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Biology - Classical (Mendelian) Genetics

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    Study Guide Biology – Classical (Mendelian) Genetics 1. Introduction to Genetics 1.1 What Is Genetics? Genetics is the study of how traits are passed from parents to their offspring. Traits are characteristics such 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 one generation to the next through the processes of meiosis an d sexual reproduction. 1.2 The Beginning of Genetics: Gregor Mendel The science of genetics began with the work of Gregor Mendel , an Augustinian monk. During the 1860s 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 called Mendelian genetics or classical genetics . This term helps distinguish it from molecular 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 factors were DNA. Modern science has since confirmed that genes are made of DNA and control specific inherited traits. 1.3 Chromosomes, Diploid Cells, and Haploid Cells Most complex organisms, including humans, have diploid cells . A diploid cell contains two sets of chromosomes — one set inherited from each parent. • Humans have 23 pairs of chromosomes , for a total of 46 chromosomes . • Because chromosomes come in pairs, diploid cells contain two genes for each characteristic .

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    Study Guide Before sexual reproduction occurs, diploid cells must reduce their chromosome number. This happens through meiosis , a special type of cell division. • Meiosis produces haploid cells , which have only one set of chromosomes . • These haploid cells are called gametes , 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 Gene Each parent contributes one gene for each trait to their offspring. Different versions of the same gene are called alleles . For example, human earlobe shape is controlled by two alleles: • One allele produces attached earlobes • Another allele produces free - hanging earlobes The combination of alleles a person inherits determines which trait they show. 1.5 Genome, Genotype, and Phenotype The complete set of genes that determines an organism’s traits is called its genome . In humans, the genome contains about 20,000 genes . • Genotype refers 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 • Phenotype is 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 only two phenotypes .

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    Study Guide 1.6 Homozygous and Heterozygous Traits An organism can have either identical or different alleles for a trait. • Homozygous means both alleles are the same. • Heterozygous means 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 Alleles When two different alleles are present and only one is expressed, that allele is called dominant . The allele that does not show its effect is called recessive . For earlobe shape in humans: • The allele for free earlobes is dominant. • The allele for attached earlobes is recessive. This means: • A person with one or two dominant alleles will have free earlobes. • A person will have attached earlobes only if they inherit two recessive alleles . Dominant alleles always show their effect when present, while recessive alleles are expressed only when no dominant allele is present. 2. Inheritance Patterns 2.1 Mendel’s Pea Plant Experiments Gregor Mendel was the first scientist to develop a reliable way to predict how traits are inherited. He carried out his experiments using pea plants , which were ideal for studying genetics. Mendel focused on seven traits in pea plants: • Plant height

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    Study Guide • Pod shape • Pod color • Seed shape • Seed color • Flower color • Flower location Pea plants usually self - pollinate , meaning they can fertilize themselves. Over many generations, this leads to plants that are homozygous for certain traits. These plants are called pure lines because they consistently pass the same trait to their offspring. 2.2 Parent, F1, and F2 Generations Mendel began by cross - pollinating pure - line plants with different traits. He called these original plants the parent generation (P generation) . For example: • He crossed pure - line tall plants with pure - line short plants . • All offspring from this cross were tall. Mendel named these offspring the F1 generation (first filial generation). Next, Mendel crossed the tall F1 plants with each other. This produced the F2 generation (second filial generation). In this generation, Mendel observed an important pattern: • Three - fourths of the plants were tall • One - fourth of the plants were short This consistent result helped Mendel understand how traits are inherited. 2.3 Mendel’s Laws of Genetics After many years of experiments, Mendel developed three key principles, now known as Mendel’s laws of genetics .

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    Study Guide 1. Law of Dominance When an organism has two different alleles for a trait, one allele is dominant and masks the expression of the other. 2. Law of Segregation During the formation of gametes (through meiosis), the two alleles for a trait separate . As a result, each gamete carries only one allele for each trait. 3. Law of Independent Assortment Each pair of alleles separates independently of other allele pairs during gamete formation. This means the inheritance of one trait usually does not affect the inheritance of another.

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