chapter 2 the human heritage: genes and environment
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Chapter 2
The Human Heritage: Genes and Environment
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Genetic Code• Chromosome: A threadlike
structure made up of genes – 46 in all human cells, except
sperm and ova which have 23• Deoxyribonucleic acid (DNA):
A long double-stranded molecule that makes up chromosomes
• Genes: The segments on a DNA molecule that act as hereditary blueprints for the organism’s development
Model of DNA
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DNA Replication• A strand of DNA replicates
by splitting down the middle of the rungs of its ladderlike structure
• Each free base picks up a new complementary partner:– Cytosine (C) pairs with
guanine (G)– Adenine (A) pairs with
thymine (T)
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Mitosis: A Process of Cell Replication
Meiosis: A Source of Variability
Sexual Determinism: A Case of Variability
Sexual Reproduction and Genetic Transmission
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Mitosis
• Mitosis is the process of cell division that generates all the cells of the body except the germ cells.
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Meiosis
• Meiosis is the process of cell division that generates the germ cells.
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Sexual Determinism: A Case of Variability
The Five Sexes Fausto-Sterling (1993)
1- Male = testes 2- Female = ovaries3- Hermaphrodites = 1 testes & 1 ovary4- Merms = Male pseudohermaphrodites i.e.,
testes & some aspect of female genitalia5- Ferms = Female pseudohermaphrodites
i.e., ovaries & some aspect of male genitalia
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An Interaction
• Genotype: An individual’s genetic endowment
• Environment: The totality of conditions and circumstances that surround an individual
• Phenotype: The observable characteristics of an individual
– Physical and psychological traits, health, behavior
Genotype + Environment Phenotype
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Laws of Genetic Inheritance
Gregor Mendel (1822-1884): Garden peas• Allele: The specific form of a gene coded for a
particular trait
• Homozygous: Having inherited two genes of the same allelic form for a trait
• Heterozygous: Having inherited two genes of different allelic forms for a trait
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Heterozygous Possibilities1. Dominant gene: The allele that
is expressed when an individual possesses two different alleles for the same trait
2. Recessive genes: The allele that is not expressed when an individual possesses two different alleles for the same trait
3. Codominance: An outcome in which a trait that is determined by two alleles is different from the trait produced by either of the contributing alleles alone
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Sex-Linked Traits• Given that the X chromosome is much larger than the Y
chromosome, most inherited sex-linked characteristics are carried on the X chromosome
• Males are more susceptible to genetic defects than are females– If a daughter has a harmful recessive gene on one X chromosome,
she will usually have a normal dominant gene on the other X chromosome to override it
– A son who inherits a harmful recessive gene on his X chromosome has no such complementary allele to override the recessive gene’s harmful effects
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Genotype and Phenotype
The Laws of Genetic Inheritance
•Sex-Linked Genetic Effects•Examples: Red-green color blindness, hemophilia, muscular dystrophy, night blindness,Hypogammaglobulin (lack of production of anitbodies to fight bacteria), Vitamin D Resistance (Rickets)
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Genes, The Organism, and the Environment
• Range of Reaction- all possible gene/env relationships compatible w/ organism’s continued life
• Canalization- certain characteristics typical of a species may be restricted to a narrow range despite wide variations in environmental conditions
• The Study of Genetic Influences on Human Behavior- heritability I.e., genetically influenced v. genetically caused
• Estimating Genetic Influence Through Kinship Studies- twin & adoption studies
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Mutations and Genetic Abnormalities
Sickle-Cell Anemia: An Example of Gene-
Environmental Interaction
Down Syndrome: A Chromosomal Error
Sex-Linked Chromosomal
Abnormalities
Phenylketonuria: A Treatable Genetic
Disease
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Down Syndrome: A Chromosomal Error
• Cause: More than 95% of children born with Down syndrome have 3 (vs. 2) copies of chromosome 21
• Traits: Mentally/physically retarded; distinctive physical characteristics (e.g., slanting eyes, rather flat facial profile, ears lower than normal, short neck, protruding tongue, dental irregularities, small curved fingers, wide-spaced toes); more likely to die young
• Incidence: 1:1000 births (increases with mother’s age: 1:100 by age 40, 1:32 by age 45, 1:12 by age 49)
• Outlook: Supportive intervention that includes special education by concerned adults can markedly improve the intellectual functioning of some of these children
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Down Syndrome: A Chromosomal Error
• Cause: More than 95% of children born with Down syndrome have 3 (vs. 2) copies of chromosome 21
• Traits: Mentally/physically retarded; distinctive physical characteristics (e.g., slanting eyes, rather flat facial profile, ears lower than normal, short neck, protruding tongue, dental irregularities, small curved fingers, wide-spaced toes); more likely to die young
• Incidence: 1:1000 births (increases with mother’s age: 1:100 by age 40, 1:32 by age 45, 1:12 by age 49)
• Outlook: Supportive intervention that includes special education by concerned adults can markedly improve the intellectual functioning of some of these children
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Sickle-Cell Anemia: Gene–Environment Interaction
• Cause: Recessive gene (victims are homozygous, but heterozygous subjects are also mildly affected)
• Traits: Abnormal blood cells cause circulatory problems (e.g., heart enlargement) and severe anemia
• Incidence: 8-9% of U.S. blacks• Outlook: Crippling, but
treatable with medication
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Klinefelter Syndrome: A Sex- Linked Abnormality
• Cause: Extra X chromosome (i.e., XXY)
• Traits: Males fail to develop secondary sex characteristics (e.g., facial hair, voice change), are sterile, and most have speech and language problems
• Incidence: 1:900 males
• Outlook: May be treatable with testosterone replacement therapy
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Prenatal Testing
• Ultrasound
• Amniocentesis
• Chorionic Villus Sampling
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Ultrasound
• Ultrasound creates images of the fetus by using high-frequency sound waves to "see" inside your body. This ultrasound image reveals the normal development of healthy twins.
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Amniocentesis
• During an amniocentesis, samples of amniotic fluid are removed using a needle and then tested for genetic abnormalities in the laboratory.
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Chorionic Villus Sampling
• During CVS, samples of the cells that line the placenta, known as chorionic villi cells, are removed and tested for genetic abnormalities in the laboratory.
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Survival Strategies: Culture• Cultural artifacts, such as tools and
clothing, greatly extend the range of environments in which human beings can reproduce and thrive
• Cultural knowledge, such as the creation of “hazard prevention strategies,” serves to protect and support children to enable them to mature and reproduce