Understanding Inheritance PDF

## Understanding Inheritance ### Key Terms - **trait**: a specific characteristic or feature exhibited by an organism - **true breeding**: organisms that exhibit the same traits, generation after generation - **cross**: the fertilization of a female gamete of specific genetic origin with a male...

## Understanding Inheritance ### Key Terms - **trait**: a specific characteristic or feature exhibited by an organism - **true breeding**: organisms that exhibit the same traits, generation after generation - **cross**: the fertilization of a female gamete of specific genetic origin with a male gamete of specific genetic origin - **P generation**: in breeding, the organisms initially crossed and are typically true breeding - **F₁ generation**: the offspring of a cross of the P generation - **monohybrid cross**: a cross of two individuals that differ by one trait - **F₂ generation**: the offspring of a cross between the F₁ generation - **dominant**: the form of a trait that always appears when an individual has an allele for it - **recessive**: the form of a trait that only appears when an individual has two alleles for it - **law of segregation**: traits are determined by pairs of alleles that segregate during meiosis so that each gamete receives one allele - **genotype**: the combination of alleles for any given trait, or the organism's entire genetic make-up - **phenotype**: the physical and physiological traits of an organism - **homozygous**: an organism that has two identical alleles of a gene - **heterozygous**: an organism that has two different alleles of a gene ### Early Ideas About Inheritance - People bred animals and plants for thousands of years without understanding the mechanisms of inheritance. - The Greek philosopher Aristotle (384-322 B.C.E.) proposed the first widely accepted theory of inheritance, called *pangenesis*. According to this theory, the egg and sperm consist of particles, called *pangenes*, from all parts of the body. Upon fertilization of the egg by a sperm, the pangenes develop into the parts of the body from which they were formed. This idea about inheritance was accepted for hundreds of years, even though no experiments were done to test its assumptions or results. - In 1677, Antony van Leeuwenhoek (1632–1723) discovered living sperm in semen, using a single-lens microscope that he designed. He believed that he saw a complete, miniature person in the head of sperm. Van Leeuwenhoek believed that this miniature person came from the father but developed in the mother. - During the 1800s, the breeding of ornamental plants became popular. Scientists observed that offspring had characteristics of both parents. The idea of *blending* became the working theory of inheritance. Scientists thought that characteristics of the parents blended in the offspring. They thought that this blending was irreversible, so that the original characteristics of the parents would not appear in future generations. - These theories were developed to explain different observations that had been made. But none was based on scientific evidence, and they were all eventually disproved over time. It was the scientific evidence of an Austrian monk, Gregor Mendel, that would eventually provide some answers to the question "How are traits inherited?" ### Developing a Theory of Inheritance: Gregor Mendel's Experiments - Gregor Mendel (1822–1884) studied botany and mathematics at the University of Vienna before entering the monastery. This knowledge proved invaluable for his studies of inheritance. Mendel was successful in sorting out some of the mystery of inheritance in large part because of the plant he chose for his study: the pea plant. Pea plants were available in many varieties and show many traits. In genetics, a *trait* is a specific characteristic or feature of an organism, such as the flower colour of a plant. The laws that Mendel developed through his work on pea plants formed the foundations of our modern theory of inheritance. ### Mendel's Pea Plants - Pea plants reproduce through sexual reproduction, but they usually self-fertilize. This means that the same plant provides both the male and female gametes. The plants that Mendel selected for his study self-fertilized to produce offspring with consistent traits generation after generation. Such plants are called *true breeding*. He obtained his true-breeding plants through selective breeding techniques until he had plants that self-fertilized to produce plants with predictable traits. Mendel also needed to be able to control his experiments. He did this by selectively fertilizing a female gamete with a specific male gamete, in a process called a *cross*. You may also see it referred to as *cross-pollination*. ### The Results of Mendel's True-Breeding Crosses - For his experiments, Mendel chose seven traits that were expressed in two distinguishable forms. These traits and their forms are listed in [Table 5.1](https://www.google.com/search?q=Table+5.1+The+Seven+Traits+of+Pea+Plants+Studied+by+Mendel&tbm=isch&ved=2ahUKEwi1kOHV44b9AhWLl4kHHf8LB9QQ2-cCegQIABAA&oq=Table+5.1+The+Seven+Traits+of+Pea+Plants+Studied+by+Mendel&gs_lcp=CgNpbWcQAzICCAAyAggAMgIIADICCAAyAggAMgIIADICCAAyAggAMgIIADICCAAyAggAMgIIADICCAAyAggAMgIIADoECAAQQzoFCAAQgAQ6BQgAELEDOgQIABBDOgUIABCxAzoICAAQgAQQsQM6CAgAELEDEIMBOgQIABBDOgUIABCxAzoICAAQgAQQsQM6CAgAELEDEIMBOgUILhC4AQoECAAQQzoFCAAQgAQ6BQgAELEDEMkDEEMyAggAOgQIABBDOgQIABBDOgQIABBDOgQIABBDOgQIABBDOgQIABBDOgQIABBDogQIABBDOgQIABBDOgQIABBDOgQIABBDOgIIAFgAogEEMgUIABCKBAgAEAcQHlDuEljWMGCSYOhpaABwAHgAgAFhiAHuBJIBAzAuNJgBAKABAaoBC2d3cy13aXotaW1nwAEB&sclient=img&ei=5k-RYqubJ-C3gQf4vK3wBQ&bih=765&biw=1440&hl=en). Mendel began each experiment with true-breeding plants, which he called the parental or *P generation*. True-breeding plants with one form of a trait were crossed with true-breeding plants with the other form of the same trait. For example, he crossed true-breeding plants with green-coloured seeds with true-breeding plants with yellow-coloured seeds. Offspring of crosses between plants of the P generation were called the first filial or *F₁ generation*. This type of cross is called a *monohybrid cross* because only one trait is monitored in the cross and *hybrid* plants (those made from parents of differing forms of traits) are produced. - As shown in [Figure 5.3](https://www.google.com/search?q=Figure+5.3+After+crossing+true-breeding+plants,+only+yellow+seeds+were+produced+in+the+F1+generation.+Note+that+a+cross+is+represented+by+the+symbol+x.+Identify+the+form+of+seed+colour+that+seems+to+have+disappeared.+Why+could+it+not+really+disappear+from+the+pea+plant+population%3F&tbm=isch&ved=2ahUKEwi1kOHV44b9AhWLl4kHHf8LB9QQ2-cCegQIABAA&oq=Figure+5.3+After+crossing+true-breeding+plants,+only+yellow+seeds+were+produced+in+the+F1+generation.+Note+that+a+cross+is+represented+by+the+symbol+x.+Identify+the+form+of+seed+colour+that+seems+to+have+disappeared.+Why+could+it+not+really+disappear+from+the+pea+plant+population%3F&gs_lcp=CgNpbWcQAzICCAAyAggAMgIIADICCAAyAggAMgIIADICCAAyAggAMgIIADICCAAyAggAMgIIADICCAAyAggAMgIIADoECAAQQzoFCAAQgAQ6BQgAELEDOgQIABBDOgUIABCxAzoICAAQgAQQsQM6CAgAELEDEIMBOgQIABBDOgUIABCxAzoICAAQgAQQsQM6CAgAELEDEIMBOgUILhC4AQoECAAQQzoFCAAQgAQ6BQgAELEDEMkDEEMyAggAOgQIABBDOgQIABBDOgQIABBDOgQIABBDOgQIABBDOgQIABBDOgQIABBDogQIABBDOgQIABBDOgQIABBDOgQIABBDOgIIAFgAogEEMgUIABCKBAgAEAcQHlDuEljWMGCSYOhpaABwAHgAgAFhiAHuBJIBAzAuNJgBAKABAaoBC2d3cy13aXotaW1nwAEB&sclient=img&ei=5k-RYqubJ-C3gQf4vK3wBQ&bih=765&biw=1440&hl=en), when Mendel grew the seeds produced from a monohybrid cross between yellow-seed plants and green-seed plants, he found that all of the offspring in the F₁ generation had yellow seeds. The green form of seed colour seemed to have disappeared. For all seven of the traits studied, Mendel noticed that when true-breeding organisms with contrasting forms of a trait were crossed, the offspring expressed only one form of that trait. ### The Results of Mendel's F₁ Crosses - Mendel also studied pea plants that result from a cross between plants of the F₁ generation. These offspring were called the second filial or F₂ generation. Mendel allowed the plants from the F₁ generation to self-fertilize, and then grew the seeds for the F2 generation. The results of one of his experiments are shown in [Figure 5.4](https://www.google.com/search?q=Figure+5.4+For+F2+generation+plants,+the+ratio+of+plants+with+yellow+seeds+to+plants+with+green+seeds+was+approximately+3:1&tbm=isch&ved=2ahUKEwi1kOHV44b9AhWLl4kHHf8LB9QQ2-cCegQIABAA&oq=Figure+5.4+For+F2+generation+plants,+the+ratio+of+plants+with+yellow+seeds+to+plants+with+green+seeds+was+approximately+3:1&gs_lcp=CgNpbWcQAzICCAAyAggAMgIIADICCAAyAggAMgIIADICCAAyAggAMgIIADICCAAyAggAMgIIADICCAAyAggAMgIIADoECAAQQzoFCAAQgAQ6BQgAELEDOgQIABBDOgUIABCxAzoICAAQgAQQsQM6CAgAELEDEIMBOgQIABBDOgUIABCxAzoICAAQgAQQsQM6CAgAELEDEIMBOgUILhC4AQoECAAQQzoFCAAQgAQ6BQgAELEDEMkDEEMyAggAOgQIABBDOgQIABBDOgQIABBDOgQIABBDOgQIABBDOgQIABBDOgQIABBDogQIABBDOgQIABBDOgQIABBDOgQIABBDOgIIAFgAogEEMgUIABCKBAgAEAcQHlDuEljWMGCSYOhpaABwAHgAgAFhiAHuBJIBAzAuNJgBAKABAaoBC2d3cy13aXotaW1nwAEB&sclient=img&ei=5k-RYqubJ-C3gQf4vK3wBQ&bih=765&biw=1440&hl=en). Based on these results, Mendel realized that the green form of seed colour had not really disappeared in the F₁ generation. Instead, it was unexpressed, since green seed colour could reappear in the F2 generation. The ratio of plants with yellow seeds to plants with green seeds in the F2 generation was 6022:2001, or 3.01:1. This is very close to a ratio of 3:1, which is called the *Mendelian ratio*. For each of the seven traits of pea plants, Mendel discovered that one form of a trait disappeared in the F₁ generation and reappeared in the F2 generation. ### The Law of Segregation - Mendel concluded that there must be two hereditary "factors" for each trait he studied. Today, we refer to these factors as *alleles*. Recall that alleles are different forms of a gene, and that diploid organisms have two alleles for each gene. So, for example, each of Mendel's pea plants had two alleles for seed colour. For the F₁ generation, although all of the seeds were yellow, they all had a copy of each form of the gene for seed colour. They each had an allele for yellow seeds from one parent, and an allele for green seeds from the other parent. Yellow is the *dominant* form of seed colour and green is the *recessive* form of seed colour. Mendel's work led him to propose the *law of segregation*. This law states that inherited traits are determined by pairs of "factors," or two alleles of a gene. These two alleles segregate into each of the gametes of the parents during meiosis, so that each gamete contains one of the alleles. Upon fertilization, each offspring contains one allele from each parent. The form of trait that is expressed in an individual depends on whether they inherit dominant or recessive alleles for the trait. If a dominant allele is present, only the dominant form of the trait will be expressed. Expression of the recessive form requires that an individual has two recessive alleles for the trait. ### Genotype and Phenotype - Alleles are often represented using upper-case and lower-case letters. A dominant allele is represented by the upper-case form of the first letter of the allele's description. The same letter in lower-case is used to represent the recessive allele. The allele for yellow seeds in Mendel's pea plants is represented by Y, and the allele for green seeds is represented by y. Since every diploid organism has two alleles for each gene, there are three possible allele combinations: two copies of the dominant form, two copies of the recessive form, and one copy of each form. For Mendel's pea plants, therefore, there are three possible allele combinations for seed colour: YY, Yy, and yy. - As shown in [Table 5.2](https://www.google.com/search?q=Table+5.2+Genotypes+and+Phenotypes+of+Pea+Plant+Seed+Colour&tbm=isch&ved=2ahUKEwi1kOHV44b9AhWLl4kHHf8LB9QQ2-cCegQIABAA&oq=Table+5.2+Genotypes+and+Phenotypes+of+Pea+Plant+Seed+Colour&gs_lcp=CgNpbWcQAzICCAAyAggAMgIIADICCAAyAggAMgIIADICCAAyAggAMgIIADICCAAyAggAMgIIADICCAAyAggAMgIIADoECAAQQzoFCAAQgAQ6BQgAELEDOgQIABBDOgUIABCxAzoICAAQgAQQsQM6CAgAELEDEIMBOgQIABBDOgUIABCxAzoICAAQgAQQsQM6CAgAELEDEIMBOgUILhC4AQoECAAQQzoFCAAQgAQ6BQgAELEDEMkDEEMyAggAOgQIABBDOgQIABBDOgQIABBDOgQIABBDOgQIABBDOgQIABBDOgQIABBDogQIABBDOgQIABBDOgQIABBDOgQIABBDOgIIAFgAogEEMgUIABCKBAgAEAcQHlDuEljWMGCSYOhpaABwAHgAgAFhiAHuBJIBAzAuNJgBAKABAaoBC2d3cy13aXotaW1nwAEB&sclient=img&ei=5k-RYqubJ-C3gQf4vK3wBQ&bih=765&biw=1440&hl=en), the combination of alleles an organism has is called the *genotype*. The expression of a genotype is called the *phenotype*. A pea plant with a seed colour genotype of YY would have a phenotype of yellow seeds. Since Y (yellow) is dominant to y (green), a pea plant with a genotype of Yy would also have a phenotype of yellow seeds. Yellow seed colour is referred to as the *dominant phenotype*. Green seed colour is the *recessive phenotype*. Only pea plants with a genotype of yy would have a phenotype of green seeds. An individual with two identical alleles is said to be *homozygous* for that trait. If the individual has two dominant alleles (YY), it is *homozygous dominant*. If the individual has two recessive alleles (yy), it is *homozygous recessive*. An individual with two different alleles of a gene (Yy) is said to be *heterozygous*. ### Activity 5.1 Tasting Is Genetic - Specific taste cells in the taste buds on our tongues detect some molecules in the food we eat as bitter. This is because the molecules stimulate specialized proteins on taste cells, called *bitter taste receptors*. However, some things taste bitter to some people but are tasteless to others. These differences are based on genetics. The genetic link has been known since the 1930s, when it was accidentally discovered that a molecule called *phenylthiocarbamide* (PTC) tasted very bitter to some people but was tasteless to others. Today, we know that this is due to a gene for a bitter taste receptor and that there is a dominant allele and a recessive allele for this gene. - Before the toxicity of PTC was as well understood as it is today, analyzing for the ability to taste this molecule became one of the first genetic testing methods. This test was even used for paternity testing before DNA testing was available. A representation of data from studies to determine the inheritance of this trait is shown on the right above. - [Table 5.1](https://www.google.com/search?q=Table+5.1+Tasting+Is+Genetic&tbm=isch&ved=2ahUKEwi1kOHV44b9AhWLl4kHHf8LB9QQ2-cCegQIABAA&oq=Table+5.1+Tasting+Is+Genetic&gs_lcp=CgNpbWcQAzICCAAyAggAMgIIADICCAAyAggAMgIIADICCAAyAggAMgIIADICCAAyAggAMgIIADICCAAyAggAMgIIADoECAAQQzoFCAAQgAQ6BQgAELEDOgQIABBDOgUIABCxAzoICAAQgAQQsQM6CAgAELEDEIMBOgQIABBDOgUIABCxAzoICAAQgAQQsQM6CAgAELEDEIMBOgUILhC4AQoECAAQQzoFCAAQgAQ6BQgAELEDEMkDEEMyAggAOgQIABBDOgQIABBDOgQIABBDOgQIABBDOgQIABBDOgQIABBDOgQIABBDogQIABBDOgQIABBDOgQIABBDOgQIABBDOgIIAFgAogEEMgUIABCKBAgAEAcQHlDuEljWMGCSYOhpaABwAHgAgAFhiAHuBJIBAzAuNJgBAKABAaoBC2d3cy13aXotaW1nwAEB&sclient=img&ei=5k-RYqubJ-C3gQf4vK3wBQ&bih=765&biw=1440&hl=en) shows the data on the PTC tasting test from the attached document.