Biology - Classical (Mendelian) Genetics - Quick Guides | Biology

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Study GuideBiology–Classical (Mendelian) Genetics1. Introduction to Genetics1.1 What Is Genetics?Genetics is the study of how traits are passed from parents to their offspring. Traits are characteristicssuch as eye color, height, or earlobe shape. These traits are controlled by genes.Genes are specific sections of DNA. Each gene contains instructions for making a particular protein,and these proteins help determine how an organism looks and functions. Genes are passed from onegeneration to the next through the processes of meiosis and sexual reproduction.1.2 The Beginning of Genetics: Gregor MendelThe science of genetics began with the work ofGregor Mendel, an Augustinian monk. During the1860s and 1870s, Mendel conducted experiments that helped explain how traits are inherited.Although Mendel’s ideas were revolutionary, they were not widely accepted until the early 1900s.Today, his discoveries form the foundation of genetics. Because of this, genetics is often calledMendelian geneticsorclassical genetics. This term helps distinguish it frommolecular genetics,which focuses on DNA and genes at a chemical level.Mendel knew that “factors” were passed from parents to offspring, but he did not know these factorswere DNA. Modern science has since confirmed that genes are made of DNA and control specificinherited traits.1.3 Chromosomes, Diploid Cells, and Haploid CellsMost complex organisms, including humans, havediploid cells. A diploid cell contains two sets ofchromosomes—one set inherited from each parent.•Humans have23 pairs of chromosomes, for a total of46 chromosomes.•Because chromosomes come in pairs, diploid cells containtwo genes for eachcharacteristic.

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Study GuideBefore sexual reproduction occurs, diploid cells must reduce their chromosome number. Thishappens throughmeiosis, a special type of cell division.•Meiosis produceshaploid cells, which have onlyone set of chromosomes.•These haploid cells are calledgametes, or sex cells (sperm and egg).When a sperm and egg join during fertilization, the diploid number of chromosomes is restored.1.4 Alleles: Different Forms of a GeneEach parent contributes one gene for each trait to their offspring. Different versions of the same geneare calledalleles.For example, human earlobe shape is controlled by two alleles:•One allele producesattached earlobes•Another allele producesfree-hanging earlobesThe combination of alleles a person inherits determines which trait they show.1.5 Genome, Genotype, and PhenotypeThe complete set of genes that determines an organism’s traits is called itsgenome. In humans, thegenome contains about20,000 genes.•Genotyperefers to the specific combination of alleles an organism has.○For earlobes, a person might have:•Two alleles for attached earlobes•Two alleles for free earlobes•One of each•Phenotypeis the physical trait that is actually expressed.○If a person has attached earlobes, the phenotype is “attached earlobes.”○If a person has free earlobes, the phenotype is “free earlobes.”Even though three different genotypes are possible for earlobe shape, there are onlytwophenotypes.

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Study Guide1.6 Homozygous and Heterozygous TraitsAn organism can have either identical or different alleles for a trait.•Homozygousmeans both alleles are the same.•Heterozygousmeans the two alleles are different.In homozygous individuals, both alleles affect the trait in the same way. In heterozygous individuals,the alleles may interact, and often only one allele determines the phenotype.1.7 Dominant and Recessive AllelesWhen two different alleles are present and only one is expressed, that allele is calleddominant. Theallele that does not show its effect is calledrecessive.For earlobe shape in humans:•The allele forfree earlobesis dominant.•The allele forattached earlobesis recessive.This means:•A person withone or two dominant alleleswill have free earlobes.•A person will have attached earlobesonly if they inherit two recessive alleles.Dominant alleles always show their effect when present, while recessive alleles are expressed onlywhen no dominant allele is present.2. Inheritance Patterns2.1 Mendel’s Pea Plant ExperimentsGregor Mendel was the first scientist to develop a reliable way to predict how traits are inherited. Hecarried out his experiments usingpea plants, which were ideal for studying genetics.Mendel focused onseven traitsin pea plants:•Plant height

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Study Guide•Pod shape•Pod color•Seed shape•Seed color•Flower color•Flower locationPea plants usuallyself-pollinate, meaning they can fertilize themselves. Over many generations, thisleads to plants that arehomozygousfor certain traits. These plants are calledpure linesbecausethey consistently pass the same trait to their offspring.2.2 Parent, F1, and F2 GenerationsMendel began bycross-pollinating pure-line plantswith different traits. He called these originalplants theparent generation (P generation).For example:•He crossedpure-line tall plantswithpure-line short plants.•All offspring from thiscross were tall.Mendel named these offspring theF1 generation(first filial generation).Next, Mendel crossed the tall F1 plants with each other. This produced theF2 generation(secondfilial generation). In this generation, Mendel observed an important pattern:•Three-fourthsof the plants were tall•One-fourthof the plants were shortThis consistent result helped Mendel understand how traits are inherited.2.3 Mendel’s Laws of GeneticsAfter many years of experiments, Mendel developed three key principles, now known asMendel’slaws of genetics.

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Study Guide1. Law of DominanceWhen an organism hastwo different allelesfor a trait,one allele is dominantand masks theexpression of the other.2. Law of SegregationDuring the formation of gametes (through meiosis), the two alleles for a traitseparate. As a result,each gamete carriesonly one allelefor each trait.3. Law of Independent AssortmentEach pair of allelesseparates independentlyof other allele pairs during gamete formation. Thismeans the inheritance of one trait usually does not affect the inheritance of another.

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