基础生物实验 22 - 实验八理论简介：孟德尔遗传学
本期内容是孟德尔遗传学的基础理论部分，实验手册与实验模拟请看后两期。本部分内容来自 University of California, Berkeley - UC Berkeley Extension, 虚拟实验的内容来自 Labster. 本部分内容均不会标记为为原创，但由于是UP主购买的课程，因此不接受非授权的转载，谢谢您的理解。
每一个生物基础实验均会分为三部分：第一部分为实验的生物理论；第二部分为实验的指导手册；第三部分为 Labster 的虚拟实验模拟。第一部分的基本信息由 Ying Liu, Ph.D. 提供，第二部分的实验手册来自 Labster, 第三部分的实验模拟过程均由UP主操作。
Lab 8 - Genetics
熊猫~
Mendel’s Peas
- Gregor Mendel: father of modern genetics;
- He crossed peas and discovered some basic rules of inheritance;
- He tracked only characters that occurred in two forms;
- Mendel used varieties that were ‘true-breeding’ (plants that produce offspring of the same variety when they self-pollinate).
Mendel’s Experiments
- Mendel mated two contrasting, true-breeding varieties = hybridization;
- True-breeding parents = P generation;
- The hybrid offspring of the P generation = F1 generation;
- When F1 individuals breed (F1 X F1), the F2 generation is produced (hybrid cross);
- Example: when he crossed and flowered pea plants, all of the F1 hybrids were purple.
杂交流程：
Mendel’s Experiments
Mendel observed the same pattern of inheritance in six other characters:
- All offspring showed the dominant trait, and none showed the recessive trait in the F1;
- The F2 generation showed a ratio of 3:1 (dominant: recessive).
Mendel’s Model
- Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring of peas:
1. Genes come in pairs (= alleles);
2. Each parent passes one of the pair to offspring;
3. In hybrids only the ‘dominant’ member of the pair is expressed;
4. The pair separates when gametes are formed.
Mendel’s peas:
- Round: produce an enzyme that converts sugar to starch, which attract water due to osMosis;
- Wrinkled: lack the enzyme (sweet peas).
Law of Dominance
For each character, an organisM inherits two alleles, one from each parent;
- The two alleles may be identical (homozygous);
- The two alleles may differ (heterozygous).
The dominant allele determines the organisM’s appearance, the recessive trait is only seen when the organisM has two recessive alleles (homozygous recessive);
Conventional: dominant allele is designated with a capital letter, recessive allele is designated with a lowercase letter.
Law of Segregation
- The two alleles for a given character separate (= segregate) during gamete formation;
- Segregation of alleles corresponds to the distribution (separation) of homologous chromosomes into different gametes during meiosis;
- The possible combinations (offspring) of sperm and egg can be shown using a Punnett square;
- How to: determine all possible gametes; show possible combinations.
Phenotype vs Genotype:
Test Cross:
Dihybrid Cross
- Cross of true-breeding (homozygous) parents differing in two characters produces dihybrids in the F1 generation (heterozygous at two loci);
- A dihybrid cross = a cross between F1 dihybrids:
- Are the traits transMitted to offspring as a package?
- Are the traits inherited independently of each other?
Assortment Hypothesis:
Law of Independent Assortment
- Generally speaking, this law applies only to genes on different chromosomes;
- In peas, seed color gene is on chromosome 1; seed shape gene is on chromosome 7.
Peas:
Meiosis
F2 generation:
Forked Line Method
- Probability method for complex problems;
- What fraction will be homozygous recessive at all loci?
Genotypes & Phenotypes:
Human Genetics
F x f:
- O blood type is recessive, but also the most common;
- Polydactyly is dominant, but not common.
Pedigree:
- Human geneticists construct pedigree from family medical history to study genetic disorders;
- Conventions: square - male; circle - female.
Autosomal Dominant: Huntington’s Disease
- Neurodegenerative disorder caused by a single dominant allele;
- Dominant trait: one copy of the ‘bad’ allele is enough to cause disease.
Autosomal Recessive: Cystic Fibrosis
- Genetic disorder caused by a recessive allele (1/2000 births);
- 1 in 29 Caucasians in the US are carriers (heterozygotes).
Autosomal Recessive:
- Cystic fibrosis: Cl- ions do not pass normally through a cell membrane, resulting in thick mucus in lungs and other places, often causing infections;
- Tay-Sachs disease: lack of the enzyme that breaks down fatty acid proteins in lysosomes results in accumulation;
Sickle Cell Anemia
HbA/HbA (AA): wild type, susceptible to malaria;
HbS/HbS (SS): has sickle cell anemia;
HbA/HbS (AS): carrier, protected from malaria.
Other Inheritance Patterns
- Incomplete dominance: curly, straight, wavy hair;
- Co-dominant alleles: blood type, sickle cell anemia;
- Polygenic inheritance: where multiple genes impact a single trait (Height, Skin color).
sex-Linked Genes
- Traits are controlled by genes on the sex chromosomes;
- X-linked: the allele is carried on the X chromosome;
- Y-linked: the allele is carried on the Y chromosome;
- Most sex-linked traits are X-linked;
- Examples of X-linked traits: Hemophilia - blood clotting disorder; Color blindness; Male pattern baldness.
sex-Linked Genes
- Color blindness;
- If mom is a carrier, and dad is normal;
- 50% of sons will be color blind;
- 50% of daughters will be carriers.
sex-Linked Pedigree
- More males than females show sex-linked traits since males only inherit one copy of the X chromosome;
- Often skips a generation (grandfather to grandson, mom is the carrier).
sex-Linked Genes
- Can a father pass a X-linked gene to his daughter? To his son?
- For a recessive X-linked trait to be expressed (color blindness, hemophilia);
- A female needs two copies of the allele;
- A male needs only one copy of the allele.
本期内容到此结束，感谢阅读！下一期为实验手册 & 下下期将进行 Labster 实验！
