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4: Mendelian Genetics

Last updated

Jan 21, 2020
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- 3.6: Errors in Meiosis
- 4.1: Introduction

Ying Liu
City College of San Francisco

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Before Mendel, the basic rules of heredity were not understood. For example, it was known that green-seeded pea plants occasionally produced offspring that had yellow seeds; but were the hereditary factors that controlled seed color somehow changing from one generation to the next, or were certain factors disappearing and reappearing? And did the same factors that controlled seed color also control things like plant height?

4.1: Introduction
This page examines Mendel's foundational laws of genetics through his pea plant experiments, detailing the Laws of Inheritance such as Segregation and Independent Assortment. It covers inheritance patterns, dominant and recessive alleles, and the application of Punnett squares in monohybrid crosses. The text also discusses pedigree trees in analyzing genetic diseases like cystic fibrosis, emphasizing Mendel's pivotal role in establishing modern genetics.
4.2: Introduction
This page summarizes Gregor Mendel's foundational principles of inheritance established through experiments with pea plants in the 1860s, demonstrating the concept of particulate inheritance and the First Law of Segregation. It also references various resources, including images and articles related to genetics, detailing Mendel's work and other genetic figures. The focus is on inheritance patterns, traits studied by Mendel, and tools like Punnett squares for illustrating allele segregation.
4.3: Mendel’s Experiments
This page covers the basics of Mendelian genetics, highlighting Mendel's pea experiments that illustrate inheritance laws. Key topics include monohybrid crosses, true-breeding plants, generation labels (P, F1, F2), trait ratios, and genetic postulates. It also discusses constructing pedigree trees for genetic disease analysis and emphasizes Mendel's hybridization methods and relevant terminology.
4.4: Punnett Squares and Test Crosses
Mendel also invented several testing and analysis techniques still used today. Classical genetics is the science of solving biological questions using controlled matings of model organisms. It began with Mendel in 1865 but did not take off until Thomas Morgan began working with fruit flies in 1908. Later, starting with Watson and Crick’s structure of DNA in 1953, classical genetics was joined by molecular genetics, the science of solving biological questions using DNA, RNA, and proteins isolated
4.5: Laws of Inheritance
This page discusses Mendel's experiments with pea plants, which established key principles of inheritance, including the Law of Segregation and the Law of Independent Assortment. It highlights how traits are passed on through alleles, with dominant alleles affecting phenotypes and a typical 3:1 ratio observed in monohybrid crosses.
4.6: Mendel’s First Law
Mendel’s First Law, also called The Law of Equal Segregation, states that during gamete formation, the two alleles at a gene locus segregate from each other; each gamete has an equal probability of containing either allele. More than one allele of a gene can be present in an individual since most eukaryotic organisms have at least two sets of homologous chromosomes. For organisms that are predominantly diploid, chromosomes exist as pairs, with one homolog inherited from each parent.
4.7: Relationships Between Genes, Genotypes and Phenotypes
A specific position along a chromosome is called a locus and each gene occupies a specific locus; each locus will have an allelic form. The complete set of alleles (at all loci of interest) in an individual is its genotype. The visible or detectable effect of these alleles on the structure or function of that individual is called its phenotype
4.8: Pedigree Analysis
4.9: Pedigrees and Punnett Squares
This page covers genetics concepts, including pedigrees and Punnett squares. Pedigrees illustrate trait inheritance across generations, while Punnett squares demonstrate offspring genotype probabilities based on parental crosses, following Mendel's laws. The text discusses both single and dihybrid crosses, explaining how to calculate the inheritance likelihood of multiple traits using pea plant genetics examples.
4.10: Biochemical Basis of Dominance
For the majority of genes studied, the normal (i.e. wild-type) alleles are haplosufficient. So in diploids, even with a mutation that causes a complete loss of function in one allele, the other allele, a wild-type allele, will provide sufficient normal biochemical activity to yield a wild type phenotype and thus be dominant and dictate the heterozygote phenotype.
4.11: Black fur color - a dominant trait
This page discusses how the coat color of labrador retrievers is influenced by mutations in the TYRP1 gene. Variants in this gene affect eumelanin production, leading to brown coats in homozygous brown allele dogs (bb) and black coats in homozygous (BB) or heterozygous (Bb) dogs. Additionally, these mutations are linked to human conditions like oculocutaneous albinism type 3, emphasizing the gene's significance in pigmentation.
4.12: Yellow fur color - a recessive trait
This page discusses how the MC1R gene influences coat color in Labrador retrievers, specifically leading to yellow labs due to variations that affect melanin production. Dogs with functional MC1R alleles produce dark eumelanin, while yellow and red labs produce light pheomelanin due to a premature stop codon. The gene's effects extend to human pigmentation variations, where the dominant allele results in darker colors and the recessive allele leads to lighter colors.
4.13: Sex-Linkage- An Exception to Mendel’s First Law
In the previous chapter we introduced sex chromosomes and autosomes. For loci on autosomes, the alleles follow the normal Mendelian pattern of inheritance. However, for loci on the sex chromosomes this is mostly not true, because most of the loci on the typical X-chromosome are absent from the Y-chromosome, even though they act as a homologous pair during meiosis. Instead, they will follow a sex-linked pattern of inheritance.
4.14: Phenotypes May Not Be As Expected from the Genotype
The phenotypes described thus far have a nearly perfect correlation with their associated genotypes; in other words an individual with a particular genotype always has the expected phenotype. However, many phenotypes are not determined entirely by genotype alone. They are instead determined by an interaction between genotype and non-genetic, environmental factors.
4.15: Phenotypic Ratios May Not Be As Expected
For a variety of reasons, the phenotypic ratios observed from real crosses rarely match the exact ratios expected based on a Punnett Square or other prediction techniques. There are many possible explanations for deviations from expected ratios. Sometimes these deviations are due to sampling effects, in other words, the random selection of a non-representative subset of individuals for observation. On the other hand, it may be because certain genotypes have a less than 100% survival rate.
4.16: Extensions of the Laws of Inheritance
According to Mendel’s law of independent assortment, genes sort independently of each other into gametes during meiosis. This occurs because chromosomes, on which the genes reside, assort independently during meiosis and crossovers cause most genes on the same chromosomes to also behave independently. When genes are located in close proximity on the same chromosome, their alleles tend to be inherited together. This results in offspring ratios that violate Mendel's law of independent assortment.
4.E: Genetic Analysis of Single Genes (Exercises)
4.S: Genetic Analysis of Single Genes (Summary)

Thumbnail: Pea plants were used by Gregor Mendel to discover some fundamental laws of genetics. (Flicker-Christian Guthier-CC:A)

Contributors

Dr. Todd Nickle and Isabelle Barrette-Ng (Mount Royal University) The content on this page is licensed under CC SA 3.0 licensing guidelines.

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