reproduction

Reproduction Overview

  • Main Topics Covered:

    • Reproductive anatomy and physiology

    • Genetics

    • Male and female reproductive systems

    • Estrous cycles

Genetics of Animal Breeding

  • Definition of Genetics:

    • Genetics is the study of heredity , focusing on how traits are transmitted from parents to offspring

  • Related Biological Concepts:

    • Organs, Tissues, Cells: The basic unit of heredity is found within cells in the nucleus.

    • Nucleus: Contains chromosomes composed of DNA, which holds the hereditary material.

Chromosomes

  • Characteristics of Chromosomes:

    • Coiled masses of DNA. All body cells of an animal (except sperm and oocytes) contain identical chromosomes.

  • Ploidy Levels:

    • Diploid (2n): 2 copies of each chromosome present in body cells.

      • Example: Dog: 78 chromosomes (39 pairs).

    • Haploid (1n): 1 copy of each chromosome found in germ cells (gametes).

      • Example: Dog: 39 chromosomes (unpaired).

  • Comparison of Species Chromosome Counts:

    • Dog: 78 (Diploid), 39 (Haploid)

    • Alpaca: 70 (Diploid), 35 (Haploid)

    • Horse: 64 (Diploid), 32 (Haploid)

    • Donkey: 62 (Diploid), 31 (Haploid)

    • Cow: 60 (Diploid), 30 (Haploid)

    • Goat: 60 (Diploid), 30 (Haploid)

    • Sheep: 54 (Diploid), 27 (Haploid)

    • Pig: 38 (Diploid), 19 (Haploid)

    • Cat: 38 (Diploid), 19 (Haploid)

  • Karyotype:

    • A visual representation of the chromosomes of a species used to determine genetic sex and chromosomal diseases.

    • In humans, pair numbers 1 to 22 are autosomes, with the 23rd pair being sex chromosomes (XX for females, XY for males).

Genetic Sex Determination

  • Genetic Female: XX chromosomes

  • Genetic Male: XY chromosomes

  • Mechanism of Determination:

    • The sex of mammals is determined by the sex chromosome carried in the sperm (X or Y).

    • Resulting pairs: XX (Female) and XY (Male).

Free Martin Syndrome

  • Definition:

    • A sterile female (XX, XY chimera) born as a co-twin to a male, commonly observed in cattle but reported in other species.

  • Cause:

    • Fusion of placentas leading to a shared blood supply between male and female twins.

Fetal Development and Phenotype

  • Gonadal Development:

    • Question arises whether the ovaries or testes develop first during fetal development.

    • Fetal testes produce Mullerian Inhibiting Hormone (MIH), inhibiting the development of the female reproductive tract.

  • Heifer Calf Phenotype:

    • Typically appears as a heifer but may exhibit underdeveloped internal organs (small ovaries and uterus, no cervix, short blindly ending vagina) or male seminal vesicles.

Chimera

  • Definition:

    • An animal containing cells from two different zygotes; each population of cells retains its own characteristics, resulting in mixed tissues.

  • Chromosome Count Comparison:

    • Normal calf: 60 chromosomes per cell.

    • Freemartin: 60 chromosomes from female cells plus 60 chromosomes from male brother (XY).

Selecting Breeding Stock

  • Key Definitions:

    • Gene: A segment of DNA located on a chromosome.

    • Genotype: The combination of genes an individual possesses.

    • Phenotype: The physical appearance of an individual as determined by its genotype.

    • Heritability Estimates: An assessment of the probability that a trait can be passed from offspring to parent.

Breeding Stock Traits in Swine

  • Trait Heritability Ratings:

    • Litter size: 5%

    • Birth weight: 20%

    • Feed efficiency: 30%

    • Percent lean cuts: 45%

    • Percent fat cuts: 60%

Genetic Effects in Animal Breeding

  • Nonadditive Gene Effect:

    • Only one or a few gene pairs control the expression of a trait (e.g., the horned condition in cattle: Polled = no horns).

  • Additive Gene Effect:

    • Multiple genes collectively influence the expression of a trait (e.g., milk production, carcass traits, weight gain).

    • Genetic testing may be utilized to identify the presence or absence of specific genes that indicate differences in disease susceptibility or production traits.

Scrapies Testing in Sheep

  • Definition of Scrapie:

    • A neurodegenerative disorder classified as transmissible spongiform encephalopathy caused by a prion (PrPSc) that alters normal proteins (PrPc) within neurons, leading to progressive brain degeneration.

  • Transmission and Symptoms:

    • Infected sheep can transmit the scrapie prion throughout their lives, even if asymptomatic. Most infections occur from the dam to offspring at or shortly after birth.

    • Clinical Signs:

      • Incubation period: 2-5 years

      • Symptoms include lethargy, hyperexcitability, unusual gait, fixed stare, ataxia, blindness, tremors, convulsions, intense itching, and ultimately death within 2 weeks to 6 months.

Genetic Susceptibility to Scrapie

  • Key Codon Variations:

    • Codon 136:

      • Valine (V) linked with susceptibility; Alanine (A) linked with resistance.

    • Codon 154:

      • Histidine (H) linked with resistance; Arginine (R) linked with susceptibility.

    • Codon 171:

      • Arginine (R) linked with resistance; Glutamine (Q) and Histidine (H) linked with susceptibility.

  • Inheritance Pattern:

    • Each sheep inherits two copies of each gene (one from each parent).

    • Genotypes AA RR are nearly resistant; genotypes AV QQ can be susceptible to certain strains.

Male Reproductive System Functions

  1. Spermatogenesis: Production of spermatozoa (sperm cells).

  2. Hormone Production: Release of hormones including testosterone.

  3. Delivery: Transport spermatozoa to the female at the appropriate reproductive time.

Female Reproductive System Functions

  1. Oogenesis: Production of eggs.

  2. Hormone Production: Releases hormones including estrogen, progesterone, and inhibin.

  3. Pregnancy Maintenance: Supports pregnancy and delivery of offspring.

Hormonal Regulation in Reproduction

  • Regulated by hormones secreted by the hypothalamus, anterior pituitary gland, testes, and ovaries:

    • **Gonadotropin-Releasing Hormone (GnRH)

    • Luteinizing Hormone (LH)

    • Follicle-Stimulating Hormone (FSH)**

Female Reproductive Anatomy

  • Components include the following:

    • Ovaries: Site of oogenesis.

    • Oviducts: Channels for the ovum.

    • Uterine Horns: Sites for potential embryo implantation.

    • Uterine Body: Central portion of the uterus.

    • Cervix: Passage between uterus and vagina.

    • Vagina: Birth canal.

    • Vestibule and Vulva: External structures of the female reproductive system.

Ovulation Process

  • Triggered by a surge of Luteinizing Hormone (LH) resulting in the rupture of the mature follicle and ovum release.

  • Following ovulation, the empty follicle fills with blood, forming the Corpus Hemorrhagicum, which transitions into the Corpus Luteum, responsible for producing progesterone.

Corpus Luteum Functions

  • Hormonal Production: Produces progesterone, which is essential for:

    • Maintenance of pregnancy.

    • Suppression of estrus behavior, follicle development, and ovulation in most species.

    • Prepares the uterus for embryo implantation.

    • Closing the cervix.

    • Encouraging mammary gland development.

  • The lifespan of the Corpus Luteum is "fixed" unless the ovum is fertilized.

Fate of the Corpus Luteum

  1. Pregnancy:

    • If the female is pregnant, the uterus signals that it is "Full!", thus extending the lifespan of the Corpus Luteum.

  2. Non-Pregnancy:

    • The uterus signals "Empty!" and releases Prostaglandin F2α (PGF2α), leading to the lysis of the Corpus Luteum and cessation of progesterone production.

Estrous Cycle Overview

  • Variability by Species:

    • The estrous cycle varies significantly among species.

  • Phases of the Estrous Cycle:

    • Proestrus

    • Estrus

    • Metestrus

    • Diestrus

    • Anestrus

  • Cycle Patterns:

    • Nonseasonally Polyestrous: Cycle year-round if not pregnant (e.g., Cattle, Pigs).

    • Seasonally Polyestrous: Cycle during specific seasons (e.g., Horses = Spring-Summer; Sheep = Fall).

    • Monoestrous: One cycle per year (e.g., Fox, Mink, Dog).

Detailed Phases of the Estrous Cycle

  • Proestrus:

    • Follicles begin developing.

    • Estrogen production increases.

  • Estrus:

    • Period of sexual receptivity.

    • Peak estrogen production.

    • Ovulation occurs near the end of this phase (in some species).

    • Induced ovulators (e.g., Cats, Rabbits) may remain in a prolonged state of estrus if not bred.

  • Metestrus:

    • Development of the Corpus Luteum takes place.

    • Progesterone from the CL prevents follicular development and prepares the uterus for implantation of a fertilized ovum.

  • Diestrus:

    • Maximum size and effect of the Corpus Luteum, secreting high levels of progesterone.

    • If fertilized ovum implants, the CL is retained far into pregnancy; otherwise, the CL degenerates with increased prostaglandin F2α production at the end of this phase.

  • Anestrus:

    • Temporary ovarian inactivity observed in some species. Ovaries effectively shut down for a specific period of time.