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.