Patterns of Inheritance Chapter 12

7 Early Ideas of Heredity 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 5. Produce many “babies”(seeds) FAST!

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9 Monohybrid Crosses Monohybrid cross: a cross to study only 2 variations of a single trait Mendel produced true-breeding pea strains for 7 different traits -each trait had 2 alternate forms (variations) -Mendel cross-fertilized the 2 true-breeding strains for each trait

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11 Monohybrid Crosses F 1 generation (1 st filial generation): offspring produced by crossing 2 opposite (Tall crossed with short) pure-bred strains All F 1 plants resembled only 1 parent -no plants with intermediate forms between the 2 parents were produced (Example: no medium heights)

12 Monohybrid Crosses F 2 generation: offspring resulting from the self- fertilization of F 1 plants F 2 plants exhibited both forms of the trait: ¾ plants with the dominant form ¼ plant with the recessive form Mendel discovered the ratio is actually: 1 pure-bred dominant plant 2 hybrid dominant plants 1 pure-bred recessive plant Ratio = 3 : 1.

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16 Monohybrid Crosses dominant: the form of each trait expressed in the F 1 plants (Capital Letters ) recessive: the form of the trait not seen in the F 1 plants (Small-case Letters)

17 Monohybrid Crosses - definitions gene: information for a trait passed from parent to offspring alleles: alternate forms of a gene homozygous: having 2 of the same allele (TT – tt – RR – rr – BB – bb) heterozygous: having 2 different alleles (Tt – Rr – Bb)

18 Monohybrid Crosses - definitions genotype: total set of alleles of an individual (Genes - letters) PP = homozygous dominant Pp = heterozygous pp = homozygous recessive phenotype: outward appearance of an individual (Physical)

19 Monohybrid Crosses – Mendel’s conclusions Principle of Segregation: Two alleles (homologous chromosomes) separate during gamete (Sperm or Egg) formation Meiosis Proves Mendel was correct!

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22 Principle of Independent Assortment: the alleles of each gene divide into gametes independently of each other Mendel was mostly right about this (show on board) Monohybrid Crosses – Mendel’s conclusions

25 Probability – Predicting Results Product Rule: the probability of 2 independent events occurring is the PRODUCT of their individual probabilities. Rr Yy x RrYy, probability of obtaining rr yy offspring is: probability of rr = ¼ probability of yy = ¼ probability of rr yy = ¼ x ¼ = 1/16

Punnett Squares Punnett squares can be used to predict the outcome of a genetic cross

27 Pedigree analysis is used to track inheritance patterns in families.

28 Testcross Testcross: a cross used to determine the genotype of an individual with dominant phenotype -cross the individual with unknown genotype (e.g. P_) with a homozygous recessive (pp) -the phenotypic ratios of offspring are different, depending on the genotype of the unknown parent

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31 Dihybrid Crosses Dihybrid cross: examination of 2 separate traits in a single cross -for example: RR YY x rryy The F 1 generation of a dihybrid cross (RrYy) shows only the dominant phenotypes for each trait.

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34 Dihybrid Crosses The F 2 generation shows all four possible phenotypes in a set ratio: 9 : 3 : 3 : 1

35 Extensions to Mendel Mendel’s model of inheritance assumes that: -each trait is controlled by a single gene -each gene has only 2 alleles -there is a clear dominant-recessive relationship between the alleles Many genes do not meet these criteria!

36 Extensions to Mendel (1) Polygenic inheritance- multiple genes control the phenotype of a trait. These traits show continuous variation Examples: Human height, Eyecolor and Skin Color - Lab Retrievers

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45 Extensions to Mendel (2) Incomplete dominance: the heterozygote is intermediate in phenotype between the 2 homozygotes.

Red X white 4 / 4 pink

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(3) Codominance: the heterozygote shows some aspect of the phenotypes of both homozygotes. (4) Multiple alleles: more than 2 possible alleles for a gene Extensions to Mendel

P generation F 1 generation F 2 generation

C.Multiple alleles: gene with more than 2 possible alleles 1.Each individual inherits only 2 2.Examples: Blood types (A B o ) + Rabbit Coats (C ch h c )

53 Extensions to Mendel The human ABO blood group system demonstrates: -multiple alleles: there are 3 alleles of the I gene (I A, I B, and i) -codominance: I A and I B are dominant to i but codominant to each other

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