Principles of Genetics

Genetics

The scientific study of genes, genetic variation, and heredity

Trait

A distinguishing quality or characteristic

Artificial selection

Human-guided mating decisions

Blending theory

Parental 'essence' blends together resulting in the appearance of their offspring being intermediate

Gregor Mendel

Father of modern genetics

Qualitative traits

Traits that can be placed into categories (i.e. horned OR polled)

Mendel's pea plants

When plants with differing traits (purple versus white flowers) pollinated each other, the resulting offspring (F1) only exhibited one appearance (purple flowers)

F1 generation

The first generation of offspring from a cross of two parental lines

F2 generation

The second generation of offspring obtained from self-pollination of the F1 generation

Mendel's conclusions

1. Expressed traits: inherited unchanged in a hybridization (dominant phenotype) 2. Latent traits: disappear in the offspring of a hybridization (recessive phenotype) 3. Because the recessive trait reappears in progeny of F1, the traits remained separate

Dominant appearance

Could mean the plant had 2 dominant 'elements' or a dominant and recessive element

Recessive appearance

Lacked a dominant 'element'

3/4 of the plants

Produced purple flowers, 1/4 produced white flowers

Number of traits observed

Similar results obtained with the other 6 traits

Element

Plants must have 2 copies of an 'element' but only pass one 'element' on to offspring

Deoxyribonucleic acid

Self replicating material present in nearly all living organisms that carries genetic information.

Nucleotides

The building blocks of DNA, comprised of adenine, thymine, cytosine, and guanine.

Chromosomes

Tightly coiled packages of SOME of an organism's DNA.

Genome

The entire 'set' of DNA, including hereditary instructions for building, running, and maintaining an organism.

Regions of DNA

DNA has coding and non-coding regions; non-coding regions are known as spacer DNA and coding regions are known as genes.

Genes

Functional unit of heredity; a small region of DNA that codes for a functional molecule (i.e. a protein).

Locus (loci)

Specific location on a specific chromosome where a gene is found.

Allele

Alternative form of a gene that arises from a mutation in the DNA.

Differentiating alleles

Example: Horned versus polled; polled is denoted P (uppercase p) and horned is denoted p (lowercase p).

Alleles of Extension gene in dogs

More than two alleles can be present in a population; an organism can only inherit two, one from each parent.

E allele

Induces production of eumelanin (black pigment).

Homozygous

An individual possesses two of the same alleles, e.g., HH or hh.

Heterozygous

An individual possesses two different alleles, e.g., Hh.

Genotype

The genetic makeup of an organism that influences how it looks/performs.

Phenotype

The physical expression of the genotype.

Punnett Square

A 2-dimensional grid used to determine the possible genotypes (and resulting phenotypes) from a mating.

Mendel's law of dominance

A certain allele will be dominant over another, expressing its phenotype and masking the phenotype of the other.

Dominant allele

Represented with an uppercase letter and expresses its phenotype.

Recessive allele

Represented with a lowercase letter and is masked by the dominant allele.

Partial dominance

A form of dominance where the phenotype is a blend of both alleles.

Co-dominance

A form of dominance where both alleles are fully expressed in the phenotype.

Overdominance

A form of dominance where the heterozygote has a phenotype that is more extreme than either homozygote.

No dominance

The heterozygous organisms possess a phenotype that is exactly intermediate to the phenotypes of the homozygous organisms.

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30 sec ¼ mile time.

Epistasis

The expression of genes at one locus depends on the alleles present at one or more other loci.

Coat color in horses

Example of epistasis where the extension gene determines if a horse will be black or red.

Extension gene

Determines if a horse will be black or red; black is dominant (BB and Bb genotypes).

Agouti gene

Restricts black pigment to the points; restriction to the points is dominant (AA and Aa genotypes).

Simply-inherited traits

Traits that are controlled by one to a handful of genes, their expression and interactions are well-understood, and tend to be categorical (either/or).

Polygenic traits

Controlled by tens-hundreds of different genes, tend to be quantitative (continuous), and are affected by the environment.

Economically important traits in animal agriculture

Traits that are significant for livestock producers, often related to production efficiency.

Pattern of inheritance

Every gene has a pattern of dominance and there are epistatic interactions.

Gene A

May determine whether or not gene X can be expressed and might be able to 'mask' gene X.

Environment

External factors that can influence the expression of traits in an organism.

Desirable alleles

Alleles that contribute positively to the performance of an organism in a trait of interest.

Carcass weight

Weight of the animal's body after slaughter, influenced by genes.

Undesirable alleles

Alleles that do not add anything to carcass weight (+0 pounds).

Yearling weight

Weight of the animal at one year of age.

Birthweight

Weight of the animal at birth.

Weaning weight

Weight of the animal when it is weaned from its mother.

Environmental effect

Impact of environmental factors on an animal's performance.

Heritability

Measure of how much of a trait's variation is due to genetics.

Chromosome

Structure within cells that contains DNA.

Gene

Basic unit of heredity that contains information for a trait.

Locus/loci

Specific location of a gene on a chromosome.

Complete dominance

Type of dominance where one allele completely masks the effect of another.

No-dominance

Type of inheritance where neither allele is dominant.

Codominance

Type of inheritance where both alleles are expressed equally in the phenotype.

Simply-inherited trait

Trait controlled by a single gene.

Polygenic trait

Trait controlled by multiple genes.