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Genetics

Genetics. Basics Heritability Environment Modification of gene expression. Genetics I. Every cell has has a copy of our genetic blueprint Except for gametes (sperm and egg) and mature red blood cells 23 pairs of chromosomes (46 chromosomes total)

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Genetics

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  1. Genetics Basics Heritability Environment Modification of gene expression

  2. Genetics I • Every cell has has a copy of our genetic blueprint • Except for gametes (sperm and egg) and mature red blood cells • 23 pairs of chromosomes (46 chromosomes total) • Each chromosome is made up of helical DNA strands • There are approximately 1000 segments – called genes - in each chromosomal DNA • Approximately 21,000 pairs of genes in humans • Genes are composed of 4 different nucleotides • Adenine, guanine, cytosine, and thymine

  3. Genetics II • Genes have two essential jobs • Make copies of themselves that are transmitted over generations • Direct the assembly of organisms • Through the production of RNA and, more indirectly, proteins • Particular proteins are produced when a particular gene is “switched on”; known as “gene expression” • What is the meaning of life? • We are machines created by our genes for the purpose of replicating our genes.

  4. Genetics III • Each gene has a partner on the paired chromosome • Genes can be dominant or recessive • Genotype: the specific sequence of nucleotides that an organism inherits • Phenotype: an organism’s observable properties • The result of what happens when genes interact with each other and the environment • There are 3 billion nucleotides • 99.9% of the nucleotide sequences are the same in all human beings • That means that there are 3 million sequences of nucleotides that are responsible for the differences amongst us • Chimps and men share 96% of the nucleotide sequences • 120 million sequence differences

  5. Genetics IV • Genotypic variation is created by sexual reproduction and mutation • A unique genotype is created at the moment of conception • Meiosis is the process whereby the gametes (sperm in male, egg in female) are formed. • During meiosis in the male, the two members of each pair of chromosomes fuse, exchange segments, duplicate, then randomly split into 4 sperm. • Males produce 300 million sperm per day. • The product of meiosis in females is a single egg (the largest cell in the human body). • Women are born with all their eggs (~ 1.5 million at birth; 40,000 at adolescence, and 350 to 450 mature during woman’s childbearing years). • Fertilized cell, known as a zygote, contains the 23 chromosomes, one half of each pair from the father and the other half from the mother

  6. Each parent contributes a random selection of half of their genes to an offspring The probability of an offspring sharing any particular gene with a parent is .5 Thus, the degree of relatedness between parent and child is .5 Genetics V

  7. Heredity is the transmission of characteristics from parents to offspring by means of genes Heritability is the degree to which individual differences in phenotype are determined by individual differences in genotype Heritability: a statistical estimate, for a given trait (like intelligence), of the percentage of variation within a group that is due to genetic factors Heritability vs. heredity

  8. Heritability II • Both height and intelligence are characteristics that vary across individuals • How heritable is height? • On the next slide, in the two left-hand columns, there are the names of six individuals living in six different environments and their corresponding heights • In the two right-hand columns is the outcome if you could give all six individuals the same genetics. • Result: Despite living in separate environments, if the six individuals had the same genetics, their heights would be almost identical. • Conclusion: Height is highly heritable. Differences in environment contribute little to differences in height. • How heritable is intelligence? Why?

  9. More on heritability: The case of height Before holding genetics constant After holding genetics constant

  10. More on heritability: The case of intelligence Before holding genetics constant After holding genetics constant

  11. The heritability of some human traits

  12. Determining heritability • Twin studies: compare monozygotic (MZ or identical) twins raised in the same environment to dyzygotic (DZ or fraternal) same sex twins raised in the same environment. • Holding environment “constant”, but varying the genetics • If identical twins are more alike on the trait than are fraternal twins, then genetics plays an important role; if not, then environment • Adoption studies: compare MZ twins raised in the same environment to MZ twins raised in separate environments. • Holding genetics constant, but varying the environment • If identical twins that are raised together are more similar than identical twins that are raised apart, then the environment plays an important role; if not, then genetics

  13. Environmental Influences • Even among MZ (identical) twins raised together, heritability of personality characteristics is less than 50%. • Meaning that environment must account for the rest of the variability. • But if identical twins are raised together, how is their environment different? • Shared vs. nonshared environments • Shared: same parents, same home, same school, etc. • Nonshared: occupy different niches within the family, may have different friends, have different experiences • Some claim that it is the nonhome, nonshared environment that is critical in shaping the adult personality • Also, genetics and environment interact • Reactive: Different genetically determined makeups react differently to the same environmental stimulation • See MAO and violent behavior example on page 88 • Evocative: Different genetically determined makeups cause people to respond differently • Proactive: When old enough, children begin to choose their own environments, which in turn will affect their development

  14. Gene Expression Can Be Modified • Pick your Mouse (Anxious, Hyperactive, etc.) • Knock-out mice: deleting genes to eliminate their effects • Knock-in mice: replace genes in other locations to enhance their effects • The Case of Transgenic Mice • Inserting strands of foreign DNA into a developing embryo • The Promise of RNA Interference • Prevents gene expression by silencing the DNA instructions to make proteins

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