CHAPTERS 12

Unit 3: Genetics

Objectives

Understanding of the formation of gametes & the role of DNA

Knowledge of genotypic and phenotypic outcomes

Mastery of mating, dominance, and incomplete dominance

Role of Biotechnology in livestock systems

Gametogenesis

Takes place in the sex cells of the male & female Male=spermatogenesis Female=oogenesis Cell division by meiosis

Cell Division by Meiosis 1 & 2 Interphase-chromosomes duplicate

“Resting Phase” Prophase Metaphase Anaphase Telophase Cytokinesis

Spermatogenesis

4 sperm are produced from each primary spermatocyte

Oogenesis

First division produces one large oocyte, and one smaller cell (polar body)

Second division results in one egg (ovum), and the second polar body

Polar bodies eventually die and get reabsorbed

Figure 12.3 Meiosis or reduction cell division in the testicle and ovary (example with two pairs of chromosomes). Source: Colorado State University.

Fertilization

Sperm & egg each provide one chromosome to each pair

Fertilized egg called a zygoteZygote is diploid (two copies of each gene)Gametes are haploid (one copy of each gene)Each event is random, provides genetic

diversity

Figure 12.4 Combining of chromosomes through fertilization (two pairs of genes used for simplification of example). Source: Colorado State University.

DNA

The genetic code for each animal is carried in the DNA Deoxyribonucleic acid Double helical structure Location of each gene on the chromosome is called a

locus

Figure 12.6 DNA helix and structure of nucleotides.

Figure 12.5 A simplified example showing a pair of chromosomes containing several pairs of genes. Source: Colorado State University.

Genes & Chromosomes

Genes & chromosomes are paired-homologous

Transmission of traits to offspring is entirely dependent upon which chromosomes are passed on

Sex Chromosomes X & Y

Male carries both X & Y chromosomes Female only carries the X chromosome

Genes & Chromosomes

Homologous chromosomes may differ in the way they influence a trait Homozygous-if they have the same effect on a trait Heterozygous-if they have a different effect on a

trait These traits are called alleles One gene is always dominant while the suppressed

gene is recessiveGenotype vs. Phenotype

Genotype refers to the genetic code of the trait Phenotype is the trait that is expressed

Fundamentals of Mating

Homozygous dom. X Homozygous dom.Homozygous dom. X HeterozygousHomozygous dom. X Homozygous rec.Heterozygous X HeterozygousHeterozygous X Homozygous rec.Homozygous rec. X Homozygous rec.Mate using a Punnet Square

Gene Interaction

Dominance interaction exists when the dominant trait suppresses the recessive trait

Incomplete dominance refers to a phenotype that is expressed differently from the dominant and recessive phenotypes

Some heterozygotes are superior to homozygotes-Hybrid Vigor

Complete dominance, lack of dominance, overdominance

Figure 12.14 Bar graphs illustrating: (A) complete dominance; (B) lack of dominance; (C) overdominance.

Genetics & the Environment

Environment can influence the expression of a genetic trait

Give an example

Biotechnology

Genetic engineering Superovulation Sexing semen Cloning ET Genetic markers Gene therapy Genetic selection Artificial insemination

Biotechnology

Applications in genetic biotechnology Genes can be removed, altered, and reinserted

into an embryo in vitro Genes can be modified or duplicated with the

help of growth medium bST

Genes from one specie can be inserted into another specie to enhance or alter a trait Transgenesis

Nuclei can be taken from one individual an put together by Nuclear Fusion

Figure 12.15 A fertilized swine egg photographed at the moment it is microinjected with new genetic material. The vacuum in the large pipette at the bottom anchors the cell while a mixture containing the genetic material is forced through the smaller pipette into one of the egg’s pronuclei. Courtesy of R. E. Hammer and R. L. Brinster, University of Pennsylvania School of Veterinary Medicine.

Figure 12.16 Somatotropin production for use in cows and pigs.