Using Mendel's laws, we can calculate and predict the ratios of traits that appear in offspring and grandchildren. In addition, modern genetics has greatly changed medicine and agriculture through DNA technology.
Basic Knowledge
Using a Punnett square makes genetic combinations easier to organize.
Example 1: Aa × Aa(F₁ individuals)
The father's reproductive cells are A:a = 1:1, and the mother's reproductive cells are also A:a = 1:1
Table:
A a
A AA Aa
a Aa aa
→ AA:Aa:aa = 1:2:1(genotype), round:wrinkled = 3:1(phenotype)
Example 2: Aa × aa(test cross)
A a
a Aa aa
→ Aa:aa = 1:1, round:wrinkled = 1:1
Using this method, if the genotypes of the parents are known, we can calculate the ratio of traits that will appear in the offspring.
Modern genetics:
・DNA testing: Used for paternity testing and as evidence in criminal cases(police and forensic medicine).
・Genetic engineering: Disease-resistant crops and insulin production.
・Human Genome Project: Decoded all human DNA(completed in 2003).
・Gene therapy: A treatment method that corrects genes causing diseases.
・iPS cells(Shinya Yamanaka): A technology that reprograms somatic cells and differentiates them into various types of cells.
Phenotype(A trait that actually appears, such as round, wrinkled, purple, or white)
Genotype(A combination of genes, such as AA・Aa・aa)
Test cross(A method for determining a genotype by crossing with a recessive homozygote(aa))
DNA testing(A technology for identifying individuals from differences in DNA)
Genetic engineering(A technology for inserting another gene into an organism's DNA)
iPS cells(Induced pluripotent stem cells. Created by Shinya Yamanaka in 2006)
Further Exploration (Background and Significance)
Examples of human heredity:
・Earwax: wet earwax(dominant) vs. dry earwax(recessive)
・Rolling the tongue: able to do it(dominant) vs. unable to do it(recessive)
・Dimples: present(dominant) vs. absent(recessive)
These can be explained by simple Mendelian inheritance. On the other hand, height and skin color involve many genes and the environment in complex ways(polygenic inheritance), so they do not follow a simple 3:1 ratio.
Blood types: The ABO blood group system has three types of alleles: A, B, and O. A and B are both dominant to each other(codominance), while O is recessive. The combination determines type A, B, AB, or O.
Genetic engineering is used in GM crops(genetically modified crops), such as herbicide-resistant soybeans and insect-resistant corn. At the same time, debates continue about their safety and effects on ecosystems.
iPS cells are a technology developed in Japan for which Shinya Yamanaka received the Nobel Prize in Physiology or Medicine in 2012. Applications in regenerative medicine(treatments for the eyes, heart, and nervous system) are advancing.
The depth of biology can be seen in how Mendel's discovery of the simple 3:1 ratio has led, 150 years later, to modern medicine and food production.
- Use Punnett squares to calculate genetic combinations
- Aa × Aa → 1:2:1(genotype), 3:1(phenotype)
- Aa × aa(test cross)→ 1:1
- Phenotype = appearance, genotype = combination of genes
- ABO blood types have three alleles; A and B are codominant
- DNA testing, genetic engineering, and genome analysis
- iPS cells = Shinya Yamanaka(2006), Nobel Prize(2012)
- Height and skin color are polygenic traits and do not show a 3:1 ratio
Points to Note (Often Confused)
① Distinguish between genotype(AA・Aa・aa) and phenotype(round・wrinkled). ② 3:1 is the phenotype ratio, while 1:2:1 is the genotype ratio. ③ In a test cross(determining whether Aa or AA by crossing with aa), a 1:1 ratio means the other individual is Aa; if all offspring show the dominant trait, the individual is AA. ④ iPS cells(Shinya Yamanaka, 2006) and ES cells(derived from fertilized eggs) are different.
Practice
- When Aa(round) and aa(wrinkled) are crossed, what is the phenotype ratio in the offspring generation?
- When the genotype ratio is 1:2:1, what is the phenotype ratio? Explain why.
- State the name of the Japanese scientist who created iPS cells.