Genetics For Dummies
Explore BookThe scientific language of genetics
- Alleles: Alternative versions of a gene
- Autosomal chromosome: A non-sex 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: An allele or phenotype that completely masks a recessive allele or phenotype
- Gene: The fundamental unit of heredity; a specific section of DNA within a chromosome that codes for a specific molecule, usually a protein
- 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: An allele or phenotype that is masked by a dominant allele or phenotype; recessive traits are exhibited only when an individual has two recessive alleles at the same locus or gene
The structure of the cell nucleus and its chromosomes
Mendel’s laws of inheritance
- 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
- A five-sided (pentose) sugar called deoxyribose
- A phosphate
- One of four nitrogen-rich bases: adenine, guanine, cytosine, or thymine
Uncover inheritance based on genotype and phenotype ratios
- Determine how many traits you’re dealing with.
- Count the number of phenotypes for each trait.
- Carefully read the problem to identify the question.
Do you need to calculate the ratios of genotype (for example, AA, Aa, or aa) or phenotype (such as yellow or green)? Are you trying to determine something about the parents or the offspring? - Look for words that mean and and or to help determine which probabilities to multiply (and) and which to add (or).
Parent Genotypes | Offspring Phenotypic Ratio | Type of Inheritance |
Aa x Aa | 3 A_ : 1 aa | Monohybrid cross, simple dominance |
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, simple dominance |
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 |
The Central Dogma of Genetics
The Central Dogma of Genetics is that the genetic information stored in genes is first transcribed into messenger RNA (mRNA) and is then translated into protein. Transcription occurs in the nucleus of a cell and uses the sequence of a gene to create an mRNA transcript. Each gene is identified by transcription machinery and includes its regulatory sequences (promoter, enhancers, silencers), exons (the sequences that code for the protein product), introns (the intervening sequences located between the exons that do not code for protein product), and the sequences that signal the end of the gene (terminator sequence).
After the mRNA is created, a cap is added to one end, a poly-A tail is added to the other end, and the introns are removed by splicing. The mRNA then moves out of the nucleus, where it is then translated. During translation, the mRNA sequence is read in 3-base pair segments called codons. Each 3-base pair codon codes for a specific amino acid (the building blocks of protein). The result of translation is a string of amino acids that are joined to create the final protein product (a polypeptide chain), which is then folded and sometimes modified to make the active form.