师：同学们，我们今天来学习高一人教版生物学必修2第一章《孟德尔的豌豆杂交实验（二）》第一课时的内容。首先回顾一下上一节课分离定律的内容。
These days, scientists know how you inherit characteristics from your parents. They're able to calculate probabilities of having a specific trait or getting a genetic disease according to the information they have from the parents and the family history. But how is this possible? To understand how traits pass from one living being to its descendants, we need to go back in time to the 19th century. A man named Gregor Mendel, an Austrian monk and biologist, loved to work with plants. By breeding the pea plants he was growing in the monastery's garden, he discovered the principles that rule heredity.
In one of the most classic examples, Mendel combined a pure - bred yellow - seeded plant with a pure - bred green - seeded plant, and he got only yellow seeds. He called the yellow color trait the dominant one because it was expressed in all the new seeds. Then he let the new yellow - seeded hybrid plants self - fertilize. And in this second generation, he got both yellow and green seeds, which meant that the green trait had been hidden by the dominant yellow. He called this hidden trait the recessive trait.
From those results, Mendel inferred that each trait depends on a pair of factors, one of them coming from the mother and the other from the father. Now we know that these factors are called alleles and represent the different variations of a gene. Depending on which type of allele Mendel found in each seed, we can have what we call a homozygous P, where both alleles are identical, and what we call a heterozygous P, when the two alleles are different. This combination of alleles is known as genotype, and its result, being yellow or green, is called phenotype.
To clearly visualize how alleles are distributed amongst descendants, we can use a diagram called the Punnett square. You just place the different alleles on both axes, and then you figure out the possible combinations. Let's look at Mendel's peas, for example. Let's write the dominant yellow allele as an uppercase Y and the recessive green allele as a lowercase y. The uppercase Y always overpowers its lowercase friend. So the only time you get green babies is if you have two lowercase ys.
In Mendel's first generation, the yellow homozygous P mom will give each P kid a yellow dominant allele and the green homozygous P dad will give a green recessive allele. So all the P kids will be yellow heterozygous. Then in the second generation, where the two heterozygous kids "marry", their babies could have any of the three possible genotypes showing the two possible phenotypes in a 3 to 1 proportion.
But even peas have a lot of characteristics. For example, besides being yellow or green, peas may be round or wrinkled. So we could have all these possible combinations: round yellow peas, round green peas, wrinkled yellow peas, and wrinkled green peas. To calculate the proportions for each genotype and phenotype, we can use a Punnett square too. Of course, this will make it a little more complex. And lots of things are more complicated than peas, like, say, people.
These days, scientists know a lot more about genetics and heredity, and there are many other ways in which some characteristics are inherited. But it all started with Mendel and his peas.
孟德尔在使用豌豆进行一对相对性状的杂交实验过程中，发现子一代只出现显性性状，子二代出现性状分离。据此，孟德尔提出了四点假说。第一点，生物的性状由遗传因子决定。第二点，在体细胞中，遗传因子成对存在。第三点，形成生殖细胞（也就是配子）的时候，成对的遗传因子彼此分离，进入不同的配子中。第四点，受精时，雌雄配子随机结合。根据这四点假说，合理地解释了实验现象。然后孟德尔设计了测交实验进行验证。实验前，孟德尔预测测交实验结果为高茎比矮茎为1 : 1的分离比。最终通过实验验证了高茎与矮茎数量比确实是1 : 1，由此得出结论，这就是孟德尔的分离定律。
在这个实验过程中，孟德尔采用了假说 - 演绎的实验方法，即在观察和分析的基础上提出问题以后，通过推理和想象提出解释问题的假说，再根据假说进行演绎推理，推出预测的结果，然后通过实验来检验。如果实验结果与预测相符，就可以认为假说是正确的，反之则认为假说是错误的，这就是假说 - 演绎法。