Presentation on theme: "Objectives 1. Be able to explain Gregor Mendel’s Law of Dominance."— Presentation transcript:
1 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.
2 Gregor Mendel – Father of Modern Genetics True-breeding - term used to describe organisms that produce offspring identical to themselves if allowed to self-pollinate.
3 The work of Mendel Mendel had true- breeding pea plants. He asked the question: What would happen if the pea plants were breed with pea plants with different traits?
4 Vocabulary WordsTrait - specific characteristic that varies from one individual to anotherGene - sequence of DNA that codes for a protein and thus determines a traitAllele - one of a number of different forms of a geneHybrid - offspring of crosses between parents with different traitsThe principle of dominance states that some alleles are dominant and others are recessive.
5 P – Parent generationF1 – first generation of offspring (f – filial from latin filius “son”)
6 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 fibrosisMonohybrid cross- a cross that involves only one trait with two phenotypesPunnett Square-device for predicting possible offspring of crosses between different genotypes
7 Punnett Squarediagram showing the gene combinations that might result from a genetic crossCross BB and bbBbBb
8 Activity #1People 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 redhair have children withred hair?4. Do three Punnett Squares:Nn x nn, NN x nn, nn x nn
9 Activity #2Long eyelashes are the result of a dominant allele, S. Short eyelashes are the result of the recessive genotype, ss.1. What is the phenotypeof the person next to you?2. What is the genotype of theperson next to you?3. Do the Punnett Square: Ss x Ss4. Calculate the ratio: homozygous dominant toheterozygous to homozygous recessive
10 Problem SolvingBlack 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
11 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% recessiveHomozygous recessive x heterozygous = 50% recessive (white)
12 Activity Using a Punnett Square, estimate the ratio of offspring in the F1 and F2 generationfrom the cross between ahomozygous dominant rabbitwith floppy ears (EE) and ahomozygous recessive rabbitwith straight ears (ee) .
13 Today’s Objectives1. 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.
14 Crossing the F1 generation Segregation - separation of alleles during gamete formationWhen 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.
15 Cross of heterozygous yellow and round peas. How many different phenotypes do we have?4What is the ratio for the phenotypes?9:3:3:1
16 Using ratios from Punnett Squares Ratio of tall to short pea plants is 3:1If 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?
17 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.
18 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.
19 Beyond Dominant and Recessive Incomplete dominance – (in between) situation in which one allele is not completely dominant over anotherThere is no white. There is no red. - new phenotype pink
20 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
21 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 allelesBlood type in humansEye color in humans
22 Polygenic Traits Means “having many genes” Two or more genes control a traitEye color in fruit flies (at least three genes)Skin color in humans (probably more than four genes)
23 ObjectivesBe able to describe Thomas Hunt Morgan’s discovery that seemed to violate Mendel’s principlesBe able to define the structures that actually assort independentlyBe able to explain how Alfred Sturtevant used gene linkage to create gene mapsBe able to explain how gene maps are produced
24 Linkage and Gene MapsIt’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.
25 Thomas Hunt Morgan Did research on fruit flies Identified more than 50 Drosophila genesDiscovered many appeared to be “linked” togetherExample: reddish-orange eyes and miniature wingsDid this violate Mendel’s principle of independent assortment?
26 Morgan’s ConclusionsEach chromosome is actually a group of linked genes.Mendel’s principle of independent assortment still holds true.It is the chromosomes, however, that assort independently, not individual genes
27 Linkage and Gene MapsJust because two genes are located on the same chromosome does not mean that they are linked together forever.Crossing-over in meiosisAlso, the further apart they are the more likely they are to separate. The closer they are the less likely they are to separate.
28 Alfred SturtevantA Columbia University student who worked in Mendel’s labUsed gene linkage to create gene mapsThe rate at which linked genes were separated and recombined could be used to create a map of distances between the genes