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Biology 3201 Unit 3 – Genetic Continuity Chapters 16, 17, & 18 Ms. K. Morris 2010-2011 Chapter 16 Genetics & Heredity p. 524-565 16.1 • Heredity- the passing of genetic traits such as the colour of hair or eyes from one generation to the next, resulting in similarities between one family or strain. • Traits- distinguishing characteristics that make an individual unique. Traits that are passed on are said to be inherited. • Genetics- the branch of biology dealing with the principles of variation and inheritance in organisms; how traits are passed from generation to generation. • Variations- in genetics, the forms of the trait. Or, also, significant deviations from the normal biological form, function, or structure. • Purebred- having descended from ancestors of a distinct type, or breed. Purebred organisms in a given species or variety all share similar traits. • True Breeding- organisms that are homozygous for a particular trait or set of traits and produce like offspring. • P generation (parent generation)- the designation for the parent generation. • Filial Generation- offspring of a cross of parent generations; the F1 generation or subsequent generations. • F1 generation (first filial generation)- offspring from the cross of the P (parent) generation. • F2 generation (second filial generation)- offspring from the cross of the F1 generation. • Hybrid- an organism heterozygous for a trait. • Monohybrid- a cross of two heterozygous individuals that differ in one trait. For example, Aa x Aa. • Dihybrid- a cross of two heterozygous individuals that differ in two traits. For example, AaBb x AaBb. • Dominant- type of trait, in which the characteristic is always expressed, or appears, in the individual. • Recessive- having an allele that is latent (present but inactive) and is therefore not usually expressed unless there is no dominant allele present. • Principle of Dominance- when individuals of contrasting traits are crossed, the offspring will express only the dominant trait. • Mendelian Ratio- ratio of dominant phenotype (homozygous dominant genotype and heterozygous genotypes) to recessive phenotype (homozygous recessive phenotype); ratio of 3 : 1 (75% to 25%). • Law of Segregation- Mendel’s first law of inheritance, in which the hereditary traits are determined by pairs of alleles from each parent. These alleles separate during gamete formation, giving each offspring only one allele from each parent. • Gene- a specific sequence of DNA that governs the expression of a particular trait and can be passed to an offspring (part of the chromosome). • Allele- alternate form of a gene. • Homozygous- describes an individual with two alleles at one locus that are identical. • Heterozygous- describes an individual with two different alleles at a locus. • Mendel’s concept of ‘unit characters’ and the ‘unit theory of inheritance’: • Unit Characters- a term describing Mendel’s “factors” of inheritance (genes), which are inherited as independent units. • Unit Theory- a term describing Mendel’s laws of inheritance, from his discovery that genes (which he called “factors”) are inherited as independent units. • Probability- the chance, or likelihood, of a particular outcome; usually expressed as a ratio. Probability = desired outcome ÷ total # of possible outcomes *Complete Questions 1-4 p. 531 (Probability Examples) • Product Rule- a rule that states that the probability, or chance, that two or more independent events will occur together is the product of their individual probabilities of occurring alone. • Punnett Square- a simple grid used to illustrate all possible combinations of gametes from a given set of parents. *Complete Questions 1-3 p. 533 (Punnett Square Practice) • Genotype- Genetic make-up of an organism; remains constant throughout an individuals life. Usually indicated by the combination of letters in a Punnett Square. • Phenotype- the physical and physiological traits of an organism. • Complete Dominance- the type of inheritance in which both heterozygotes and dominant homozygotes have the same phenotype. • Test Cross- cross of an individual of unknown genotype with a homozygous recessive individual, used as a method to determine the unknown genotype. • Explain the significance of a test cross. Use a test cross to determine the unknown genotype of a dominant organism. (p.533-534) • It is impossible to determine the genotype of an organism by simply looking at its appearance. • “How would you determine the unknown genotype?” • Note: the absence of the homozygous recessive trait in the offspring does not confirm that the unknown parent is homozygous dominant, especially in small samples of offspring. • Complete “Thinking Lab” p. 534 • The test cross is a way to set up a cross to figure out an unknown genotype. The only unknown genotype is if a phenotype is dominant. • Basically, if someone expresses the dominant phenotype, you often will not know whether that person's genotype is homozygous dominant (DD) or heterozygous (Dd). • To do the test cross, you cross the individual with the dominant phenotype (D?) (and unknown genotype) with a homozygous recessive individual (dd). • If any of the offspring appear with the recessive phenotype (dd), you know that your parent with the dominant phenotype that you crossed is actually heterozygous (Dd). – See ex. p. 534 ___________________________________ • Complete 16.1 Review Questions – P. 535 # 1-17 16.2 Recall: • Monohybrid Cross- a cross of two heterozygous individuals that differ in one trait. – For example, Aa x Aa. • Dihybrid Cross- a cross of two heterozygous individuals that differ in two traits. – For example, AaBb x AaBb. • Law of Independent Assortment- (p. 537) Mendel’s second law of inheritance, stating that inheritance of alleles for one trait does not affect the inheritance of alleles for another trait. *Do Sample Problem page 540 (The Two-Trait Cross) Incomplete Dominance (p. 541) - Is inheritance in which an active allele does not entirely compensate for an inactive allele. - Examples include: snapdragon flowers (heterozygous is pink) and Japanese 4 o’clock flowers (heterozygous is pink). - Blending of the traits of two different alleles at one locus that occurs when neither allele is dominant. *Locus- the location of a gene on a chromosome. • Predict the outcome of monohybrid and dihybrid crosses for incomplete dominance: Ex. For incomplete dominance for flower colour in snapdragons the following can be used: (i) R – red (ii) FR – red (iii) R – red R’ – white (see figure 16.15 p. 541) FW – white W – white *Be able to solve crosses involving one completely dominant trait with one other trait that is not. Co-dominance (p. 541) - Is the condition in which both alleles of a gene are expressed. - Examples include: Roan horses (red and white hair) and barred plumage chickens (black and white feathers). - In genetics, to be “co-dominant” describes a situation in which two alleles may be expressed equally. - The situation occurs when two different alleles for a trait are both dominant. • Predict the outcome of monohybrid and dihybrid crosses for co-dominance: Ex. For co-dominance, blood type may be represented as follows: (i) IA – type A (ii) A – type A IB – type B B – type B *Be able to solve crosses involving one completely dominant trait with one other trait that is not. Multiple Alleles (p. 542) - A pattern of inheritance when a gene may have more than two alleles for any given trait. - Examples include: the human ABO blood type, eye colour in drosophila (fruit flies), and colour patterns in ducks. - Human Blood Types: Phenotype (blood type) Genotypes A IAIA or IAi B IBIB or IBi AB IAIB O ii *Do Sample Problem page 542 (Human Blood Types) Outcome: Demonstrate the inheritance of traits governed by multiple alleles by predicting the genotypic and phenotypic ratios in crosses involving human blood types (ABO groups). Carrier – an organism that is heterozygous for the given trait but does not show the recessive trait. • Do Thinking Lab p. 543 (Inheritance of Coat Colour in Rabbits) • Do 6.2 Review Questions p. 544 # 1-8 16.3 The Chromosome Theory of InheritanceStates that genes are located on chromosomes, and that chromosomes provide the basis for the process of segregation and independent assortment of these genes. Chromosome theory can also account for patterns in inheritance that do not follow Mendel’s laws. Chromosome Theory of Inheritance… (summarized): - Mendel’s factors (genes) are carried on chromosomes. - It is the segregation and independent assortment of chromosomes during meiosis that accounts for the patterns of inheritance. Crossing-Over – (recall from 14.2) can occur among chromosomes during cell division. In cellular reproduction, the process in which non-sister chromatids exchange genes (during prophase I of meiosis) allowing for the recombination of genes. Genes located very close together on a chromosome will almost always be inherited together, while genes located some distance apart are more likely to be separated by a crossing-over event. The emphasis on crossing-over is how it breaks gene-linkages and creates variation. Gene Linkage (linked genes) – genes are carried on chromosomes. Gene linkage is a way of expressing that genes are linked to specific chromosomes. Gene-Chromosome Theory of Inheritance– A theory which states that genes exist at specific sites arranged in a linear fashion along chromosomes. • Genes exist on specific sites on chromosomes. When pairs of homologous chromosomes separate during gamete formation, they form two gametes. Each gamete will contain a separate allele for each trait. During fertilization, chromosomes from one gamete will combine with another gamete. • When Mendel did his experiments with pea plants, he did not know that chromosomes existed in cells. In the early 1900s, chromosomes were discovered and observed in cells. • In 1902, two scientists Walter Sutton and Theodor Boveri were studying meiosis (cell division) and found that chromosomes behaved in a similar way to the factors (genes) which Mendel described. • Sutton observed behaviour of the chromosomes during meiosis which accounts for Mendel’s observations and conclusions concerning segregation and independent assortment. Sutton and Boveri made three observations: • Chromosomes occur in pairs and these pairs segregate during meiosis. • Chromosomes align independently of each other along the equator of the cell during meiosis. • Each gamete (sex cell) receives only one chromosome from each pair. • From the above observations, they formed the chromosome theory of inheritance (which we defined in a previous note). • The Law of Independent Assortment in modern terms includes gene linkage and crossing over in its explanation. (Mendel’s second law of inheritance, stating that inheritance of alleles for one trait does not affect the inheritance of alleles for another trait). Morgan’s Discoveries • In 1910, an American scientist called Thomas Morgan made a very important discovery from his work with fruit flies. • Normal fruit flies have red eye color. • Morgan crossed two red eyed parent flies and obtained a white eyed male. • In other crosses, he obtained red eyed females, red eyed males and white eyed males. • Since the white eye color was only present in the male flies, Morgan concluded that eye color was linked to an organisms sex. • Thus, the gene for eye color in fruit flies was located on the sex chromosome, in this case the X chromosome. • Such genes are called sex-linked genes. • Morgan also stated that genes which are located on the same chromosomes are linked to each other and usually do not segregate (separate) when inherited. • These are called linked genes. • Morgan found that certain genes did not follow the Law of Independent Assortment but instead tended to be inherited together. • However, Morgan found that some linked genes do segregate. • From his work, Morgan created the genechromosome theory which states that genes exist at specific sites and are arranged in a linear fashion along chromosomes. • Morgan’s experiments restated Mendel’s Law of Independent Assortment by including crossing over. Sex Chromosome – X or Y chromosome that carries the genes involved in determining the sex of an individual. Sex-Linkage (sex-linked inheritance) – The transfer of genes involved in determining the sex of an individual. Sexlinked inheritance involves pairs of genes on the X chromosome in the female, and a single gene on the X chromosome in the male. In this case, gender is important in gene expression and gender must be considered part of the phenotype. Autosomal Inheritance- typically involves pairs of genes, with gender being irrelevant to gene expression. Autosomes are chromosomes that are not directly involved in determining the sex of an individual. • Distinguish between genotypes and phenotypes evident in autosomal and sex-linked inheritance. Note: Genes which are located on the X chromosome are called X-linked while those on the Y chromosome are called Y-linked. Most sex-linked genes are located on the X chromosome. Females: XX Males: XY Chromosomes & Gene Expression *Chromosome Inactivation p. 547-548 • Males and females produce the same amounts of proteins. This is coded by genes which are located on the X chromosome. • Females have two X chromosomes in their cells while males have only one X chromosome. • From experiments, scientists have shown that one of the two female X chromosomes is inactivated and this inactivated chromosome is called a Barr body. • Ex. The tortoiseshell coat colour in cats is an ex. Of the presence of an inactivated X chromosomes. *Modifier Genes p. 549-550 • Certain genes, called modifier genes, work with other genes to control the expression of a particular trait. • In humans, modifier genes help control the trait of eye color. In this case, modifier genes influence the level of melanin present in the human eye to provide a range of eye colors from blue to brown. *Polygenic Inheritance (multiple gene inheritance) p.549 • Most traits are controlled by one gene, however, some traits are controlled by more than one gene, this is called polygenic inheritance; a pattern of inheritance in which a trait is controlled by more than one gene • Polygenic genes cause a range of variation in individuals called continuous variation. • In humans, traits such as height, skin color, etc. are polygenic traits. • traits are determined by a number of different contributing genes present at different locations, and expression depends on the sum of all the influences of all of these. Outcomes: Questions… • Predict the outcome of monohybrid and dihybrid crosses involving sex-linked traits. • Predict the genotypes, phenotypes and ratios among offspring and compare specific genotypes and phenotypes for males and females. • Solve dihybrid crosses involving one trait that is completely dominant with one other trait that is sex-linked. • Explain why sex-linked defects are more common in males than females. • True or False? Males are biologically stronger than females. Changes In Chromosome Structure Changes in the physical structure of chromosomes can occur: 1. Spontaneously 2. As a result of irradiation 3. After exposure to certain chemicals Different types of chromosome mutations (factors that may lead to mutations in a cell’s genetic information) (p. 550-553) (i) deletion (ii) inversion (iii) duplication (iv) translocation * nondisjunction (monosomy, trisomy) • In a deletion, a piece of a chromosome gets lost. • The lost piece contains genes and when they are lost, genetic information is also lost. • Viruses, irradiation, or certain chemicals can cause pieces of a chromosome to be broken off. • Example : Cri-du-chat (a piece of chromosome 5 is lost and facial abnormalities and an abnormality in the larynx cause the infants cries to sound like a cat mewing) • In an inversion, a piece of a chromosome separates, flips over, and rejoins. • A certain gene segment becomes free from its chromosome momentarily before being reinserted in reverse order. • This completely changes the position and order of the genes on a chromosome, and can alter gene activity. • Example : Certain forms of Autism. • In a duplication, a sequence of genes is repeated one or more times within one or several chromosomes. • The greater the repetition of genes, the greater the chance of a problem occurring. • Too many repeats affect the functioning of the gene (even though some gene sequences can be repeated thousands of times in normal chromosomes). • Example : Fragile X Syndrome (1/1500 males, 1/2500 females). A duplication occurs in chromosome X. Normal people have 29 repeats, and people with Fragile X Syndrome have about 700 repeats of this sequence. • In translocation, a piece of one chromosome changes places with a piece of another chromosome, or another part of that same chromosome. • Examples: Cancers- if a part of chromosome 14 exchanges places with a part of chromosome 8, cancer can occur in the affected individual. • Some occurrences of Down Syndrome are related to translocation between chromosomes 14 and 21. • One kind of leukemia can be traced to translocation between chromosomes 22 and 9. Nondisjunction • Sometimes, chromosomes (chromatids) fail to separate from each other during meiosis. This produces gametes (sex cells) which have either too many or too few chromosomes. • If a gamete which does not have the correct number of chromosomes is involved in fertilization, an embryo will be produced which has either too many or too few chromosomes (other than 46). • When an individual inherits an extra chromosome, the condition is called trisomy. • If an individual inherits one less chromosome, the condition is called monosomy. • There are relatively few syndromes in the human population involving nondisjunction because most cases of nondisjunction prove to be fatal. • Human embryos with too many or too few autosomes rarely survive. • These embryos are usually aborted by the mother, but some survive and have genetic disorders. Complete Thinking Lab p. 552 Monosomy & Trisomy Human genetic diseases caused by chromosomal mutations (p. 553) Chromosomal mutations are more serious than gene mutations because they involve a larger portion of genetic material. 1. Down syndrome (Trisomy 21) 2. Turner syndrome 3. Klinefelter syndrome (XXY syndrome) 4. Jacobs syndrome (XYY syndrome) 5. Triple X syndrome (XXX syndrome) 1. Down Syndrome • This disorder is also called trisomy 21. • This occurs when an individual receives three copies of chromosome 21 instead of the normal two (it results from nondisjunction). • Individuals who have this syndrome have the following symptoms: – Mild to moderate mental impairment – A large, thick tongue, resulting in speech defects – A poorly developed skeleton; short, stocky body structure – Thick neck – Abnormalities in one or more vital organs – About 40% have heart defects – May survive into 30’s or 40’s and beyond – Have a greater chance of becoming senile (similar to Alzheimer’s) – Almond-shaped eyes – Prone to respiratory problems – They have 47 chromosomes instead of 46. Down Syndrome Karyotype Down Syndrome 2. Turner Syndrome (XO) • In this disorder, an individual inherits only a single X chromosome, as well the Y chromosome is missing. Results from nondisjunction. • This results in a female with the genotype XO, O represents a missing chromosome. The female has 45 chromosomes. • It occurs in about 1/2000 live female births • These females will exhibit a number of symptoms including ; – Infertile – fail to develop secondary sex characteristics (external female genitalia, but no ovaries) – typically do not experience puberty without estrogen therapy, – normal in childhood – normal intelligence (learning difficulties in math) – Webbed neck (sometimes) – Skeletal abnormalities (very short stature, 4’8”) – principal difficulty is acceptance by the peer group. Most women with Turner Syndrome lead typical lives, including normal family relationships. Turners Karyotype (XO) Turners Syndrome 3. Klinefelter Syndrome (XXY) • This disorder results in a male who has an extra X chromosome (results from nondisjunction). • 47 chromosomes instead of 46. • These individuals have the genotype XXY instead of XY. • Symptoms of this disorder include: – usually normal in appearance – normal intelligence – tall – underdeveloped testes, sterile – may also cause female characteristics (breast development, feminine body shape, lack of facial hair). Klinefelter XXY Karyotype Klinefelter XXY 4. Jacobs Syndrome (XYY) – results from nondisjunction – extra Y in male – low mental ability – speech and reading problems – normal appearance – persistent acne Jacob’s Karyotype (XYY) 5. Triple X Syndrome (XXX) - results from nondisjunction 47 chromosomes female with an extra X chromosome normal intelligence normal in appearance may be sterile XXX Karyotype Outcomes/Questions… • Is it possible for a person born with a chromosomal abnormality (such as Down Syndrome) to have a ‘normal’ child? • Given the high cost of health care, should forced sterilization be mandatory for individuals with genetic diseases? • Complete 16.3 Review Questions p. 552 #1-11 16.4 The study of human genetics is a complicated field. This is due to a number of reasons: – Humans have long life spans. – We produce very few offspring. – Most people do not keep very accurate records of their family history. However, there are certain patterns of inheritance which scientists have determined for particular human genetic disorders. These include (p.555-559): 1. Autosomal Recessive Inheritance (Tay Sachs, PKU) 2. Codominant Inheritance (Sickle Cell Anemia) 3. Autosomal Dominant Inheritance (Progeria, Huntington’s) 4. Incomplete Dominance (FH) 5. X-linked Recessive Inheritance (colour blindness, hemophilia, muscular dystrophy) 1. Autosomal Recessive Inheritance • An autosomal recessive disorder is carried on the autosomes ( body chromosomes ) and are not specific to the sex of a person. • Examples include ; • Tay-Sachs disease • Phenylketonuria ( PKU ) • Albinism Tay-Sachs Disease • This is a disease in which individuals lack an enzyme in the lysosomes which are located in their brain cells. Because of this, the lysosomes are unable to break down specific lipids. Thus the lipids build up inside the lysosomes and eventually destroy the brain cells. • Children with Tay-Sachs disease appear normal at birth, but experience brain and spinal cord deterioration around 8 months old. • By 1 year of age, the children become blind, mentally handicapped, and have little muscular activity. Most children with their disorder die before age 5. • There is no treatment for this disorder. Phenylketonuria ( PKU ) • In this disorder an enzyme which converts a substance called phenylalanine to tyrosine is either absent or defective. • Phenylalanine is an amino acid which is needed for regular growth and development and protein metabolism. • Tyrosine is another amino acid which is used by the body to make the pigment melanin and certain hormones. • When phenylalanine is not broken down normally, harmful products accumulate and cause damage to the individual’s nervous system. This is called phenylketonuria ( PKU ). • Babies who develop PKU appear normal at birth. However, within a few months they can become mentally handicapped. • Today, testing and proper diet can prevent PKU from occurring in children. Albinism • This is a genetic disorder in which the eyes, skin and hair have no pigment. • People with this disorder either lack the enzyme necessary to produce the melanin pigment in their cells or lack the ability to get the enzyme to enter the pigmented cells. • Albinos face a high risk of sunburns and eye damage from exposure to the Sun. 2. Codominant Inheritance • An example of codominant inheritance is sickle cell anemia. In this disorder, individuals have a defect in the hemoglobin of their RBCs and therefore the shape of the RBC changes from a normal round shape to an abnormal sickle shape. { See Fig. 16.33, P. 557 } • Symptoms of this disorder include ; • Blood clots • Reduced blood flow to vital organs • Lack of energy • Suffering from various illnesses • Constant pain • Premature death • In this trait, individuals can be either normal, have sickle cell disease, or carry the sickle cell trait. • Individuals who are heterozygous for the disorder have what is called heterozygous advantage. These individuals will carry one sickle cell allele as well as one normal allele for the trait, but have a better chance of survival than those individuals who carry two sickle cell alleles. { See Fig. 16.34, P. 557 } 3. Autosomal Dominant Inheritance • Genetic disorders which are caused by autosomal dominant alleles are very rare in humans, but they do exist. • Some of these disorders are caused by chance mutations. Others arise only after individuals have passed their child bearing age. • Two examples of this type of disorder are ; • Progeria • Huntington’s disease Progeria • This is a rare disorder which causes a person to age very rapidly. • It affects only 1 / 8,000,000 newborns. • This disorder results from a random, spontaneous mutation of a gene. • The mutated gene dominates the normal gene and this accelerated the ageing of an individual. Huntington’s Disease • This is a lethal disorder in which the brain progressively deteriorates over a period of about 15 years. • It begins to appear after the age of 35. • Symptoms include ; • Early symptoms include ; – irritability, – mild memory loss, – involuntary arm and leg movements. • Later symptoms include ; – – – – Loss of muscular coordination Loss of memory Loss of speech Death in the forties or fifties 4. Incomplete Dominance • In this situation an individual which a disorder exhibits a phenotype which is midway between the dominant and recessive traits. • An example of this type of disorder is Familial Hypercholesterolemia ( FH ). • Normal cells have surface receptors which absorb lowdensity lipoproteins ( LDLs ) from the blood. • Individuals who have the FH disorder have cells which only have half the normal number of LDL receptors on their surface. • Thus, the LDL molecules do not get absorbed from the blood and the individual has a high level of cholesterol in his / her blood. • This increased cholesterol level can build up on the walls of arteries and cause atherosclerosis which leads to heart attacks and strokes. 5. X - Linked Recessive Inheritance • These types of disorders occur from genes which are located on the X chromosome. • Disorders of this type are due to the recessive form of the gene and only occurs if there is no dominant form of the gene present. • An example is a disorder called red-green colour blindness. Here, an individual is unable to distinguish between the colors red and green. • About 8% of men and 0.04% of women suffer from this disorder. – Do colorblindness problems on the board. • Inheritance of certain characteristics through sex chromosomes: – Red-green colour blindness – Hemophilia – Muscular dystrophy • An example of such a disorder is hemophilia. This is a disorder in which a person’s blood lacks certain clotting factors, thus the blood will not clot. Because of this, a small cut or bruise may kill an individual. Human Genetic Analysis • Geneticists are able to analyze the patterns of human inheritance using two methods; – Examination of karyotypes – Construction of pedigrees Core Lab #6 – Karyotype Lab (Appendix B) • Analyze and interpret models of human karyotypes. (p. 553-560) The Human Karyotype • Within our body cells, humans normally possess 46 chromosomes. • 44 of these are autosomes (body chromosomes) and 2 are sex chromosomes. • A karyotype is a photograph of the chromosomes which are located in the nucleus of a somatic cell (body cell). • Once a photograph has been taken of the chromosomes in a cell’s nucleus, they are cut out and arranged in pairs according to their size, shape, and appearance. • By observing the karyotype, disorders may become apparent. • See Fig. 16.37, P. 560 Constructing Pedigrees • A pedigree is a chart which shows the genetic relationships between individuals in a family. • Using a pedigree chart and Mendelian genetics, scientists can determine whether an allele (gene) which is responsible for a given condition is dominant, recessive, autosomal, sex-linked, etc. • A pedigree can also be used to predict whether an individual will inherit a particular genetic disorder. • When studying human genetic inheritance it is not possible to perform experimental crosses. • Because of this, human geneticists use data such as medical, historical, and family records which provide information on different generations of humans. • Using this information, they create a pedigree chart which shows the genetic relationships among a group of related individuals. • See Fig. 16.17, P. 544 (& p. 558, 560-562) Outcome… • Draw and interpret pedigree charts from data on single and multiple allele inheritance patterns. • Be able to analyze inheritance data and infer the method of inheritance (dominant, recessive, sexlinked). • Compare pedigree charts for the inheritance of non sex-linked and sex-linked conditions. • Demonstrate the inheritance of autosomal traits determined by single and multiple alleles, and sexlinked traits. – Ex. Freckles, right or left handed.