Genetics and Molecular Biology: Key Concepts and Laws

What is a gene?

A section of DNA that codes for a protein.

What is an allele?

A different version of the same gene.

What is a character?

A heritable feature such as flower color.

What is a trait?

A specific variation of a character such as purple flower color.

Define genotype

The genetic makeup of an organism (AA, Aa, or aa).

Define phenotype

The observable traits of an organism.

Homozygous dominant

Two dominant alleles (AA).

Homozygous recessive

Two recessive alleles (aa).

Heterozygous

One dominant and one recessive allele (Aa).

Law of Segregation

The two alleles for a trait separate during gamete formation.

Law of Independent Assortment

Alleles of different genes separate independently during gamete formation.

Law of Dominance

The dominant allele masks the recessive allele in a heterozygote.

Incomplete dominance

Heterozygotes show a blended phenotype such as red plus white equals pink.

Codominance

Both alleles are expressed equally such as AB blood type.

Pleiotropy

One gene influences multiple traits such as sickle cell disease.

Epistasis

One gene can mask or modify the expression of another gene.

Polygenic traits

Traits controlled by many genes such as height or skin color.

Sex-linked genes

Genes located on a sex chromosome such as color blindness.

Purpose of a Punnett square

To predict genotype and phenotype ratios of offspring.

Example cross AA × aa

All offspring are Aa heterozygous.

Aa × Aa genotype ratio

1 AA : 2 Aa : 1 aa.

Aa × Aa phenotype ratio if A is dominant

3 dominant : 1 recessive.

DNA full name

Deoxyribonucleic acid.

RNA full name

Ribonucleic acid.

DNA sugar

Deoxyribose.

RNA sugar

Ribose.

DNA bases

Adenine, Thymine, Cytosine, Guanine.

RNA bases

Adenine, Uracil, Cytosine, Guanine.

DNA base-pairing

A pairs with T, G pairs with C.

RNA base-pairing

A pairs with U, G pairs with C.

DNA strand type

Double-stranded.

RNA strand type

Usually single-stranded.

Semi-conservative replication

Each new DNA molecule has one old strand and one new strand.

Gene expression

Process by which information in DNA directs protein synthesis.

Transcription

DNA is copied into mRNA in the nucleus.

Translation

mRNA is read by ribosomes to make a protein in the cytoplasm.

Start codon

AUG which codes for methionine.

Stop codons

UAA, UAG, UGA.

Ribosome function

Reads mRNA codons and links amino acids into a protein.

Golgi apparatus function

Modifies, packages, and ships proteins.

Prokaryotic gene expression

Transcription and translation occur together in the cytoplasm.

Eukaryotic gene expression

Transcription occurs in the nucleus and translation occurs in the cytoplasm.

Point mutation

A change in a single nucleotide in DNA.

Silent mutation

DNA change that does not alter the amino acid.

Missense mutation

DNA change that alters one amino acid in the protein.

Nonsense mutation

DNA change that creates a stop codon, ending translation early.

Deletion

Loss of a chromosome segment.

Duplication

Repetition of a chromosome segment.

Inversion

A chromosome segment is reversed in orientation.

Translocation

A chromosome segment moves to another chromosome.

Nondisjunction

Failure of chromosomes or chromatids to separate during meiosis.

Why not all genes are active at once

To save energy and resources.

Constantly expressed genes

Example: hair growth.

Rarely expressed genes

Example: adrenaline.

Cyclically expressed genes

Example: estrogen.

Levels of gene regulation

Packing or unpacking DNA, transcription, mRNA processing, translation.

Traditional breeding

Crossing parents with desired traits naturally.

Genetic engineering

Directly altering an organism's DNA in a laboratory.

Benefit of GMOs

Increased yield, drought resistance, or better nutrition.

Risk of GMOs

Possible allergies or reduced biodiversity.

Karyotype

A picture showing the number and shape of chromosomes in a cell.

Character vs trait

Character is a heritable feature such as eye color; trait is the specific version such as blue eyes.

Polygenic traits result in

Continuous variation such as height or skin color.

Epistasis example

A gene preventing pigment can mask the effect of pigment color genes.

Sex-linked traits often affect

Males more than females because they have only one X chromosome.

Transcription location

Nucleus of eukaryotic cells.

Translation location

Ribosomes in the cytoplasm.

Importance of AUG

It is the start codon for translation.

Effect of nondisjunction

Produces gametes with abnormal chromosome numbers such as trisomy 21.

Function of ribosomes

Site of protein synthesis.

Function of Golgi apparatus

Packages and modifies proteins for export.