Lecture 10 notes, General Biology
Lecture 10 notes, General Biology 101-NYA-05
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This 23 page Class Notes was uploaded by CatLover44 on Sunday October 16, 2016. The Class Notes belongs to 101-NYA-05 at Dawson Community College taught by Virginia Hock in Fall 2016. Since its upload, it has received 5 views. For similar materials see General Biology 1 in Biology at Dawson Community College.
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Date Created: 10/16/16
General Biology Lecture 10 Wednesday, October 12, 2016 Topics Covered: Mendelian genetics (chromosomes, genes, gene locus, alleles, genotype, phenotype, dominant and recessive genes, mono hybrid and dihybrid cross). Event Mitosis Meiosis DNA replication Occurs during Occurs once during interphase before interphase, before nuclear division meiosis 1 begins, and doesn't happen again before metaphase 2. Number of One, which includes Two, each including the chromosomes prophase, metaphase, prophase, metaphase, anaphase and anaphase and telophase. telophase. Pairing of homologous Doesn't happen. Occurs during prophase chromosomes 1, homologous chromosomes join, forming tetras, groups of 4 chromatids. During this period, crossing over happens. Number of resulting Two. Four. daughter cells Genetic composition of Each daughter cell is Each sperm or egg is daughter cells diploid (2N) and haploid (N) and genetically identical to genetically non-identical the parent cell. to the parent cell, and to each other. Role in animal body Enables multicellular Produces gametes, and organism to arise from introduces genetic the zygote, and allows variability among for growth and repair. gametes. 2 - Genetics is the branch of biology that deals with the way biological characteristics are inherited. It involves sexual reproduction (meiosis). - The human genome project involved sequencing all the DNA in a human cell. The human genome consists of 20,000-25,000 genes. Humans are identical in 99.9% of the sequences of their genes. - Genes are located on chromosomes. - Genes contain information needed to produce proteins o Expression of a gene results in the creation of a protein o They effect the events that go on in a cell, so they effect the organism itself o They play a role in determining traits like height, hair colour, and blood type - Each gene occurs on a specific chromosome and at a specific place on that chromosome o The gene locus has a predictable location o The gene locus is sort of like a street address - The locus is the location of that gene on a chromosome 3 - Homologous chromosomes are homologous pairs of chromosomes that are similar to each other, they code for the same genes, but they have different alleles for that gene - Alternate versions of a gene are called alleles, which control the same character, but don't contain the same information. For example, with respect to eye colour, the gene from mom for blue eyes = 1 allele, and the gene from dad for brown eyes = another allele. Dominant and Recessive Alleles Dominant alleles mask the presence of other (recessive) alleles. They are represented by capital letters. Recessive alleles are hidden by dominant alleles, and are shown by lowercase letters. 4 Example: the allele for brown eyes is dominant, and the allele for blue eyes is recessive, so the allele for brown eyes is written as B, and the allele for blue eyes is written as b. There are three possible combinations of alleles 1) Heterozygous dominant: Bb 2) Heterozygous recessive: bB 3) Homozygous: bb (homozygous recessive), BB (homozygous dominant) Genotypes and Phenotypes Genotypes are a short way of writing the combination of alleles that denote a specific characteristic in an organism. 5 Phenotypes are the appearance of a characteristics (for example, eye colour, flower colour, etc.). Phenotype doesn't always determine genotype, so don't try to guess what the phenotype of an organism is based only on its colour. Behaviour of Recessive Alleles - Recessively inherited disorders only occur in people who have homozygous recessive alleles for that disorder (they have ff, pp, bb genotypes). - Someone who has the phenotype for a a recessively inherited disorder, but is heterozygous for it, is a carrier. They don't show the trait, but they carry it. - Close relatives tend to have the same recessive alleles. 6 Behaviour of Dominant Alleles - Dominantly inherited disorders occur in people who are homozygous dominant, or heterozygous dominant for that trait (they have FF, Ff. BB, or Bb genotypes). - People with homozygous recessive alleles for that trait wouldn't have that trait (opposite of recessive disorder). Mendelian Genetics - Gregor Mendel (1822-1884) was an Austrian priest who developed a formal explanation of how characteristics are passed from one generation to the next. - He did this by working mostly with pea plants, and chose to focus of the colour of their flowers since this trait is easy to quantify. - He used math to come to his conclusions about genetics (Mendel’s principles gave rise to Mendel’s Model). - The Mendel Model can be divided into four concepts. o There are alternative forms of genes (alleles) o For each characteristic, an organism has two genes, one from each parent. § These genes may be the same, or they can be different alleles. 7 - When the two genes of a pair are different alleles, one can be dominant and the other can be recessive. - A sperm or egg cell carries only one alleles for each inherited trait because allele pairs separate (law of segregation) from each during the production of gametes. - When sperm and egg cells unite during fertilization, each contributes its allele, giving the new cell two sets of 23 chromosomes again (restoring the paired condition in the offspring). Mendel’s First Experiment - There are two colours of pea plants, purple and white. - Mendel used true-breeding plants (purple flowers will always have offspring with purple flowers when they're self-pollinated, and white flowers will always have offspring with white flowers when they're self-pollinated). - Mendel asked himself what would happen if a plant with a purple flower was artificially crossed with a plant that has white flowers? (Purple x White). - Mendel removed the stamens from purple flowers, took pollen from the plant with the white flower, and fertilized the purple plant with the pollen from the white flower. 8 - He collected the seeds that resulted from this pollination, planted them the following year, and all the offspring had purple flowers. - The parents were homozygous: the purple parent was homozygous dominant (PP, purple allele) and the white parent was homozygous recessive (pp, white allele). - The first generation of flowers was heterozygous (Pp), with one P allele from one parent and another p allele from the other. - This means that the purple genotype is dominant over the white genotype. Mendel's Second Experiment - Mendel asked himself what would happen if the offspring of the first cross were mated with each other? - The result was 75% of the flowers were purple, and the other 25% were white. - Pp x Pp produces both Pp and pp genotypes. 9 Mendel’s Results (2 alleles per chromosome, dominant vs recessive, and segregation) - After many crosses, Mendel concluded that the factor that determined flower colour occurred in two forms: purple and white, and if the two forms were present in one offspring, the purple form was somehow dominant over the white form. - The factors that controlled flower colour somehow separated from each other during meiosis (when sex cells are formed). - Despite separating, each factor kept its ability to control flower colour in the offspring, which is the basis of the Law of Segregation. Law of Segregation - Mendel didn't know about meiosis, so he developed this law mathematically. The modern explanation for the Law of Segregation is based on meiosis: diploid organisms produce haploid gametes, resulting in 4 sex cells with 1 set of chromosomes each. - The Law of Segregation states that both alleles will always end up in different gametes after meiosis. 10 Monohybrid Cross - A monohybrid cross is a cross between two individuals who are heterozygous at one locus, and only one trait is compared. The Punnet Square - Determines probability of an offspring having a particular genot ype. It's a simple grid (or matrix) that shows the expected frequencies of genotypes. - The rows represent the possible gametes of one parent, and the columns represent the possible gametes of the other parent. - The boxes inside the Punnet square represent the possible combinations of gametes. - You draw a Punnet square by comparing all the possible combinations of alleles from the mother, to all the possible combinations of alleles from the father. - Since Mendel’s first experiment yielded only purple flowers, the corresponding Punnet square for this experiment would only have Pp genotypes. The Punnet square for his second experiment would contain Pp and pp genotypes. - Note: the ratios that were obtained were the statistical results of large samples, so they don't represent the results that would be obtained from a single pair of parents. 11 Rules of Probability: addition rule and multiplication rule What is the probability of having a girl or a bo y? - Probability of having a girl = ½ - Probability of having a girl second = ½ - Probability of having a girl and a girl = ½ x ½ = ¼ - Probability of having a girl or a boy = ½ + ½ = 1 Chance has no memory, so even if you have a girl three times in a row, this doesn't increase your chances of having a girl a fourth time. Solving Genetics Problems 1. Read the problem carefully, find out what questions to be answered. 2. Decide on notation for the gene and alleles, and write them down. Example, P and p for purple and white. 3. Decide on the genotype for the two phenotypes, for example PP = dominant/purple, pp = recessive / white. 4. Decide on genotypes for each parent and write them down. For example, PP = homozygous dominant, Pp = heterozygous, pp = homozygous recessive. 5. Make the required genetic crosses using a Punnet square, and record the results of the crosses. 12 Example: Faye is an albino (which is a recessive trait) while Fred has normal pigmentation, but his father is albino. 1) What is the probability that Faye and Fred will have an albino child? 2) What is the probability that they will have a child that's a carrier for the albino allele? AA, Aa = normal pigmentation; genotypes: AA, Aa (normal pigmentation), aa = albino; parents: Faye = aa, Fred = Aa. Solution: there is a 50% / 50% chance of them having a normal or albino child. 13 Test Cross - You can test whether an individual with a dominant phenotype is heterozygous or homozygous by using a test cross. For example, is a purple flower ’s genotype PP or Pp? Run a test cross to answer! Cross P? X pp. 14 So far, we've gone over monohybrid and dihybrid crosses. - Monohybrid crosses have to do with the Law of Segregation (sex cells separate during their production) by performing breeding experiments where only a single characteristic is monitored. - Dihybrid crosses have to do with the Law of Independent Assortment, which allows us to consider two characteristics at the same time. o Dihybrid crosses occur when different alleles are located on 2 gene loci. o Each pair of alleles is inherited independently, so each pair segregates independently of the other pairs of alleles during sex cell formation (meiosis). Law of Independent Assortment (2 traits that aren't on the same chromosome) - An organism with 2 pairs of chromosomes (diploid, 2N = 4) has 1 pair of long homologous chromosomes, and one pair of short homologous chromosomes. - There are two ways for homologous chromosomes to align during Metaphase 1: 1. Long blue with long red, short blue with short red 2. Long blue with long red, short red with short blue. 15 - During meiosis, the genes end up on different cells. Diagram of Two Possible Alignments of Homologous Chromosomes During Metaphase 1 Diagram of Possible Haploid Gametes of Plant Seeds (R = round, dominant allele; Y = yellow, dominant allele) 16 Independent Assortment and Gamete Production Example: how many gametes will an individual with the following genotype produce? Genotype: AaBBccDdEE (5 genes in total). Solution: the genotype is only heterozygous for two genes, so the individual will make 4 gametes: ABcDE, aBcDE, ABcdE, and aBcdE. If you count the heterozygous chromosomes, and remember there are 2 of every gene, the number of gametes = 2 = 2 = 4. 2 Independent Assortment and Probabilities Example: what fraction of the offspring of parents each with the genotype Kk, Ll, Mm will be kk ll mm? Solution: each of the characters, K, L, and M, will assert independently. The chance of any specific homozygous recessive genotype appearing is ¼. The chance 17 of all three appearing is therefore (1/4) = 1/64 (multiply them together for the probability). Punnet Square for a Dihybrid Cross Example: Dihybrid Cross in Guinea Pigs 2 pairs of alleles on non-homologous chromosomes. - Black fur is dominant over brown fur o Black = BB or Bb, brown = bb - Short fur is dominant over long fur o Short = SS or Ss, long = ss 18 - If a homozygous black, homozygous short haired guinea pig is crossed with a brown, long haired guinea pig, all the offspring will be black with short hair: BBSS x bbss = BbSs. Note: the phenotypic ratio for a dihybrid cross is 9:3:3:1. This is the expected ratio in a dihybrid F2 chromosomes (for example, B and S alleles are on different chromosomes, so they can assort themselves independently). 19 Example: dihybrid cross George has phenylketonuria (PKU), a recessive trait, and is also myopic, also a recessive trait. Marilyn is a carrier for both genes. What is the probability that George and Marilyn will have a son who is myopic but doesn't have PKU? PKU = pp, no PKU = Pp or PP; myopia = mm, no myopia = MM or Mm. George = ppmm, Marilyn = PpMm; ppmm X PpMm = P?mm 1. Write all the combinations that both parents can have , starting with Marilyn since she is the only heterozygote. You don't need to write all the combinations for George because he only has 1 combination (he's homozygous recessive for both traits). 2. Fill in the Punnet square. 3. Find the % of offspring corresponding to a myopic son that doesn't have PKU (P?mm). You're looking for the son with myopia, but no PKU, so you have to multiply the probabilities. Probability of P?mm = probability of Ppmm or PPmm = 25% Probability of having a son = 50% 20 So the probability of having a son with P?mm = 4/16 = 1/4 , or 25% 0.50 x 0.25 = 0.125 Since Marilyn is PpMm, and George is ppmm, we need to find Ppmm. George is homozygous recessive (only has pp). Example: dihybrid cross and cocker spaniels Black coat colour in cocker spaniels is determine by a dominant allele B and red coat colour is determined by the recessive allele b. Solid pattern is determined by the dominant allele of an independently assorting locus S, and spotted pattern is determined by the recessive allele s. 21 A solid black male is mated with a solid red female and produced a litter of six pups, 2 solid black, 2 solid red, 1 black and spotted, and 1 red spotted. Determine the genotypes of the parents. What do we know? The male must be B?S? (Solid black), and the female has to be bbS? (Solid red). Since there are solid pups and spotted pups, both parents must have had at least 1 copy of the recessive genes, s and r. This means that the father must be heterozygous. 2 pups are solid black (B?S?), 2 are solid red (ssS?), 1 is black and spotted (B?ss), and the last pup is red and spotted (rrss). 22 Review: Laws of Segregation and Independent Assortment 23