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Chapter 14 The Patterns of Inheritance Father of genetics Gregor Mendel Mendel was a monk who was trained in science at the University of Vienna He observed patterns in the growth and development of pea plants From these findings he was able to predict their future offspring 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 Alleles Alternative versions of genes found at the same gene locus are called alleles Alleles are formed by mutations Genes for a characteristic found on homologous chromosomes may not be identical o If a mutation occurs in gametes the cells that become sperm or eggs then the mutation can be passed on from parent to offspring The damage DNA has to be in a sperm or egg in order to be passed down Otherwise it has no effect on offspring An organism s two alleles may be the same or different o 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 o If two homologous chromosomes carry different alleles at a given locus the organism is heterozygous at that locus also called hybrid The Relationship Among Genes Alleles and Chromosomes Figure 10 1 o Homologous chromosomes control the same characteristics o When the alleles are the same the individual is homozygous at that locus o When there are different alleles the individual is heterozygous at that locus Punnett Square Method The Punnett Square Method predicts offspring genotypes and phenotypes from combinations of parental gametes o Assign letters to the different alleles of the characteristic under consideration uppercase for dominant lowercase for recessive o Determine the possible gametes from both parents o Write the gametes from each parent along each side of a 2x2 grid Punnett square o Fill in the genotypes of each pair of combined gametes in the grid Example Mom has Bb and Dad has Bb There is a 50 chance of the offspring being Bb There is a 50 chance of the offspring being bb o Determine the genotypic ration based on the outcome of the Punnett square o 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 plants for his experiment This allowed him to see patterns in the way plant characteristics were inherited Why Pea Plants o Pea flowers have stamens the male structures that produce pollen Pollen contains the sperm male gametes sperm are gametes and pollen is the vehicle o Pea flowers have carpels female structures housing the ovaries which produce the eggs female gametes o Pea flower petals enclose both male and female flower parts and prevent entry of pollen from another pea plant Flowers of the Edible Pea Plant How Are Single Traits Inherited o Mendel knew how to dissect the flower and expose both the female and male reproductive structures He did this to cross fertilize the plants so that they would have various DNA o Because of their structure pea flowers naturally self fertilize Pollen from the stamen of a plant 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 another selected plant o Pea plants that are homozygous for a particular characteristic always produce the same phenotype If a plant is homozygous for purple flowers it will always produce offspring with purple flowers Plants that are homozygous for a characteristic are described as true breeding o P purple flower PP p white flower pp 100 of the phenotypes were Pp o A genetic cross is the mating of pollen and eggs from same or different o The parents used in a cross are part of the parental generation known as P o The offspring of the P generation are members of the first filial generation parents F1 o 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 with a true breeding homozygous white flower plant the P generation o The F1 generation consisted of all purple flowered plants What had happened to the white flowered trait since it wasn t visible It became the recessive trait in the pea plant o Figure 14 3 o Mendel allowed the F1 generation to self fertilize and create the F2 generation The F2 was composed of 3 4 purple flowered plants and white flowered plants a ratio of 3 1 The results showed that the white trait had not disappeared in the F1 but merely was hidden 75 of the flowers were purple and 25 of the flowers were white o Mendel then allowed the F2 generation to self fertilize o In the F3 generation self fertilized purple flowered F2 plants produced 3 offspring The other 2 were heterozygous and produced both purple and 1 out of the 3 were true breeding for purple white flowered offspring So Mendel was able to determine that out of the 4 offspring in the F2 generation 1 was homozygous for purple 1 was homozygous for white and the other 2 were heterozygous The Distribution of Alleles in Gametes In a homozygous parent the gametes produced will each have the same allele In a heterozygous parent the gametes produces will each have a different allele for the gene The particular combination of the two alleles carried by an individual is called the genotype The physical expression of the genotype is known as the phenotype for example purple or white flowers Genes determine physical characteristics A test cross is used to deduce whether an organism with a dominant phenotype is homozygous or heterozygous recessive organism pp o Cross the unknown dominant phenotype organism P with a homozygous Example cross a purple flower whose phenotype is unknown with a white flower whose phenotype is known to be pp o If the dominant phenotype organism is homozygous dominant PP only dominant phenotype offspring will be produced Pp Example continues if the offspring produced are all purple you know the phenotype of the purple flower is PP because all of the offspring are purple purple is dominant and white is recessive so white will not show in the offspring o If the dominant phenotype organism is heterozygous Pp approximately half the offspring will be of recessive phenotype pp Example continued if the offspring produced are 50 purple and 50 white then you know the


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LSU BIOL 1201 - Chapter 14: The Patterns of Inheritance

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