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Chapter 2

Chapter 2 - Heredity and the Environment.docx

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University of Toronto Scarborough
Mark Schmuckler

By: Paraniya Heredity and the Environment o Concepts of genotype and phenotype a framework for exploring the interactions of genes and environment. Genotype is the particular set of genes that a person inherits from his or her parents; exception of twin, no two people have exactly the same genotype Phenotype is the observable and measurable expression of an individuals physical and behavioral characteristics o In the moist environment of a womans oviduct, the sperm and the egg unite to create a new living organism that has the potential to develop into a human being. New organism is referred to as the zygote The egg, or the ovum, is the largest human cell, is about 90,000 times as heavy as the sperm that penetrates it The sperm is the smallest of all human cells, is the shape of a head, where the hereditary information is, with a whip-like tail, which it uses to propel itself through the womans reproductive system in search of the ovum o At the moment of human conception, when the sperm and egg unit, 23 chromosomes from each of parent cell is joined together to create 23 chromosome pairs, or 46 chromosomes in all The threadlike chromosomes carry genetic information that helps direct development Half of each pair from the father and half from the mother Each chromosome is homologous (similar in shape and function) to one of the chromosomes combined with the mothers egg Copies of these original 23 homologous pairs of chromosomes are passed on to every cell in a persons body with one exception the reproductive cells. Reproductive cells only contain 23 chromosomes instead of the usual 46 chromosomes because during its development, it undergoes a special form of cell division called meiosis, 23 chromosome pairs have halved. Halving process mixes the chromosomes that originated from the individuals father with the chromosomes that originate from the individuals mother mixing process is totally random There are 8 million different chromosome combinations Genetic variability is added during meiosis by a process called crossing over, in which equivalent sections of homologous chromosomes randomly switch places so that genetic information is shuffled even more o Mitosis, the process in which a body cell divides into two, first duplicating its chromosomes so that the new, daughter cells contain the usual 46 chromosomes. It occurs in all autosomes which are chromosomes that contain matching pairs and sex chromosomes, a cell duplicates its chromosomes and then divides By: Paraniya into daughter cells that have the exact same number of chromosomes as their parent cell gradually becomes a multi-cellular organism o The binding element of a chromosome is a long, thin molecule of deoxyribonucleic acid (DNA). This stores genetic information and transmits it during reproduction, is made up of building blocks called nucleotides which is a compound consistent of nitrogen base, a simple sugar, and a phosphate group, that are held together by two long, twisted parallel stands that resemble the two side rails of a spiral staircase Only bases that are compatible with each other will bond together adenine and thymine form a bond, as do cytosine and guanine Portions of chromosomes DNA molecule, called genes, are located at particular sites on the chromosome where they code for the production of certain proteins Genes are a portion of DNA that is located at a particular site on a chromosome and that codes for the production of certain kinds of proteins Proteins are fundamental components of all living cells, and are any of a group of complex organic molecules containing carbon, hydrogen, oxygen, nitrogen, and usually sulphur, and that are composed of one or more chains of amino acids At some point, the gene, or DNA segment, splits down the middle so that its pairs of bases are no longer joined, and bases bond with other, free nucleotides to form new pairs Resulting copy of the gene then acts a template for building protein molecules o Genes never work in isolation, but always in combination with environmental influences. A gene alone is useless. o Gregor Mendel, Austrian monk, who from his observations of pea plants, worked out the basic principles that govern the inheritance of characteristics from one generation to the next Worked out the mechanisms, or laws, of inheritance of characteristics: a. Principle of segregation states that each inherited trait comes from ones parent as a separate unit (ex. flower color, stem height) b. Principle of independent assortment states that the inheritance of various traits occurs independently of one another (ex. inheritance of flower color has nothing to do with inheritance of stem height) Mendel crossed a tall pea plant with a short pea plant. When looking at the offspring produced from such mating, Mendel found that the next generation of plants was tall. Although most of this generation was tall, some turned out short! Thus, the characteristics of short stem had reappeared in a subsequent generation By: Paraniya First, at any given genes position on two homologous chromosomes, there can be more than one form of that gene. These gene alternative forms are called the genes alleles, with one allele coming from the organisms mother and the other from the father. Two parent traits are the same; the organism is homozygous for that gene and the trait that goes with it. If the alleles are different, then the organism is heterozygous for that characteristic If A represents one allele and a another, there are clearly three possible combinations of alleles: AA, aa, or Aa (aA) When an organism is homozygous for a trait (AA or aa), then that organism will simply inherit the coded-for characteristics Sometimes the combination of two dissimilar alleles will produce an outcome intermediate between the traits, for which each single allele is codes Second possibility is that both alleles will express their traits simultaneously; that is, the two traits will combine but will not blend. This pattern is called co-dominance which is a genetic pattern in which heterozygous alleles express the variants of the trait for which they code simultaneously and with equal force Exemplified by blood type. Dominant allele describes more powerful of two alleles in a heterozygous combination Recessive allele describes the more weaker of two alleles in a heterozygous combination Many inherited characteristics follow this dominant- recessive type of relation, such as curly(dominant) and straight(recessive) hair, and dark(dominant) and light or blond (recessive) hair Many harmful alleles are recessive rd o Sex chromosomes are in both males and females, the 23 pair of chromosomes, which determine the individuals gender and are responsible for sex-related characteristics; in females, this pair normally comprises of two X chromosomes, in makes an X and a Y chromosome Because an X chromosome is about five times longer than a Y chromosome, it carries more genes. Thus n males the X-linked genes which are genes carried on the X chromosome will not have an analogous genes on the Y chromosome In females though, X-linked recessive genes are expressed much less frequently because females, who have two X chromosomes, have a chance of inheriting a dominant counteracting allele on the other X chromosome.
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