Transcript Genetics: The Science of Heredity

Genetics: The Science of Heredity - Mendel’s Work
Crossing Pea Plants
Gregor Mendel crossed pea plants that had different traits.
The illustrations show how he did this.
Genetics: The Science of Heredity - Mendel’s Work
Mendel’s Experiments
In all of Mendel’s crosses, only one form of the trait appeared
in the F1 generation. However, in the F2 generation, the “lost”
form of the trait always reappeared in about one fourth of the
plants.
Genetics: The Science of Heredity - Mendel’s Work
Dominant and Recessive Alleles
Mendel studied several traits in pea plants.
Genetics: The Science of Heredity - Probability and Heredity
Percentages
One way you can express a probability is as a percentage. A
percentage (%) is a number compared to 100. For example,
50% means 50 out of 100.
Suppose that 3 out of 5 tossed coins landed with heads up.
Here’s how you can calculate what percent of the coins
landed with heads up.
1. Write the comparison as a fraction.
3 out of 5 = 3/5
2. Multiply the fraction by 100% to express it as a
percentage.
3/5 x 100%/1 = 60%
Genetics: The Science of Heredity - Probability and Heredity
Percentages
Practice Problem
Suppose 3 out of 12 coins landed with tails up. How can you
express this as a percent?
25%
Genetics: The Science of Heredity - Probability and Heredity
A Punnett Square
The diagrams show how to make a Punnett square. In this
cross, both parents are heterozygous for the trait of seed
shape. R represents the dominant round allele, and r
represents the recessive wrinkled allele.
Genetics: The Science of Heredity - Probability and Heredity
Probability and Genetics
In a genetic cross, the allele that each parent will pass on to
its offspring is based on probability.
Genetics: The Science of Heredity - Probability and Heredity
What Are the Genotypes?
Mendel allowed several F1 pea
plants with yellow seeds to selfpollinate. The graph shows the
approximate numbers of the F2
offspring with yellow seeds and
with green seeds.
Genetics: The Science of Heredity - Probability and Heredity
What Are the Genotypes?
Reading Graphs:
How many F2 offspring had
yellow seeds? How many had
green seeds?
Yellow–6,000; green–2,000
Genetics: The Science of Heredity - Probability and Heredity
What Are the Genotypes?
Calculating:
Use the information in the
graph to calculate the total
number of offspring that
resulted from this cross. Then
calculate the percentage of
the offspring with yellow peas,
and the percentage with
green peas.
8,000; 75% have yellow peas
and 25% have green peas.
Genetics: The Science of Heredity - Probability and Heredity
What Are the Genotypes?
Inferring:
Use the answers to Question
2 to infer the probable
genotypes of the parent
plants. (Hint: Construct Punnett
squares with the possible
genotypes of the parents.)
Both parents probably had
the genotype Bb.
Genetics: The Science of Heredity - Probability and Heredity
Phenotypes and Genotypes
An organism’s phenotype is its physical appearance, or
visible traits, and an organism’s genotype is its genetic
makeup, or allele combinations.
Genetics: The Science of Heredity - Probability and Heredity
Codominance
In codominance, the alleles are neither dominant nor
recessive. As a result, both alleles are expressed in the
offspring.
Genetics: The Science of Heredity - The Cell and Inheritance
Meiosis
During meiosis, the chromosome pairs separate and are
distributed to two different cells. The resulting sex cells have
only half as many chromosomes as the other cells in the
organism.
Genetics: The Science of Heredity - The Cell and Inheritance
Punnett Square
A Punnett square is actually a way to show the events that
occur at meiosis.
Genetics: The Science of Heredity - The Cell and Inheritance
A Lineup of Genes
Chromosomes are made up of many
genes joined together like beads on a
string. The chromosomes in a pair may
have different alleles for some genes
and the same allele for others.
Genetics: The Science of Heredity - The Cell and Inheritance
Identifying Supporting Evidence
As you read, identify the evidence that supports the
hypothesis that genes are found on chromosomes. Write
the evidence in a graphic organizer.
Grasshoppers:
24 chromosomes in body
cells, 12 in sex cells.
Chromosomes are
important in
inheritance.
Fertilized egg has
24 chromosomes.
Alleles exist in
pairs in
organisms.
Genetics: The Science of Heredity - The Cell and Inheritance
Chromosomes
Click the Video button to watch a movie about chromosomes.
Genetics: The Science of Heredity - The DNA Connection
The DNA Code
Chromosomes are made of DNA. Each chromosome
contains thousands of genes. The sequence of bases in a
gene forms a code that tells the cell what protein to produce.
Genetics: The Science of Heredity - The DNA Connection
How Cells Make Proteins
During protein synthesis, the cell uses information from a
gene on a chromosome to produce a specific protein.
Genetics: The Science of Heredity - The DNA Connection
Mutations
Mutations can cause a cell to produce an incorrect protein
during protein synthesis. As a result, the organism’s trait, or
phenotype, may be different from what it normally would
have been.
Genetics: The Science of Heredity - The DNA Connection
Sequencing
Sequence is the order in which the steps in a process occur.
As you read, make a flowchart that shows protein synthesis.
Put each step in the flowchart in the order in which it occurs.
Protein Synthesis
DNA provides code to form messenger RNA.
Messenger RNA attaches to ribosome.
Transfer RNA “reads” the messenger RNA.
Amino acids are added to the growing protein.
Genetics: The Science of Heredity - The DNA Connection
Protein Synthesis
Click the Video button to watch a movie
about protein synthesis.
Genetics: The Science of Heredity
Graphic Organizer
RNA
includes
Messenger RNA
Transfer RNA
functions to
functions to
Copy the coded
message from the
DNA
Carry the message
to the ribosome in
the cytoplasm
Carry amino acids
Add amino acids
to the growing
protein