Chapter 2: Genetics Genetic Foundations Heredity & Environment

Genetic Foundations DNA (deoxyribonucleic acid) a complex molecule containing the genetic information that makes up the chromosomes has two strands-forming a “double helix”- held together by bonds between pairs of nucleotides (spiral staircase) Chromosomes threadlike structures made of DNA molecules that contain the genes

Genetic Foundations Chromosomes Human beings have 46 chromosomes (23 pairs) Entire DNA code/set of chromosomes is repeated in each of the cells Define the limits of species variation; separate species

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 Genes the biochemical units of heredity that make up the chromosomes a segment of DNA capable of synthesizing a *protein Genome the complete instructions for making an organism *Proteins are the cell building blocks and bodily process regulators.

Genetics and Behavior NucleusChromosomeGene CellDNA

Genes: Our Biological Blueprint Human Genome Project Completed about the year 2000 Humans have 20,000 – 25, 000 genes (21,667) There are far more proteins than genes – 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 Expression Influenced by the environment hormones light nutrition behavior stress (cortisol may cause a fivefold increase in DNA damage) Sources of Variation

Genetic Foundations Genotype – genetic composition Phenotype – observable characteristics In the eye-color example, genotypes BB, Bb, and bB would all have the same brown-eyed phenotype

Patterns of Genetic Inheritance Dominant-recessive: the dominant gene (allele) will determine the characteristic The other allele is recessive or recedes into the background with its effects not being shown. Sources of 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 Inheritance Dominant-recessive inheritance

Examples of recessive genes: Cystic fibrosis, PKU, Tay-sachs disease. Sickle-cell anemia Patterns of Genetic Inheritance Dominant-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 Inheritance Co-dominance and Additive

X-linked or Sex-linked Inheritance Humans have 23 pairs (46 total) of chromosomes: Pairs 1 – 22 are autosomes; they have exactly the same number of locations for genes. The 23 rd pair does not. The X and Y chromosomes do not have equal numbers of gene locations. Females are XX on this pair, and males are XY.

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

Chromosomal Abnormalities: Down’s Syndrome Trisomy 21: extra copy of a chromosome on the 21 st pair Round face, flattened skull, protruding tongue, extra fold of skin on eyelids, short limbs Mental and motor retardation 1 in 500 live births Most common overall cause of mental retardation. More common to older mothers, rare among African Americans

Gene damaged on X chromosome Most common inherited cause of mild to moderate mental retardation Linked to autism More common in males Chromosomal Abnormalities Sex-linked - Fragile X

XXY (Klinefelter) may have verbal difficulties. Tall, underdeveloped testes, possible breasts. 1/400 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 A bnormalities

Gene-linked Abnormalities Over 7000 known (most rare), including: Cystic fibrosis Diabetes Hemophilia Huntington PKU (phenylketonuria) Sickle-cell anemia Spina bifida Tay-sachs disease

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 Identical twins Fraternal twins Same sex only Same or opposite 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 (laboratory biology) Human Genome Project Behavioral genetics (psychology) Twin studies  Equal environment assumptions Adoption studies Concordance rates