THE BIOLOGICAL AND ENVIRONMENTAL CAUSES

OF DEVELOPMENTAL DISABILITIES

Chapter 4

By: Mrose

The Basic Concepts of Human Reproduction

Heredity- the mechanism for the transmission of human characteristics from one generation to the next.

Genome- genetic code; complete set of coded instructions for making and maintaining an organism. Inherited from both parents.

Chromosomes- are thread-like structures located inside the nucleus of animal and plant cells. Each chromosome is made of protein and a single molecule of deoxyribonucleic acid (DNA). Passed from parents to offspring, DNA contains the specific instructions that make each type of living creature unique.

Deoxyribonucleic acid (DNA) – long threadlike molecule and genetic substance. Is a complex molecule that contains the genome.

Double helix- two strands of twisted ladder-shaped structure that wrap around each other.

DNA

Gene – specific sequence of the four nucleotide bases whose sequences carry the information for constructing proteins.

Some Principles of Genetic

Determination

Dominant-Recessive Genes Principle

The principle that if one gene of a pair is dominant and the other is recessive, the dominant gene exerts its effect, overriding the potential influence of the recessive gene. A recessive gene exerts its influence only if both genes in the pair are recessive. (66)

Sex-linked Genes Principle A particularly important category of genetic linkage

has to do with the X and Y sex chromosomes.  These not only carry the genes that determine male and female traits but also those for some other characteristics as well.  Genes that are carried by either sex chromosome are said to be sex linked.

Men normally have an X and a Y combination of sex chromosomes, while women have two X's.  Since only men inherit Y chromosomes, they are the only ones to inherit Y-linked traits.  Men and women can get the X-linked ones since both inherit X chromosomes.

Polygenic Inheritance Principle Poly (many) genic (genes) inheritance

describes the interaction of many genes to produce a particular characteristic. Considering that there are as many as 140,000 genes, the huge number of combinations possible is hard to imagine. Traits that are produced by the mixing of genes are sais to be polygenically determined.

Genetoype The Genotype is that part (DNA sequence) of

the genetic makeup of a cell, and therefore of an organism or individual, which determines a specific characteristic (phenotype) of that cell/organism/individual. Genotype is one of three factors that determine phenotype, the other two being inherited epigenetic factors, and non-inherited environmental factors. DNA mutations which are acquired rather than inherited, such as cancer mutations, are not part of the individual's genotype.

Phenotype A phenotype (from Greek phainein, meaning "to show",

and typos, meaning "type") is the composite of an organism's observable characteristics or traits, such as its morphology, development, biochemical or physiological properties, phenology, behavior, and products of behavior (such as a bird's nest). A phenotype results from the expression of an organism's genes as well as the influence of environmental factors and the interactions between the two. When two or more clearly different phenotypes exist in the same population of a species, the species is called polymorph.

The Biological Sources of Developmental Disabilities

Basic terms in Human Reproduction

Gametes- are human reproduction cells which are created in the reproductive organs. Ovum- egg cells; which the ovaries of the female produceSpermatozoa- sperm cells; which the testicles/testes of the male produce

Meiosis The process of cell division in which each

pair of chromosomes in the cell separates, with one member of each pair going into each gamete or daughter cell. Thus, each gamete, the ovum and the sperm, has twenty-three unpaired chromosomes.

Fertilization - is the union of a human egg and sperm, usually occurring in the ampulla of the uterine tube. The result of this union is the production of a zygote, or fertilized egg, initiating prenatal development.zygote- fertilization results in the formation of a

single cell. In the zygote, 2 sets of 23 unpaired chromosomes, one set each from the male and female combine to form one set of paired chromosomes. In this manner, each parent contributes 50 % or half of the zygote’s genetic code or genome.

zygote

Critical periods and developmental vulnerability

during pregnancy Vulnerability- refers to how susceptible

the organism is to being injured or altered by a traumatic accident.

A traumatic incident includes such broad occurrences as teratogens or toxic agents.

Deviancy – from the normal course of prenatal development results to the occurrence of developmental disabilities.

The Course of Prenatal

Development

GERMINAL PHASE

-the initial stage of prenatal development covers first two weeks after fertilization.

3 significant developments :1. Creation of zygote- reproduction begins

with the fertilization of a female’s ovum by a male sperm

Ovulation- is when a mature egg is released from the ovary, pushed down the fallopian tube, and is available to be fertilized. Approximately every month an egg will mature within one of your ovaries.

Fertilization- takes place when the union of the genetic materials in the ovum and sperm cells.

Zygote- new cell w/c results from the transmission of the genetic materials 24 to 23 hours after fertilization.

2. Continuous cell division and cell tissue differentiationCell division- occurs very rapidly in the first few days and progress with considerable speed. Cell differentiation- continues as the inner and outer layers of the organism are formed.

Blastocyst- inner layer of cells w/c develop into the embryo later on

Trophoblast- outer layer of cells that provides nutrition and support for the embryo.

3. Implantation or attachment of the zygote to the uterine wall

What can go wrong during the germinal phase?Genetic disorders can be transmitted, such as:

1. Dominant and recessive diseases like:

Genetic mutation

2. Sex-linked inheritances such as:

3. Polygenic inheritances

4. Chromosomal deviations

5. Sex chromosomal anomalies like:

Klinefelter syndrome

6. Cranial or skull malformations such as:

significant developments in the germinal period

EMBRYONIC PHASE The second phase of human development

occurs from the end of the germinal phase to the second month of pregnancy. The mass of cells is now called the embryo.

3 main process during this phase :1. Intensification of cell differentiation

a. ectoderm –outermost layer of cells that will develop into the surface body parts

b. mesoderm- the middle layer that will develop into the body parts surrounding the internal areas

c. endoderm- the inner layer of cells that will develop into the epithelium

2. Development of the life support systems

3. Organogenesis – is the process of organ formation and the appearance of body organs during the first two months.

What can go wrong during the embryonic phase?Physical abnormalities can result. At birth, there are infants born with extra or missing limbs and fingers, ears and other body parts, a tail-like protrusion, heart or brain, digestive or respiratory organs outside the body.

Fetal Phase The third phase covers seven months that

lasts from the third to ninth month of pregnancy on the average.

Caucasian baby

At 3 months, the fetus is about 3 inches long and weighs about one ounce.

At 4 months, the fetus is five and a half inches long. Weighing about four ounces.

At 5 months, the fetus is ten to twelve inches long and weighs one half to one pound or almost half a kilo.

At six months, the fetus is 14 inches long and has gained one half to one pound.