Mendel s Methods: Monohybrid Cross Mendel investigated whether the white-flowered form disappeared entirely by breeding the F1 purple flowers with each other. Crossing two purple F1 monohybrid plants is called a monohybrid cross. The resulting offspring were called the F2 generation and surprisingly didn t breed true. Some were purple, while others were white. The reappearance of the white form raised the question of how a form can disappear from parent (P) to offspring (F1) but then reappear in the next generation (F2). The F1 plants are considered monohybrids. Monohybrids: Hybrid offspring of two true-breeding parent plants differing in phenotypes of only one trait. In this case, the trait is flower colour and the different parent phenotypes were purple and white. In looking at the large datasets resulting from his monohybrid crosses, Mendel found that approximately three-quarters of the offspring took one form and approximately onequarter took the other form, giving a three-to-one ratio (3:1). This ratio was consistent in the monohybrid crosses for each of the seven traits Mendel examined. Mendel's careful record-keeping and patience in repeating the crosses were critically important for his resulting dataset. For example, in the F 2 generation of an experiment examining flower colour, there were 705 purple-flowered and 224 white-flowered offspring (Figure 3). Likewise, in an experiment examining seed color, he found 6,022 plants with yellow seeds and 2,001 plants with green seeds. If Mendel had not bred so many plants and examined so many different traits, he might have missed a central discovery. Due to these large datasets, he could begin to observe patterns in proportion and variability in pea traits across generations. Page 3 of 22
BIOL1020 Week 10 Lecture Note Summary Lecture 18- Mendel s 2 nd Law-Independent Assortment 08.10.14 Objectives: The difference between an allele and a gene The principle of independent assortment of alleles How Punnett squares can be used to predict outcomes of complex crosses The concept of linked genes How recombination breaks linkage (except in special cases) Important Message: Independent Assortment - Alleles of genes on different chromosomes segregate independently of one another Some traits are sex-linked on X or Y chromosomes Mendel s Conclusion: Genes on the same chromosomes will assort independently to gametes, if they are far apart along the chromosome. Difference Between Genes and Alleles A gene is a unit of hereditary which codes for a specific trait. All individuals of a species have the same genes (where variation occurs due to gene mutation), but the variants of the same genes are the alleles. Even if there were no mutations, still many combinations would result to produce, still, many variations. A single amino acid difference could create a different protein and/ or function = different allele. Eg. Purple and white flowers. An allele is a variant or alternative form of the same gene. Variation between individuals results from different combinations of alleles. Assuming there are 30 000 genes with 2 alleles per gene, there would be 2 30 000 possible unique gametes ~ 10 10000 >>> 10 10 people on Earth Mendel s Laws 1. Law of Segregation 2. Law of Independent Assortment Mendel s Conclusions so far He observed that the progeny of ALL possible phenotypes resulted when he self-fertilised a plant that was heterozygous for any two of following traits (see pic) and that F2 generation recovered all traits. 1. From last lecture: Alternative versions of genes account for variations in inherited characters (different alleles confer different traits). Individuals inherit 2 alleles for each character, one from each parent and alleles segregate in progeny does not result in a blend 2. This lecture: Alleles of different genes assort independently of each other (2 or more loci combine what happens on 1 locus doesn t mean it happens at another gene locus) He concluded that: Each gene controls a particular trait: Eg, Gene A controls flower colour, Gene B controls seed colour Each gene is inherited by progeny independently of the other. This was true for all 7 of the genes he studied. From these 7, he concluded that ALL genes were inherited independently of each other this was the case for many genes but not all Page 6 of 22
Incomplete Dominance When the resulting offspring looks like neither of the parents but is a blend of the two. Neither allele is completely dominant over the other. Co-dominant When heterozygous, both alleles will be expressed equally but would not be a blend of the two alleles. Overdominance Occurs when a heterozygote expresses a phenotype that is more extreme than either parent. Plant breeders have long known that a hybrid cross of two heterozygous strains of crops can sometimes result in offspring that produce a higher yield than their truebreeding parents. Example: In nature, the size and number of fruits a plant makes in any given time is constrained by the balance between allocating resources to reproduction (fruits) versus allocating resources to survivorship (leaves). On the left is the wild type, which balanced flower production and plant growth. A mutation in this gene shifted the balance and heterozygous individuals with one functional/ normal copy of this gene, shifted away from growing leaves towards growing fruit and had a higher yield than the wild-type (homozygous for non-mutant allele). However, plants that were homozygous for the mutant allele overproduced leaves and allocated less to fruit production than the wild-type plants. Page 14 of 22
The following are diagrams which I drew to review on the day of the BIOL1020 Final Exam. Page 18 of 22
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