Hillcrest Middle School Eighth Grade Science Instructor
Cells and Heredity Chapter 3 Genetics: The Science of Heredity
I. Mendel’s Work
A. 1851 priest G. Mendel went to U. of Vienna
1. 2 yrs. later returned to monastery & taught h.s.
2. Cared for monastery’s garden
a. curious about traits (phys. char.) of pea plants
b. some resembled parent plants some did not
c. passing of traits from parent to offspring called heredity
3. Mendel discovered genetics, the scientific study of heredity
B. Mendel’s Peas
1. used peas because they have many traits existing in
only two forms
a. tall/short, no medium
b. wrinkled/smooth
2. also used because of large # offspring in 1 generation
3. in plants pistil is female part and stamen is male
4. in nature pea plants pollinate themselves, Mendel devised a system to cross pollinate plants
C. Mendel’s Experiments
1. started with purebred plant
a. purebred - organism that produces offspring with the same form of trait as parents
b. e.g. tall plants produce tall plants, short plants produce short plants
2. in first experiment Mendel crossed purebred tall with
purebred short plants
a. called these the parental generation or P generation
b. offspring of P generation are 1st filial
generation (filial is son in Latin) or F1
c. all F1 plants were tall, short trait disappeared
d. Mendel let this generation self pollinate & offspring were 3/4 tall & 1/4 short
D. Other Traits
1. Mendel studied 6 other traits: seed shape & color, seed coat color, pod color, flower position
2. results were the same as w/tall/short cross
E.Dominant and Recessive Alleles
1. Mendel thought factors that controlled traits must exist in
2. female contributes one factor, male the other
3. these factors are genes - factors that control traits
a. alleles are a different form of the same gene
b. e.g. gene that controls stem height has 1 allele for tall
1 allele for short - offspring receives one allele from each parent - could be 2 for tall, 2 allele for short or 1 of each
c. dominant allele-allele whose trait always shows in organism when allele is present
d. recessive allele-allele that is masked or covered up when dominant allele is present
II. Probability and Genetics
A. Principles of Probability
1. probability is the likelihood an event will occur
2. coin toss
a. if a coin is tossed 2 things could happen
1. land heads up
2. land tails up
b. there is a 1 in 2 chance for either (50%)
c. the results of 1 toss does not affect the next
B. Mendel & Probability
1.after Mendel bred thousands of plants he could say if he crossed Tt x Tt the probability of tall offspring was 3 out of 4, and short 1 out of 4
2.Mendel was the first to see how probability could be used
to predict genetic crosses C. Punnett Sqares
1.Punnett square-chart that shows all combinations of alleles in a genetic cross
2.used by scientists to determine the probability
of an outcome
3.using a Punnett square
a.alleles are based on chance (like coin toss)
b.there are 4 possible combinations of alleles
1.female has 2 alleles
2.male has 2 alleles
3.therefore there are 4 possible combinations

D. Phenotype & Genotype
1.phenotype - physical appearance
2.genotype – genetic make-up, combination of alleles
3.if pea plant’s genotype is Tt then phenotype is tall
4.homozygous – 2 identical alleles – TT, tt, BB, bb
5.heterozygous - 2 different alleles – Tt, Bb
E. Codominance 1.codominance – neither allele is dominant over the other
2.this means neither allele is masked and both are
My Quia activities and quizzes
Science Skills Word Searchnw
Word Search for Science Skills Unit
Scientific Inquiry and Skills Ch. 8 LE Review
Game from Prentice Hall review.
Rags to Riches Science Skills
Study practice for unit test.
Cell Review
Cell Review Vocabulary
Chemical Building Blocks Chapter 1 Matching, Flash Cards, etc.
Useful links
Last updated  2008/09/28 02:43:10 PDTHits  360