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Chapter 2

CH.2 Heredity.docx

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University of Toronto Scarborough
Mark Schmuckler

Twinkle Arora (PSYB20) CHAPTER 2: HEREDITY AND THE ENVIORNMENT - Genotype – the particular set of genes that a person inherits from her parents - With the exception of identical twins, no two people have exactly the same genotypes - Genotype interacts with the environment to produce the phenotype - Phenotype – created by the interaction of a person`s genotype or genetic makeup, with the environment; the visible expression of the person’s particular physical and behavioural characterises - Examples of the phenotype – intellectual skills, motor abilities and personality traits - Genetic transmission in a female`s oviduct – the sperm and the egg (male and female gametes reproductive cells) unite to create a zygote - Ovum – the female germ cell, or egg - Sperm – the male germ cell , the smallest of all human cells is the shape of a head - After 9 months – baby – 7 to 8 pounds and are about 50 cm long - Chromosomes and genes are located inside the nucleus or centre of the cell - 23 chromosome pairs – 46 in total - Chromosomes – threadlike structures that carry genetic information to help direct environment - Each pair is homologous – similar in shape and function to one of the chromosomes - These chromosomes are passed on to every cell in a person`s body with the exception of the reproductive cells - Meiosis - reproductive cells divide to produce new germ cells with half the normal complement of chromosomes ; thus male and female germ cells (sperm and egg) each contain only 23 chromosomes so that when they unite, the new organism they form will have 46 chromosomes, half from each parent - Crossing over – the process by which equivalent sections of homologous chromosomes switch places randomly, shuffling the genetic information each carries - Mitosis – the process in which a body cell divides in two, first duplicating its chromosomes so that the new daughter cells contain the usual 46 chromosomes - Autosomes – the 22 paired non – sex chromosomes 1 Twinkle Arora (PSYB20) - Deoxyribonucleic acid (DNA) - a ladder like molecule that stores genetic information in cells and transmits it during reproduction - DNA is made out of nucleotides that are held together by 2 parallel strands - Nucleotide – a compound containing a nitrogen base, a simple sugar and a phosphate group - BASES - ADENINE and THYMINE – Bind together - BASES – CYTOSINE and GUANINE – Bind together - Portions of the chromosome’s DNA molecule called genes are located at particular sites on the chromosome where they code for the production of certain kinds of protein - Proteins – are fundamental components of all living cells, and are any of a group of complex organic molecules containing O C N H 2 , 2, , 2 and composed of amino acids - The genetic code is written in the order in which the 4 bases are included in the gene, the DNA then splits down the middle so that its pairs of bases are no longer joined and the bases bond w. other free nucleotides - Gregor Mendel – spent years cross breeding pea plants – he observed traits of the plants varied from one generation to another with a trait that had been present in one generation disappearing in the next generation but then repapering again in the following generations - he came up with principals – principle of segregation and then principal of independent assortment - principle of segregation – states that each inherited trait comes from one’s parent as a separate unit (e.g. flower colour, stem height etc) - principal of independent assortment – states that inheritance that the inheritance of various traits occurs independently of one another - allele – an alternative forms of a gene, typically a gene has 2 alleles, one inherited from the individual’s mother and one from the other - homozygous – two of the same alleles - heterozygous – two different alleles - co-dominance – a genetic pattern in which heterozygous alleles for a particular trait for which they code simultaneously and with equal force - dominant – describing the more powerful of two alleles in a heterozygous combination - recessive - describing the weaker of two alleles in a heterozygous combination - societies prohibit marriage among close blood relations because of harmful recessive genes - sex chromosomes – in both males and females, the 23 pair of chromosomes which determine the individuals gender and are responsible for sex-related characteristics ; in females( XX) chromosomes and in males (XY) - Chromosome X comes from the child’s mother and Chromosome Y comes from the father - Chromosome X is five times longer than chromosome Y thus carries more genes - X – linked gene – genes that are carried on the X chromosome and that may have no analogous genes on the Y chromosome in males - Hemophilia- a disorder caused by an X linked recessive gene in which the blood fails to clot; found more often in males than females - The allele for haemophilia is recessive, a female who inherits it will normally clot blood as long as her second allele inherited from her other parent does NOT code for haemophilia. Only if she is homozygous for the recessive allele will her blood clotting be impaired 2 Twinkle Arora (PSYB20) - If a male receives the haemophilia on his X chromosome he is in greater danger of developing haemophilia. Like the female he will develop the disorder if he receives another haemophilia allele on his Y chromosome however, he will also develop haemophilia if he receives no counterbalancing gene on his Y chromosome - X linked recessive gene - Colour-blindness – more common in males than females - Modifier genes – genes that exert their influence indirectly by affecting the expression of still other genes - Example of modifier gene – Cataract - a condition in which the lens of the eye becomes clouded obscuring vision determined by the dominant gene Genetic Disorders - Heterozygous state – when a person inherits a normal allele and recessive one e.g. PKU - Phenylketonuria (PKU) – a disease caused by a recessive allele that fails to produce an enzyme necessary to metabolize the protein phenylalanine; if untreated immediately at birth, it damages the nervous system and causes mental disability (diagnose method – blood test) - 1 out of 20 people of European ancestry carries the recessive PKU gene - Sickle cell anemia (refer box 2.1 pg. 54)– a disease caused by a recessive gene in which red blood cells become distorted when low in oxygen causing fatigue, shortness of breath and severe pain and posing a threat to life from blockage of crucial blood vessels - Sickle cell anemia to which some individuals of African descent inherit can actually help some people survive malaria - Almost 1% of all newborns have diagnosable chromosome abnormalities (leads to 60% abortions) - Down Syndrome (DS) – a form of chromosome abnormality in which the person suffers disabling physical and mental development and is highly susceptible to such illnesses as leukemia, heart disorders and respiratory infections (usually have almond shaped eyes) - 70% of down syndrome patients live up to their 60s increasing the risk of Alzheimer’s disease - Down syndrome is caused by a deviation of chromosome 21 (usually comes from the mother`s egg) - the patient has 3 chromosomes which is why the disorder is called trisomy 21 - Down syndrome births are higher for men over 40 esp if the mother is over 35 - Infants with DS may develop normal in their first 6 months but unless they receive special therapy their rate of intellectual growth begins to decline after a year - Turner syndrome – a form of abnormality of the sex chromosomes found in females in which secondary sex characteristics develop only if female hormones are administered and in which abnormal formation id internal reproductive organs leads to permanent sterility - XXX pattern – in which a female inherits XXX (3 X chromosome) instead of normal XX. These triple X girls appear normal physically and have normal secondary sexual development but their cognitive abilities are affected esp their short term memory - Klinefelter`s Syndrome – a form of chromosome abnormality, in which a male inherits an
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