Principles of Genetics and Breeding Systems

Mendel’s Law

Principle of Independent Assortment – Inheritance of one trait has no effect on the inheritance of another trait.

Principle of Segregation

each gene has two copies (alleles), and a parent will give only one copy to a child. The other parent will give another copy, and thus the child will receive two copies (alleles)

Principle of Independent Assortment

different genes are inherited separately (mostly).

Genetics

• The study of structure, composition and function of genes

• principles of heredity and variation

Heredity

includes those traits or characteristics which are transmitted from generation to generation

Chromosome

• Strands of DNA

• Located in cell nucleus

• Genes in linear order (but not numerical)

• Characteristic number fixed with species

• Originate in pairs: one from each parent

Autosomes

Chromosomes other than sex chromosomes

Sex chromosomes

Carry material to determine secondary sex characteristics/gamete production

• X Chromosome

• Y Chromosome

Homogametic sex Chromosome

female XX

• All normal egg cells carry one X

heterogametic sex

Male XY

• ½ normal sperm cells – carry one X

  •½ normal sperm cells – carry one Y

Z/W System

Females are heterogametic; Males homogametic - Birds, some reptiles, some fish

The X-0 System

• Females have two copies of the sex chromosome (XX), but males have only one (X0)

• The 0 denotes the absence of a second sex chromosome

• Sex is determined by number of genes expressed across the two chromosomes

Haplo-diploid system

• Females are diploid and are developed from fertilized eggs. They contain twosets of chromosomes.

• Males are haploid and are developed from an unfertilized egg. The males possess only one set of chromosomes

DNA

The life instructions of the cell

Gene

Segment of DNA that tells the cell how to make a certain protein

Alleles

A variant of a gene controlling the same trait

• Different possibilities for a given trait

• usually exists in two different forms (dominant and recessive)

Dominant allele

• will always express over recessive

• mask or suppress the expression of recessive alleles are classically symbolized with Capital Letters

Recessive alleles

• will express in the absence of dominant alleles

• symbolized with small letters

Homozygous

A condition in which both the members of an allelic pair are identical

Heterozygous

A condition in which the members of an allelic pair are NOT identical

Genotype

the genetic make up of an organism

Phenotype

physical features/ appearance of an organism

• The expression of a genotype

• Produced by the interaction between genotypes and environmental factors

Codominance

Both alleles are expressed in the phenotype when present in the heterozygous state

Incomplete Dominance

Condition in the heterozygote where both genes are expressed in a way that differs from either homozygous condition

Punnett Square

a tool that allows you to see the different gene combinations that are possible when two parents of any species create offspring.

Hybridization

the process of crossing two genetically different individuals

Hybrid

The progeny of a hybridization

Monohybrid

An organism which is heterozygous with respect to only ONE pair of allele at a locus under study

Dihybrid

An organism which is heterozygous with respect to TWO pairs of

alleles at two loci under study

Reciprocal Cross

Two reverse crosses in which the sexes of the parents are interchanged

Reasons to crossbreed

• To take advantage of heterosis/hybrid vigor

• To produce animals which blend the characteristics of two or more breeds into a more desirable combination not available in any one breed