Genetics For Dummies
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.
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
One of four nitrogen-rich bases: adenine, guanine, cytosine, or thymine
Nucleotides are joined together by phosphodiester bonds. Nucleotide chains are antiparallel and complementary.
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|