Genetics (Lupe Anaya)
Title: Mendelian Genetics Using Corn Seeds
Principle(s) Investigated:
- Mendelian genetics
- Heredity
- Punnett squares
HS-LS3-2 Make and defend a claim based on evidence that inheritable genetic variations may result from: (1) new genetic combinations through meiosis.
HS-LS3-3 Apply concepts of statistics and probability to explain the variation and
distribution of expressed traits in a population.
Materials:
1. Corn Seeds ordered from:_______(green and white)
2. Small plastic cups
3. Potting soil
4. Water
5. Handout
Procedure:
(Plant the seeds about two weeks before you plan to do the assignment)
1. Give students 3 seeds
2. Have students fill their cup halfway with soil
3. Have students plant their 3 seeds in the soil about halfway
4. Water the plants
Student prior knowledge:
Students have prior knowledge that a gene can exist in alternate forms, or alleles, such as green or white plants. When an individual has two identical alleles for a gene it is homozygous for that gene, and heterozygous if it has two different alleles. Students also know that the allele that masks the effect of the other is dominant, and the masked allele is recessive. If both alleles are recessive, the individual is homozygous recessive, if both are dominant it is homozygous dominant, and it is heterozygous if the individual has one allele of each. The genotype describes the alleles present, and the phenotype describes the expression, or physical characteristics of the gene combination.
Punnett squares are used to predict the possible genotypes of offspring of known parental genotypes. Through Punnett squares, students can observe that fertilization occurs at random as Mendel’s law of segregation states. A monohybrid cross (two heterozygous individuals) yields a genotypic ratio of 1:2:1 and a phenotypic ratio of 3:1.
Explanation: Give a thorough explanation of the experiment or demonstration. Your explanation should be written to give your fellow teachers a solid understanding and include greater detail than what you might provide for your secondary students. Make certain to include equations whenever pertinent.
Questions & Answers:
1. Punnett squares provide predicted outcomes, while your observations represent actual outcomes. Why are actual outcomes sometimes different than mathematical predictions?
2. Why do you think the green plants are mostly tall and the white plants are mostly short? What can you assume about their position on chromosomes? Traits that often appear together may be genes located on the same chromosome, called linked genes. Linked genes will end up on the same gamete unless crossing over occurs. The farther apart the two genes are on the chromosome, the more likely crossing over will occur between the two genes.
3. A couple knows from a blood test that they are both heterozygous for the autosomal recessive gene that causes sickle cell disease. If their first three children are healthy, what is the probability that their fourth child will have the disease?
Applications to Everyday Life:
1. The 1:2:1 genotypic ratio that results from a monohybrid cross (two heterozygous individuals) is also seen in people. If parents that are both carriers for the same autosomal recessive trait or disorder, such as cystic fibrosis, have children, each child has a 25% risk on inheriting the condition, 50% of being a carrier of the condition, and 25% of inheriting two dominant alleles for that trait.
2. PTC gene: The ability to taste the bitterness of PTC is coded by a single gene that codes for a taste receptor on the tongue. In 2003 the PTC gene TAS2R38 was discovered and if the individual has one dominant allele they have the ability to taste it. Non-tasters have two copies of the non-tasting allele.
a. Natural selection: PTC is not found in nature but many plants produce bitter substances that can be toxic. Being able to differentiate the bitter taste evolved as a mechanism to prevent humans from eating the poisonous plants.
b. Smoking: Studies show that people who have a PTC gene and have a strong sense for tasting bitterness are more likely to find cigarettes bitter and less likely to smoke.
c. Foods: Other studies show that people might have food preferences based on whether they have the gene. For example, some tasters might think that broccoli is too bitter.
Photographs:
Videos: Include links to videos posted on the web that relate to your activity. These can be videos you have made or ones others have made.