Lesson 11: Patterns of Inheritance April 8,

Terms Allele – variations of a gene –Brown eyes vs. Blue eyes Homozygous – diploid cell containing two identical alleles for any one gene (both Brown eyes or Blue eyes) Heterozygous – diploid cell containing two different alleles for any one gene (Brown eyes gene and Blue eyes gene) 2

3 Gregor Mendel Chose to study pea plants because: 1.Other research showed that pea hybrids could be produced 2.Many pea varieties were available 3.Peas are small plants and easy to grow 4.Peas can self-fertilize or be cross-fertilized

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5 Experimental Method 1.Produce true-breeding (homozygous) strains for each trait he was studying 2.Cross-fertilized true-breeding strains having alternate forms of a trait –Also perform reciprocal crosses 3.Allowed the hybrid offspring to self-fertilize for several generations and count the number of offspring showing each form of the trait

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7 Monohybrid Crosses Cross to study only 2 variations of a single trait Mendel produced true-breeding pea strains for 7 different traits –Each trait had 2 variants

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9 F 1 Generation F 1 = First Filial Generation Offspring produced by crossing 2 true- breeding strains of differing phenotype For every trait Mendel studied, all F 1 plants resembled only 1 parent –Referred to this trait as dominant –Alternative trait was recessive No plants with characteristics intermediate between the 2 parents were produced

10 F 2 Generation Second filial generation Offspring resulting from the self- fertilization of F 1 plants Although hidden in the F 1 generation, the recessive trait had reappeared among some F 2 offspring Counted proportions of traits –3:1 ratio (phenotypic ratio)

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12 3:1 is 1:2:1 F 2 plants –¾ plants with the dominant form –¼ plants with the recessive form –The dominant to recessive ratio was 3:1 Mendel discovered the ratio is actually: –1 true-breeding dominant plant (homozygous) –2 not-true-breeding dominant plants (heterozygous) –1 true-breeding recessive plant (homozygous)

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14 Conclusions His plants did not show intermediate traits –Each trait is intact, discrete For each pair, one trait was dominant, the other recessive Alternative traits were expressed in the F 2 generation in the ratio of ¾ dominant to ¼ recessive

15 Five Element Model of Heredity 1.Parents transmit discrete factors (genes) 2.Each individual receives one copy of a gene (an allele) from each parent 3.Not all copies of a gene are identical –Allele – alternative form of a gene –Homozygous – 2 of the same allele –Heterozygous – different alleles

4.Alleles remain discrete – no blending 5.Presence of allele does not guarantee expression –Dominant allele – expressed –Recessive allele – hidden by dominant allele Genotype – total set of alleles an individual contains Phenotype – physical appearance 16

17 Principle of Segregation Two alleles for a gene segregate during gamete formation (meiosis) and are rejoined at random, one from each parent, during fertilization Physical basis for allele segregation is the behavior of homologous chromosomes during Anaphase I of Meiosis Mendel had no knowledge of chromosomes or meiosis – had not yet been described

Punnett Squares Cross purple-flowered plant with white-flowered plant P is dominant allele – purple flowers p is recessive allele – white flowers True-breeding white-flowered plant is pp –Homozygous recessive True-breeding purple-flowered plant is PP –Homozygous dominant Pp is heterozygote purple-flowered plant 18

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20 Dihybrid Crosses Examination of 2 separate traits in a single cross RR YY x rryy (doubly homozygous) –R-round; r-wrinkled; Y-yellow; y-green The F 1 generation of a dihybrid cross (RrYy) shows only the dominant phenotypes of each trait Allow F 1 to self-fertilize to produce F 2

21 F 1 generation –RrYy x RrYy (genotype) –All offspring appear Round and Yellow (phenotype) The F 2 generation –9:3:3:1 (phenotype) –9 Round yellow:3 round green:3 wrinkled yellow:1 wrinkled green

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24 Polygenic Inheritance Occurs when multiple genes are involved in controlling the phenotype of a trait The phenotype is an accumulation of contributions by multiple genes These traits show continuous variation and are referred to as quantitative traits –Ex. Human Height

25 Pleiotropy Refers to an allele which has more than one effect on the phenotype Pleiotropic effects are difficult to predict, because a gene that affects one trait often performs other, unknown functions This can be seen in human diseases such as cystic fibrosis or sickle cell anemia –Multiple symptoms can be traced back to one defective allele

26 Incomplete Dominance –Heterozygote is intermediate phenotype between the 2 homozygotes –Red flowers x white flowers = pink flowers Codominance –Heterozygote shows some aspect of the phenotypes of both homozygotes –Type AB blood