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Chapter 16 – Genetics Pg’s 526 - 544 Genetics – the branch of biology dealing with the principles of variation and inheritance in animals and plants. Who is Gregor Mendel? LINK - Mendel is known as the ‘father of genetics’. - He was a monk in an Austrian monastery, who had studied botany and mathematics. This background helped him formulate the basics behind today’s study of genetics, using experiments performed on common pea plants. Why was Mendel’s use of the Garden Pea ideal? • They were commercially available. • Easy to grow and matured quickly. • Enclosed sex organs in the flower. • 7 Easily identifiable traits with ONLY two possible variations. Mendel’s First Crosses • Mendel first developed true (pure) breeding lines for each of the 7 pea plant traits by cross pollinating plants with the same characteristics over many generations until they only yielded on form of a trait. • Pure Bred = Homozygous Dominance & Recessive • Crossing these parents he created the First Filial (F1) generation, that all possessed the same trait variation. • What did this tell him about yellow vs. green color? Principle of Dominance • Dominant forms of a trait will always be expressed when it is inherited. – Found in hybrids ( heterozygous individuals) – Found in pure dominant ( homozygous dominant ) – GG = Yellow – Gg = Still Yellow – gg = green (recessive) Genes, Alleles, and Chromosomes Crossing the F1’s • His second experiment: • He took 2 of the plants produced during his first test (F1 Generation), and breed them together to make another generation of offspring (F2 Generation). • When two of these F1 hybrids (dominant looking heterozygous plants) are crossed, the result is an F2 generation showing 3 dominant for every 1 recessive...a “Mendelian Ratio” (expressing 75% dominant, 25% recessive) • Mendelian Ratio: – 3:1 phenotype ratio - dominant to recessive – Phenotype (what you can Physically see) Genotype – the GEnetic makeup of the alleles. 1 homozygous dominant : 2 heterozygous : 1 homozygous recessive 1:2:1 Genotype Ratio What do the numbers tell??? • Based on the repeated pattern of results after completing thousands of crosses, Mendel derived the 1st Law of genetics. • Law of Segregation Mendel’s Conclusions – Law of Segregation • Each parent in any generation starts with two hereditary factors. – (Either both dominant, both recessive, or a combination of dominant & recessive.) • Only one factor from each parent is contributed to the offspring. Cont…. • Each offspring inherits only one factor from each parent. If the dominant factor is inherited, it will be expressed. However, the recessive factor will only be expressed if the dominant trait is not present – (Principle of dominance) Unit Theory • The genes / factors for one trait are inherited as a unit ....the “Unit Theory of Inheritance”. • Why Units?? • Remember the inner workings of a cell, i.e. chromosomes/DNA have not been identified yet! Law of Segregation – Demystified using the understanding of chromosome behavior. Segregation NOW WE KNOW: The Law of segregation is explained by the behavior of homologous chromosomes at meiosis. Genes and Environment Determine Characters The final phenotype expressed in many cases involves the interaction between your genotype and the environment conditions together. Ex. For humans: Skin color, Eye color, Height, Weight Genetically identical hydrangeas growing in soils of different acidity (different environments). A Punnett Square is a Handy Way of Analyzing Crosses Possible offspring In a Punnett square for a monohybrid cross, the possible gametes of the parents are put on the side of the table and the Principle of Segregation is applied. Let’s try a cross using two of the heterozygous plants from Mendel’s F1 generation Always show the parents genotype first: P - ____ _____ x ____ _____ • Phenotype = • Genotype = Consistency is Good No matter what physical trait Mendel observed a 3:1 phenotypic ratio of characteristics was always found in the F2. Characters investigated by Mendel Monohybrid Crosses Yielded Consistent Results Therefore, the Law of Segregation indeed is a general Law of genetics. What Works for Peas Also Works for Humans In the cross Aa x Aa, where A is a dominant allele for wild type (standard) pigmentation and a is a recessive allele for no pigmentation (albinism): Legend: A – a- P - ____ _____ x ____ _____ ¾ of offspring will be wild type and ¼ will be albino An albino woman Do this monohybrid cross • In pea plants, round seeds are dominant to wrinkled seeds. If a homozygous round seed plant is crossed with a heterozygous round seed plant, what is the expected phenotypic and genotypic ratios in the F1 ? • R• rP - ____ _____ x ____ Results - _____ Answer P – RR x Rr Alleles produced for gametes are R or R from one parent and R or r from the other parent R r R R RR RR Rr Rr Phenotypic Ratio: All Round Genotypic ratio: 2 RR : 2 Rr Try This one ! Fruit fly wing length is controlled by a dominant allele for long wings (L), short is recessive. If a heterozygous long winged fly is mated to a homozygous short winged fly, what is the expected phenotypic and genotypic ratio in their offspring? Legend: P - ____ _____ x ____ _____ • Do both sheets from webpage. – Monohybrid Practice – Spongebob genetics Then Pg, 535 # 10,11,12 and 14. Review - Understand these concepts pg 529-530 • Principle of Dominance – • Law of Segregation – • Unit Theory of Inheritance – Probability & Product Rule • Probability – is the chance of an event occurring, usually represented by a ratio. • Product Rule – states the chance that two or more independent events will occur TOGETHER. – This is calculated by finding the product of their individual probabilities occurring alone. Flip a coin • Probability of Heads?? - - ½ (50%) • Probability of flipping Two (2) heads Together? Try this • What is the probability of rolling a regular die and getting a 6? • What is the probability of rolling a pair of dice and getting “snake-eyes” (a pair of 1’s)? Test Cross – pg 534 • To determine the UNKNOWN genotype of an organism that shows the dominant phenotype, we perform a TEST CROSS. • This is done by taking the organism with the unknown genotype (TT or Tt) and cross it with a purebred (homozygous) recessive and look at the results. • If any of the offspring show the recessive condition then the unknown parent MUST have been heterozygous. Extensions to Mendel: (The Exceptions) Complexities in Relating Genotype to Phenotype Extensions to Mendel’s analysis • In Single-gene inheritance: – Sometimes pairs of alleles show deviations from complete dominance and recessiveness – Sometimes more than two different variations (alleles) exist for a gene – Sometimes one gene may determine more than one trait • In Multifactorial inheritance: • the phenotype expressed arises from the interactions between one or more genes, the environment, and chance events. Dominance is not always complete • Crosses between true-breeding strains can produce hybrids with phenotypes different from both parents • Ex. • Incomplete Dominance • Co-Dominance • Multiple Alleles Incomplete dominance • This occurs in the case where two pure breeding (homozygous) parents that show two completely different phenotypes have F1 hybrids that show a completely different phenotype from either parent. • The F1 expresses an intermediate phenotype. Neither allele is dominant or recessive to the other. • Phenotypic ratios are same as genotypic ratios Incomplete Dominance for Flower Color in Snapdragon Codominance • Co-Dominance occurs when parents have different traits and the alleles for these traits are equally dominant to the other. The F1 hybrid offspring of a cross between two purebreds will express the phenotype of both parents equally. • Phenotypic ratios are same as genotypic ratios Codominance Three Different Forms of Dominance Multiple Alleles • In many species a gene may have more than two possible alleles for any given gene. – Ex. Human Blood Groups (A, B, and O) – (A & B are co-dominant and O is recessive) Codominance of IA and IB Blood Group Alleles There Are Often More Than Two Alleles of a Gene Blood groups are determined by one gene with three alleles. Also that the blood groups show both complete dominance and codominance. Multiple Alleles Can be Grouped into a Dominance Series Dominance series for lentil bean coat color. Try: -Practice Problem from Pg 542 -Thinking Lab pg 543. - Then pg 544 - #2, 5, 6 a & b, Do variations on dominance relations negate Mendel’s law of segregation? • Dominance relations only affect the relationship between genotype and phenotype • Alleles still segregate randomly and unite randomly • Gene products (plus external factors) control the expression of phenotypes What about Multiple traits? • Does being tall (dominant) give you more of a chance at also being yellow (dominant). • Mendel performed a series of experiments like before, but tracked two different traits at the same time. • Two purebred parents when crossed still produced a heterozygous F1 • Note: Notice the numbers for the phenotype and genotype of the F2 • Anything similar? Law of Independent Assortment • Phenotype: 9:3:3:1 • Genotype 1:1:2:2:4:2:2:1:1 • These ratios were obtained repeatedly which helped Mendel in the statement of the Law of Independent Assortment – The genes/factors for one trait are inherited as a single unit and are inherited separately (independently) from other traits. Step by step!! • #1 - Make a legend • #2 – Show parents 4 dashes times 4 dashes. • ____ ____ ____ ____ x ____ ____ ____ ____ – NOTE- Remember two dashes for each trait!! • #3 Perform “FOIL” on each parent to make gametes. • #4 make your Punnett square. • Tip- Keep Same letter together in the boxes and always put the capital letter first (when possible) Try it • Tall is dominant over short. • Yellow is dominant over green. • Cross a homozygous tall, homozygous green plant with a plant that is heterozygous for both traits. • REVIEW LINK Chromosomes & Heredity Pg’s 545 - 555 Chromosome Theory of Inheritance • 1902 - Walter Sutton & Theodor Boveri – Observed that the behavior of the chromosomes during Meiosis was similar to the behavior that Mendel predicted of his ‘inheritance factors’. • Three Key Observations: 1. Chromosomes occur in pairs. 2. Paired chromosomes segregate during Anaphase 1. 3. Chromosomes align independently at the cell’s equator. Definition • The Chromosome Theory of Inheritance – Genes (Mendel’s factor’s) are carried on chromosomes and it is the segregation and independent assortment of these chromosomes during the stages of meiosis that accounts for the patterns of inheritance shown in offspring. Think about this – do the cross • Color-blindness in humans is recessive. When you cross two (2) parents with normal color vision you produce an offspring that is color-blind. – So far so good…….??????? • Now if you were to take a normal color vision female (from the offspring above) and cross her with a male having normal color vision and produced the following: – offspring where ALL the females have normal color vision, but the males are 50% normal vision: 50% color blind. – ?????? EXPLAIN ??????????? Review - The Human • 46 Chromosomes that occur in 23 pairs of homologous chromosomes. - 1 pair of sex chromosomes (X and Y) - 22 pairs of autosomes Autosome – non-sex determining chromosomes, responsible for containing the remaining traits of the human being. Each chromosome contains anywhere from hundreds to thousands of genes for particular traits. X-linked Inheritance • Red-green colorblindness is a sex linked (often called X-linked) condition. • The allele for color vision is located on the X chromosome. – This explains why it is so much more common in men (8%) than women (0.04%). • For a woman to be colorblind, her father had to be colorblind and her mother had to be colorblind or a carrier of the recessive allele. • For a man to be colorblind the mother only has to provide one recessive allele on her X chromosome. A male will get a Y from his father. Morgan’s Discoveries • In 1910, an American scientist called Thomas Morgan made a very important discovery from his work with fruit flies Fruit Flies • Normal fruit flies have red eyes • 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. Gene Linkage • Thomas Morgan also found out that some other genes (other than sex chromosomes genes) did not seem to follow Mendel’s Laws. – some genes would tend to be inherited together. • Further understanding of the genechromosome theory (pg 546) described that genes found very close together on the same chromosome were rarely separated by crossing over and would therefore be inherited together. (called LINKED) Gene Linkage explained • Note: all genes are located in a linear fashion from one end of the chromosome to the other. • Crossing over occurs during Prophase 1, however if two genes are very close together they have a high probability of not being separated. – (Red hair and freckles) Mendel’s Law of Independent Assortment - Restated • If Crossing over does not occur, genes that are located on different chromosome will assort independently while genes that are located on the same chromosome will be inherited together. • Try #4 (NOT sex-linked) & 8 (Sex-link) on Pg 554. The Human Karyotype • The human karyotype is an illustration or photograph of the chromosomes in the nucleus of a somatic cell in an organism. • Creating a Karyotype involves growing cells and stopping the division process during the metaphase stage. The chromosomes are then separated, stained and photographed. The chromosomes are then cut out and arranged in pairs according to size, shape and appearance. Do Karyotype lab Chromosomes and Gene Expression Exceptions to the Rule… other than sex linked traits Chromosome Inactivation • Where a girl has two X chromosomes, do they produce more proteins than boys with only one X chromosome??? • No – One of the X chromosomes from the female pair ‘turns off’, or becomes inactive. • The inactivated chromosome becomes called the BARR BODY. Barr Body • The inactivation of the X chromosome in different cells is random. So different cells may have a different X chromosome activated. • This is responsible for producing the tortoiseshell coat color in female cats. Polygenic Inheritance • Literally – Many genes control the one product. – Ex. The ear length of corn, height and skin color of humans. • Where multiple genes are contributing to the final product we arrive at a continuous variation. Modifier Genes • Genes that work with other genes to influence the final product. - Eye color in humans. • Arguably, there should be only two types of eye colors: – Brown (dominant) and Blue (recessive) • However modifying genes act together to create the final range of colors. Major Changes - Mutation • Other than crossing over, changes can occur in the actual chromosomes code (mutations) causing profound effects on the possible outcomes!!! • Changes can occur: 1. Spontaneously 2. When a cell becomes irradiated 3. When exposed to certain chemicals. Deletion • Deletion – a portion of the chromosome is lost or removed due to irradiation, viruses or various chemicals. – These pieces coded for genes, so when they are lost so is the genetic trait it coded for. • Ex. – deletion in a specific area of chromosome #5 causes the condition Cri du chat. Inversion • Inversion – a piece of the chromosome becomes free momentarily before being reinserted in the reverse order. This completely changes the order of genes that this chromosome coded for. Example - Autism has been linked to chromosomal inversions. Duplication • Duplication – is when multiple copies of a gene sequence occur. For the most part this can have no affect on a human, but in some cases too many repeats can be detrimental. – Ex. Fragile X - syndrome Translocation • Translocation – occurs when part of one chromosome changes places with part of a nonhomologous chromosome. • Example – If part of chromosome 14 exchanges places with #8 then cancer can occur. • Down syndrome – linked to translocation between chromosome 14 & 21. • A type of Leukemia has been traced to translocation between #22 and #9. Many other defects occur due to Nondisjunction • Nondisjunction – is the result of chromosomes not separating during Anaphase 1 or 2 of meiosis. As a result the daughter cells will either have too many or not enough chromosomes. – inheriting an extra chromosome is referred to as trisomy (3 copies) – inheriting only one chromosome is referred to as monosomy (only 1 copy) • Embryos with either condition rarely survive to birth due to the large number of genes being affected. • Many miscarriages can be linked to nondisjunction events taking place. • Down Syndrome – is probably one of the most common nondisjunction syndromes. It occurs when an individual receives 3 copies of chromosome 21. • -Symptoms include: mild to moderate mental impairment and a thick tongue that can create speech defects. Skeleton may not develop properly resulting in a short stocky body type. • Turner Syndrome – results from a monosomy where a person has only one X sex chromosome. The woman will have external female genitalia, but will lack ovaries. They are therefore infertile and not mature sexually. Other defects include heart, kidney and skeletal defects. • A single Y chromosome individual is not possible. This embryo would not survive where they would be lacking vital genetic information. • Klinefelter syndrome – is a trisomy condition where an extra X chromosome occurs in a male (XXY). This individual will have immature sex organs and will not grow facial hair. They are also likely to develop some breasts. • XXX females do not show any at all symptoms. Why????? • Jacobs syndrome – occurs in males with an extra Y (XYY). These individuals show speech and reading problems and have persistent acne. A study once found that there seemed to be an extremely high occurrence of this condition amongst prisoners compared to the rest of society. Do page 554 - # 1,2,3, and 9 • What is the fastest way to determine the sex of a chromosome? • Pull down its genes. • LOL Human Genetics Distinguish among the different types of inheritance patterns Pg’s 555 - 565 Autosomal Recessive Inheritance Disorders carried on autosomes and are not related to the sex of the individual. Tay-Sachs disease – body lacks the enzymes required to break specialized lipids of the nervous system. – These individuals are normal at birth, but by 8 months cells are swollen with undigested lipids, rupture and kill the cells. – By their 1st birthday they will usually be blind, mentally handicapped, and display little muscle activity. Most die before the age of 5. A.R.I.’s cont. • Phenylketonuria (PKU) – PKU is caused by the lack of an enzyme that converts phenylalanine to tyrosine. – A child with PKU will breakdown phenylalanine to abnormal products which damage the nervous system causing mental retardation and seizures. – Luckily there is a routine test and treatment in place for babies with this condition. If the condition is not detected the baby with become severely mentally handicapped within a few months. A.R.I.’s cont • Albinism – A condition where the hair, skin and eyes have no pigment. – A normal individual is capable of producing different colors in our bodies due to a varying amounts of the brown pigment called melanin. – Where an albino individual lacks this pigment they lack the ability to tan, thereby lacking the ability to naturally protect their bodies from the sun’s powerful UV rays. Codominant Inheritance • where an individual carries 2 different copies of an allele, but both are expressed (Co-dominant). • Sickle Cell Anemia – Affected individuals have a defect in the hemoglobin of their red blood cells causing an irregular shape which can clog up capillaries and lead to blood clots. These individuals tend to lack energy, suffer from various illnesses and are in constant pain. • This recessive allele is believed to have originated from Africa. Sickle Cell • Until recently homozygous recessive individuals never lived to adulthood. Therefore you would think the presence of the allele should have decreasing each generation. However in some African regions nearly half of an entire population is heterozygous for the condition. • How could this be possible???? The answer - Heterozygous Advantage • The answer was found while studying the leading cause of illness and death in Africa. (Malaria) • Children that were heterozygous for the sickle cell gene were less likely to contract malaria and survived to adulthood. • Heterozygous Advantage – where an individual with two different alleles for the same trait have a better chance of survival. Autosomal Dominant Inheritance • Due to the behaviour of dominant traits in Mendelian genetics, we can trace dominant disorders 2 ways: - since both heterozygous and homozygous individuals show a trait, the trait should be seen in every generation. • if one parent is heterozygous and crosses with a homozygous recessive individual, then the trait should still be present 50% of the time. Dominant Disorders - Although rare they do exist! • Some can be caused by random mutations of a gene sequence. • In most cases these conditions do not become prevalent until the individual has already reached reproductive age and have already conceived, potentially passing on the gene to their offspring. Two examples • Progeria – a rare disorder that causes an individual to age rapidly. Usually death occurs by age 10 - 15 • It occurs in newborns at a rate of 1 in 8 million, and does not run on families. It is therefore linked to a rare random mutation. • 15 year old male • 16 year old female Huntington’s Disease • Huntington’s Disease – a lethal disorder where the brain progressively deteriorates over time. Symptoms generally begin around age 35, after the individual has already had children. Incomplete Dominance • A condition where having even one copy of the affected gene (dominant or recessive) leads to the condition. • Familial hypercholesterolemia (FH) – is a condition that affects heterozygotes (1 : 500). The cell produces less receptors for LDL (lipids) that are required to take these lipids into the cell. Without them these lipids build up in the arteries and lead to a heart attack or stroke by around the age of 35. Homozygous individuals for this recessive condition can die of a heart attack by the age of 2 Analysis of Human Genetics • Two techniques have been used to examine human genetics: - Karyotypes - Pedigrees Pedigrees • A pedigree shows the genetic relationships between individuals in a family, • Through many years of tracing a family history and applying this data to Mendelian genetics, scientists can determine if a condition is dominant, recessive, autosomal, or sex-linked. • Pedigrees can also be used to predict the possible inheritance of a disorder. – Ex. Page 560 shows a pedigree tracing Hemophilia throughout Queen Victoria’s family. Go to “Pedigree Presentation” Different powerpoint • Do Page 559 – Mini Lab Karyotype • Page 561 – Solve the Case of the Caped Murderer • Pg 562 - #1, 7, 9, 10, 11 and 12