Mendel and heredity pdf

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.