chapter 2: genetics genetic foundations heredity & environment
TRANSCRIPT
Genetic Foundations
Chromosomes: limits
“Each of us carries a “genetic code” that we inherited from our parents. Because a fertilized egg carries this human code, a fertilized human egg cannot grow into an egret, eagle, or elephant.”
Genes: Our Biological Blueprint
Human Genome Project Initial phase completed about the year 2000 Humans have 20,000 – 25, 000 genes (21,667) There are far more proteins than genes – 10-20 million Genes (DNA) are dependent- collaborate with other
sources of information Gene expression/activity is affected by context or
environment Context is affected by hormones, light, nutrition, etc.
Sex cells are formed by meiosis rather than mitosis.
Gametes (sperm and ova) have only 23 chromosomes total.
At conception, these two unite resulting in a full complement of 46 chromosomes (23 pairs).
A fertilized egg is called a zygote.
The Sex Cells
• Alleles are normal variations of a gene, found at the same location.
• A child who inherits the same allele (type of gene) from both parents is homozygous for that trait.
• A child who inherits different alleles from each parent is heterozygous for that trait.
Sources of Genetic Variation
Genetic ExpressionInfluenced by the environment
hormoneslightnutritionbehaviorstress (cortisol may cause a fivefold increase in DNA damage)
Sources of Variation
• Patterns of Genetic Inheritance•
Dominant-recessive: the dominant gene (allele) will determine the characteristic
Sources of Genetic Variation
• Examples of dominant genes Dark hair, curly hair, dimples,
types A & B blood (vs. type O), traits for normality in vision, hearing, pigmentation, etc.
Huntington’s Disease
Patterns of Genetic InheritanceDominant-recessive inheritance
• Examples of recessive genes: Cystic fibrosis, PKU, Tay-sachs
disease. Sickle-cell anemia
Patterns of Genetic InheritanceDominant-recessive inheritance
• Co-dominance: both alleles contribute to the phenotype, although not to the same degree.
• Additive: They contribute about equally (50%-50%).
• Example of Co-dominance; Sickle-cell anemia
Patterns of Genetic InheritanceCo-dominance and Additive
• Female children receive an X chromosome from the father which matches locations on the mother’s X.
• Male children receive a Y from the father, which does not have all the gene locations of an X.
• The defective gene on the mother’s X is offset by the gene on the normal X in females, but not in males.
• So, males will show evidence of the defective gene (e.g., hemophilia, RG colorblindness).
• Females will be normal, but carriers of the defective gene.
X-linked (sex-linked) inheritance
Polygenic Inheritance
Many genes interact to influence the characteristic
Most psychological characteristics are polygenic
(Where environmental factors are included, traits are said to be multi-factorial.)
Usually happen during meiosis
Involve breakage and failure to separate
Usually result in miscarriage
Those most commonly survived are:Down syndrome (trisomy 21)
Sex-linked abnormalities
Chromosomal Abnormalities
XXY (Klinefelter) may have verbal difficulties. Tall, underdeveloped testes, possible breasts. 1/800 live male births.
XO (Turner) have trouble with math and spatial skills. Short and have webbed neck; may be infertile. 1/2500 live female births
XYY (Are they more aggressive, antisocial?)
Sex Chromosome Abnormalities
Gene-linked Abnormalities
Over 7000 known (most rare), including: Cystic fibrosis Diabetes Hemophilia Huntington PKU (phenylketonuria) Sickle-cell anemia Spina bifida Tay-sachs disease
Genetic Counseling – for whom?
Family history of disease, mental retardation, physical defects
History of miscarriages
Mother over age 35 (rate of abnormality begins to rise sharply)
May cause miscarriage (except ultrasound, maternal blood samples)
Is the problem correctible?
Genetic engineering is still in the future.
Often the only decision is whether or not to abort the fetus.
Prenatal Diagnostic Methods
Chorionic villi sampling (6-8 weeks);detects genetic defects; risk of miscarriage, limb deformity
Amniocentesis – (11 weeks, best after 15 weeks); detects genetic defects; smaller risk of miscarriage
Prenatal Diagnostic Methods
Infertility
1 in 6 couples in U.S. Waiting too late Sexually transmitted diseases
Fertility technology (IVF, donors) Adoption
Babies culturally unavailable
Environmental Influence
Rats reared in an environment enriched with playthings show increased development of the cerebral cortex
Impoverished environment
Enriched environment
Twins Identical Twins
develop from a single fertilized egg that splits in two, creating two genetically identical organisms
Fraternal Twins develop from separate eggs genetically no closer than
brothers and sisters, but they share a fetal environment
Identicaltwins
Fraternaltwins
Samesex only
Same oropposite sex
Dizygotic (two zygotes)
Share approximately 50% of their genetic heritage like any two siblings.
Major causes are maternal age and fertility drugs.
Twinning dramatically on the increase since the 1970s.
Multiple Births – fraternal twins
Multiple Births – identical twins
Monozygotic – one zygote (same fertilized egg)
Share 100% of genetic heritage
Occurs about 3 per 1000 live births worldwide
Factors may include temperature and oxygen levels and late fertilization
Genetics Research
Behavior Genetics
study of the power and limits of genetic and environmental influences on behavior
Molecular Genetics subfield of biology that studies the
molecular structure and function of genes
Nature-nurture Research
• Molecular genetics Human Genome Project
• Behavioral genetics Twin studies
Equal environment assumptions Adoption studies Concordance rates