Transcript Heredity Influences on Development Chapter 3

Heredity Influences on
Development
Chapter 3
Dr. Pelaez
Principles of Heredity Transmission
The 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 Behavior
Behavioral 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)
2)
3)
4)
5)
6)
Selective breeding experiment
Family studies
Estimating the Contribution of Gene and Environment
Gene Influences
Nonshared Environmental Influences (NSE)
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)
2)
3)
There are multiple pathways along which an individual might
develop.
Nature and nurture combine to determine these pathways.
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