Download - Mendel and heredity
Mendel and HeredityMendel and Heredity
•7-1 Fundamental of Genetics•7-2 Analyzing Heredity•7-3 Human Genetics
•7-1 Fundamental of Genetics•7-2 Analyzing Heredity•7-3 Human Genetics
Fundamentals of Genetics
• List four characteristics that make Pisum sativum a good subject for genetic studies.
• Outline the three major steps of Gregor Mendel’s garden pea experiments.
• Explain how Mendel derived ratios from his observations.
• Compare and contrast Mendel’s two laws 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 – used 7• More than one form of the same trait• Small, easy to grow, matures quickly,
large numbers of offspring…• Ability to cross-pollinate and self-
pollinate
Experimental Design
• Step 1 – allow self-pollination to produce plants that are true-breeding. P generation.
• Step 2 – cross-pollination. F1 generation.
• Step 3 – self-pollination of F1 generation. F2 generatoin.
Two ratios observed
• Dominant trait – expressed trait in F1 generation.
• Recessive trait – not expressed in F1 generation.
• Self-pollination of F1 yields a 3:1 phenotypic ratio, but a 1:2:1 genotypic ratio.
Mendel’s proposed Theory
• Parent’s pass factors(genes) to their offspring 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 each pair of alleles are separated when gametes are formed.
• Law of Independent Assortment – pairs of alleles separate independently of one another during gamete formation.
Interpreting Mendel’s Model
• 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 take place.
• = number of one kind of possible outcome
total number of possible outcomes
Monohybrid Crosses
• Provide information about one pair of contrasting traits.
Dihybrid Crosses
• Involve two pairs of contrasting traits.
Review Questions:
1. Write the formula used to determine the likelihood that an event will occur. How is probability used in genetics?
2. Construct a Punnett Square to predict the outcome of a monohybrid cross between two heterozygous tall pea plants. What are the expected phenotypic and genotypic ratios?
3. What is a dihybrid cross? What is the expected phenotypic ratio for a heterozygous dihybrid cross?
5 Factors that Influence Heredity
• A trait is displayed that is intermediate between two parents.
• Flower color in snapdragons– Red + White = Pink
#1 Incomplete Dominance
#2 Codominance
• Two dominant alleles are expressed simultaneously
• Different from incomplete dominance because both traits are displayed
• Example) roan coat in horses is a result of a cross between a homozygous red and homozygous white horse…resulting in a horse with a coat of red and 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 one extreme to another
#5 Environmental Influences
• Phenotypes being influenced by the environment
• Example) the coat color of the arctic fox in winter
Review Questions
1. What are dominant and recessive alleles?
2. Why were true-breeding plants important 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.