Download Chapter Three: Heredity and Environment

Chapter Three: Heredity and Environment
Genes
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Genes play a leading role in the journey of human development.
Parents what to know what traits their children will inherit.
Physicians question patients about close relatives and distant ancestors.
Scientists have been working for decades to discover how seriously to take genetics.
What Genes Are
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Genes are made up of DNA: the complex protein code of genetic information
DNA directs the form and function of each body cell as it develops
Each molecule of DNA is called a chromosome
Chromosomes contain instructions to make all the proteins a living being needs
Each person has 23 sets of chromosomes, or 46 chromosomes that each cell contains that contain all
human genes.
The human genome contains 30,000 genes – allows cells to make the proteins that help make each
person unique and sustain out development.
The Beginnings of Human Life
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Gamete: reproductive cell that directs process by which genetic information combined and transmitted
Father gametes—sperm
Mother gametes—ovum
Zygote: the single cell formed from the fusing of a sperm and an ovum.
Male and female gametes fuse and become a zygote
The Beginnings of Human Life
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Two reproductive cells have literally become one, and the 23 chromosomes from the father match up
with the 23 chromosomes from the mother – so that the zygote contains 46 chromosomes arranged in
pairs.
The genetic information contained on these 46 chromosomes constitutes the organism’s genetic
inheritance (Genotype) – which is set at conception and endures throughout life.
See “Mapping the Karyotype” page 61.
Sex Determination and Sex Ratio
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Of 22 out of 23 pairs of human chromosome, the matching chromosomes are very closely matched
– but not identical
• some genes come in slight, normal variations called alleles
The 23rd pair is different
– in females, it is designated XX
– in males, it is designated XY
Sex Determination and Sex Ratio, cont.
– Current sex ratio in United States is 52 males to 48 females
– Historically, wars and diseases sometimes killed most of a given population before they could
reproduce, with girls more likely to survive than boys.
– Problems in china?
– Worldwide, men are particularly likely to prefer sons over daughters.
Multiple Zygotes
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Monozygotic twins: identical twins (or quadruplets, etc.) originate from one zygote.
Because they originate from the same zygote and have the same genotype, monozygotic multiple births:
– share identical instructions for physical appearance, psychological traits, vulnerability to diseases,
etc.
– 1/3 of twins are monozygotic
Multiple Zygotes, cont.
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Dizygotic twins (fraternal twins) —begin life from two separate zygotes
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Such twins share about half their genes, like any other siblings.
Dizygotic births occur once in every 60 births
Occurs as frequently as 1 in 6 pregnancies, but usually only 1 twin develops past embryo stage
Variation among ethnic groups.
Multiple Zygotes, cont.
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Dizygotic twins
– women in late 30’s are three times more likely to have dizygotic twins as women in their early 20s.
– share no more genes than other offspring of the same parents (about 50 percent of the genes
representing individual difference).
• 50 percent of the time one twin is male
• They may look different from each other or look a great deal alike.
Health Benefits of Genetic Diversity
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Genetic diversity safeguards human health
Genetic diversity maintains the species
The more closely related the organisms, the more genes they have in common.
For example, humans and chimpanzees have 99 percent of their genes in common.
From Genotype to Phenotype
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Every psychological characteristic is genetically influenced
Every psychological characteristic and personal trait is affected by the environment
Nature always interacts with nurture.
From Genotype to Phenotype, cont.
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Genotype: genetic potential – the total of all the genes a person inherits.
Phenotype: combination of genetic potential and expressed traits.
• What can be observed by looking at a person, by analyzing the person’s body chemistry, and the
noting the person’s behavior.
• Physical traits (such as bushy eyebrows) and nonphysical traits (such as a hunger for excitement)
are parts of the phenotype.
Behavior Genetics
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Behavior genetics: study of effects of genes on behavior
– personality patterns, psychological disorders, and intellectual abilities
– Without genes, no behavior could exist.
– Without environment, no gene would be expressed.
Senility Caused by Alzheimer’s Disease
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Most common and feared type of senility is Alzheimer’s disease
– amyloid B protein accumulates in the brain, leading to dysfunction and destruction of brain cells and
disruption of the mind
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Can be genetic—but only when
“early-onset” beginning before age 50.
Senility Caused by Alzheimer’s Disease, cont.
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If “late-onset,” (more prevalent as more people live to age 80) may be a combination of genes and
environment
It is easy to overestimate the role of genes in almost any condition known to be genetic.
– other predictors that affect the mind may include hypertension, diabetes, high cholesterol, diet,
exercise, not smoking, weight control, mental alertness, and physical health
– Thus, being mentally alert as well as physically healthy also puts off senility.
Schizophrenia
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Psychopathologies (depression, antisocial behavior, phobias, and compulsions) are genetically based
traits that are subject to strong environmental influence.
Relatives of people with schizophrenia have a higher-than-normal risk of developing the illness
themselves.
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Schizophrenia is multifactorial, with environmental elements (show-acting virus, head injury,
inadequate oxygen at birth)
Alcoholism
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Inherited biochemistry makes some people highly susceptible to alcohol addiction
Anyone can abuse alcohol, but each person’s genetic makeup creates an addictive pull can be
overpowering, or weak, or something in the middle
– may explain ethnic variations
Alcoholism, cont.
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Examples of traits – quick temper, a readiness to take risks, and a high level of anxiety.
Culture counts too (whether alcohol is present in environment) - if a person with a strong genetic
tendency toward alcoholism spends a lifetime in an environment where alcohol is unavailable, the
genotype will never be expressed in the phenotype.
Chromosomal and Genetic Abnormalities
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We now give attention to these because we can recognize
3 reasons why chromosomal and genetic abnormalities are the focus of attention:
– disruptions of normal development
– origins of genetic and chromosomal abnormalities
– misinformation and prejudice add to problems of people with these abnormalities
Chromosomal Abnormalities
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A gamete with more than or less than 23 chromosomes creates a zygote with chromosomal
abnormalities
– most likely variable that creates chromosomal abnormalities is mother’s age (over 35)
– father’s age (over 40) also a variable
Chromosomal Abnormalities, cont.
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Most zygotes with chromosomal abnormalities never come to term because of spontaneous abortions or
miscarriages.
– spontaneous abortion occurs in about one-half of all fetus with chromosomal abnormalities
Down Syndrome
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The most common extra-chromosome condition.
Three chromosomes at gene #21 (trisomy-21)
A few decades ago, almost all surviving Down Syndrome infants died in early childhood, but advances
in treatment mean that now most survive well into adulthood.
Characteristics:
Abnormalities of the 23rd Pair
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Location of sex chromosome – about 1 in every 500 infants is missing a sex chromosome.
Kleinfelters syndrome —XXY
– Characteristics?
Fragile X syndrome
– Caused by the mutation of a single gene on an X chromosome
– intensifies from generation to generation
Genetic Testing and Genetic Counseling
• Genetic counseling: a process of consultation and testing that enables individuals to learn about their
genetic heritage, including conditions that might harm any children they may have.
• Recommended for:
• Individuals with a parent, sibling, or child with a serious genetic condition known to be dominant or
recessive
• Couples with history of early spontaneous abortions, stillbirths, or infertility
• Couples from the same ethnic group or subgroup—especially if closely related
• Women over 35 and men over 40
The Process of Genetic Counseling
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Counselor constructs couples’ family history
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Some tests provide information before conception
– charts patterns of health and illness over generations
The Process of Genetic Counseling, cont.
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Other tests are prenatal
– alpha-fetoprotein assay (a sample of the mother’s blood is tested for the level of alpha-fetoprotein.
– ultrasound (AKA sonogram) (high-frequency sound waves are used to produce a “picture” of the
fetus.
– Amniocentesis (about ½ an ounce of the fluid inside the placenta is withdrawn).
The Process of Genetic Counseling, cont.
– Chorionic villi sampling (a sample of the placental tissue that surrounds the fetus is obtained and
analyzed).
A Basis for Decision
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Many want to know ahead of time, some do not
Genetic testing is a tool, not an answer.
Alternatives
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If both partners are carriers of a serious condition or are at high risk because of age or family
characteristics, they may turn to:
- artificial insemination with donor
sperm
- in-vitro fertilization (IVF) with a donor ovum
– artificial insemination donor (AID)
– postponement of pregnancy until promising treatments are further developed
Alternatives
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Decisions about conception are not based on genetic analysis.
Two couples with identical odds may make quite different choices – depending on age, ethnicity,
religion, finances, ethics, personal relationships, and the number and health status of any other children
they may have.
Alternatives
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Ethics?
There is no right or wrong decision.
Every decision about prenatal testing is difficult, and each choice has its costs
Ultimately, a child with special needs is ultimately the primary responsibility of the parents.