Section 6-4
“Traits &
genes”
Write everything that is
BLACK
6.4 Traits, Genes, and Alleles
KEY CONCEPT
Genes encode proteins that produce a
diverse range of traits
6.4 Traits, Genes, and Alleles
State Standards
• 4. Genes are a set of instructions encoded in the DNA
sequence of each organism that specify the
sequence of amino acids in proteins characteristic of
that organism. As a basis for understanding this
concept:
– D. Students know specialization of cells in multi
cellular organisms is usually due to different
patterns of gene expression rather than to
differences of the genes themselves.
6.4 Traits, Genes, and Alleles
The same gene can have many versions
• Alleles are made up of
genes. A Gene is a piece of
DNA that directs a cell
to make a certain protein
• Each gene has a locus,
a specific position
on a pair of
homologous chromosomes
6.4 Traits, Genes, and Alleles
• Again a gene is a piece of DNA
that directs a cell to make a
certain
protein
DNA Brain POP w/
activity
6.4 Traits, Genes, and Alleles
• An allele is an alternative form of a gene
• Each parent donates one allele for every
gene
• Pea plants have 2
alleles for shape - they
receive one from each
parent
• For shape, there is a
wrinkled allele and a
round allele
6.4 Traits, Genes, and Alleles
• What are proteins made of?
• Proteins are made up of smaller building blocks
called amino acids,
joined together in
chains. There are 20
different amino acids.
Some proteins are just
a few amino acids long,
while others are made
up of several
thousands.
6.4 Traits, Genes, and Alleles
6.4 Traits, Genes, and Alleles
Gene Expression: Transcription
The majority of genes are expressed as the proteins they encode. The
process occurs in two steps:
Transcription = DNA → RNA
Translation = RNA → protein
Taken together, they make up the "central dogma" of biology:
DNA → RNA → protein.
6.4 Traits, Genes, and Alleles
Genes influence the development of traits
• All of an organism’s genetic
material is called the genome
• A genotype refers to the actual
genes
• A phenotype is the physical
expression of a trait
6.4 Traits, Genes, and Alleles
Genotype:
• The gene combination of an
organism
• It consists of 2 alleles
• For example:
– Pure dominant, 2 dominant genes
– Pure recessive, 2 recessive genes
– Hybrid, 1 dominant and 1 recessive
gene
6.4 Traits, Genes, and Alleles
Phenotype:
• The way an organism
looks
• No matter what genes are
present, phenotype of a
tall pea plant is tall and a
short pea plant is short
6.4 Traits, Genes, and Alleles
– A dominant allele is expressed
as a phenotype when at least
one allele is dominant
– A recessive allele is expressed
as a phenotype only when two
copies are present
–Dominant alleles are
represented by
uppercase letters
–Recessive alleles by
lowercase letters
6.4 Traits, Genes, and Alleles
Mendel’s observed traits
• Dominant traits: covers up the
other traits
• Recessive traits: traits that
“disappear”
• Rule of Dominance: When a
dominant trait is present the
recessive trait is hidden
6.4 Traits, Genes, and Alleles
Step 1:
Label the dominant
and recessive traits
R = Red flower
r = Purple flower
6.4 Traits, Genes, and Alleles
Step 2:
Write the genotypes of each
parent to be crossed
Purple
Hybrid Red
(means heterozygous)
Rr
(since it is recessive it
has to be homozygous)
Rr x rr
rr
6.4 Traits,
and Alleles
Step Genes,
3:
Set up the punnett square.
Each parent will give up one gene
or the other so there will be one
gene per box for each parent
R
r Rr
r
rr
r Rr
rr
6.4 Traits, Genes, and Alleles
Step 4:
Analyze the genotypes
R
r Rr
r Rr
r
rr
rr
2 of the 4 (50%)
offspring will be
pure recessive
2 of the 4 (50%)
will be hybrids
6.4 Traits, Genes, and Alleles
Step 5:
Analyze the phenotypes
R
r Rr
r
rr
r Rr
rr
Punnett Square Handout
50% of the
offspring
will be red
50% of the
offspring
will be purple
6.4 Traits, Genes, and Alleles
DNA Structure and Function
DNA Model Lab
• DNA contains the genetic information that codes for the RNA
and proteins necessary for cell function.
• All DNA in the chromosomes has to be copied (replicated) and
transmitted to daughter cells via mitosis.
• Non-faithful replication or inability to correct errors and damage
to DNA results in mutations. A= Adenine
• Structure of DNA
T= Thymine
Four nucleotides (A,T,C,G) are C= Cytosine
chemically joined through sugar
and phosphate molecules in the G= Guanine
backbone.
RNA ONLY
During synthesis of mRNA
U= Uracil
(where U is used instead of T)
6.4 Traits, Genes, and Alleles
6.4 Traits, Genes, and Alleles
RNA carries DNA’s instructions
• The central dogma is the
basic concept proposed
by Crick (one of the
scientists that discovered
DNA)
• It states that:
information flows in
one direction from
DNA to RNA to
proteins
6.4 Traits, Genes, and Alleles
• The central dogma includes three processes:
Brain POP- Mutation
– Replication
– Transcription
– Translation
Transcription = DNA → RNA
Translation = RNA → protein
replication
transcription
translation
• RNA is a link
between DNA and
Mistake in Replication = Mutation
proteins
6.4 Traits, Genes, and Alleles
• RNA differs from DNA in 4 ways:
– RNA has a ribose sugar
– RNA has uracil instead of thymine
– RNA is a single-stranded structure
– RNA can leave the nucleus and go into the
cytoplasm DNA can’t
6.4 Traits, Genes, and Alleles
Transcription makes three types of
RNA
• Transcription copies
DNA to make a strand
of RNA
6.4 Traits, Genes, and Alleles
– The RNA strand detaches from
the DNA once the gene is
transcribed
RNA
6.4 Traits, Genes, and Alleles
Amino acids are coded by mRNA base sequences
• Translation converts mRNA messages into
polypeptides (protein)
• A codon is a sequence of three nucleotides that
codes for an amino acid
codon for
codon for
Methionine (Met) Leucine (Leu)
6.4 Traits, Genes, and Alleles
6.4 Traits, Genes, and Alleles
• A change in the order in which codons are read
changes the resulting protein
• Regardless of the organism, codons code for the
same amino acid
6.4 Traits, Genes, and Alleles
Amino acids are linked together by a peptide
bond to become a protein
• An anticodon is a set of three nucleotides that is
complementary (opposite) to an mRNA codon
• An anticodon is carried by a tRNA transfer RNA
(translation)
6.4 Traits, Genes, and Alleles
Transcription vs. Translation Review
Transcription
• Process by which
genetic
information
encoded in DNA
is copied onto
messenger RNA
• Occurs in the
nucleus
• DNA
mRNA
(DNA unzipped)
Translation
• Process by which
information encoded
in mRNA is used to
assemble a protein
at a ribosome
• Occurs on a
Ribosome
• mRNA
protein
6.4 Traits, Genes, and Alleles
Why are proteins important?
• Well, for starters, you are made of proteins. 50% of
the dry weight of a cell is protein of one form or
another. Meanwhile, proteins also do all of the heavy
lifting in your body: digestion, circulation, immunity,
communication between cells, motion-all are made
possible by one or more of the estimated 100,000
different proteins that your body makes.
• Here are several examples of how RNA codes for
amino acids. These codons are for tryptophan, the
amino acid that contributes to the sleepy feeling you
may have after eating turkey; phenylalanine, an
amino acid used in the aspartame sweetener in diet
soda; tyrosine, an important amino acid in
intracellular signaling processes; and cystine.
6.4 Traits, Genes, and Alleles
• Genes control the genetic traits, and genes are DNA,
which is organized into chromosomes
6.4 Traits, Genes, and Alleles
Codon
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Lab
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6.4 Traits, Genes, and Alleles
Brain POP’s
• Genetics w/ vocab sheet
• Genetic Mutations w/ vocab sheets