CHAPTER 14 MENDEL AND THE GENE IDEACopyright 2002 Pearson Education, Inc., publishing as Benjamin CummingsSection A: Gregor Mendels Discoveries1.Mendel brought an experimental and quantitative approach to genetics2. By the law of segregation, the two alleles for a character are packaged into separate gametes3. By the law of independent assortment, each pair of alleles segregates into gametes independently4. Mendelian inheritance reflects rules of probability5. Mendel discovered the particulate behavior of genes: a review

Every day we observe heritable variations (eyes of brown, green, blue, or gray) among individuals in a population.These traits are transmitted from parents to offspring.One mechanism for this transmission is the blending hypothesis.This hypothesis proposes that the genetic material contributed by each parent mixes in a manner analogous to the way blue and yellow paints blend to make green.Over many generations, a freely mating population should give rise to a uniform population of individuals.IntroductionCopyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

However, the blending hypothesis appears incorrect as everyday observations and the results of breeding experiments contradict its predictions.An alternative model, particulate inheritance, proposes that parents pass on discrete heritable units - genes - that retain their separate identities in offspring.Genes can be sorted and passed on, generation after generation, in undiluted form.Modern genetics began in an abbey garden, where a monk names Gregor Mendel documented the particulate mechanism of inheritance.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Mendel grew up on a small farm in what is today the Czech Republic.In 1843, Mendel entered an Augustinian monastery.He studied at the University of Vienna from 1851 to 1853 where he was influenced by a physicist who encouraged experimentation and the application of mathematics to science and a botanist who aroused Mendels interest in the causes of variation in plants.These influences gelled in Mendels experiments.Mendel brought an experimental and quantitative approach to genetics

Mendel chose garden peas as his experimental organism because:They were available in many easily distinguishable varieties. Strict control over mating was possible to ensure the percentage of new seeds. Pea flowers enclose the carpel and stamens, which prevents cross-pollination. Immature stamens can be removed to prevent self-pollination. Mendel hybridized pea plants by transferring pollen from white flower to a purple flower. Mendel chose to track only characters in pea plants that varied in an either or (or clear cut) manner. For example, his plants had either purple or white flowers; there was nothing in between these two varieties. Character: Detectable inheritable feature of an organism.eg. color Trait: Variant of an inheritable character.

Fig. 14.1

Copyright 2002 Pearson Education, Inc., publishing as Benjamin CummingsMendel also started his experiments with varieties that were true breeding plant, which he hybridized (cross pollinated) in experimental crosses: True breeding: always producing offspring with the same traits as the parents when the parents are self-fertilized. The true breeding parental plants of such a cross are called the P generation (Parental) The hybrid offspring of the P generation are called the F1 generation (first filial or generation )Allowing F1 generation plants to self-pollinate, produces the next generation, the F2 generation (second filial).

Mendel observed the transmission of selected traits for at least three generations and arrived at two principles of heredity that are now known as: (1) The law of segregation and (2) The law of independent assortment.

Another advantage of peas is that Mendel had strict control over which plants mated with which.Each pea plant has male (stamens) and female (carpal) sexual organs.In nature, pea plants typically self-fertilize, fertilizing ova with their own sperm.However, Mendel could also move pollen from one plant to another to cross-pollinate plants.Copyright 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 14.1

In a typical breeding experiment, Mendel would cross-pollinate (hybridize) two contrasting, true-breeding pea varieties.The true-breeding parents are the P generation and their hybrid offspring are the F1 generation.Mendel would then allow the F1 hybrids to self-pollinate to produce an F2 generation.It was mainly Mendels quantitative analysis of F2 plants that revealed the two fundamental principles of heredity: the law of segregation and the law of independent assortment. Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

When Mendel crossed true-breeding plants with different character traits, he found that the traits did not blend. Using the scientific process, Mendel designed experiments in which he used large sample sizes and kept accurate quantitative records of the results. For example, a cross between true-breeding varieties, one with purple flowers and one with white flowers produced F1 progeny (offspring)with only purple flowers. Hybridization: Mating or crossing of two varieties. Monohybrid cross: A cross that tracks the inheritance of a single character. By the law of segregation, the two alleles for a characters are packaged into separate gametesFig. 14.2

EXPERIMENT: When Mendel allowed the F1 plants to self-fertilize (self-pollinate). RESULTS: Based on a large sample size, Mendel recorded 705 purple-flowered and 224 white-flowered in the F2 generation a ratio of 3:1 The inheritable factor for white flowers was not lost (reappeared in the F2). , so the hypothesis was rejected. Fig. 14.2HYPOTHESIS: Mendel hypothesized that if the inhertitable factor for white flowers had been lost, then a cross between F1 plants should produce only purple flowered plants. CONCLUSION: From these types of experiments and observations, Mendel concluded that since the inheritable factor for white flowers was not lost in the F1 generation, it must have been masked by the presence of the purple flowers factor.

Mendles factor are now called genes and in Mendels terms Purple flower is the dominant traitWhite flower is the recessive trait. Mendel repeated these experiments with six other characters and found similar 3 to 1 ratios in the F2 generations.

Copyright 2002 Pearson Education, Inc., publishing as Benjamin CummingsTable 14.1

This cross produced a three purple to one white ratio of traits in the F2 offspring, Mendel reasoned that the heritable factor for white flowers was present in the F1 plants, but it did not affect flower color.Purple flower is a dominant trait and white flower is a recessive trait.The reappearance of white-flowered plants in the F2 generation indicated that the heritable factor for the white trait was not diluted or blended by coexisting with the purple-flower factor in F1 hybrids. Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Mendel found similar 3 to 1 ratios of two traits among F2 offspring when he conducted crosses for six other characters, each represented by two different varieties.For example, when Mendel crossed two true-breeding varieties, one of which produced round seeds, the other of which produced wrinkled seeds, all the F1 offspring had round seeds, but among the F2 plants, 75% of the seeds were round and 25% were wrinkled. Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Mendel developed a hypothesis to explain these results that consisted of four related ideas.1. Alternative version of genes (different alleles) account for variations in inherited characters. For example the gene for flower color in pea plants exists in two alternative (forms); one for purple color and one for white color. These alternative versions for a gene are now called alleles. Fig. 14.3 Today we know that each gene resides at a specific locus on a specific chromosome. The DNA at that locus can vary somewhat in its sequence of nucleotides, and hence in its information content. The purple-flower allele and the white flower allele are two DNA variations possible at the flower-color locus on one chromosome of one pea plant.

2. For each character, an organism inherits two alleles, one from each parent. Mendel deduced that each parent contributes one factor even though he did not know about chromosomes or meiosis.We now know that Mendels factors are genes. Each genetic locus is represented twice in diploid organisms, which have homologous pairs of chromosomes, one set from each parent These homologous loci may be identical,(have the same allele,AA or aa) as in the true-breeding plants of the P generation.Alternatively, the two alleles may differ ( Aa )In the flower-color example, the F1 plants inherited a purple-flower allele from one parent and a white-flower allele from the other.

3. If two alleles differ, then one, the dominant allele, is fully expressed in the organisms appearance, the other is completely masked (recessive allele) and has no noticeable effect on the organism appearance. Mendels F1 plants had purple flowers because the purple-flower allele is dominant and the white-flower allele is recessive. Dominant alleles are designated by a capital letter. A purple flower color Recessive alleles are designated by a lowercase letter. a white flower color Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

4. The two alleles for each character segregate (separate) during gamete production. Without any knowledge of meiosis, Mendel deduced that a sperm cell or ovum carries only one allele for each inherited characteristic, because allele pairs separate (segregate) from each other during gamete production (meiosis)Gametes of true breeding plants will all carry the same allele. If different alleles are present in the parent, there is a 50% chance that the gametes will receive the dominant allele and a 50% chance that it will receive the recessive allele.THE SEPARATION OF ALLELES INTO SEPARATE GAMETES IS SUMMARIZED AS MENDELS LAW OF SEGREGATION.

Mendels law of segregation accounts for the 3:1 ratio that he observed in the F2 generation.The F1 hybrids (Aa) will produce two classes of gametes, half with the purple-flower allele (A) and half with the white-flower allele (a).During self-pollination, the gametes of these two classes unite randomly. Ova containing purple-flower alleles have equal chances of being fertilized by sperm carrying purple-flower alleles or sperm carrying white-flower alleles. Since the same is true for ova containing white flower alleles, there are four equally likely combinations of sperm and ovum. Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The combinations resulting from a genetic cross may be predicted by using a Punnett square of the plants with two alleles for purple flowers (AA) of the plants with one allele for purple flowers and one allele for white flowers (Aa). Since the purple-flower allele is dominant, these plants have purple flowers. of the plants with two alleles for white flower color (aa), which will have white flowers since no dominant allele is present.Copyright 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 14.4

A Punnett square analysis of the flower-color example demonstrates Mendels model.One in four F2 offspring will inherit two white-flower alleles and produce white flowers.Half of the F2 offspring will inherit one white-flower allele and one purple-flower allele and produce purple flowers.One in four F2 offspring will inherit two purple-flower alleles and produce purple flowers too.Mendels model accounts for the 3:1 ratio in the F2 generationCopyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Genetics has some unique, useful vocabulary.Homozygous: having two identical alleles for a given character or gene (e.g. AA or aa).All gametes carry that alleleHomozygotes are true breeding Heterozygous: having two different alleles for a character or gene (e.g. Aa).Half the gametes carries one allele (A) and the remaining half carries the other (a)Heterozygotes are not true- breeding. The pattern of inheritance for all seven of the characteristics studied by Mendel was the same: one parental trait disappeared in the F1 generation and reappeared in of the F2 generation

Because of dominance and recessiveness, an organisms appearance does not always reveal its genetic composition, therefore we should distinguish between: Phenotype: an organisms expressed trait or organisms appearance (e.g. purple or white flower)Genotype: An organisms genetic makeup (AA, Aa or aa)Fig. 14.5For flower color in peas, both PP and Pp plants have the same phenotype (purple) but different genotypes (homozygous and heterozygous).AA and Aa plants have the same phenotype (purple) but different

The Test cross: because some alleles are dominant over others, the genotype of an organism may not be apparent,eg:a pea plant with purple flowers may be either homozygous dominant,AA or heterozygous,Aa. So we useA test cross, breeding a homozygous recessive (aa) with dominant phenotype, but unknown genotype,AA or Aa, so we can determine the identity of the unknown allele. Copyright 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 14.6

Mendels experiments that followed the inheritance of flower color or other characters focused on only a single character via monohybrid crosses.He conduced other experiments in which he followed the inheritance of two different characters, a dihybrid cross.By the law of independent assortment, each pair of alleles segregates into gametes independentlyCopyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

In one dihybrid cross experiment, Mendel studied the inheritance of seed color and seed shape.The allele for yellow seeds (Y) is dominant to the allele for green seeds (y).The allele for round seeds (R) is dominant to the allele for wrinkled seeds (r).Mendel crossed true-breeding plants that had yellow, round seeds (YYRR) with true-breeding plants that has green, wrinkled seeds (yyrr). The resulting F1 dihybrid progeny were heterozygous for both traits (RrYy) and had round yellow seeds, the dominant phenotypes. From the F1 generation, Mendel could not tell if the two characters were inherited independently or not, so he allowed the F1 progeny to self-pollinate.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Mendel considered two alternate hypothesis: Hypothesis 1: If the two characters segregate together, the F1 hybrids can only produce the same two classes of gametes (RY and ry) that they received from the parents, and the F2 progeny will show a 3:1 phenotypic ratio. Hypothesis 2: If the two characters segregate independently, the F1 hybrids will produce four classes of gametes (RY, Ry, rY, ry)And the F2 progeny will show a 9:3:3:1 ratioExperiment: Mendel performed a dihybrid cross by allowing self-pollination of the F1 plants (RrYy X RrYy)Results: Mendel categorized the F2 progeny and determined a ratio of 315:108:101:32 which approximates 9:3:3:1. Conclusion: The experimental results supported the hypothesis that each allele pair segregates independently during gamete formation

Fig. 14.7aFig. 14.7b

Mendel repeated the dihybrid cross experiment for other pairs of characters and always observed a 9:3:3:1 phenotypic ration in the F2 generation.Each character appeared to be inherited independently.The independent assortment of each pair of alleles during gamete formation is now called Mendels law of independent assortment.One other aspect that you can notice in the dihybrid cross experiment is that if you follow just one character, you will observe a 3:1 F2 ratio for each, just as if this were a monohybrid cross.

One possibility is that the two characters are transmitted from parents to offspring as a package.The Y and R alleles and y and r alleles stay together.If this were the case, the F1 offspring would produce yellow, round seeds.The F2 offspring would produce two phenotypes in a 3:1 ratio, just like a monohybrid cross.This was not consistent with Mendels results.Copyright 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 14.7a

An alternative hypothesis is that the two pairs of alleles segregate independently of each other.The presence of one specific allele for one trait has no impact on the presence of a specific allele for the second trait.In our example, the F1 offspring would still produce yellow, round seeds.However, when the F1s produced gametes, genes would be packaged into gametes with all possible allelic combinations.Four classes of gametes (YR, Yr, yR, and yr) would be produced in equal amounts.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

When sperm with four classes of alleles and ova with four classes of alleles combined, there would be 16 equally probable ways in which the alleles can combine in the F2 generation.These combinations produce four distinct phenotypes in a 9:3:3:1 ratio.This was consistent with Mendels results.Copyright 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 14.7b

Mendel repeated the dihybrid cross experiment for other pairs of characters and always observed a 9:3:3:1 phenotypic ration in the F2 generation.Each character appeared to be inherited independently.The independent assortment of each pair of alleles during gamete formation is now called Mendels law of independent assortment.One other aspect that you can notice in the dihybrid cross experiment is that if you follow just one character, you will observe a 3:1 F2 ratio for each, just as if this were a monohybrid cross.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

*After the university, Mendel taught at the Brunn Modern School and lived in the local monastery.The monks at this monastery had a long tradition of interest in the breeding of plants, including peas.Around 1857, Mendel began breeding garden peas to study inheritance.Pea plants have several advantages for genetics.Pea plants are available in many varieties with distinct heritable features (characters) with different variants (traits).

*If the blending model were correct, the F1 hybrids from a cross between purple-flowered and white-flowered pea plants would have pale purple flowers.Instead, the F1 hybrids all have purple flowers, just a purple as the purple-flowered parents.

*A diploid organism inherits one set of chromosomes from each parent.Each diploid organism has a pair of homologous chromosomes and therefore two copies of each locus.

*If an organism has identical allele for a particular character, then that allele exists as a single copy in all gametes. This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis.If an organism has identical allele for a particular character, then that allele exists as a single copy in all gametes.