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Chapter 2

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Elizabeth Johnson

January 24/2012 PSY210 Notes: Chapter 2 • Genotype: particular set of genes person inherits form parents • Phenotype: created by the interaction of genotype (genetic makeup) with the environment. It’s the visible expression of physical/behavioural characteristics. • Ovum: female germ cell, egg. Largest human cell, 90,000 times heavier than the sperm. • Sperm: male germ cell. Sperm is the smallest of all human cells. • Chromosomes: thread like structures that are in the nucleus. They carry genetic information. Of the 46 pairs of chromosomes in you, 23 come from mom and 23 come from dad. Each chromosome is homologous (similar in shape and function). Each cell contains these 43 pairs, EXCEPT. The reproductive cells, which go through a process called meiosis to divide up the 23 pairs and end up with only 23 chromosomes in total. • Meiosis: the reproductive cells divide to produce new germ cells with half the normal complement of chromosomes (23) • The random assortment of 46 homologous chromosomes in meiosis makes possible the production of about 8 million different chromosome combinations in male and female germ cells. Further diversity is added by a process during meiosis called crossing over. • Crossing over: when equivalent sections of homologous chromosomes randomly switch places with each other so the genetic information is shuffled even more. • Mitosis: process in which the body cell divides in two, duplicating its chromosomes and giving the daughter cell 46 chromosomes total. • Autosomes: chromosomes that contain matching pairs. 22 paired non sex chromosomes. Vs sex chromosomes. • Binding element of a chromosome is DNA (deoxyribonucleic acid). Its building blocks are called nucleotides. • Nucleotides: a compound containing a nitrogen base, simple sugar, and a phosphate group. 4 different nitrogen contaning bases: adenine, thymine, cytosine, guanine. A forms a bond with T, and C forms a bond with G. • Chromosomes carry hereditary information through genes, which code for production of certain kinds of proteins. • The genetic code is dependent on the order of the bases, for this to happen. The DNA segment splits down the middle and the base pairs are no longer bonded with each other, and they bond with other free nucleotides. The resulting code of bases is a template for the new protein molecules. • Protein: complex organic molecules containing C, H, O, N and usually Sulphur. Proteins are composed one or more chains of amino acids and are fundamental components of living cells. Genes and Environment • A gene alone is useless, its coded message cannot be read unless it is placed in an environment that signals when and how it should respond. Therefore, genes never work in isolation it is always combined with environmental influences. Transmission of traits • Gregor Mendel, worked with pea plants to discover how traits are transmitted, he worked out two mechanisms. Principle of segration and principle of independent assortment. • Principle of segregation: each inherited trait comes one parent as a separate unit e.g. flower colour, stem height. • Principle of independent assortment: inheritance of different traits occurs independently of one another so inheritance of flower colour has nothing to do with stem height. • Allele: a gene has two alleles, 1 from mom, and 1 from dad. Therefore, allele is the alternate form of a gene present on two homologous chromosomes. If two alleles from mom and dad are the same then the organism is homozygous, if they are different then the organism is heterozygous. • If an organism is heterozygous then 3 possibilities: two traits blend e.g. - dark mom, light dad = medium kid. OR Co dominance where both traits will express themselves, BUT will not blend. E.g. - blood type: mom =A, dad= B. kid = AB. No blending. 3 possibility= only one of the alleles will express itself. The more powerful allele is called dominant and weaker is called recessive. Sex Chromosomes: • In males and females the 23 pair of chromosomes determines the gender and is responsible for sex related characteristics. • Female has two homologous XX chromosomes and male has XY chromosomes. • Some genes on the X of a male have no equivalent genes on the Y, so recessive X linked genes (genes carried on the X with no analogous genes on the Y) will automatically be expressed. The male's Y has no counteracting dominant gene. While for females, they have more of a chance of inheriting a dominant and counteracting allele on the other X chromosome. • Haemophilia is one X linked recessive disorder where blood fails to clot. So for a female to get it she needs to be homozygous. For a male to NOT get it, he needs to have the gene for normal blood clotting on his small Y so it can counteract the other one on X. • Men have higher rates of colour blindness, night blindness, atrophy of the optic nerve etc due to x linked recessive genes. Interactions among genes • Complicated traits such as intelligence, creativity, sociability are actually affected by interaction of multiple genes and not just one gene. This explains why normal parents could give birth to a genius and that genius would still have ordinary kids. • Further more, there are modifier genes which are genes that influence the expression of ANOTHER gene, exerting their influence indirectly. E.g. - early development of cataract (condition in which the lens of the eye become clouded), although the occurrence of cataract is determined by a dominant gene. The nature of cataract formation is influenced by modifier genes so if the cloudiness forms along the periphery of the lens or in the centre. Genetic Disorders • Refer pg 51. Table of genetic disorders. Why harmful alleles survive • Harmful alleles survive mainly because they are not harmful in a heterozygous state which allows them to passed on. E.g. - Allele for phenylketonuria (PKU) where you fail to metabolize protein phenylalanine which is in milk (basic diet for infants) just as long as the child as a normal allele it will survive. BUT if the child is homozygous, then this fail of metabolizing the protein can cause damage to nervous system and mental retardation. So, this allele survives mostly in heterozygous individuals allowing it to be passed on even though it is activated only 25% of the time if both the parents are carriers. • Some potentially harmful alleles survive because they are beneficial in combination with a normal allele. E.g. - sickle anaemia (red cells become distorted as when low in oxygen and cause severe pain shortness of breath and blockage of blood vessels) which affects people from Canada and USA BUT majority of people affected are in Africa. This reason this allele not only survives but also flourishes is because it provides built in resistance against malaria which prevalent in Africa but only if the person is heterozygous(1 normal allele, 1 sickle cell anaemia allele) allowing these individuals to survive better than others. But the built in protection for malaria does little when the person is homozygous because sickle cell anaemia it self is dangerous and can kill. Chromosomal abnormalities • Developmental disorders are not only caused by genes or gene groups but entire chromosomes. These chromosomal abnormalities arise during the process of meiosis and are not present in the parents of the child. • E.g. - Down syndrome: physical and mental retardation and unique physical appearance (short stature, almond shaped eyes etc). They have heightened susceptibility to leukemia, heart disorders and respiratory infections. Down syndrome is caused by having 3 set of chromosomes in chromosome 21. So this disorder is also called trisomy 21. This extra chromosome most often comes from the mother, where her homologous pair of 21rst chromosome failed to divide. The chance of this happening during pregnancy increase with the mothers age AND the fathers age. Father over 40, mother over 35 have a higher risk. Kids with Down syndrome develop normally until 6 months, but then their development slows down and they may have trouble speaking complex sentences and even analyzing their surroundings. But with enough care, it is possible for a kid with Down syndrome to hold a job and become competent. Sex chromosomal anomalies • Sometimes abnormalities happen within the sex chromosomes. For example, females born with only 1 X, so XO this happens because the fathers sperm did not contain an X or a Y. This is called Turner syndrome (characteristics include: short, stubby fingers, webbed necks, and unusually shaped mouths and ears). Normal intelligence with deficits in visual and spatial processing. They are sterile. Also cant interpret social cues as well • Females can also have three Xs instead; appear normal and normal sexual development. But their short term memory and verbal skills are affected. • When a male inherits an extra X, so XXY, its called Klinefelters syndrome. Sterile, with breast development. Verbal language deficits like XXX female and reading/memory/reasoning problems. Also sometimes can be retarded. • Men can also inherit an extra Y, so XYY. Generally taller than other men. • Fragile X syndrome: more common in males than in females. X chromosome is narrowed in some areas making it fragile. Can lead to retardation. People with this have physical abnormalities, psychological and social problems (cleft palate, seizures, disorders of the eye, anxiety, hyperactivity, attention deficits and abnormal communication problems) Genetic Counselling • Health care service that provides medical info about genetic disorders and risks • Preventive genetic counselling is when couples can themselves be tested for defects and if there are defects they can go thro adoption or one of the assisted reproductive techniques in which a donors egg or sperm may be substituted for one of their own germ cells. Prenatal Diagnostic techniques • Amniocentesis: technique for assessing fetal cells by inserting a needle in the amniotic sac which is filled with amniotic fluid. The amniotic fluid must have fetus’s skin cells which the pathologist can then analyze for chromosomal and genetic makeup. Optimum time for this is 16 week of pregnancy because by then their should be enough fetal cells in the amniotic fluid and the fetus is still small enough that it doesn’t hurt it. There is a risk of miscarriage though. • Chorionic villi sampling: assessing cells from the chorionic villi (projethions fthm the chorion that surround the amniotic sac). Can be done as early as 9 or 10 week of pregnancy. The villi help the zygote to embed itself in the uterine lining and then multiply to form the placenta. Even though the villi are not part of the embryo, they have identical genes and chromosomes because they all arise from the same fertilized egg. • The cells can be examined by a high powered microscope to see missing or extra chromosomes. Also, genetic markers (pieces of DNA) serve as indicators for many disorders caused by one or more defective genes. E.g. - gene for cystic fibrosis is on chromosome 7, gene for Alzheimer’s is on the long arm of chromosome 21. • Nancy Wexler, found the genetic marker for Huntington disease (fatal deterioration of the nervous system that begins in mid- adulthood) by sampling 5000 Venezuelans who were all descendents of a woman who had died of Huntington more than 100 years ago. The genetic marker is located at one end of chromosome 4. • Two other tests routinely used during pregnancy: 1. Alpha-fetoprotein assay (AFP) - maternal blood test that can reveal fetal problems such as Down syndrome or defects of nervous system and as well as presence of more than one embryo. 2. Ultrasound- method of visualizing deep body structures to detect physical abnormalities in a fetus. It scans the uterus by using sound waves then produces a sonogram/film that shows the size and structure of the developing fetus. It can also determine the gender. • All of these tests come with the issue of when is it okay to abort? (Down syndrome, Klinefelters turner etc are not lethal conditions) and other issues of how potential employers and insurance companies might decide to require in- depth genetic screening procedures. Gene Therapy • Its process to alleviate/cure inherited disorders by inserting normal alleles into patient cells to compensate for defective alleles. They use modified viruses to do this (modified meaning that the harmful properties are removed) these viruses carry the new genes into the patient’s cells. Scientists use viruses for gene therapy because viruses are adapted to penetrate another organisms cells. So, the procedure is, target cells are removed from the patient’s body, infused with the new gene using the virus, then returned to the body. How the environment influences the expression of genes • Range of reaction: the notion the human beings genetic makeup establishes a range of possible developmental outcomes, and the environment forces actually determine how the individual actually develops. • So genetics set boundaries, and within those boundaries the environment determines how the person is shaped. • Canalization: genetic restriction of a phenotype to a small number of developmental outcomes making environment
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