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Cheat Sheet

Genetics For Dummies

From Genetics For Dummies, 2nd Edition by Tara Rodden Robinson

Genetics is a complex field with lots of details to keep straight. But when you get a handle on some key terms and concepts, including the structure of DNA and the laws of inheritance, you can start putting the pieces together for a better understanding of genetics.

The Scientific Language of Genetics

From chromosomes to DNA to dominant and recessive alleles, learning the language of genetics is equivalent to learning the subject itself. The following key terms are guaranteed to appear frequently in your study of all things genetic:

  • Alleles: Alternative forms of a gene

  • Autosomal chromosome: A nonsex chromosome

  • Chromosome: A linear or circular strand composed of DNA that contains genes

  • Diploid: An organism with two copies of each chromosome

  • DNA: Deoxyribonucleic acid; the molecule that carries genetic information

  • Dominant: A phenotype or allele that completely masks the presence of the other, recessive allele in the heterozygote

  • Gene: The fundamental unit of heredity; a specific section of DNA within a chromosome

  • Genotype: The genetic makeup of an individual; the allele(s) possessed at a given locus

  • Heterozygote: An individual with two different alleles of a given gene or locus

  • Homozygote: An individual with two identical alleles of a given gene or locus

  • Locus: A specific location on a chromosome

  • Phenotype: The physical characteristics of an individual

  • Recessive: A phenotype or allele exhibited only when homozygous

The Structure of the Cell Nucleus and Its Chromosomes

If you could open the nucleus of a cell and peek inside, you’d find chromosomes — the strands of DNA where genes reside. This figure helps you see how all the parts of a chromosome relate to one another.

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Mendel’s Laws of Inheritance

Genetic inheritance boils down to three simple concepts put forth by Gregor Mendel, a humble monk and part-time scientist who founded the entire discipline of genetics:

  • Segregation: In diploid organisms, chromosome pairs (and their alleles) are separated into individual gametes (eggs or sperm) to transmit genetic information to offspring.

  • Dominance: A dominant allele completely masks the effects of a recessive allele. A dominant allele produces the same phenotype in heterozygotes and in homozygotes.

  • Independent assortment: Alleles on different chromosomes are distributed randomly to individual gametes.

The Structure of DNA

DNA is made up of long chains of nucleotides. To make a complete DNA molecule, single nucleotides join to make chains that come together as matched pairs and form long double strands. Each nucleotide is comprised of the following:

  • A five-sided (pentose) sugar called deoxyribose

  • A phosphate

  • One of four nitrogen-rich bases: adenine, guanine, cytosine, or thymine

Nucleotides are joined together by phosphodiester bonds. Nucleotide chains are antiparallel and complementary.

image0.jpg

Uncover Inheritance Based on Genotype and Phenotype Ratios

When solving genetics problems, it pays to know what patterns to look for. The parent genotypes and offspring phenotypic ratios in this table can help you figure out what kind of inheritance is at work.

Parent Genotypes Offspring Phenotypic Ratio Type of Inheritance
Aa x Aa 3 A_ : 1 aa Simple dominance, monohybrid cross
Aa x Aa 1 AA : 2 Aa : 1 aa Incomplete dominance
AaBb x AaBb 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb Dihybrid cross
AaBb x AaBb 9 A_B_ : 3 A_bb : 4 aaB_ : aabb Recessive epistasis
AaBb x AaBb 12 A_B_ : A_bb : 3 aaBb : 1 aabb Dominant epistasis
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