M1 L1: Mendelian Genetics

• Mendel used the scientific approach to identify two laws of inheritance. Modern genetics began during the mid-1800s with a monk named Gregor Mendel, who discovered the basic principles of heredity by breeding garden peas in carefully planned experiments.
• Mendel's Experimental, Quantitative Approach:

  Mendel grew up on his parent's small farm in a region of Austria that is now part of the Czech Republic. Mendel received agricultural training in schoolalong with a basic education in this agricultural area. As an adolescent, Mendel overcame financial hardship and illness to excel in high school and, later, at the Olmutz Philosophical Institute. In 1843, at the age of 2 1, Mendel entered an Augustinian monastery, a reasonable choice at that time for someone who valued the life of the mind.
• Mendel's Experimental, Quantitative Approach:

  In 1851, he left the monastery to pursue two years of study in physics and chemistry at the University of Vienna. He considered becoming a teacher but failed the necessary examination. Mendel's development as a scientist, is in large part due to the strong influence of two professors: physicist Christian Doppler, who encouraged his students to learn science through experimentation and trained Mendel to use mathematics to help explain natural phenomena.
• Mendel's Experimental, Quantitative Approach:

  The other was a botanist named Franz Unger, who aroused Mendel's interest in the causes of variation in plants. After attending university, Mendel returned to the monastery and was assigned to teach at a local school, where several other instructors were enthusiastic about scientific research. In addition, his fellow monks shared a long-standing fascination with the breeding of plants. 
• Mendel's Experimental, Quantitative Approach:

  Around 1857, Mendel began breeding garden peas in the abbey garden to study inheritance. Although the question of heredity had long been a focus of curiosity at the monastery, Mendel's fresh approach allowed him to deduce principles that had remained elusive to othersbegan breeding
• Mendel's Experimental, Quantitative Approach:

  One reason Mendel probably chose to work with peas is that there are many varieties; for example, one variety has purple flowers, while another variety has white flowers. A heritable feature that varies among individuals, such as flower color, is called a character. Each variant for a character, such as purple or white color for flowers, is called a trait. Other advantages of using peas are their short generation of 3 times and a large number of offspring from each mating Furthermore, Mendel could: strictly control mating between plants.
• Mendel's Experimental, Quantitative Approach:

  Each pea flower has both pollen-producing organs (stamens) and an egg-bearing organ (carpel). Pea plants usually self-fertilize: Pollen grains from nature, the stamens and on the carpel of the same flower, and sperm released from the pollen grains fertilize eggs present in the carpel
• Mendel's Experimental, Quantitative Approach:

  To achieve cross-pollination of two plants, Mendel removed their immature stamens of a plant before they produced pollen and then dusted pollen from another plant onto the altered flowers. Each resulting zygote then developed into an embryo encased in a seed (a pea). His method allowedplantMendel to always be sure of the parentage of new seeds.
• Mendel's Experimental, Quantitative Approach:

  Mendel chose to track only those characters in two distinct, alternative forms, such as purple or white flower color. He also made sure that he started his experiments with varieties that were true-breeding that is, over many generations of self-pollination, these plants had produced only the same variety as the parent plant. For example, plants with purple flowers are true breeding if the seeds produced by self-pollination in successive generations all give rise to plants that also have purple flowers.
• Mendel's Experimental, Quantitative Approach:

  In a typical breeding experiment, Mendel contrasted two true-breeding pea varieties-for example, purple-flowered plants, and white-flowered plants. This mating or crossing of varieties is called hybridization. The true-breeding parents are referred to as the P generation (parental generation), and their hybrid offspring are the F1 generation or first filial generation, the word filial from the Latin word for "son".
• Mendel's Experimental, Quantitative Approach:

  Allowing these F hybrids to self-pollinate or to cross-pollinate with other F1 hybrids) produces an F2 generation (second filial generation). Mendel's quantitative analysis of the F2 plants from thousands of genetic crosses like these allowed him to deduce two fundamental principles of heredity, now called the law of segregation and the law of independent assortment.
• Figure 14.2: Crossing Pea Plants

  1. Removed stamens from purple flower
  2. Transferred bearing pollen from stamens of white flower to egg-bearing carpel of purple flower
  3. Waited for pollinated carpel to mature into pod
  4. Planted seeds from pod
  5. Examined offspring: all had purple flowers

  
• The Law of Segregation:

  The explanation of heredity most widely in favor during the 1800s was the "blending" hypothesis, the idea that genetic material contributed by the two parents mixes just as blue and yellow paints blend to make green. This hypothesis predicts that over many generations, a freely mating population will give rise to a uniform population of individuals, something we don't see.
• The Law of Segregation:

  If the blending model were correct, the F1 hybrids from Mendel's cross between purple-flowered and white-flowered pea plants would have been pale purple flowers, a trait intermediate between those of the P generation. Notice that the experiment produced a very different result: All the F1 offspring had flowers of the same color as the purple-flowered parents. What happened to the white-flowered plants' genetic contribution to the hybrids? If it were lost, then the F1 plants could produce only purple-flowered offspring in the generation.
• The Law of Segregation:

  But when Mendel allowed the F1 plants to self- or cross-pollinate and planted their seeds, the white-flower trait reappeared in the F2 generation. The blending hypothesis is also inconsistent with this reappearance of traits after they've skipped a generation
• Law of Segregation:

  Mendel used very large sample sizes and kept accurate records of his results: 705 of the F2 plants had purple flowers, and 224 had white flowers. These data fit a ratio of approximately three purple to one white. Mendel reasoned that the heritable factor for white flowers did not disappear in the F1 plants but was somehow hidden, or masked when the purple-flower factor was present. In Mendel's terminology, purple flower color is a dominant trait, and white flower color is a recessive trait.

  
• Law of Segregation:

  The reappearance of white plants in the generation was evidence that the heritable factor causing white flowers had not been diluted or destroyed by coexisting with the purple-flower factor in the F1 hybrids. Mendel observed the same pattern of inheritance in six other characters, each represented by two distinctly different traits. For example, when Mendel crossed a true-breeding variety that produced smooth, round pea seeds with one that produced wrinkled seeds, all the Fi hybrids produced round seeds; this is the dominant trait for seed shape.
• Mendel's Model (explain the 3:1 inheritance pattern):

  First, alternative versions of genes account for variations in inherited characters. The gene for flower color in pea plants exists in two versions: purple and white flowers. These alternative versions of a gene are called alleles. Each gene is a sequence of nucleotides at a specific place, or locus, along a particular chromosome. The DNA at that locus can vary slightly in its nucleotide sequence. The purple-flower allele sequence allows the synthesis of purple pigment, and the white-flower allele sequence does not.
• Mendel's Model (explain the 3:1 inheritance pattern):

  Second, for each character, an organism inherits two versions of a gene, one from each parent (without knowing about the existence of chromosomes) Each somatic cell in a diploid organism has two sets of chromosomes, one set inherited from each parent. Thus, a genetic locus is represented twice in a diploid cell, once on each homolog of a specific pair of chromosomes. The two alleles at a particular locus may be identical, as in the true-breeding plants (P generation) or the alleles may differ (F1 hybrids)
• Mendel's Model (explain the 3:1 inheritance pattern):

  Third, if the two alleles at a locus differ, then one, the dominant allele, determines the organism's appearance; the other, the recessive allele, has no noticeable effect on the organism's appearance. Accordingly, Mendel's F1 plants had purple flowers because the allele for that trait is dominant the allele for white flowers is recessive.
• Mendel's Model (explain the 3:1 inheritance pattern):

  The fourth and final part of Mendel's model, the law of segregation states that the two alleles for a heritable character segregate (in other words, separate from each other) during gamete formation and end up in different gametes. Thus, an egg or sperm gets only one of the two alleles that are present in the diploid cells of the organism making the gamete. In terms of chromosomes, this segregation corresponds to the distribution of copies of the two members of a pair of homologous chromosomes to different gametes in meiosis
• Mendel's Model (explain the 3:1 inheritance pattern):

  Note that if an organism has identical alleles for a particular character, then that allele is present in all gametes. Because it is the only allele that can be passed on to offspring when the plant self-pollinates, the offspring always look the same as their parents in regard to that characteristic; this explains why these plants are true-breeding. But if different alleles are present, as in the F; hybrids, then 50% of the gametes receive the dominant allele and 50%% receive the recessive allele particular character

  
• Mendel's Model

  For the flower-color, two different alleles present in an F1 individual will segregate into gametes such that half the gametes will have the purple-flower allele and half will have the white-flower allele. During self-pollination, the gametes of each class unite randomly. An egg with a purple-flower allele has an equal chance of being fertilized by a sperm with a purple-flower allele or by a sperm with a white-flower allele. Since the same is true for an egg with a white-flower allele, there are four equally likely combinations of sperm and egg. 
• Punnett square, a handy diagrammatic device for predicting the allele composition of offspring from a cross between individuals of known genetic makeup. Notice that we use a capital letter to symbolize a dominant allele and a lowercase letter for a recessive allele. In our example, P is the purple-flower allele, and p is the white-flower allele; it is often useful as well to be able to refer to the gene itself as the P/p gene.

  
• Useful Genetic Vocabulary

  An organism that has a part of identical alleles for a gene encoding a character is called a homozygote. In the parental generation, the purple-flowered pea plant is homozygous for the dominant allele (PP), while the white plant is homozygous for the recessive allele (pp). Homozygous plants "breed true" because all of their gametes contain the same allele--either P or p. If we cross dominant homozygotes with recessive homozygotes, every offspring will have two different alleles-Pp in the case of the F1 hybrids of our flower-color experiment
• Useful Genetic Vocabulary

  An organism that has two different alleles for a gene is called a heterozygote and is said to be heterozygous for that gene. Unlike homozygotes, heterozygotes produce gametes with different alleles, so they are not true-breeding. For example, P- and p-containing gametes are both produced by our F hybrids. Self-pollination of the F1 hybrids thus produces both purple-flowered and white-flowered offspring.
• Useful Genetic Vocabulary

  Because of the different effects of alleles, an organism's traits do not always reveal its genetic composition. Therefore, we distinguish between an organism's appearance or observable traits, called its phenotype, and its genetic makeup, its genotype. Phenotype refers not only to traits that relate directly to physical appearance but also to physiological traits. For example, one pea variety lacks the normal ability to self-pollinate, which is a phenotypic trait (called non-self-pollination).

  
• The Testcross:

  Given a purple-flowered pea plant, we cannot tell if it is homozygous (PP) or heterozygous (Pp) because both genotypes result in the same purple phenotype. To determine the genotype, we can cross this plant with a white-flowered plant (pp), which will make only gametes with the recessive allele (p). The allele in the gamete contributed by the purple-flowered plant of unknown genotype will determine the appearance of the offspring.

  
• The Testcross:

  If all the offspring of the cross have purple flowers, then the purple-flowered mystery plant must be homozygous for the dominant allele, because a PP x pp cross produces all Pp offspring. But if both the purple and the white phenotypes appear among the offspring, then the purple-flowered parent must be heterozygous. The offspring of a Pp x PP Cross will be expected to have a 1:1 phenotypic ratio. Breeding an organism of an unknown genotype with a recessive homozygote is called a test cross because it can reveal the genotype of that organism.
• The Law of Independent Assortment:

  Mendel derived the law of segregation from experiments in which he followed only a single character, such as flower color. All the F1 progeny produced in his crosses of true-breeding parents were monohybrids, meaning that they were heterozygous for the one particular character being followed in the cross. We refer to a cross between such heterozygotes as a monohybrid cross.
• The Law of Independent Assortment:

  Mendel worked out the second law of inheritance by following two characters at the same time, such as seed color and seed shape. Seeds (peas) may be either yellow or green. They also may be either round (smooth) or wrinkled. From single-character crosses, Mendel knew that the allele for yellow seeds is dominant (Y), and the allele for green seeds is recessive (y). For the seed-shape character, the allele for the round is dominant (R) and the allele for wrinkled is recessive (r).
• The Law of Independent Assortment:

  Imagine crossing two true-breeding pea varieties that differ in both of these characters-a cross between a plant with ye-low round seeds (YYRR) and a plant with green wrinkled sed(yyrr). The F1 plants will be dihybrids, individuals heterozygous for the two characters being followed in the cross (YyRr)

  
• The F1 plants, of genotype YyRr, exhibit both dominant phenotypes, yellow seeds with round shapes, no matter which hypothesis is correct. If the hybrids must transmit their alleles in the same combinations in which the alleles were inherited from the P generation, then the F1 hybrids will produce only two gametes: YR and yr hybrids this "dependent assortment" hypothesis predicts the phenotypic ratio of the F2 gen will be 3:1 just like monohybrid cross

  
• Law of Independent Assortment:

  The alternative hypothesis is that the two pairs of alleles segregate independently of each other. An F1 plant will produce four classes of gametes in equal quantities: YR, Yr, yR, and yr. If sperm of the four classes fertilize eggs of the four classes, there will be 16(4x4) equally probable ways in which the alleles can combine in the F2 generation. = four phenotypic categories with a ratio of 9:3:3:1 The alleles for one gene, seed color segregate into gametes independently of the alleles of any other gene, such as the gene for seed shape.
• Law of Independent Assortment:

  Mendel tested his seven pea characters in various dihybrid combinations and always observed a 9:3:3:1 pheno-typic ratio in the F2 generation. The results of Mendel's dihybrid experiments are the basis for the law of independent assortment, where two or more genes assort independently = each pair of alleles segregates independently during gamete formation. This law applies only to genes (allele pairs) located on different chromosomes (that is, on chromosomes that are not homologous) or, alternatively, to genes that are very far apart on the same chromosome.