Development and Heredity Overview

The Process of Development & Heredity

Prenatal Development

  • Definition:

    • Begins with fertilization

    • Ends with birth

    • Lasts about 38 weeks after conception (266 days)

  • Divisions:

    • Preembryonic Period

    • Embryonic Period

    • Fetal Period

Fertilization

  • Definition:

    • The union of sperm and egg

  • Process:

    • Sperm undergo capacitation while moving through the female reproductive tract.

    • Capacitation: Weakening of the membrane around the tip of the sperm head, enabling the release of enzymes.

    • When sperm reach the egg in the fallopian tube, enzymes from thousands of sperm break down the outer layers of the egg to form an opening for a single sperm to enter.

  • Changes Upon Entry:

    • Membrane changes prevent entry of additional sperm.

    • The sperm nucleus moves to the center of the egg cell, membranes of their nuclei disintegrate, and chromosomes combine.

  • Result:

    • Formation of a single cell known as the zygote, containing 46 chromosomes (23 from sperm, 23 from egg).

    • Zygote is the first cell of the future individual.

    • The ovum remains viable for about 24 hours; sperm are fertile in the female for approximately 2-3 days.

    • For pregnancy to occur, intercourse must happen between 3 days before and 1 day after ovulation.

  • Location of Fertilization:

    • Occurs in the fallopian tube.

Early Development

Preembryonic Period

  • Duration:

    • Lasts about 2 weeks following fertilization.

  • Cell Divisions:

    • Early divisions are called cleavage.

    • First cleavage division completed within 36 hours, resulting in two cells, called blastomeres.

    • By the end of the third day, there are 16 cells in a ball, known as the morula.

    • By the fifth day, it forms a hollow sphere of cells, called the blastocyst, which is ready for implantation.

Implantation

  • Timeline:

    • By the 21st day of the cycle (7th day after ovulation), the endometrium is prepared to receive the blastocyst.

  • Process:

    • The blastocyst attaches to the endometrium and burrows into it.

    • The endometrial cells grow over the blastocyst until it is completely implanted by the 14th day after ovulation.

  • Hormonal Changes:

    • The blastocyst secretes human chorionic gonadotropin (hCG), which acts like LH, traveling through blood to the ovary.

    • This secretion causes the corpus luteum to continue secreting progesterone, maintaining the endometrium.

    • hCG levels decline as the placenta develops and begins secreting its own hormones.

  • Germ Layer Formation:

    • During implantation, three primary germ layers develop: ectoderm, mesoderm, endoderm.

    • This germ layer formation indicates the end of the preembryonic period (approximately 14 days).

Embryonic Development

  • Duration:

    • Begins at the start of the 3rd week and ends at the 8th week, referred to as an embryo.

    • Lasts for 6 weeks during which the placenta and extraembryonic membranes are formed.

  • Key Developments:

    • Skin forms during the 3rd week.

    • The heart starts pumping blood by the end of the 4th week.

Extraembryonic Membranes

  • Components:

    • Amnion: forms the amniotic sac, which is filled with amniotic fluid (the bag of waters).

    • Purpose: Surrounds the embryo, cushions and protects it, allows for symmetrical development and movement, ruptures naturally or surgically.

    • Chorion: Develops villi that penetrate the endometrium and assist in forming the placenta.

    • Eventually fuses with the amnion.

Formation of Placenta

  • Source:

    • Develops from the endometrium and chorion.

  • Function:

    • Fully functional by the end of the embryonic period.

    • Plays a nutritive, respiratory, and excretory role for the embryo/fetus and functions as a temporary endocrine gland secreting hCG, estrogen, and progesterone.

  • Structure:

    • Chorionic villi become highly vascular and extend to umbilical vessels.

    • The umbilical cord connects to the fetus, consisting of 2 arteries and 1 vein.

    • The placenta acts as a barrier preventing maternal and fetal blood mixing, offering protection from harmful substances, though some (e.g., alcohol, drugs, infectious agents) can cross this barrier.

Embryonic Development (cont.)

  • Critical Phase:

    • Most critical time for development as tissues differentiate and organ systems are formed by the end of this period (8th week).

  • Metrics:

    • The embryo measures about 1 inch (2.5 cm) long and weighs approximately 1 gram.

    • By this time, the embryo appears distinctly human.

Fetal Development

  • Duration:

    • Begins at the 9th week and continues until birth.

    • This is a period of growth and maturation for the fetus.

  • Sex Determination:

    • Can be established by the end of 16 weeks.

  • Maternal Experience:

    • By the 20th week, the mother can feel the fetus moving (termed quickening), which is considered a sign of viability.

  • Physical Changes:

    • The skin is covered with a cheese-like substance known as vernix caseosa and fine hair recognized as lanugo.

  • Pregnancy Gestation:

    • Commonly divided into three trimesters and lasts about 40 weeks from the beginning of the last menstrual period (280 days).

    • Definitions:

    • Less than 37 weeks: premature

    • 37-40 weeks: term

    • More than 41-42 weeks: postmature

Heredity

  • Chromosomal Contribution:

    • Each parent contributes 23 chromosomes, containing approximately 1.5 billion bits of genetic information.

  • Genetics & Genes:

    • Gregor Mendel's studies of garden peas led to the discovery of independent units of genetic code, termed genes, which are segments of DNA located on chromosomes.

    • Each chromosome can contain thousands of genes, each encoding for a specific trait.

Function of Meiosis in Reproduction

  • Chromosome Configuration:

    • Chromosomes exist in 23 pairs: 1 pair of sex chromosomes and 22 pairs of autosomes, with each pair matching in gene types.

  • Total Cell Composition:

    • A typical somatic cell contains 46 chromosomes; sex cells (gametes) contain 23 chromosomes since they undergo two cell divisions.

  • Genetic Inheritance:

    • This process enables offspring to inherit genetic information from both parents.

Meiosis (cont.)

  • Independent Assortment:

    • Each chromosome assorting independently during meiosis results in unique gametes.

    • During one phase of meiosis, chromosomes exchange genes, ensuring genetic variation among sperm and ovum.

  • Genetic Uniqueness:

    • Each child from the same set of parents is likely to be genetically unique due to these processes.

Dominant/Recessive Genetic Traits

  • Trait Control:

    • Genetic traits are governed by pairs of similar genes, one from each parent.

  • Alleles:

    • Alleles are genes situated at the same locus on homologous chromosomes, which can be expressed differently in offspring.

    • Genetic Variations:

    • Homozygous: alleles are the same (e.g., AA or aa).

    • Heterozygous: alleles are different (e.g., Aa).

  • Expression of Traits:

    • Mendel found that some genes are dominant while others are recessive.

    • A dominant gene's effects can mask those of a recessive gene:

    • Dominant allele denoted with a capital letter (e.g., B for brown eyes).

    • Recessive allele denoted with the same letter in lowercase (e.g., b for blue eyes).

  • Phenotype Implications:

    • A dominant trait is expressed even when one allele is recessive (e.g., BB or Bb results in brown eyes).

    • A recessive trait is expressed only if both alleles are recessive (e.g., bb results in blue eyes).

Examples of Dominant/Recessive Traits

  • A blue-eyed man marries a brown-eyed woman, and they have 6 brown-eyed children.

    • The mother’s genotype is likely Bb (heterozygous for brown eyes).

    • If a 7th child has blue eyes, it implies the mother is indeed Bb.

  • Curly Hair Trait:

    • Curly hair is an autosomal dominant trait; a person with genotype Cc would have curly hair (phenotype).

  • Albinism:

    • Total lack of melanin in skin and eyes is caused by recessive genes (e.g., aa) and is visible only in homozygous recessive individuals.

Carrier of a Genetic Trait

  • Definitions:

    • Homozygous dominant (AA) or heterozygous (Aa) individuals express a normal phenotype.

    • A genetic carrier has a heterozygous genotype and can pass on the gene for a recessive condition without having the condition themselves (e.g., 2 normal parents Aa can produce normal children or children with albinism).

Blood Types

  • Overview:

    • Blood types are determined by three genes: A, B, and O.

    • Both A and B are dominant genes (codominance).

  • Possible Genotypes:

    • Type A can have genotype AA or Ao.

    • Type AB will have genotype AB.

    • Type O will have genotype oo.

Sex Determination

  • Chromosomal Basis:

    • Females have XX chromosomes while males have XY chromosomes.

    • The X chromosome is larger and contains genes unrelated to sex traits, while the Y chromosome is smaller and primarily dictates male characteristics.

  • Normal X Requirement:

    • Both males and females require at least one normal X chromosome to express necessary traits (e.g., clotting factors, photopigments).

Sex-linked Traits

  • Definition:

    • Traits linked to sex chromosomes are termed sex-linked or X-linked traits.

  • Female Expression:

    • In females, recessive X-linked traits are masked by dominant counterparts on the other X chromosome.

  • Male Expression:

    • Males need only one recessive X-linked gene to express the trait since there is no corresponding allele on the Y chromosome.

  • Frequency:

    • X-linked recessive traits appear more frequently in males compared to females (e.g., hemophilia, red-green color blindness).

Hereditary Traits

  • Complex Traits:

    • Many inherited characteristics (e.g., skin color, height) are polygenic, influenced by multiple gene pairs.

  • Genetic Mutation:

    • Changes in an individual's genetic code can occur spontaneously or due to external factors (mutagenic chemicals, radiation, viruses).

    • Mutations in reproductive cells may lead to heritable changes.

Sexual Sources of Genetic Variation

  • Unique Genetics:

    • Each person is genetically unique due to:

    1. Independent assortment of chromosomes

    2. Crossover of homologues

    3. Random fertilization of eggs by sperm

Chromosome Segregation and Independent Assortment

  • Key Processes in Meiosis I:

    1. Segregation:

    • Separation of alleles during meiosis to different daughter cells (e.g., Bb leads to B in one cell and b in another).

    • Errors in segregation can lead to major health issues (e.g., cancer, infertility, Down syndrome).

    1. Independent Assortment:

    • Alleles of different traits are distributed independently of one another.

    • Example: Hair color and thumb joint traits are distributed without dependency.

  • Genetic Variety:

    • Independent assortment and random fertilization produce trillions of zygote combinations, greatly enhanced by crossing over variations.