7 SCIENCE
CHAPTER 5 - NOTES
GENETICS
Lesson 1: Mendel and his peas
Heredity: the passing of traits from parents to offspring.
Genetics: The study of how traits are passed from parents to offspring.
True-Breeding Plants: when a true-breeding plant self-pollinates, it always produces offspring with
traits that match the parent.
Mendel’s Conclusions: TWO genetic factors control each inherited trait. Hel also proposed that when
organisms reproduce, each reproductive cell-sperm or egg- contributes one factor for each trait.
Dominant trait: a genetic factor that blocks another genetic factor; it is the only factor seen or
expressed.
Recessive trait: a genetic factor that is blocked by the presence of a dominant factor; this is only
observed when two recessive genetic factors are present in offspring.
Lesson 2 – Understanding Inheritance
Chromosomes: contain genetic information that controls traits, exist in pairs, and one chromosome
within the pair is from each parent.
Gene: a section on a chromosome that has genetic information for one trait.
Alleles: different forms of a gene.
Phenotype: how a trait appears or is expressed.
Genotype: the two alleles that control the phenotype of a trait.
Homozygous: when the two alleles or a gene are the same (ie: both RR or rr).
Heterozygous: the two alleles of a gene are different (ie: Rr).
Punnett Squares: a model used to predict possible genotypes and phenotypes of offspring.
Example:
Genotype of parent 1: Rr
Genotype of parent 2: Rr
R
RR
Rr
R
r
r
Rr
rr
The offspring in the above example can have one of three genotypes-RR, Rr, or rr.
The ratio of genotypes is written as 1:2:1.
The ratio of phenotypes is written as 3:1 because R is dominant and expressed in three of the
offspring.
Incomplete Dominance: Alleles show incomplete dominance when the offspring’s phenotype is a
combination of the parent’s phenotypes (Red parent crossed with a White parent and the offspring is
pink).
Codominance: when both alleles can be OBSERVED in a phenotype, this type of interaction is called
codominance.
Polygenetic inheritance: occurs when multiple genes determine the phenotype of a trait.
Lesson 3 – DNA and Genetics
DNA: Chromosomes are made of proteins and deoxyribonucleic acid (DNA), which is an organism’s
genetic material. A gene is a segment of DNA on a chromosome
A DNA double helix is made of two strands of DNA. Each strand is a chain of nucleotides.
Nucleotide: a molecule made of a nitrogen base, a sugar, and a phosphate group.
Sugar-phosphate groups form the sides of the DAN zipper
Nitrogen bases bond and form the teeth of the zipper
Four nitrogen bases:
A.
B.
C.
D.
Adenine (A)
Cytosine (C)
Thymine (T)
Guanine (G)
A and T always bond together, and C and G always bond together.
DNA replication
Every time a cell divides, all chromosomes must be copied for the new cell. The new DNA is identical to
existing DNA.
Replication: the process of copying a DNA molecule to make another DAN molecule.
The steps of DNA replication:
1. DNA strand separates and nitrogen bases are exposed.
2. Nucleotides move into place and form new nitrogen base pairs.
3. Two identical strands of DNA are produced.
Role of RNA in making proteins
Proteins are made with the help of Ribonucleic Acid (RNA) – a type of nucleic acid that carries the code
for making proteins from the nucleus to the cytoplasm. RNA also carries amino acids around inside a
cell and forms a part of ribosomes.
RNA is made of nucleotides – key differences between RNA and DNA:
1. RNA is single stranded while DNA is double stranded.
2. RNA has the nitrogen base uracil (U) instead of thymine (T) and the sugar ribose instead of
deoxyribose.
Transcription: the first step in making a protein is to make mRNA (messenger RNA) from DNA, this
process is called transcription.
Three types of RNA:
1. Messenger RNA – mRNA
2. Transfer RNA – tRNA
3. Ribosomal RNA – rRNA.
Translation: The process of making a protein from RNA.
Proteins are made of amino acids.
The order of the nitrogen bases in mRNA determines the order of the amino acids in a protein.
Three nitrogen bases on mRNA form the code for one amino acid.
Each series of three nitrogen bases on mRNA is called a codon.
There are 64 codons, but only 20 amino acids.
One codon signals that translation should start,
Three codons do not code for any amino acid – they code for the end of the proteinthey signal that translation should stop.
Mutation: A change in the nucleotide sequence of a gene.
Types of Mutations:
1. Deletion - one or more nitrogen bases are left out of the DNA sequence.
2. Insertion - one or more nitrogen bases are added to the DNA.
3. Substitution – one nitrogen base is replaced by a different nitrogen base.
Genetic Engineering: a change to an organism’s genetic material that does not naturally occur.