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Genes and Heredity Classical Genetics Heredity Genetics is the study of heredity. Heredity – the passing of traits from parents to offspring Heredity ensures that you have characteristics similar to your parents (but not exact copy) Genes – units of instruction (located on chromosomes) that produce or influence a specific trait in the offspring (ie. Eye color) Genome – a cell’s total hereditary endowment of DNA Biological traits are controlled by genes of both parents but are unique to individual ~8,000,000 combinations are possible between 23 chromosomes from each parent Alleles – two or more alternate forms of a gene like brown hair vs. blonde hair Gene locus – location of gene on a chromosome Alleles Gene Locus History Scientists noticed traits were passed on, but how? Aristotle – suggested through blood (bloodlines) Early Naturalists – believed in hybrids where species result from breeding between other species Buffon (1700’s) – suggested head and limbs came from father, the rest from the mother 1800’s – a blending of traits from both parents Very late 1800’s – microscopes allowed for meiosis to be observed, there was speculation about chromosomal involvement Gregor Mendel Austrian Monk 1822 – 1884 Worked with garden peas to explain gene inheritance for plants and heredity “Father of Genetics” https://www.youtube.com/watch?v=Mehz7tCxjSE How Mendel's pea plants helped us understand genetics Ted Ed Why Peas??? 1) Have number of visible traits that are explainable in one of two ways – – – Ex) green vs. yellow peas Ex) tall vs. short plants Ex) wrinkled vs. smooth peas 2) Garden peas are self-fertilizing and cross-fertilizing Self-fertilization (Pollination) – pollen produced from male stamens attaches to female pistil inside same flower. Cross-fertilization – pollen from male stamen attaches to female pistil inside another flower. Mendel used cross-fertilization (removed stamen and transferred pollen from one plant to another plants pistil) He combined male and female sex cells of different plants Mendel’s experiments involved the crossing of pea plants with different traits to see what traits the offspring would have Mendel's Experiment Parental Cross (P) Round x Wrinkled Seed First Filial Generation (F1) All Round Second Filial Generation (F2) 3:1 ratio of round to wrinkled Mendel’s Thoughts… Why did the F1 generation produce all round seeds? Why did the F2 generation produce more round that wrinkled seeds? Does it make a difference if the characteristic came from the male or female plant? In order to answer these questions, Mendel repeated the procedure with other characteristics Mendel’s Discoveries Discovered that one trait dominates another (no matter what characteristic came from the male or female sex cell) Mendel established the general principles of genetics (later to become his laws of Inheritance) Mendel’s Laws of Inheritance Law of Unit Characteristics: traits are controlled by factors called genes which occur in pairs (one from each parent). Ex) RR, Rr, rr Law of Dominance: the dominant form of a trait prevents or masks the expression of the recessive form. The dominant form is represented by capital letters (R) and the recessive form by lower case letters (r) Law of Segregation: during gamete formation (meiosis), homologous pairs separate and gametes have one of each homologue Law of Independent Assortment: genes which are located on separate chromosomes assort (separate) independently during meiosis. One gene does not effect where any other gene will move. In other words, all dominant genes do not end up in the same gamete. NOTE: Just because a trait is dominant, it does not necessarily mean the majority of a population will express that phenotype Most Dutch people are blue-eyed and blonde which are both recessive traits. Dominant genes – determine the expression of the genetic trait in offspring (capital letter) Recessive genes – overruled by dominant genes (lower case letter) *Normally the letters of genes follow the dominant gene name (capital) and will stay the same for recessive genes (lower case) Genetic Terminology Phenotype: the observed or displayed form of the genes carried by an individual (what you see); results from interaction between genes and the environment. – Ex. Round seeds or wrinkled seeds Genotype – the combination of the genes that are carried on the chromosomes for a given trait – Ex. Round seeds = RR, Rr – Wrinkled Seeds = rr Heterozygous – refers to the genotype in which the gene pair are different. Often called a hybrid or carrier. They would express the dominant phenotype – Ex. Rr = Round Homozygous – refers to the genotype in which both genes are identical. Often called pure bred. – Ex. Homozygous dominant = RR – Ex. Homozygous recessive = rr RATIOS In terms of possible offspring phenotypes and genotypes, you may find it easier to write them as a ratio – Genotypic ratio: homo dominant : heterozygous : homo recessive – Ex. 9 RR, 3Rr, 1 rr 9:3:1 – Phenotypic ratio: Dominant: Recessive – Ex. 3 Round, 1 wrinkled 3:1 Monohybrid Crosses Genetics problems are sometimes complex, so we simplify them using punnet squares In a punnet square, the gametes from one parent are arranged along one side of the square and the gametes of the other parent are arranged along a second side With this, you can derive all possible combinations of gametes Monohybrid crosses (cross of a single trait) are the easiest to study Ex) a plant heterozygous for seed shape is crossed with another plant which is also heterozygous Parents: Rr x Rr Possible gametes: R r R r Punnet Square R r R RR Rr r Rr rr Genotypic Ratio: RR: Rr: rr = 1:2:1 Phenotypic Ratio: Round:wrinkled = 3:1 Thus, there is a 3 in 4 or 75% chance of producing a round seed plant when two heterozygous plants are crossed (or a 1 in 4 chance of producing a wrinkled seed plant) Practice Examples 1) Using the trait for seed shape (round dominant to wrinkled), determine the phenotypic and genotypic ratios for the following crosses: a) Homozygous dominant x homozygous dominant b) Homozygous dominant x heterozygous Practice Examples c) Homozygous dominant x homozygous recessive d) Heterozygous x homozygous recessive e) Homozygous recessive x homozygous recessive 2) Determine the phenotypic and genotypic ratios for a cross between a homozygous dominant red flower and a recessive white flower 3) Determine the phenotypic and genotypic ratios for a cross between a man who is heterozygous for a skin pigment and an albino woman (no skin pigment is a recessive trait) 4) Determine the phenotypic and genotypic ratios for a cross between two plants which are heterozygous for height (tall is dominant). 5) Determine the phenotypic and genotypic ratios for both the F1 and F2 generations for a cross between a homozygous tall plant and a short plant 6) Determine the phenotypic and genotypic ratios for a cross between a heterozygous hornless cow and a horned bull. What are the chances of producing a horned offspring? Test Cross Performed to determine the genotype of a dominant phenotype (by looking at them we cannot tell if they are homozygous or heterozygous) Test cross is always performed between the unknown genotype and a homozygous recessive genotype If the unknown is heterozygous, half of the offspring will be dominant and half will be recessive If the unknown is homozygous dominant, all the offspring will be dominant Ex. A farmer wants to know if his white sheep is pure bred or a hybrid so he crosses it with a black sheep White sheep = W ___ Black sheep = ww w w W w Ww ww Ww ww w w W W Ww Ww Ww Ww Multiple Alleles In Mendel’s experiments there were only two possible alleles and the dominant allele controlled the trait. However, it is common for there to be more than two alleles for a particular trait This is called multiple alleles Ex) Fruit Flies (drosophila) can have eyes colored red, apricot, honey, and white (but it is only possible to have two of these different genes at one time) There is a dominance hierarchy Red is dominant to apricot, is dominant to honey, is dominant to white We don’t use capital and lower-case letters. Instead we use capital letters with superscript numbers to express different genes and their combinations E1 – Red E2 – Apricot E3 – Honey E4 – White Determine the phenotypic ratios from a cross between a fly with wild-type eye color (E1E4) to one with apricot eyes (E2E3) Note: Wild-type refers to the ‘normal’ allele (the one found in majority of wild populations (not necessarily the most dominant though) Incomplete Dominance Sometimes hybrids have a blending of traits (not found by Mendel) When two traits are equally dominant, they interact to produce a new phenotype – this is known as incomplete dominance Two types of incomplete dominance: – Intermediate inheritance or non-dominance – Codominance Intermediate inheritance Results when neither allele is dominant Hybrid phenotype is a blend or mixture of two phenotypes Ex) Snapdragons – Red x White = Pink This is also notated differently (with superscripts) FRFR x FW FW = FRFW Determine the F1 phenotypic ratio from the crossing of two pink snapdragons Codominance Results when both alleles are dominant Hybrid expresses both phenotypes Ex) Shorthorn cattle = Red bull x White Cow = Roan Calf Roan calf has intermingled white and red hair Both genes expressed at same time Also expressed in notation with superscript HRHR x HWHW = HRHW If a roan shorthorn cow is crossed with a white shorthorn bull, what is the probability that the offspring will be roan? Blood Types Most familiar example of multiple alleles and codominance in humans is blood type Most common typing system divides individuals into four classes: A, B, AB, and O There are three alleles for this: IA IB i IA and IB are codominant i is recessive to both of these Phenotype A Genotype IAIA, IAi B IBIB, IBi AB IAIB O ii If one parent has blood type A and the other has blood type B, determine the parental genotypes if the offspring had the following blood types: A) ½ AB, ½ B B) all AB If one parent has blood type A and the other has blood type B, determine the parental genotypes if the offspring had the following blood types: C) ½ AB, ½ A D) ¼ A, ¼ B, ¼ AB, ¼ O Lethal Alleles When the expression or phenotype of a gene causes death, it is called a lethal allele Lethal genes are usually recessive or mutant genes with a low frequency of occurrences in the population Occasionally a fully dominant lethal allele can form Examples Dihybrid Crosses What’s the probability of producing a yellow if two heterozygous plants (Yy) are crossed? What’s the probability of producing a round seed if two heterozygous plants (Rr) are crossed? So what’s the probability of producing a yellow, round seeded pea plant if two plants heterozygous for both traits are crossed? A cross that deals with two different traits is called a dihybrid cross. Dihybrid crosses can be solved with a larger 16 square punnet square Mendel’s law of Independent Assortment was created from his study of dihybrid crosses – this law states that genes assort independently (one gene does not influence the inheritance of another when located on different chromosomes) Mendel cross-pollinated pure-breeding plants that produced yellow, round seed coats with pure-breeding plants that produced green, wrinkled seed coats – – – – Pure breeding round coat = RR Pure breeding wrinkled coat = rr Pure yellow = YY Pure green = yy Genotypes: – Pure yellow, round parent = YYRR – Pure green, wrinkled parent = yyrr The F1 offspring produced from this cross are heterozygous for both yellow and round genotypes (YyRr) In a dihybrid cross, there are 4 different possible phenotypes Ex) Cross YyRr x YyRr – There are four possible gamete combinations – Therefore our punnet square is no longer 2 x 2, but it is 4 x 4 – Remember that seed coat gene is independent of seed color Probability Probability = # of chances for event/Total # of combinations Two rules to remember: 1) Rule of Independent events – Previous events do not affect future events Ex) Tossing two heads in a row – chances of tails is still 50% 2) Product Rule – Probability of independent events occurring simultaneously is equal to the product of these events occurring separately Ex) Chances of tossing heads after two tosses is ½, but chances of tossing heads three times in a row is: ½ x ½ x ½ = 1/8 You can calculate dihybrids by using two monohybrids and using the product rule Y y Y YY Yy y Yy yy R r R RR Rr r Rr rr Chances of getting yyrr is ¼ x ¼ = 1/16 Chances of getting YyRr is ½ x ½ = 1/4 You can also calculate dihybrids using a dihybrid punnet square Dihybrid Punnet square YR Yr yR yr YR YYRR YYRr YyRR YyRr Yr YYRr YYrr YyRr Yyrr yR YyRR YyRr yyRR yyRr yr YyRr Yyrr yyRr yyrr YYRR – 1/16 (6.25%) YYRr – 2/16 = 1/8 (12.5%) YYrr – 1/16 (6.25%) YyRR – 2/16 =1/8 (12.5%) YyRr – 4/16 = 1/4 (25%) Yyrr – 2/16 = 1/8 (12.5%) yyRR – 1/16 (6.25%) yyRr – 2/16 = 1/8 (12.5%) Yyrr – 1/16 (6.25%) Remember that all fractions should equal 16/16 when added together (or all percentages should add to 100%) Ratios for Dihybrid crosses For Dihybrid crosses you MUST identify what the ratio stands for! Ex) Genotype: YYRR:YYRr:YYrr:YyRR:YyRr:Yyrr:yyRR:yyRr:yyrr 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1 (all add to 16) Phenotype: Yellow,round:yellow,wrinkled:green, round:green, wrinkled 9 : 3 : 3: 1(all add to 16) NOTE: A dihybrid cross of two parents heterozygous for both traits will always give a 9:3:3:1 phenotypic ratio! If n = number of traits, 2n = number of phenotypes and 3n = number of genotypes Examples What are the gametes produced from the following genotypes (You should list four for each even if they are the same): – BbRR – BBrr What is the probability of producing a tall, red flowered plant if two plants heterozygous for both traits are crossed (tall and red are both dominant traits)? What is the probability of having a blue eyed, blonde haired baby if both parents are heterozygous for eye trait and one is heterozygous for hair color and the other is blonde? (Reminder: Blue eyes and blonde hair are recessive traits) When a fruit fly of genotype Mm Nn Oo is mated to another fruit fly of identical genotype what is the probability of the offspring having the following genotypes: a) MM Nn OO b) mm NN Oo c) Mm Nn Oo Gene Interaction Some traits are regulated by more than one gene These are called polygenic genes – Ex) Skin color, eye color, height Polygenic Characteristics 1) Epistatic gene interaction – action of one gene (or set of genes) masks the expression of other, nonallelic genes Ex) Dog coat B = Black b = brown But: W = prevents pigment forming w = doesn’t prevent pigment forming Therefore genotype wwBb = Black, but WwBb = white *W is an epistatic gene (interferes with B) Another example: - at least 2 pairs of genes control eye color - B = brown - b = blue - Another pair is - A = pigment - a = no pigment (albino) - Gene pair aa is epistatic to B and b because it produces no pigmented eyes Epigenetics http://www.youtube.com/watch?v=kp1bZEUgqVI In humans, there is a dominant allele that causes vitiligo, where small-unpigmented spots appear on the body. Also, there is a recessive allele for another gene that causes albinism, which causes the entire body to be unpigmented. Vitiligo cannot be seen in albinos Using the Dog coat example: Give the genotypes/phenotypes of the possible offspring of the mating of bbAa x Bbaa 2) Complementary Interaction – produce an effect (phenotype) that neither can produce by itself Ex) Chicken combs - R = Rose comb - P = Pea comb (located on different chromosome) - R + P = Walnut comb - Absence of R + P = Single comb * This is different from intermediate inheritance because those were genes located on the same chromosomes (alleles) Enzymes Enzyme – protein molecule that speeds up chemical reaction in cells Ex) Clover plant – production of cyanide (regulated by two genes) – G = provides info for production of enzyme 1 (glucoside) – g = mutation – enzyme not produced – H = provides info for production of enzyme 2 (turns glucocide into cyanide) – h = mutation – enzyme not produced – GgHh will produce Cyanide. ggHH or GGhh will not. 3) Pleiotropic genes – affect more than one trait Ex) mutated gene causing sickle cell anemia (blood disorder) - HbA – Normal Hemoglobin - HbS – mutation – causes sickle cell anemia Homozygous HbS hemoglobin can still carry oxygen, but when it gives oxygen to the body, the molecules become interlocked and change the shape of red blood cells (sickle shape) These blood cells do not pass through capillaries, oxygen is not delivered, normal organ function is impaired. In addition to RBCs changing shape, sickle cell people have other symptoms: – Enlarged spleens, rheumatism, pneumonia, and heart, kidney, lung, and muscle damage Ex) Marfan’s syndrome Inability to produce normal connective tissue due to one gene Symptoms: Eye, skeleton, cardiovascular defects http://www.youtube.com/watch?v=ab_B0lZqq6M Breeding Patterns Artificial selection (Selective Breeding) – crossing of desired traits from plants or animals to produce offspring with both characteristics Used by farmers and ranchers for years Improves domestic varieties of plants and animals – Ex) Canola that can germinate and grows quickly in cold climates Purebreds – genotypes are regulated by inbreeding (homozygous traits). Similar phenotypes are selected for breeding Also, new varieties of plants or animals can be created through hybridization – this is an attempt to blend desirable but different traits Heredity: Crash Course Biology #9 http://www.youtube.com/watch?v=CBezq1fFUEA The Source of Heredity The Chromosomal Theory – Chromosomes carry genes (units of heredity structure) – Paired chromosomes segregate during meiosis. Each gamete has ½ the number of chromosomes as in a somatic cell – Somatic cell – all cells except sex cells – Chromosomes assort independently during meiosis – Each chromosome contains many genes – Autosomes – all body chromosomes EXCEPT sex chromosomes Morgan’s experiments Thomas Hunt Morgan (1866 – 1945) – American geneticist who used fruit flies (Drosophila melanogaster) to study Mendel’s principles of inheritance Why Fruit Flies? Rapid Reproduction – >100 eggs/mating – Reproduce soon after leaving eggs – Good for genetics probability! Short Life cycle (10 – 15 days) – Study many generations in short period of time Small size – Many housed in one test tube with little food supply Males easily distinguished from females Discovered mutations linked to certain traits. Supported the theory that genes responsible for traits were located on the same chromosome He found a white-eyed male among many redeyed offspring – Conclusion: white-eyed trait = mutation Mated white-eyed male with red-eyed female and all F1 generation had red eyes (red dominant). But when crossing two F1 hybrids for F2 generation, he got ¾ red and ¼ white. This was explained by Mendel, but he also found that only males had white eyes (no females) After further crosses, found females could express the white mutation, but not in the F1 or F2 generation To explain this, Morgan studied cells from salivary glands of Drosophila (8 chromosome) He stained chromosomes and viewed under a microscope Found males had only 3 homologous chromosome pairs (sex chromosomes not identical) and females had 4 – XX = female – XY = male Conclusion: Because X and Y are not completely homologous, they must contain different genes (unlike autosomes) Drosophila eye color located on X chromosome. Because Y is not identical, it does not carry the trait. Sex linked Traits – controlled by genes located on the sex chromosomes Therefore: – Red-eyed female = XRXR or XRXr – White-eyed female = XrXr – White-eyed male = XrY **XR – Red gene dominant and located on X chromosome Punnet Square Explaination XR XR Xr XRXr XRXr Y XRY XRY XR Xr XR XRXR XRXr Y XRY XrY F1 F2 Male offspring always inherit a X-linked trait from their mother (father supplies Y chromosome each time) Notice the females can have 3 possible genotypes and males only have 2 – Males CANNOT be heterozygous for a sex-linked trait (not a carrier – either has it or doesn’t) To get a white-eyed female, a female with at least one white gene must be crossed with a white-eyed male Sex Determination Sex-linked genes are found in humans (pg 601) – Ex) Red-green color blindness – Determined by a recessive trait located on the X chromosome – Males colorblind more than females because females need 2 recessive genes and males only need one (other gene is Y) – 10% of males experience color blindness compared to only 0.4% of females Examples 1) Colorblindness is inherited as a sex-linked recessive disease. An affected male marries a heterozygous female. – Draw the Punnett square of the possible offspring – What is the chance that they will have an affected child? – Could any of their daughters be affected? 2) Suppose hairy ears is inherited as a Y-linked trait. A man with hairy ears marries a woman with normal ears. – Draw a punnett square of the possible offspring – Could any of their children have hairy ears? 3) This disease is inherited as a sex-linked dominant disease. An affected male marries a homozygous recessive female. – Draw a punnett square of the possible offspring – What is the likelihood that any child they will have will be affected – Would the parents of the father (grandparents of these offspring) have been affected? – What is the likelihood that their male children will be affected? – What is the likelihood that their female children will be affected? Barr Bodies Small dark spots of chromatin, located in the nuclei of female mammalian cells (not found in male cells) 1961 – Mary Lyon – Proposed dark spot was a dormant X chromosome – One of the X chromosomes become inactive (but this inactive chromosome varies between cells) – This means that not all female cells are identical (depending on which of the two X chromosomes is active) – Heterozygous females who carry a lethal gene on their Xchromosome may escape disease because gene is only active in 50% of cells. Chimera Cats and Your Mom https://www.youtube.com/watch?v=eSMQcy5Re QQ&sns=em Calico Cats Example Male Cats = XBY – Black or XOY – orange Female Cats = XBXB – Black or XOXO – Orange or XBXO – black and orange patchwork Areas which are orange – XO active; areas which are black – XB active. Males cannot be calico unless nondisjunction occurs and an extra X chromosome appears (XXY) – These cats are infertile. TDF Humans = 46 chromosomes Female = 23 pairs homologous chromosomes (22 autosomes, 2 X sex chromosomes) Male = 22 pairs of homologous chromosomes and one X and one Y sex chromosomes Human X chromosome carries 100 – 200 different genetic traits Y chromosome codes only for gender – this gene is called TDF TDF TDF = Testes determining factor TDF is not activated until the 6th or 7th week of pregnancy therefore a male fetus is no different from a female fetus until then The testes develop in the body like ovaries until TDF activated and then they descend Testicular feminization syndrome XY individual appear female Positive test for male Y chromatin Negative test for female chromatin Syndrome caused by gene mutation on the X chromosome that acts only in the XY zygotes These individuals do not react to injections of male sex hormones – therefore they don’t display muscle development of males Fragile X syndrome A person with fragile X syndrome has a mutation in the X chromosome. Negative test for female chromatin Symptoms of mental disability range depending on whether the person is male (more severe) or female (less severe) Can be inherited from either the mother or the father Crossing over Morgan discovered that specific genes are located on specific chromosomes – Ex) Eye color for Drosophila on X chromosomes Remember: Two or more genes that are on different chromosomes segregate independently But what about genes on the same chromosome? Morgan concluded that the gene for wing shape and gene for body color were located on the same chromosome (They do not segregate independently) Linked Genes – located on the same chromosome and tend to be transmitted together Crossing-over can provide new combinations of genes – Occurs during synapsis of meiosis – Single chromosome can change as it passes from generation to generation – Figure 21.9 Gametes with chromosomes that recombine (through crossing over) have sections that are maternal and other sections that are paternal Example A = wild-type body color a = black body color B = Straight wings b = curled wings AABB x aabb = Expect all AaBb - But instead find some Aabb (wild-type, curled) and aaBb (black, straight) - This is due to crossing over Frequency of Crossing over Crossover % = number of recombinations x 100 Total number of offspring AABB x aabb F1 – 282 – wild-type, straight wings (expected) 9 – black, straight (crossing over result) 9 – wild-type, curled (crossing over result) = 18/300 x 100 = 6% Mapping Chromosomes Genes on the same chromosome segregate together Gene Marker – recessive traits that are expressed in the recessive phenotype of an organism. Markers used to identify other genes on the same chromosome Ex) Appearance of white eyes (recessive) informs us of other genes located on the same chromosome. Crossing over alters gene linkages along a chromosome. If new segments of DNA are exchanged at marker sites, mapping is impossible Geneticists determine the position of the genes by their crossing over frequency – 1% frequency = genes close together – 12% frequency = genes much farther apart The greater the frequency, the greater the map distance! Crossover frequency of 5% means two genes are 5 map units apart 16% frequency is much farther apart (16 map units) Special stains have been developed to create bands of color used to identify positions of genes along a chromosome. Examples Assume crossover frequency between gene A and B is 12%, between B and C is 7%, and between A and C is 5%. Create the genetic map. – If we assume gene A is in the middle, then B – A and A – C is equal to B – C • 12 + 5 = 7 – Therefore A is not in the middle – If we assume gene B is in the middle, then A – b and B – C is equal to A – C. • 12 + 7 = 5 – Therefore B is not in the middle – If we assume gene C is in the middle, then A – C and C – B is equal to A – B • 5 + 7 = 12 – Therefore C is in the middle