MCAT Genetics

The observable physical characteristics of an organism.

Phenotype

The genetic makeup of an organism.

Genotype

An allele that is expressed whenever it is present.

Dominant Allele

An allele that is expressed only when two copies are present.

Recessive Allele

An individual with two identical alleles for a gene.

Homozygous

An individual with two different alleles for a gene.

Heterozygous

The different versions of the same gene.

Alleles

Alternative form of a gene found at a specific locus.

Allele

The physical location of a gene on a chromosome.

Locus

The passing of traits from parents to offspring.

Heredity

The branch of biology that studies inheritance.

Genetics

The scientist known as the father of genetics.

Gregor Mendel

Mendel's first law stating that two alleles separate during gamete formation.

Law of Segregation

Mendel's second law stating that genes assort independently if they are on different chromosomes.

Law of Independent Assortment

The production of sex cells containing one copy of each chromosome.

Meiosis

Cells containing two sets of chromosomes.

Diploid

Cells containing one set of chromosomes.

Haploid

The chromosome number in human somatic cells.

46

The chromosome number in human gametes.

23

A pair of chromosomes containing the same genes.

Homologous Chromosomes

Identical copies of a chromosome produced during DNA replication.

Sister Chromatids

Exchange of genetic material between homologous chromosomes during meiosis.

Crossing Over

Creation of new combinations of alleles through crossing over.

Genetic Recombination

The random orientation of homologous chromosome pairs during meiosis I.

Independent Assortment

Random fertilization further increases this property among offspring.

Genetic Variation

A trait controlled by a single gene.

Mendelian Trait

A trait controlled by multiple genes.

Polygenic Trait

A trait influenced by both genes and the environment.

Multifactorial Trait

A dominant allele that is not always expressed.

Incomplete Penetrance

The degree to which a genotype is expressed in an individual's phenotype.

Variable Expressivity

Inheritance pattern in which the heterozygote displays an intermediate phenotype.

Incomplete Dominance

Inheritance pattern in which both alleles are fully expressed.

Codominance

The classic example of codominance in humans.

AB Blood Type

The inheritance pattern of the ABO blood group.

Multiple Alleles

Blood type that has both A and B antigens.

Type AB

Blood type that has neither A nor B antigens.

Type O

Universal donor for red blood cells.

Type O Negative

Universal recipient for red blood cells.

Type AB Positive

Protein found on red blood cells that determines positive or negative blood type.

Rh Factor

A cross used to determine the genotype of an individual showing the dominant phenotype.

Test Cross

A cross between two individuals differing in one trait.

Monohybrid Cross

A cross between two individuals differing in two traits.

Dihybrid Cross

Probability of offspring inheriting a particular genotype.

Punnett Square

A chromosome that is not a sex chromosome.

Autosome

The human sex chromosomes.

X and Y Chromosomes

Inheritance pattern in which the gene is located on the X chromosome.

X linked inheritance

Inheritance pattern in which the gene is located on the Y chromosome.

Y

Most X

linked disorders are inherited in this manner.

A male has only one copy of each X

linked gene because he is this.

The probability that two genes will be inherited together increases when they are this.

Linked

Genes that are located close together on the same chromosome.

Linked Genes

The farther apart two genes are on a chromosome, the more likely this process will separate them.

Crossing Over

The percentage of recombinant offspring used to estimate gene distance.

Recombination Frequency

One percent recombination equals this unit of genetic distance.

One Centimorgan

The inheritance pattern in which mitochondrial DNA is passed only from mothers to offspring.

Maternal Inheritance

The organelle that contains its own circular DNA.

Mitochondria

The failure of chromosomes to separate properly during meiosis.

Nondisjunction

Nondisjunction most commonly occurs during this process.

Meiosis

Having an abnormal number of chromosomes.

Aneuploidy

The presence of three copies of one chromosome.

Trisomy

The presence of only one copy of a chromosome.

Monosomy

Genetic disorder caused by Trisomy 21.

Down Syndrome

Genetic disorder caused by Monosomy X.

Turner Syndrome

Genetic disorder caused by XXY chromosomes.

Klinefelter Syndrome

A population that is not evolving is said to be in this state.

Hardy

The Hardy

Weinberg equation for allele frequencies.

The Hardy

Weinberg equation for genotype frequencies.

Frequency of the dominant allele in a population.

p

Frequency of the recessive allele in a population.

q

Frequency of homozygous dominant individuals.

p²

Frequency of heterozygous individuals.

2pq

Frequency of homozygous recessive individuals.

q²

The five assumptions required for Hardy

Weinberg equilibrium.

The evolutionary force that introduces new alleles into a population.

Mutation

Movement of alleles into or out of a population.

Gene Flow (Migration)

Random fluctuations in allele frequencies due to chance.

Genetic Drift

Genetic drift has the greatest effect in populations of this size.

Small Populations

Genetic drift resulting from a natural disaster.

Bottleneck Effect

Genetic drift occurring when a small group establishes a new population.

Founder Effect

Evolutionary mechanism in which individuals with advantageous traits reproduce more successfully.

Natural Selection

Selection favoring one extreme phenotype.

Directional Selection

Selection favoring the intermediate phenotype.

Stabilizing Selection

Selection favoring both extreme phenotypes.

Disruptive Selection

A change in the DNA nucleotide sequence.

Mutation

A mutation involving a single nucleotide substitution.

Point Mutation

A point mutation that changes one amino acid to another.

Missense Mutation

A point mutation that creates a premature stop codon.

Nonsense Mutation

A point mutation that does not change the amino acid sequence.

Silent Mutation

Mutation caused by insertion or deletion of nucleotides not divisible by three.

Frameshift Mutation

Mutation caused by inserting DNA bases into a sequence.

Insertion

Mutation caused by removing DNA bases from a sequence.

Deletion

Replacement of a purine with another purine or a pyrimidine with another pyrimidine.

Transition Mutation

Replacement of a purine with a pyrimidine or vice versa.

Transversion Mutation

DNA repair mechanism that removes incorrect bases after replication.

Mismatch Repair

DNA repair mechanism that removes damaged nucleotides such as thymine dimers.

Nucleotide Excision Repair

DNA repair mechanism that replaces a single damaged base.

Base Excision Repair

DNA damage commonly caused by ultraviolet radiation.

Thymine Dimers

Enzyme that joins DNA fragments by forming phosphodiester bonds.

DNA Ligase

Enzyme that cuts DNA at specific nucleotide sequences.

Restriction Endonuclease

DNA produced by combining DNA from different sources.

Recombinant DNA