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Mendel and Heredity
Fundamentals of Genetics
• List four characteristics that makePisum sativum a good subject forgenetic studies.
• Outline the three major steps of GregorMendel’s garden pea experiments.
• Explain how Mendel derived ratios fromhis observations.
• Compare and contrast Mendel’s twolaws of heredity.
Gregor Mendel• Began experiments in
1857• Austrian monk• Studied genetics of peas• Father of genetics• Failed his math/science
teacher exam• Attended Univ. of Vienna
- experimentation
Why the Garden Pea?
• Many varieties – 32 to start – used7
• More than one form of the sametrait
• Small, easy to grow, maturesquickly, large numbers ofoffspring…
• Ability to cross-pollinate and self-pollinate
Experimental Design
• Step 1 – allow self-pollination toproduce plants that are true-breeding. P generation.
• Step 2 – cross-pollination. F1generation.
• Step 3 – self-pollination of F1generation. F2 generatoin.
Two ratios observed
• Dominant trait – expressed trait in F1generation.
• Recessive trait – not expressed in F1generation.
• Self-pollination of F1 yields a 3:1phenotypic ratio, but a 1:2:1 genotypicratio.
Mendel’s proposed Theory
• Parent’s pass factors(genes) to theiroffspring that produce traits.
• Two alleles for each trait –heterozygous (Tt) or homozygous (TT, tt)
• Phenotype = physical appearance• Genotype = set of alleles• Dominant is expressed over recessive
Mendel’s Laws
• Law of segregation – member of eachpair of alleles are separated whengametes are formed.
• Law of Independent Assortment –pairs of alleles separate independentlyof one another during gameteformation.
Interpreting Mendel’sModel
• Capital letters= dominant alleles• Lower case letters = recessive alleles• Two alleles for each trait• Homozygous dominant = DD• Heterozygous dominant = Dd (hybrid)• Homozygous recessive - dd
Probability
• The likelihood that an event will takeplace.
• = number of one kind of possible outcome total number of possible outcomes
Monohybrid Crosses
• Provide information about one pair ofcontrasting traits.
Dihybrid Crosses
• Involve two pairs of contrasting traits.
Review Questions:
1. Write the formula used to determine thelikelihood that an event will occur. How isprobability used in genetics?
2. Construct a Punnett Square to predict theoutcome of a monohybrid cross between twoheterozygous tall pea plants. What are theexpected phenotypic and genotypic ratios?
3. What is a dihybrid cross? What is theexpected phenotypic ratio for a heterozygousdihybrid cross?
5 Factors that InfluenceHeredity
• A trait is displayed thatis intermediate betweentwo parents.
• Flower color insnapdragons–Red + White = Pink
#1 Incomplete Dominance
#2 Codominance
• Two dominant alleles areexpressed simultaneously
• Different from incompletedominance because bothtraits are displayed
• Example) roan coat inhorses is a result of across between ahomozygous red andhomozygous whitehorse…resulting in ahorse with a coat of redand white hair.
#3 Multiple Alleles
• Genes with more than one allele• Example) three alleles can determine• blood type (A, B, and O)
– A and B are dominant over O– Neither A nor B are dominant over each
other– When A and B are present…they are
codominant
#4 Continuous Variation
• Several genes influence a trait
• Examples) height and weight
• A variety of phenotypes exist from oneextreme to another
#5 Environmental Influences
• Phenotypes being influenced by theenvironment
• Example) the coat color of the arcticfox in winter
Review Questions
1. What are dominant and recessivealleles?
2. Why were true-breeding plantsimportant in Mendel’s experiments?
3. What is a Punnett Square?4. What is the difference between
Phenotype and Genotype?5. Describe two inheritance patterns
besides simple dominance.