5 Heredity

Explain how meiosis results in the transmission of chromosomes from one generation to the next.

• Meiosis is a process that ensures the formation of haploid gamete cells, sometimes referred to as daughter cells, in sexually reproducing diploid organisms.
• Meiosis I involves the following steps:
  • i. Prophase I: Homologous chromosomes pair up and condense, synapsis occurs and then chiasmata may form, meiotic spindle begins to form, centrosomes move to opposite poles of the cell, and the nuclear envelope breaks down.
  • ii. Metaphase I: Meiotic spindle fibers align homologous pairs of chromosomes along the equator of the cell at the metaphase plate.
  • iii. Anaphase I: Homologous chromosomes separate, while sister chromatids remain attached, as meiotic spindle fibers pull chromosomes toward poles.
  • iv. T elophase I: Meiotic spindle breaks down, a new nuclear envelope develops, a cleavage furrow (animal cell) or cell plate (plant cell) forms, and cytokinesis occurs. Two haploid daughter cells are formed (at the end of meiosis I).
• Meiosis II involves the following steps:
  • i. Prophase II: Meiotic spindle forms; sister chromatids connected at the centromere attach to meiotic spindle.
  • ii. Metaphase II: Chromosomes align along the metaphase plate; the kinetochore of each chromatid is attached to a microtubule extending from the poles.
  • iii. Anaphase II: Proteins at the centromeres break down, and sister chromatids are pulled apart and toward opposite poles in the cell.
  • iv. T elophase II: Meiotic spindle breaks down, a new nuclear envelope develops, a cleavage furrow (animal cell) or a cell plate (plant cell) forms, chromatids begin to decondense, and cytokinesis occurs. Four haploid daughter cells are formed, each with an unduplicated chromatid.

Describe similarities and differences between the phases and outcomes of mitosis and meiosis.

• Mitosis and meiosis are similar in the use of a spindle apparatus to move chromosomes but differ in the number of cells produced and the genetic content of the daughter cells.

Explain how the process of meiosis generates genetic diversity.

• Correct separation of the homologous chromosomes in meiosis I and sister chromatids in meiosis II ensures that each gamete receives a haploid (1n) set of chromosomes that comprises an assortment of both maternal and paternal chromosomes. When incorrect separation occurs (nondisjunction), gametes are no longer haploid.
• During prophase I of meiosis, non-sister chromatids exchange genetic material via a process called crossing over (recombination), which increases genetic diversity among the resultant gametes.
• Sexual reproduction in eukaryotes increases genetic variation, including crossing over, random assortment of chromosomes during meiosis, and subsequent fertilization of gametes.
  • Exclusion: Knowledge of the details of sexual reproduction cycles in various plants and animals is beyond the scope of the AP Exam.

Explain the inheritance of genes and traits as described by Mendel’s laws.

• Mendel’s laws of segregation and independent assortment can be applied to genes that are on different chromosomes.
• In most cases, fertilization involves the fusion of two haploid gametes, restoring the diploid number of chromosomes and increasing genetic variation in populations by creating new combinations of alleles in the zygote.
  • i. Rules of probability can be applied to analyze the passing of single-gene traits from parent to offspring.
  • ii. Monohybrid, dihybrid, and test crosses can be used to determine whether alleles are dominant or recessive.
  • iii. An organism’s genotype is the set of alleles inherited for one or more genes by an individual organism. An organism’s genotype can be homozygous or heterozygous for each gene.
  • iv. An organism’s phenotype is the observable expression of the inherited traits.

Explain deviations from Mendel’s model of the inheritance of traits.

• Patterns of inheritance of many traits do not follow the ratios predicted by Mendel’s laws and can be identified by quantitative analysis, when the observed phenotypic ratios statistically differ from the predicted ratios.
  • i. Genes located on the same chromosome are referred to as being genetically linked. The probability that these linked genes segregate together during meiosis can be used to calculate the map distance (or map units) between them on a chromosome. This calculation is called gene or genetic mapping.
  • ii. Codominance occurs when the phenotype from both alleles is expressed such that the heterozygote would have a different phenotype than either homozygote.
  • iii. Incomplete dominance occurs when neither allele of a gene can mask the other, so the phenotype of the heterozygote is a blended version of the dominant and recessive phenotypes.
• Some traits, known as sex-linked traits (X- or Y-linked), are determined by genes on sex chromosomes. The pattern of inheritance of sex-linked traits can often be predicted from data, including pedigrees, indicating the genotypes and phenotypes of both parents and offspring.
• Pleiotropy is a phenomenon in which the expression of a single gene results in multiple traits or effects; these traits therefore do not segregate independently.
• Some traits result from non-nuclear inheritance.
  • i. Chloroplasts and mitochondria are randomly assorted to gametes and daughter cells; thus, traits determined by chloroplast and mitochondrial DNA do not follow simple Mendelian rules.
  • ii. In animals, mitochondria are usually transmitted by the egg and not by sperm; thus, traits determined by the mitochondrial DNA are typically maternally inherited.
  • iii. In plants, mitochondria and chloroplasts are transmitted in the ovule and not in the pollen; as such, mitochondria-determined and chloroplast-determined traits are typically maternally inherited.

Explain how the same genotype can result in multiple phenotypes under different environmental conditions.

• Environmental conditions influence gene expression and can lead to phenotypic plasticity (e.g., the ability of individual genotypes to produce different phenotypes).