Meiosis and Heredity

• Meiosis is a type of cell division that produces daughter cells with half the number of chromosomes as the parent cell, while mitosis produces identical cells.
• Humans have a total of 46 chromosomes, with 23 inherited from each parent.

Asexual vs. Sexual Reproduction

• Asexual reproduction involves a single animal passing its genes to offspring, creating a clone.
• Sexual reproduction combines genes from two parents.

Gametes and Heredity

• Gametes are sperm and egg cells.
• Heredity is the passing of traits from one generation to the next; the study of heredity is genetics.
• A locus is the location of a gene on a chromosome.
• A chromatid is coiled, packed chromatin; a duplicated chromatid is called a sister chromatid.

Somatic Cells and Homologous Chromosomes

• Somatic cells are all cells except sex cells, and they contain 46 chromosomes in the nucleus.
• Sex cells (gametes) contain 23 chromosomes.
• Homologous chromosomes are chromosome pairs that are the same length and have the same centromere position and staining patterns; they carry genes controlling the same inherited characters.

Karyotypes and Sex Chromosomes

• A karyotype is an ordered display of chromosomes from a mitotic cell in metaphase.
• Sex chromosomes determine an individual's sex (XX for females, XY for males).
• Autosomes are chromosomes 1-22, with the 23rd pair being the sex chromosomes.

Diploid vs. Haploid Cells

• A diploid cell (2n) has two sets of chromosomes.
• A haploid cell contains a single set of chromosomes, such as a gamete.

Life Cycle and Fertilization

• A life cycle is the generation-to-generation sequence in the history of reproduction.
• Fertilization is the union of gametes (egg and sperm).
• Meiosis reduces the number of chromosome sets from diploid to haploid.

Meiosis I and II

• Mitosis results in two cells, whereas meiosis results in four.
• Meiosis 1 starts with one diploid stem cell with homologous pairs and ends with two haploid cells with sister chromatids.
• Meiosis 2 starts with haploid cells with sister chromatids and ends with four haploid cells.

Meiosis Stages

• Interphase includes G1 (intense growth, new proteins), DNA replication, and more growth.
• Prophase 1 involves the breakdown of the nucleus, formation of spindle fibers, condensation of chromatin into chromosomes, and crossing over.
• Chiasmata are regions where crossing over occurred.
• Synapsis is the pairing of homologous chromosomes in early prophase 1.
• A tetrad is a grouping of four chromatids, aided by synaptonemal complexes during prophase 1.
• Crossing over occurs within a tetrad, where chromosomes exchange pieces.
• Metaphase 1 involves pairs of homologs lining up at the metaphase plate.
• Anaphase 1 involves homologous pairs being pulled apart, with sister chromatids remaining attached.
• Telophase 1 involves the reformation of the nucleus and the disappearance of spindle fibers; animal cells form cleavage furrows, while plant cells form cell plates.
• DNA is not copied again before meiosis 2.
• Prophase 2 starts with haploid stem cells, the nucleus breaks down, and spindle fibers form.
• Metaphase 2 involves sister chromatids lining up in the middle.
• Anaphase 2 involves sister chromatids separating.
• Telophase 2 involves the nucleus reforming and spindle fibers disappearing, resulting in four daughter cells.

Genetic Diversity

• Mutations are changes in an organism's DNA and are the original source of genetic diversity.
• Independent assortment of chromosomes is where homologous pairs randomly align during metaphase 1 of meiosis (2^23 possible combinations).
• Crossing over occurs in prophase 1, producing recombinant chromosomes (mixed-up chromosomes) about 2-3 times per chromosome.
• Random fertilization adds to genetic variation because any sperm can fuse with any egg.

Mitosis vs. Meiosis & Plant life cycle

• Diploid cells can divide by mitosis or meiosis, but haploid cells can only divide by mitosis.
• Zygotes give rise to multicellular, diploid sporophytes, and spores give rise to multicellular, haploid gametophytes.
• Cytokinesis is the division of the cytoplasm, creating two cells.

Medelian Genetics

• Mendel discovered the principles of heredity by studying pea plants.
  • Pea plants had distinct varieties, short generation times, and controlled mating.
• A character (gene) is a feature that varies among individuals.
• A trait (allele) is each variant for a character.
• True breeding refers to homozygous plants that produce the same offspring.
• The blending theory is the outdated idea that reproduction was a blend of both parents' traits.
  • When both alleles are the same it's called, true-breeding, purebred, or homozygous.
  • When both alleles are different it's called, hybrid, cross-bred, heterozygous, or carrier.

Medallion model of inheritance

• Alternative versions of genes exist.
• An organism inherits two alleles for each character, one from each parent.
• The law of dominance states that dominant alleles mask recessive ones.
• The law of segregation states that two alleles separate during anaphase 2.
• A monohybrid cross determines the dominant relationship between two traits (alleles) of one character (gene).

Phenotype vs Genotype

• Phenotype is physical appearance.
• Genotype is the genetic makeup.
• A test cross involves breeding a mystery individual with a homozygous recessive individual.
  • Phenotypic refers to observable characteristics.
  • Genotypic refers to genetic makeup.

Probability Rules

• Probability is the chance that an event will occur.
• Independent assortment states that one event has no effect on another.
• The addition rule states that the probability of one of two or more mutually exclusive events occurring is the sum of their individual probabilities.
• The multiplication rule states that the probability of two or more independent events occurring together is the product of their individual probabilities.
  • Genotypic ratio 1:2:1
  • Phenotypic ratio 3:1
  • Dihybrid ratio 9:3:3:1 (if both parents are heterozygous)

Dominance Patterns and examples

• Complete dominance occurs when phenotypes of the heterozygotes and dominant homozygotes are identical.
  • Polydactyly (extra fingers or toes) and Achondroplasia (dwarfism) are dominant genetic disorders.
  • Huntington's disease is a nervous system disorder.
• Codominance is when both alleles are expressed and both are dominant.

Genetic Interactions

• Pleiotropy is when one gene has multiple effects (or features).
  • Sickle-cell disease affects red blood cell shape, oxygen amount, and blood cell lifespan, straining the liver and spleen.
• Polygenic inheritance involves multiple genes affecting a single trait.
• Quantitative characters (genes) vary in the population along a continuum, indicating polygenic inheritance; height is affected by over 180 genes.
• Epistasis is when one gene at one location affects another gene at another location.
  • For example, the E gene which decides whether or not the color will appear, and the B gene determines what color. So BBee will result in a yellow dog since the e gene effects it.

Pedigrees and Genetic Testing

• A pedigree is a genetic family tree used for predictions.
• Carriers carry an allele but do not express it.
• Multifactorial genes are "kicked off" based on lifestyle.
• Amniocentesis involves taking amniotic fluid and testing it for genetic disorders.

Sex Linked Genes

• A sex-linked gene is a gene that appears on either sex chromosome.
• A Y-linked gene is a gene on the Y chromosome, including the sex-determining gene.
• A Barr body is a condensed, inactive X chromosome.

Genetic Recombination and Chromosomal Alterations

• Genetic recombination is when genes recombine due to crossing over.
• Genes that are far apart are recombined 50% of the time.
• Large-scale chromosome alterations can cause miscarriages.
• Nondisjunction occurs when pairs of homologous chromosomes do not separate during anaphase 1.
• Aneuploidy results from having a non-normal number of chromosomes.
  • Monosomic: A zygote has only one copy of a particular chromosome (3 copies is called a trisomic)
• Polyploidy is when an organism has more than two complete sets of chromosomes.

Chromosomal Disorders

• Klinefelter syndrome occurs when a male is born with XXY chromosomes (1 in 500/1000 births).
  • XYY occurs in 1/1000 males, and XXX occurs in 1/1000 females.
• Turner syndrome is when a female has a single X chromosome (1/2000-4000 births).

Chromosomal Alterations

• Deletion: abcd → acd
• Duplication: abcd → abbcd
• Inversion: abcd → adcb
• Translocation: A non-homologous chromosome crosses with another.
• Deletion of chromosome 5 causes cri du chat or cry of the cat.
• Certain cancers, such as chronic myelogenous leukemia (CML), are caused by translocation of chromosomes 9 and 22.