GENETICS: Mendel and Meiosis
Genetics—the scientific study of heredity
Modern genetics is based on the knowledge that traits are controlled by
genes located on chromosomes.
CHARACTERISTICS OF MEIOSIS:
1. Occurs only in gonads
2. Produces cells called gametes that have half the normal chromosome
number for that species. Haploid is the term used to describe the
chromosome number in this type of cell.
3. A two stage cell division.
a. Meiosis one—involves the formation of tetrads
and is the reduction division, that is, the division
that makes the resulting daughter cells haploid.
Chromosomes do not break apart at the
centromeres during the first meiotic division as
they do in mitosis. Crossing over may occur early
in this stage of meiosis.
b. Meiosis two—involves the separation of the
chromatids and formation of the actual daughter-
cells or gametes.
4. Types of meiosis include:
a. Spermatogenesis—production of the male gametes in
humans, called sperm. Four full-sized daughter cells
produced per mother cell (primary spermatocyte) in this type
b. Oogenesis—production of the female gametes in
humans, ova (eggs). Produces only one full-sized
daughter cell per mother cell (primary oocyte) in this type of
5. The union of a haploid sperm and a haploid ovum
produces a diploid (containing the normal number of
chromosomes for a species) zygote which develops into
a new individual of that species.
MENDEL & MEIOSIS
Gregor Mendel—father of modern genetics. Austrian monk that discovered
the basic principles of heredity in his work with garden peas.
Why did Mendel choose peas?
1. Readily available
2. Could be cultivated quickly
3. Produced large numbers of offspring
4. Easily cross/selectively bred
5. Displayed several traits in one of two contrasting forms
1. Cross-pollinated two plants with contrasting traits (P generation)
2. One of the traits seemed to have disappeared in offspring (F1
3. Cross pollinated members of the F1 generation
4. Trait that had disappeared, presented itself once more in the
offspring of the second cross (F2 generation)
5. Mendel reasoned that each trait was controlled by two “factors”,
one being hidden in the F1 generation. Today we call these
Mendel’s three principles of inheritance:
Principle of Dominance—one gene in a pair may prevent the other
gene form being expressed. This gene is said to be dominant. The
gene that is hidden is said to be recessive. Terms associated with this
1. Homozygous (purebred)—both genes in a pair
2. Homozygous dominant—two dominant genes in a
3. Homozygous recessive—two recessive genes in a
4. Heterozygous—one dominant and one recessive
gene in a pair (hybrid)
Principle of Segregation—two genes for the same trait segregate or
separate during meiosis (gamete formation).
Principle of Independent Assortment—genes segregate independently
of each other during meiosis (gamete formation)
Terminology associated with Mendelian genetics:
1. Allele—alternate gene form for each trait
2. Genotype—combination of alleles present for a trait
3. Phenotype—appearance of a trait as determined by a certain
4. Monohybrid cross—a cross involving the study of the inheritance
of one trait
5. Dihybrid cross—a cross involving the study of the inheritance of
Incomplete dominance—occurs when neither of the alleles for a trait is
dominant over the other. In incomplete dominance, the traits seem to blend
to form an intermediate form of the trait, such as the pink flower of the four-
Multiple alleles—although each organism normally has only two alleles for
each trait, more than two alleles may be possible for that trait in a
population. In humans, blood type is controlled by three alleles. The alleles
for type A and type B blood are both dominant over the allele for type 0
blood, therefore, types A and B are said to be codominant.
Test cross—the cross of an organism with an unknown dominant genotype
with an organism that is homozygous recessive for that trait