Mendel’s Genetics
Chapter 3: Sections 1 & 2
Section 1. Mendel’s Work
1. Gregor Mendel is the Father of Genetics. He was the first to record the passing of traits
from one organism to its offspring. His work was the foundation for understanding of
heredity.
2. Heredity is the passing of traits from an organism to its offspring
3. Genetics is the study of heredity.
4. Traits are inherited characteristics or features that an organism has and can pass on to its
offspring through its genes.
5. A gene is a section of DNA that codes for an expression of a trait.
6. Traits are controlled by alleles. An allele is one of a pair of genes for the same trait.
7. Organisms inherit one allele from each parent.
8. Some alleles are dominant over others. A dominant allele is one whose trait always shows
up in the organism when the allele is present.
9. A recessive allele is one whose trait is masked or covered up when the dominant allele is
present.
10. Mendel began his study of genetics with pea plants. He worked to produce plants that were
true breeders – with specific controllable traits generation after generation. Mendel studied
pea plants because pea plants have traits that exist in only two forms (tall stems or short
stems; yellow pods or green pods); they produce a great number of offspring; many
generations can be observed in a short period of time; pea plants can easily be crossed.
11. Purebreds are organisms that always produce offspring with the fame form of a trait as its
parents.
12. Mendel created hybrids by crossbreeding different purebreds. Hybrids are organisms that
have two different alleles for a trait.
13. Mendel used 7 main traits in his genetics experiments. The following are some of the traits
he used: seed shape: Round (R) vs. wrinkled seeds (r), seed color: Yellow (Y) vs. green (y),
Pod shape: smooth (S) vs. pinched (s), stem height: tall (T) vs. short (t).
Section 2. Probability & Genetics
1. Probability is the likelihood that a particular event will happen.
2. Mendel was the first scientist to interpret his findings on the passing of traits of pea plants
from parents to offspring using the principles of probability
3. Geneticists use Punnett squares, charts that show all of the possible outcomes of a genetic
cross. (See your worksheets on Mendel’s Work and Probability for examples. These will be
on the test.)
4. A phenotype is an organism’s physical appearance. (A plant may be tall.)
5. A genotype is an organism’s genetic make-up or combination of alleles. (The genotype for a
tall plant might be TT or Tt.)
6. Organisms are said to be homozygous for a trait if both alleles for the trait are the same. (TT
for a tall plant and tt for a short plant.)
7. Organisms are said to be heterozygous for a trait if each allele for the trait is different. (Tt)
8. Organisms are said to be codominant for a trait when the alleles are neither dominant nor
recessive. As a result, neither allele is masked in the offspring. Codominance creates
organisms like roan cattle whose parents have either red or white hair. The roan offspring
have both white and red hairs and look pinkish brown.
Section 3. The Cell & Inheritance
1. The Chromosome Theory of Inheritance states that genes are carried from parents to their
offspring on chromosomes.
2. Chromosomes are rod-like bundles of coiled DNA.
3. Meiosis is the process by which the number of chromosomes is reduced by half to form sex
cells – sperm and eggs. During meiosis chromosome pairs separate.
4. Sex cells are haploid. They have half the number of chromosomes as body cells.
(Advanced Information: Sex cells are called gametes. Other body cells are called
autosomes.) Body cells are diploid. They have the full number of chromosomes.
5. Only one chromosome from each chromosome pair ends up in each sex cell.
6. A karyotype is a picture of a complete set of an organism’s chromosomes.
Section 4. The DNA Connection
1. The nitrogen bases, Adenine, Thymine, Cytosine and Guanine, form the rungs of the
DNA double helix. These nitrogen bases are arranged to form codes.
2. A, T, C, and G are the letters of the genetic alphabet. In different sequences, these four
letters create the codes that spell out the 20 different amino acids that serve as the
building blocks for all of the proteins that are needed for the functioning of a living
organism.
3. The codes direct the order in which amino acids are put together to form specific proteins.
4. Amino acids are small molecules that are linked together chemically to form proteins.
Amino acids are referred to as the building blocks of proteins.
5. A codon is a group of 3 nitrogen bases that make up the code for a specific amino acid. (See
the amino acid chart given out in class.)
6. Protein synthesis is the process by which cells use information from a gene on a
chromosome to produce a specific protein.
7. During protein synthesis, messenger RNA copies the coded message from the DNA in the
nucleus and carries the message into the cytoplasm.
8. Transfer RNA adds amino acids to form the string-like structure of proteins.
9. Proteins are large organic molecules that are made up of smaller molecules called amino
acids. Although there are only 20 different amino acids they can combine in many ways to
form thousands of different proteins.
10. Cells use proteins for many different things. Proteins form part of the cell membrane.
Proteins make up many of the cell’s organelles. Proteins are used by the body for growth and
repair. Proteins make up certain body parts such as hair. Enzymes, which control many of
life’s activities, are proteins.
11. A mutation is a change in a gene or chromosome. Some mutations are harmful, some are
helpful, and some are neutral.