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Chapter 2


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University of Toronto Scarborough
Mark Schmuckler

PSYB20 Chapter 2 Heredity and the Environment The diversity of newborns: From the moment they are born, babies differ from one another in: Physical appearance & behaviour What contributes to these individual differences at such a young age? Transactions between hereditary and environmental factors begin before birth Such transactions between genes and the environment make each newborn unique The interaction of genes and environment: - Genotype: the particular set of genes that a person inherits from her parents - Phenotypes: the visible expression of the persons particular physical and behavioural characteristics - Phenotypes are created by the interaction of a persons genotype, with the environment What do psychologists study? - Phenotypes - Ex. motor abilities, intellectual skills, social behaviour, and personality traits THE PROCESS OF GENETIC TRANSMISSION Oviduct is where sperm and egg unite to create baby The female egg: (ovum) The largest human cell 90 000X heavier then sperm Has genetic info (23 chromosomes) - The male sperm: Smallest of all human cells A head with a whip-like tail Has genetic info (23 chromosomes) Chromosomes and Genes Each chromosome is homologous: the same in size and functions of 23 females are = to 23 males - Homologous- similar in shape and size TF allow pairing to be possible In the womens oviduct Female egg and male sperm unite to create: zygote which has 23 chromosome pairs OR 46 chromosomes Copies of these 23 PAIRS of chromosomes are passed on to every cells in a persons body EXCEPT: the reproductive cells - Each reproductive cell only contains 23 SINGLE chromosomes instead of 46 single ones This is because during development, the reproductive cell undergoes a special form of cell division called meiosis So the when the egg and sperm unite, 23 + 23 can combine to give 46!! What is meiosis? The process by which a germ cell divides to produce new germ cells w/ only half of the normal complement of chromosomes. Thus, male and female germ cells (sperm and ovum) each contain only 23 x-somes so that they unit, the new organism they form will have 46 x-somes, half from each parent. How is the tremendous diversity of genetic combinations facilitated in organisms? Crossing over: equivalent sections of homologous chromosomes randomly switch places to shuffle genetic info Halving of the chromosomes during meiosis mixes the chromosomes that originated from the mother with the chromosomes that originated from the father How does the single zygote become a complex human being? By a process called mitosis: the cell duplicates its chromosomes and then divides into daughter cells that have the same # of chromosomes Autosomes 22-paired non-sex x-somes Genes, DNA, and Proteins What is the chromosome? A thin molecule of deoxyribonucleic acid, DNA - A ladder like molecule that stores genetic information in cells and transmits it during reproduction - DNA is made up of building blocks od nucleotides - Each nucleotide: is a compound consistent of a Nitrogen-containing base A simple sugar, And a phosphate group (A-T or C-G) Nucleotide Compound containing a Nitrogen base, simple sugar, and a phosphate group Only bases that are compatible w/ each other will bond- no other combination possible - Only A-T - Only C-G Genes A portion of DNA that is located at a particular site on an x-some and that codes for production of certain kinds of proteins Proteins Fundamental components of living cells A part of a complex organic molecule containing carbon, hydrogen, oxygen, nitrogen and sulphur all of which are composed of one or more chains of amino acids. How do chromosomes carry the units of hereditary information? Portions of the chromosomes called genes, are located a particular sites on the chromosome, and code the production of certain kinds of proteins Specific bases put in a certain order code for a protein Proteins serve multiple functions in the organism: - Give cells their characteristic physical properties - Trigger chemical rxn - Carry chemical msgs - Regulate genes All of which composes a living organism. GENETIC INFLUENCES ON DEVELOPMENT Genes never work in isolation, but always in combination w/ environmental influences The transmission of Traits: A Basic Model How are characteristics inherited? Gregory Mendel: observed generations of peas and worked the mechanisms, or laws of the inheritance of characteristics- cataloguing the result of hybridizations Two Principals he carried out 1. The principle of segregation: Each inherited trait comes from ones parent as a separate unit (Ex. flower colour, stem height) 2. The principle of independent assortment: the inheritance of various traits occurs independently of one another (Ex. inheritance of colour has nothing to do with height) Mendels principals cover the concepts of disappearance and reappearance of short and tall stems. Thus There are two concepts crucial to understanding genetic influences on development: a. Genes typically have two forms: alleles, one from the mother and one from the father b. If the alleles from the two parents are the same, the individual is homozygous, if the alleles are different, the organism is heterozygous So why do certain characteristics disappear than reappear in later generations? Must understand how alleles can combine AA or aa (Aa, AA, aA, aa) The combinations of alleles from both parents can have different results: Mom + dads allele combines and only one trait shows up = dominant allele, (hidden trait is the one with the recessive allele) Mom + Dads allele combine to produce intermediate traits = Co-dominance Co-dominance A genetic pattern in which heterozygous alleles express the variants of the trait for which they code simultaneously and with equal force Both alleles will express their traits simultaneously, where two traits will combine but will not blend (think of blood types mom is blood type A and dad is blood type B= baby is blood type AB blood antigens) Dominant Describing the more powerful of two alleles in a heterozygous combination In heterozygous combination, the characteristic associated w/ only 1 of the alleles is expressed, in this case, the more powerful allele is dominant over the weaker- recessive allele Recessive Weaker of two alleles in a heterozygous combination Fortunately, many harmful alleles are recessive TF reducing incidence of genetic abnormalities
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