Genetics Modified by Georgia Agricultural Education Office June
Genetics Modified by Georgia Agricultural Education Office June, 2002
Genetics • the study of heredity • the way in which traits of parents are passed on to offspring
Father of Genetics • Gregor Mendel • Austrian monk • did experiments with peas and proved that certain characteristics……. .
………. • Such as color and height, are passed from parent to offspring
Differences • all differences in organisms are not caused by genetics • some are caused by the environment • the conditions under which the animals are raised
Parts of the cell • cell membrane - contains the cytoplasm and the nucleus
Cytoplasm • protects and cushions the nucleus
Nucleus • contains the genetic material
Chromosomes • carry the genes • exist in pairs in all cells except sperm and egg cells
Genes • single determiner of a hereditary trait
Cell Division • two types • Mitosis • Meiosis
Mitosis • cell division for growth • this division never ends • chromosome pairs are duplicated exactly alike
Mitosis • each new cell is exactly like the old ones • asexual division • chromosomes are in pairs
Mitosis • steps of mitosis • Prophase (Interphase) • Metaphase • Anaphase
Steps • Telophase • Cytokinesis • ** PMATC
Meiosis • cell division of reproductive cells (sperm and egg) • each new cell is not exactly like the old one
Meiosis • in each new cell chromosomes are not in pairs • each new cell contains half of the original number of chromosomes
Meiosis • allows for random assortment of parental genes
Gamete • mature egg or sperm cell • each gamete has half the original number of chromosomes
Dominant gene • in a pair, hides the effect of another gene
Recessive gene • the gene which is hidden by a dominant gene
Examples • of dominant genes • tongue rolling • free ear lobes • polled cattle
Examples • of recessive genes • color red in Holsteins • dwarfism
Symbols • dominant genes • capital letters
Symbols • recessive genes • lower case letters
Homozygous • gene pairs • one which carries two genes for a trait • both represented by capital or lower case letters
Homozygous • polled - PP • horned - pp
Heterozygous • gene pairs • one which carries one dominant and one recessive gene
Heterozygous • Pp
Genotype • genetic makeup of an animal or a given trait expressed by symbols • Ex: PP, AA, Pp, Aa, aa
Phenotype • physical appearance expressed in words • Ex: polled, horned
T t T Mitosis Meiosis t t t Mitosis
Steps of Meiosis • Prophase I ( Interphase) • Metaphase I • Anaphase I • Telophase I
Steps of Meiosis • Prophase II (Interphase) • Metaphase II • Anaphase II • Telophase II • Cytokinesis
If • two heterozygous animals are mated • the number of traits being selected is six • the total number of offspring is over 16, 000
Incomplete Dominance • when one gene does not hide the effect of the other in a gene pair • result is a mixture of the two traits
Incomplete Dominance • Roan coloring in cattle • RR X WW = RW • Red X White = Roan • Roan - mixture of red and white hairs
Sex determination • male sex chromosomes • male mammals have two different sex chromosomes • XY • females are XX
Sex determination • poultry • the female determines the sex of the offspring • female is ZW • male is ZZ
Sex Linked Traits • genes which are carried only on the sex chromosomes • ex: red green color blindness • 2: 25 males • 1: 150 females
Sex Linked Traits • hemophilia • bleeders’ disease • more common in males than in females
Sex Linked Traits • barred feather pattern vs black in chickens
Mutation • when a new trait is shown that did not exist in either parent
Mutation • the new breed of polled Herefords • which resulted from the crossing of two horned animals
Causes of Mutations • radiation (nuclear accident or x-ray) • chemicals
Systems of breeding • purebreeding • inbreeding • outcrossing • grading up • crossbreeding
Purebreeding • purebred is defined as a member of a breed • the animals of which possess a common ancestry • distinctive characteristics
Purebreeding • is registered or eligible for registry
Outcrossing • mating of animals of different families within the same breed • relatively safe system of breeding
Outcrossing • unlikely that two unrelated animals would carry the same undesirable genes and pass them on
Grading up • mating of purebred sires to grade females • relatively inexpensive and is dependent upon the quality of the sire
Inbreeding • mating of related animals • closebreeding - a form of inbreeding • most intensive form • animals are very closely related
Closebreeding • sire to daughter • son to dam • brother to sister
Closebreeding • was used on foundations but is not used much today • used in the development of the Quarter Horse breed
Closebreeding • animals are more uniform • more chance for undesirable recessive gene expression
Linebreeding • refers to the mating of animals which are more distantly related • cousins, grandparents, half sibling
Linebreeding • used favorably today because there is less chance of expression on undesirable recessive traits
Linebreeding • less chance of genetically superior stock • considered middle of the road
Crossbreeding • mating of animals from different breeds • often causes hybrid vigor
Hybrid Vigor • causes crossbreds to outproduce the average of their parents • some breeds cause hybrid vigor better than others
Hybrid Vigor • breeding a British breed, Angus, to an Indian Breed< Brahman • compared to an Angus X Shorthorn cross
Hybrid Vigor • Angus X Brahman would have more hybrid vigor
Selection • based upon • type • pedigree • showring winnings • production records
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