level, they call for

the production of enzymes and other proteins that are necessary for the formation and functioning of new cells.

Some genes are responsible for regulating the pace and timing of development (i.e., the genes responsible for the growth spurt we experience as adolescents, and then shut these growth genes down in adulthood).

Environmental factors clearly influence how messages coded in the genes are carried out (i.e., a child who inherits genes for tall stature but has poor nutrition for a prolonged period early in life,

5

he could end up being only average or even below average in height).

G.) How Are Genes Expressed? Simple Dominant-Recessive Inheritance

1.) Many human charcteristics are influenced by only one pair of genes (called alleles): one from the mother, one from the father.

2.) Gregor Mendel contributed greatly to our knowledge of single gene-pair inheritance by cross-breeding different strains of peas and observing the outcomes.

3.) Simple dominant-recessive inheritance is a pattern of inheritance in which one allele dominates another so that only its phenotype is expressed.

4.) A dominant allele is a relatively powerful gene that is expressed phenotypically and masks the effect of a less powerful gene (i.e., a gene calling for normal vision).

5.) A recessive allele is a less powerful gene that is not expressed phenotypically when paired with a dominant allele (i.e., a gene calling for nearsightedness).

6

Codominance1.) Alternative forms of a gene do not always

follow the simple dominant-recessive pattern described by Gregor Mendel.

2.) Some are codominant: The phenogype they produce is a compromise between the two genes.

3.) Another type of codominance occurs when one of two heterozygous alleles is stonger than the other but fails to mask all of its effects (i.e., sickle-cell anemia).

Sex-linked Inheritance1.) Some traits are called sex-linked

characteristics because they are determined by genes located on the sex chromosomes (i.e., red/green color blindness).

Genetic Imprinting1.) “Parent-specific” patterns of inheritance are

due to genetic imprinting – a process in which particular gene pairs are biochemically marked so that only one parent’s allele (either the mother’s or father’s) is expressed, regardless of its composition.

Polygenic Inheritance

7

1.) Most important human characteristics are influenced by many pairs of alleles and are called polygenic traits (i.e., height, weight, intelligence, skin color, temperamental attributes, susceptibility to cancer).

8

CHROMOSOMAL AND GENETIC ABNORMALITIES

Congenital defects are those that are present at birth, although many of these conditions are not detectable when the child is born (i.e., Huntington’s disease).

A.) Chromosomal Abnormalities Abnormalities of the Sex Chromosomes

1.) Many chromosomal abnormalities involve the 23rd pair – the sex chromosomes.

2.) Occasionally, males are born with an extra X or Y chromosome, producing the genotype XXY or XYY, and females will often survive if they inherit a single X chromosome (XO) or even 3, 4, or 5 X chromosomes. Each of these conditions has somewhat different developmental implications.

9

3.) About 1 individual in 1000 has an X chromosome that is brittle in places and may even have separated into two or more pieces – a condition known as Fragile-X syndrome.

Autosomal Abnormalities1.) Several hereditary abnormalities are

attributable to the autosomes – that is, the 22 pairs of chromosomes that are similar in males and females.

2.) The most common type of autosomal abnormality occurs when an abnormal sperm or ovum carrying an extra autosome combines with a normal gamete to form a zygote that has 47 chromosomes.

3.) Down syndrom , or trisomy-21, a condition in which the child inherits all or part of an extra 21st chromosome. Children with Down syndrome are mentally retarded, with IQs that average 50.

Causes of Chromosomal Abnormalities1.) Most chromosomal abnormalities result

from the uneven segregation of chromosomes during the meiosis of male and female germ cells.

10

2.) Another cause is found in offspring of older mothers, where the likelihood of Down syndrome and other chromosomal abnormalities increases.

3.) The aging-ova hypothesis is the hypothesis that an older mother is more likely to have children with chromosomal abnormalities because her ova are degenerating as she nears the end of her reproductive years.

11

B.) Genetic Abnormalities Genetic abnormalities may result from mutations –

that is, changes in the chemical structure of one or more genes that produce a new phenotype. Mutations can also be induced by environmental hazards such as toxic industrial waste, radiation, agricultural chemicals that enter the food supply, and possible even some of the additives and preservatives in processed foods.

APPLICATIONS: GENETIC COUNSELING, PRENATAL DETECTION, AND TREATMENT OF HEREDITARY DISORDERS

A.) Genetic Counseling Genetic counseling is a service that helps

prospective parents to assess the likelihood that their children will be free of hereditary defects.

Genetic counselors normally begin by obtaining a complete family history, or pedigree, from each prospective parent to identify relatives affected by hereditary disorders.

B.) Prenatal Detection of Hereditary Abnormalities Since the overall rate of chromosomal

abnormalities dramatically increases after age 35, older mothers often undergo a prenatal screening known as amniocentesis. This is a method of

12

extracting amniotic fluid from a pregnant woman so that fetal body cells within the fluid can be tested for chromosomal abnormalities and other genetic defects.

A major disadvantage of amniocentesis is that it is not easily performed before the 11th to 14th week of pregnancy, when amniotic fluid becomes sufficiently plentiful to withdraw for analysis.

An alternative procedure known as chorionic villus sampling (CVS) collects tissue for the same tests as amniocentesis does an can be performed during the 8th or 9th week of pregnancy.

13

Another very common and very safe prenatal diagnostic technique is ultrasound, a method of scanning the womb with sound waves that is most useful after the 14th week of pregnancy.

C.) Treating Hereditary Disorders Today, the potentially devastating effects of many

other hereditary abnormalities can be minimized or controlled.

For example, new medical and surgical techniques, performed on fetuses in the uterus, have made it possible to treat some hereditary disorders by delivering drugs or hormones to the unborn organism by performing bone marrow transplants, or surgically repairing womb genetically transmitted defects of the heart, neural tube, urinary tract, and respiratory system.

14

Advances in genetic engineering are raising the specter of germline gene therapy – a process by which harmful genes are altered or replaced with healthy ones in the early embryonic stage, thereby permanently correcting a genetic defect.

HEREDITY INFLUENCES ON BEHAVIOR

Behavioral genetics is the scientific study of how genotype interacts with environment to determine behavioral attributes such as intelligence, personality, and mental health.

A.) Methods of Studying Heredity Influences Heritability is the amount of variability in a trait

that is attributable to hereditary factors.

1.) Selective breeding experiment is a method of studying genetic influences by determining whether traits can be bred in animals through selective mating. (I.e., Tryon (1940) attempted to show that maze-learning ability is a heritable attribute in rats.)

2.) Family studies are used with persons who live together and they are compared to see how similar they are on one or more attributes. If the attributes in question are heritable, then the similarity between any two pairs of individuals

15

who live in the same environment should increase as a function of their kinship.

There are two kinds of family studies: twin design and adoption design.

3.) Estimating the Contribution of Genes and Environment

When studying traits that a person either does or does not display (for example, a drug habit or clinical depression), researchers calculate and compare concordance rates – the percentages of pairs of people (i.e., identical twins, parents and their adoptive children) in which both members of the pair display the trait if one member has it.

16

4.) Gene Influences Behavioral geneticists use statistical

techniques to estimate the amount of variation in a trait that is attributable to hereditary factors. This index, called a heritability coefficient, is calculated as follows from twin data.

H = (r identical twins – r fraternal twins) x 2

5.) Nonshared Environmental Influences (NES) are unique to the individual – experiences that are not shared by other members of the family and, thus, make family members different from each other.

NSE = 1 – r (identical twins reared together)

6.) Shared Environmental Influences (SE) are experiences that individuals living in the same home environment share and that conspire to make them similar to each other.

SE = 1 – (H + NSE)

B.) Heredity Influences in Intellectual Performance According to Plomin et al. (1997), as children

mature, genes actually seem to contribute more (rather than less) to individual differences in their IQs.

17

Wilson (1978) found that identical twins were no more similar than fraternal twins on tests of infant mental development during the first year of life. By age 18 months, however, genetic influences were already detectable.

Adoption studies paint a similar picture. The IQs of adopted children are correlated with the intellectual performances of both their biological parents (suggesting a genetic influence) and their adoptive parents (indicating effects of shared family environment).

18

C.) Heredity Contributions to Personality Although psychologists have typically assumed

that the relatively stable habits and traits that make up our personalities are shaped by our environments, family studies and other longitudinal projects reveal that many core dimensions of personality are genetically influenced (i.e., introversion-extroversion and empathic concern).

1.) How Much Genetic Influence? If we use twin data to estimate the genetic

contribution to personality, we might conclude that many personality traits are moderately heritable (i.e., H = +.40).

One implication of moderate heritability coefficient is that personality is heavily influenced by environmental factors.

19

2.) Which Aspects of Environment Influence Personality?

Nonshared environmental influences contribute most heavily to personality (Rowe, 1994).

For example, parents often treat sons differently than daughters, or first-born children differently than later-borns.

D.) Heredity Contributions to Behavior Disorders and Mental Illness

Schizophrenia is a serious mental illness characterized by severe disturbances in logical thinking, emotional expression, and social behavior, which typically emerges in late adolescence or early adulthood. There are strong indications that schizophrenia is genetically influenced.

20

In recent years, it has also become quite clear that heredity contributes to abnormal behaviors and conditions such as alcoholism, criminality, depression, hyperactivity, manic-depressive psychosis and a number of neurotic disorders.

HEREDITY AND ENVIRONMENT AS DEVELOPMENTAL CO-CONSPIRATORS

Å.) The Canalization Principle This principle illustrates that (1) there are multiple

pathways along which an individual might develop, (2) nature and nurture combine to determine these pathways, and (3) either genes or environment may limit the extent to which the other factor can influence development.

An example of a highly canalized human attribute is babbling in infancy.

21

Examples of less canalized attributes such as intelligence, temperament, and personality can be deflected away from their genetic pathways in any of several directions by a variety of life experiences.

B.) The Range-of-Reaction Principle According to Gottesman (1963), genes typically do

not rigidly canalize behavior. Instead, an individual genotype establishes a range of possible responses to different kinds of life experiences: the so-called range of reaction.

Gottesman claims that genotype sets boundaries on the range of possible phenotypes that one might display to different environments.

C.) Genotype/Environment Correlations1.) Passive Genotype/Environment Correlations

is the notion that the rearing environments that biological parents provide are influenced by the parents’ own genes, and hence are correlated with the child’s own genotype.

2.) Evocative Genotype/Environment Correlations assumes that a child’s genetically influenced attributes affect the behavior of others toward him or her.

22

3.) Active Genotype/Environment Correlations is the notion that our genotypes affect the types of environments that we prefer and seek out.

CONTRIBUTIONS AND CRITICISMS OF THE BEHAVIORAL GENETICS APPROACH

Critics argue that the behavioral genetics approach is merely a descriptive overview of how development might proceed rather than a well-articulated explanation of development.

23