Heredity Influences on Development

Chapter 3

Dr. Pelaez

Principles of Heredity TransmissionThe Genetic Code

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

• The zygote contains 46 chromosomes, consisting of thousands of chemical segments, or genes.

• Genes are hereditary blueprints for development that are passed on unchanged from generation to generation.

• Chromosomes come in matching pairs. (23 chromosomes from each parent.)

Growth of the Zygote and Production of Body Cells

• The zygote moves through the fallopian tube toward the uterus, and begins the process of mitosis.

• The zygote first divide into 2 cells, then 2 become 4, 4 become 8, 8 become 16, etc.

• Before each division, the zygote duplicates its 46 chromosomes.

Germ (or Sex) Cells

• The germ cells are the sex cells of the zygote.

• The germ cells’ function is to produce gametes, which are either sperm in males or ova in females.

• Meiosis occurs when the male germ cell in the testes produces sperm and when the female germ cell in the ovaries produces ova.Often cross-over takes place. This is when there is an exchange of genetic material.

Multiple Births

• Multiple births occur when the are two people who share a genotype. When the zygote begins the process of mitosis it may begin to split into identical cells. At this moment two individuals have been formed. This is an example of monozygotic twins.

• Dizygotic twins are more common. These are twins that result when a mother releases two ova at the same moment and each is fertilized by a different sperm.

The 23rd pair of chromosomes• The sex of the zygote is determined by the 23rd pair of

chromosomes.• For a female sex, both of these sex chromosomes are an X

chromosome.• For a male sex, the 23rd pair consists of one X chromosome and one

Y chromosome.

The purpose of our genes…..• Our genes are responsible for the construction of our enzymes and

other proteins.• These genes are essential in order to form new cells and their

specific functions.• Other genes are accountable for regulating the pace of

development.

How are genes expressed?

Simple Dominant-Recessive Inheritance

Alleles: human characteristics that are influenced by one pair of genes (one from the mother, one from the father).

1) Dominant-recessive: a pattern of inheritance in which one allele dominates another so that its phenotype is only expressed

2) Dominant: a powerful gene expressed phenotypically masking the effect of a less powerful gene (i.e., a gene for normal vision)

3) Recessive: a less powerful gene not expressed phenotypically when paired with a dominant allele (i.e., a gene for nearsightedness)

Codominance

Codominance: The phenotype produced is a compromise between the two genes.

Sex linked Inheritance

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

Genetic Imprinting

A process in which particular gene pairs are biochemically marked so that only one parent’s allele is expressed, regardless of its composition.

Polygenic Inheritance

Human characteristics that are influenced by many pairs of alleles (i.e., height, weight, intelligence, skin color, temperamental attributes, susceptibility to cancer).

Chromosomal and Genetic Abnormalities

Congenital defects are those that are present at birth, although many are not detectable when the child is born.

Chromosomal Abnormalities

Abnormalities of Sex Chromosomes• Involve the 23rd pair – sex chromosomes.• Males may be born with an extra X or Y chromosome.• About 1 in every 1,000 individual has an X chromosome that is

fragile and may have separated into two or more pieces, known as the Fragile-X syndrome.

Autosomal Abnormalities• 22 pairs of chromosomes that are similar in males and females• The most common occurs when an abnormal sperm/ovum carrying

an extra autosome combines with a normal gamete to form a zygote that has 47 chromosomes (i.e., Down syndrome, or trisomy-21).

Causes of Chromosomal Abnormalities• Most chromosomal abnormalities result from the uneven

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

• Aging-ova hypothesis is another cause of chromosomal abnormalities found in the offspring of older mothers.

Genetic Abnormalities

Mutations: changes in chemical structure of one or more genes that produce a new phenotype.

Genetic abnormalities may occur from mutations.

Mutations may be induced by:• toxic industrial waste• radiation• agricultural chemicals that enter the food supply• additives and preservatives in processed foods

Applications: Genetic Counseling, Prenatal Detection, and

Treatment of Hereditary Disorders

Genetic Counseling

Genetic counseling is a service that helps prospective parents to assess the likelihood that their children will be free of hereditary defects.

• Counselors usually begin by obtaining a complete report of family history.

Prenatal Detection

• The overall rate of chromosomal abnormalities dramatically increases after the age of 35. A prenatal screening known as amniocentesis is a method performed by extracting amniotic fluid from a pregnant woman that can be tested for chromosomal abnormalities and other genetic defects.

Disadvantage

Amniocentesis is not easily performed before the 11th – 14th week of pregnancy.

Alternative procedures

1) Chorionic villus sampling (CVS): collects tissue and can be performed during the 8th or 9th week of pregnancy

2) Ultrasound: scanning the womb with sound waves most useful after the 14th week of pregnancy

Treating Hereditary Disorders

New medical and surgical techniques, performed on fetuses have made it possible to treat some hereditary disorders:

• Delivering drugs or hormones to the unborn organism by performing bone marrow transplants

• Surgically repairing womb genetically transmitted defects of the heart, neural tube, urinary tract, and respiratory system.

• Germline gene therapy

Hereditary Influences on BehaviorBehavioral Genetics is the scientific study of how genotype interacts

with the environment in order to determine behavioral attributes such as intelligence, personality, and mental health.

Methods of Studying Heredity Influences

Heritability is the amount of variability in a trait that is attributable to hereditary factors.

1) Selective breeding experiment

2) Family studies

3) Estimating the Contribution of Gene and Environment

4) Gene Influences

5) Nonshared Environmental Influences (NSE)

6) Shared Environmental Influences (SE)

Heredity Influences in Intellectual Performance

• Plomin et al (1997)• Wilson (1978)

Heredity Contributions to Personality

• Twin data may be used to estimate genetic contributions • There is a heavy influence by the environment

(i.e., Nonshared Environmental influences (Rowe, 1994))

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

Heredity Contributions to Behavior Disorders and Mental Illness

There is strong indication that schizophrenia is genetically influenced.

Schizophrenia is a serious mental illness characterized by severe disturbances in logical thinking, emotional expression, and social behavior.

Heredity may also contribute to abnormal behaviors such as alcoholism, criminality, depression, hyperactivity, manic-depressive psychosis, and a number of neurotic disorders.

Heredity and Environment as Developmental Co-conspirators

The Canalization Principle

1) There are multiple pathways along which an individual might develop.

2) Nature and nurture combine to determine these pathways.

3) Either genes or the environment may limit the influence on development by other factors.

The Range-of-Reaction Principle

Range of reaction is when an individual genotype establishes a range of possible responses to different kinds of life experiences.

• Gottesman (1963)

Genotype/Environment Correlations• Passive Genotype/Environment Correlations• Evocative Genotype/Environment Correlations• Active Genotype/Environment Correlations