Mendel and the Gene Idea 10 21 2013 The father of genetics Gregor Mendel A monk who was trained in science at the University of Vienna Genes Alleles and Chromosomes Homologous chromosomes carry the same kinds of genes for the same characteristics Genes for the same characteristic are found at the same loci on both homologous chromosomes Genes for a characteristic found on homologous chromosomes may not be identical Alternative versions of genes found at the same gene locus are called alleles o Alleles are formed by mutations o If a mutation occurs in gametes sperm or egg then the mutation can be passed on from parent to offspring o An organism s alleles may be the same or different If both homologous chromosomes carry the same allele gene form at a given gene locus the organism is homozygous or true breeding at that locus If two homologous chromosomes carry different alleles at a given locus the organism is heterozygous at that locus also called hybrid The Punnett Square Method predicts offspring genotypes and phenotypes from combinations of parental gametes 1 Assign letters to the different alleles of the characteristic under consideration uppercase for dominant lowercase or recessive 2 Determine the possible gametes from both parents 3 Write the gametes from each parent along each side of a 2x2 grid Punnett square 4 Fill in the genotypes of each pair of combined gametes in the grid 5 Determine the genotypic ratio based on the outcome of the Punnett square 6 Based on which traits are dominant and which are recessive calculate the predicted phenotypic ratio How were the Principles of Inheritance Discovered Gregor Mendel Chose edible pea plant for his experiments o o o o o Allowed him to see patterns in the way plant characteristics were inherited Why pea plants Pea flowers have stamens male structures that produce pollen pollen contains the sperm male gametes sperm are gametes and pollen is the vehicle Pea flowers have carpels female structures housing the ovaries which produce the Pea flower petals enclose both male and female flower parts and prevent entry of pollen eggs female gametes from another pea plant o Because of their structure pea flowers naturally self fertilize Pollen from the stamen transfers to the carpel of the same plant where the sperm then fertilizes the plant s eggs o Mendel was able to mate two different plants by cross fertilization Female parts carpels were dusted with pollen from other selected plants How are Single Traits Inherited Pea plants that are homozygous for a particular characteristic always produce the same phenotype flowers If a plant is homozygous for purple flowers it will always produce offspring with purple Plants that are homozygous for a characteristic are described as true breeding A genetic cross is the mating of pollen and eggs from same or different parents The parents used in a cross are part of the parental generation known as P The offspring of the P generation are members of the first filial generation F1 Offspring of the F1 generation are members of the F2 generation Mendel s flower color experiments o Crossed a true breeding homozygous purple flower plant which a true breeding homozygous white flower plant the P generation F1 all purple flowered plants o Mendel allowed the F1 generation to self fertilize F2 purple and white 3 1 ratio The results showed that the white trait didn t disappear in F1 but was hidden o Mendel allowed F2 to self fertilize F3 all white flowered F2 produced white flowered offspring proved to be true breeding or homozygous F3 purple flowered F2 plants produced 3 offspring 1 out of 3 were true breeding for purple offspring The others were heterozygous producing both white and purple o Mendel determined that out of 4 offspring in F2 1 was homozygous for purple 1 was homozygous for white and the other two were heterozygous Genotype the particular combination of the two alleles carried by an individual Phenotype the physical expression of the genotype ex purple or white flowers Test cross used to deduce whether an organism with a dominant phenotype is homozygous or heterozygous 1 Cross the unknown dominant phenotype organism P with a homozygous recessive organism 2 If the dominant phenotype organism is homozygous dominant PP only dominant phenotype pp offspring will be produced Pp 3 If the dominant phenotype organism is heterozygous Pp approximately half the offspring will be of recessive phenotype pp How is sex determined in the offspring Mammals have a set of sex chromosomes that dictate gender Females two X chromosomes o Males an X and a Y chromosome o o o o o The rest of the non sex chromosomes occur in identical pairs and are called autosomes For organisms with males XY and females XX the sperm sex chromosome determines o During sperm formation each receives the X or Y chromosome along with a copy of all the sex of the offspring autosomes The female has only X so the unfertilized egg will always have just an X chromosome Egg sperm with a Y male Egg X bearing sperm a female is produced Many traits don t follow simple Mendelian rules of inheritance Not all traits are completely controlled by a single gene A trait may not be completely dominant to another Incomplete dominance the pattern of inheritance when the heterozygous phenotype is intermediate between the two homozygous phenotypes o Human hair texture is influenced by a gene with two incompletely dominant alleles C1 and C2 Two copies of the C1 allele gives curly hair Two copies of the C2 allele gives straight hair Heterozygotes C1C2 genotype have wavy hair If two wavy haired people mate their children could have any of the three hair types C1C1 curly C1C2 wavy straight C2C2 A single gene may have multiple alleles An individual may have at most two different gene alleles A species may have multiple alleles for a given characteristic o However each individual still carries two alleles for this characteristic Human blood types Human blood group genes produce blood types A B AB and O three alleles A B and o o Alleles A and B code for enzymes that add different sugar molecules to the ends of glycoproteins that protrude from red blood cells o Allele o codes for a nonfunctional enzyme that doesn t add any sugar molecules o Blood types A B AB and O arise as a result of the actions of these alleles o Alleles A and B are both dominant to o but are not dominant to each other co dominance AA or Ao genotypes type A BB or Bo genotypes type B oo genotypes type O o
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