Transcript Gene
Objectives 1. Be able to explain Gregor Mendel’s Law of Dominance. 2. Be able to do a monohybrid cross and calculate the ratios. 3. Be able to apply the use of a test cross. Gregor Mendel – Father of Modern Genetics True-breeding term used to describe organisms that produce offspring identical to themselves if allowed to selfpollinate. The work of Mendel Mendel had truebreeding pea plants. He asked the question: What would happen if the pea plants were breed with pea plants with different traits? Vocabulary Words Trait - specific characteristic that varies from one individual to another Gene - sequence of DNA that codes for a protein and thus determines a trait Allele - one of a number of different forms of a gene Hybrid - offspring of crosses between parents with different traits The principle of dominance states that some alleles are dominant and others are recessive. P – Parent generation F1 – first generation of offspring (f – filial from latin filius “son”) Mendel’s Law of Dominance Law states that the dominant trait is the only trait to appear in F1 generation in a cross between two pure lines (BB x bb) Albinism, Rh blood factor, Cystic fibrosis Monohybrid cross- a cross that involves only one trait with two phenotypes Punnett Square-device for predicting possible offspring of crosses between different genotypes Punnett Square diagram showing the gene combinations that might result from a genetic cross Cross BB and bb B B b Bb Bb b Bb Bb Activity #1 People with red hair have the recessive genotype, nn. Those with any other color hair have at least one dominant allele, N. 1. What is your phenotype? 2. What is your genotype? 3. Will a person with red hair have children with red hair? 4. Do three Punnett Squares: Nn x nn, NN x nn, nn x nn Activity #2 Long eyelashes are the result of a dominant allele, S. Short eyelashes are the result of the recessive genotype, ss. 1. What is the phenotype of the person next to you? 2. What is the genotype of the person next to you? 3. Do the Punnett Square: Ss x Ss 4. Calculate the ratio: homozygous dominant to heterozygous to homozygous recessive Problem Solving Black coat color in guinea pigs is dominant over white coat color. How can you determine if a black guinea pig is pure or a hybrid? Use B – black and b - white Test cross Is a black guinea pig pure or hybrid? Do a test cross by mating the animal with pure recessive (white) Homozygous recessive x homozygous dominant = all offspring 0% recessive Homozygous recessive x heterozygous = 50% recessive (white) Activity Using a Punnett Square, estimate the ratio of offspring in the F1 and F2 generation from the cross between a homozygous dominant rabbit with floppy ears (EE) and a homozygous recessive rabbit with straight ears (ee) . Today’s Objectives 1. Be able to explain Gregor Mendel’s explanation of segregation. 2. Be able to explain Gregor Mendel’s principle of independent assortment. 3. Be able to do a two-factor cross and calculate the ratios. 4. Be able to write a summary of Mendel’s Principles. Crossing the F1 generation Segregation - separation of alleles during gamete formation When each F1 plant flowers, the two alleles are segregated from each other so that each gamete carries only a single copy of each gene. Therefore, each F1 plant produces two types of gametes—those with the allele for tallness and those with the allele for shortness. Cross of heterozygous yellow and round peas. How many different phenotypes do we have? 4 What is the ratio for the phenotypes? 9:3:3:1 Using ratios from Punnett Squares Ratio of tall to short pea plants is 3:1 If we breed heterozygous tall pea plants with each other and in one generation we made 1000 plants, how many tall pea plants and short pea plants should we have? Summary of Mendel’s Principles The inheritance of biological characteristics is determined by individual units known as genes. In organisms that reproduce sexually, genes are passed from parents to their offspring. In cases in which two or more forms of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive. In most sexually reproducing organisms, each adult has two copies of each gene—one from each parent. These genes are segregated from each other when gametes are formed. The alleles for different genes usually segregate independently of one another. Objectives 1. Be able to explain incomplete dominance. 2. Be able to explain codominance. 3. Be able to explain multiple alleles. 4. Be able to explain polygenic traits. Beyond Dominant and Recessive Incomplete dominance – (in between) situation in which one allele is not completely dominant over another There is no white. There is no red. new phenotype pink Codominance situation in which both alleles of a gene contribute to the phenotype of the organism Red and white hair color in roan cattle, black and white feather color in certain varieties of chickens Multiple alleles - three or more alleles of the same gene Coat color in rabbits is determined by a single gene that has at least four different alleles Blood type in humans Eye color in humans Polygenic Traits Means “having many genes” Two or more genes control a trait Eye color in fruit flies (at least three genes) Skin color in humans (probably more than four genes) Objectives Be able to describe Thomas Hunt Morgan’s discovery that seemed to violate Mendel’s principles Be able to define the structures that actually assort independently Be able to explain how Alfred Sturtevant used gene linkage to create gene maps Be able to explain how gene maps are produced Linkage and Gene Maps It’s easy to see how genes located on different chromosomes assort independently, but what about genes located on the same chromosome? Wouldn’t they generally be inherited together? Thomas Hunt Morgan’s studies back in 1910 helped us to answer this question. Thomas Hunt Morgan Did research on fruit flies Identified more than 50 Drosophila genes Discovered many appeared to be “linked” together Example: reddish-orange eyes and miniature wings Did this violate Mendel’s principle of independent assortment? Morgan’s Conclusions Each chromosome is actually a group of linked genes. Mendel’s principle of independent assortment still holds true. http://www.zerobio.com/independent_assortm ent.htm It is the chromosomes, however, that assort independently, not individual genes Linkage and Gene Maps Just because two genes are located on the same chromosome does not mean that they are linked together forever. Crossing-over in meiosis http://www.zerobio.com/crossin g_over.htm Also, the further apart they are the more likely they are to separate. The closer they are the less likely they are to separate. Alfred Sturtevant A Columbia University student who worked in Mendel’s lab Used gene linkage to create gene maps The rate at which linked genes were separated and recombined could be used to create a map of distances between the genes Drosophila melanogaster Gene Maps Chromosome 2: http://www.pearsonsuccessnet.com/ebook/products/0-13036843-1/sb04069.pdf