BIOB11 – Lecture 1 - Chromosomes, Heredity, Meiosis and Recombination
Diploid: Containing both members of each pair of homologous chromosomes, seen in most somatic cells. Diploid cells are
produced from diploid parental cells during mitosis. Two copies of each gene.
Haploid: Containing only one member of each pair of homologous chromosomes. Haploid cells are produced during
meiosis as seen in sperm. (gametes are haploid sperm and egg donate only one)
Allele: Alternate forms of the same gene.
Genotype: Genetic composition of organism.
Phenotype: Observable traits or morphology
Dominant: “wildtype” RR
Meiosis: part of gametogenesis in which sperm and eggs are produced.
Homologous Chromosomes: Paired chromosomes of diploid cells, each carrying one of the two copies of the genetic
material carried by that chromosome.
Bivalent (tetrad): The complex formed during meiosis by a pair of synapsed homologous chromosomes. Paired
Recombinant DNA: Molecules containing DNA sequences from more than one source.
Reciprocal genetic exchange: Reciprocal patterns of labelling on two non-sister chromatids
is indicative of breakage and reunion of exchanged segments.
Homozygous: both alleles are identical (eg: dominant). If dominant, generally the “wildtype” RR
Heterozygous: alleles differ (one dominant, one recessive = Rr)
Nulls: Homozygous recessive (rr). Generally associated with the”mutant” trait.
Conduct a cross: sperm and eggs get together. This is easy to do in plants as many are self fertile and will do the work for
Synaptonemal Complex (SC): A ladderlike structure composed of three parallel bars with many cross fibers. The SC holds
each pair of homologous chromosomes in the proper position to allow the continuation of genetic recombination
between strands of DNA.
Non-disjunction: improper segregation
Down Syndrome or Trisomy 21: A chromosome complement that has one extra chromosome; ie. A third homologous
chromosome (causing Down Syndrome)
Autosomes: not a sex chromosome
Brief History of Genetics (will not ask dates and names)
-2013: many genomes are completely sequenced
-“bioinformatics” comes of age
The earliest studies revealed genes to be discrete factors
that were retained throughout the life of an organism
and then passed on to each of its progeny. Over the
following century, these hereditary factors were shown
to reside on chromosomes and to consist of DNA, a
macromolecule with extraordinary properties.
-Genome: the collective body of genetic information that
is present in a species.
-Most organisms are diploid
10.1 The Concept of a Gene as a Unit of Inheritance
-In the 1860’s, Gregory Mendel was the start of the science of
genetics. His goal was to mate, or cross, pea plants having different
inheritable characteristics and to determine the pattern by which
these characteristics were transmitted to the offspring. Mendel chose
to focus on seven clearly definable traits, including height, seed color,
and seed shape, each of which occurred in two alternate and clearly
identifiable forms as the Dominant Allele and the Recessive Allele
forms (eg: for the seed shape, the dominant allele = round (R) and the
recessive allele = wrinkled (r)).
- After several years of research, Mendel drew the following conclusions:
1. The characteristics of the plants were governed by distinct factors (or units) of inheritance, which were later termed
genes. An individual plant possessed two copies of a gene that controlled the development of each trait, one derived
from each parent. The two copies could be either identical to one another or nonidentical. Alternate forms of a gene are
called alleles. For each of the seven traits studied, one of the two alleles was the dominant over the other. When both
were present together in the same plant, the existence of the recessive allele was masked by the dominant one.
2. Each reproductive cell (or gamete) produced by a plant contained only one copy of the gene for each trait. A particular
gamete could have either the recessive or dominant allele for a given trait, but not both. Each plant arose by the union of
a male and female gamete. Consequently, one of the alleles that governed each trait in a plant was inherited from the
female parent, and the other allele was inherited from the male parent.
3. Even though the pair of alleles that governed a trait remained together throughout the life of an individual plant, they
became separated (or segregated) from one another during the formation of the gametes. This finding formed the basis
of Mendel’s law of segregation.
4. The segregation of the pair of alleles for one trait had no effect on the segregation of alleles for another trait. A
particular gamete, for example, could receive a paternal gene governing seed color and a maternal gene governing seed
shape. This finding formed the basis of Mendel's Law of independent assortment.
- Mendel: Following the round vs wrinkled seed phenotype 10.2 Chromosomes: The Physical Carriers of the Genes
The Discovery of Chromosomes:
- All the genetic information needed to build and maintain a complex plant or animal had to fit within the
boundaries of a single cell.
- During cell division, the material of the nucleus became organized into visible “threads,” which were named
chromosomes, meaning “colored bodies.”
- Even though the sperm is a tiny cell, it was known to be as important genetically as the much larger egg.
- The most apparent feature in common between both the egg and the sperm were the nucleus and its
- Theodore Boveri: a Sea urchin egg had been fertilized by two sperm rather than just one, as is normally the case
(this is called polyspermy and is characterized by disruptive cell division and early death of the embryo.