Chapter 13: Patterns of Inheritance 1
Gregor Mendel (1822-1884) Between 1856 and 1863 28,000 pea plants Called the Father of Genetics" 2
Site of Gregor Mendel s experimental garden in the Czech Republic 3
Particulate Inheritance Mendel stated that physical traits are inherited as particles factors 4
DNA Only part of DNA in a chromosome codes for proteins. Only 1.5% of human DNA is expressed as protein Bacteria have one single circular piece of DNA and 90% of it is translated. Some bacteria have an additional circular piece of DNA called a PLASMID. It can move from one bacterium to another through transformation. PLASMID 5
Genetic Terminology Trait - any characteristic that can be passed from parent to offspring Heredity - passing of traits from parent to offspring Genetics - study of heredity 6
Genetic Terminology Cont. Alleles - two forms of a gene (dominant & recessive) alleles are represented by capital or lower case letters (RR) (Rr) (rr) Dominant - stronger of two genes expressed in the hybrid; represented by a capital letter (R) Recessive - gene that shows up less often in a cross; represented by a lowercase letter (r) 7
Genetic Terminology cont. Genotype - gene combination for a trait (e.g. RR, Rr, rr) Phenotype - the physical feature resulting from a genotype (e.g. red, white) 8
Genotypes Homozygous genotype - gene combination involving 2 dominant or 2 recessive genes (e.g. RR or rr); also called pure Heterozygous genotype - gene combination of one dominant & one recessive allele (e.g. Rr); also called hybrid 9
Types of Genetic Crosses Monohybrid cross - cross involving a single trait e.g. flower color Dihybrid cross - cross involving two traits e.g. flower color & plant height 10
Genotype & Phenotype in Flowers Genotype of alleles: R = red flower r = yellow flower All genes occur in pairs, so 2 alleles affect a characteristic Possible combinations are: Genotypes RR Rr rr Phenotypes RED RED YELLOW 11
Mendel s Pea Plant Experiments 12
Why peas, Pisum sativum? Can be grown in a small area Produce lots of offspring Produce pure plants when allowed to selfpollinate several generations Can be artificially crosspollinated 13
Reproduction in Flowering Plants Pollen contains sperm Produced by the stamen Ovary contains eggs Found inside the flower Pollen carries sperm to the eggs for fertilization Self-fertilization can occur in the same flower Cross-fertilization can occur between flowers 14
Mendel s Experimental Methods Mendel hand-pollinated flowers using a paintbrush He could snip the stamens to prevent self-pollination Covered each flower with a cloth bag He traced traits through the several generations 15
How Mendel Began Mendel produced pure strains by allowing the plants to self-pollinate for several generations 16
Eight Pea Plant Traits Seed shape --- Round (R) or Wrinkled (r) Seed Color ---- Yellow (Y) or Green (y) Pod Shape --- Smooth (S) or wrinkled (s) Pod Color --- Green (G) or Yellow (g) Seed Coat Color ---Gray (G) or White (g) Flower position---axial (A) or Terminal (a) Plant Height --- Tall (T) or Short (t) Flower color --- Purple (P) or white (p) 17
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Probability & Genetics Can be applied to predict the outcomes of genetic crosses. Calculating the probablity of making a specific gamete is just like calculating the probability in flipping a coin. Tossing heads = 50% Making a R gamete from Rr = 50% 20
Possible Gametes Geneticists can use laws of prob. to predict alleles of the offspring of crosses or matings. Predicted results what you expect to occur Observed results what actually occurs One trait: Rr = R can go into a gamete and r can go into a gamete. Two traits: RrYy = RY can go into a gamete, Ry, ry or ry can go into gametes. (foil method) Think Meiosis 21
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Generation Gap Parental P 1 Generation = the parental generation in a breeding experiment. F 1 generation = the first-generation offspring in a breeding experiment. (1st filial generation) From breeding individuals from the P 1 generation F 2 generation = the second-generation offspring in a breeding experiment. (2nd filial generation) From breeding individuals from the F 1 generation
Following the Generations Cross 2 Results Cross 2 Hybrids Pure in all get Plants Hybrids 3 Tall & 1 Short TT x tt Tt 25 TT, Tt, tt
The Punnett Square Used to help solve genetics problems/ predict outcomes of crosses 26
Steps to doing a Monohybrid Cross A one trait cross 1. Determine the genotypes of the parents genotypes XX x XY 2. Segregate the alleles to determine the possible gametes that can form from each parent 3. X, X and X, Y 4. Draw a punnett square - gamete possibilities from parents should be placed on the top and left hand side of the square 5. Match them up to complete the square these are the possible offspring from these parents. 6. Write the Genotypic Ratio = ½ XX: ½ XY 7. Write the Phenotypic Ratio = 50% Female : 50%Male 27
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P 1 Trait: (Parental Seed first Shape generation)monohybrid Cross Alleles: R Round r Wrinkled Cross: Round seeds RR x rr x Wrinkled seeds r r Genotypic Ratio: 100% Rr R R Rr Rr Rr Rr Phenotypic Ratio: 100% Round seeds 30
P 1 Monohybrid Cross Review Homozygous dominant x Homozygous recessive Offspring all Heterozygous (hybrids) Offspring called F 1 generation Genotypic and Phenotypic Ratios =100% Rr and 100% Round seeds 31
F 1 Monohybrid Cross Trait: Seed Shape Alleles: R Round Cross: Round seeds Rr x Rr r Wrinkled x Round seeds R r Genotypic Ratio: 1RR:2 Rr:1rr R RR Rr Phenotypic Ratio: 3 Round:1wrinkled r Rr rr
F 1 Monohybrid Cross Review Heterozygous x Heterozygous Offspring: 25% Homozygous dominant RR 50% Heterozygous Rr 25% Homozygous Recessive rr Offspring called F 2 generation Genotypic ratio is 1RR:2Rr:1rr Phenotypic Ratio is 3 Round:1wrinkled
What Do the Peas Look Like? 34
And Now the Test Cross Mendel then crossed a pure & a hybrid from his F generation 2 This is known as an F or test cross 2 There are two possible testcrosses: Homozygous dominant x Hybrid Homozygous recessive x Hybrid
F 2 Monohybrid Cross (1 st ) Trait: Seed Shape Alleles: R Round Cross: Round seeds RR x Rr r Wrinkled x Round seeds R r Genotypic Ratio: 1RR:1 Rr R RR Rr Phenotypic Ratio: 100% Round R RR Rr 36
F 2 Monohybrid Cross (2nd) Trait: Seed Shape Alleles: R Round r Wrinkled Cross: Wrinkled seeds x Round seeds rr x Rr R r Genotypic Ratio: 2 Rr:2rr r Rr rr P.Ratio: r Rr rr 2 Round: 2wrinkled 37
F 2 Monohybrid Cross Review Homozygous x heterozygous(hybrid) Offspring: 50% Homozygous RR or rr 50% Heterozygous Rr Phenotypic Ratio is 1 round:1 wrinkled 38
Results of Monohybrid Crosses Inheritable factors or genes are responsible for all heritable characteristics Phenotype is based on Genotype Each trait is based on two genes, one from the mother and the other from the father True-breeding individuals are homozygous ( both alleles) are the same 39