1. The science of heredity
Chapter 3
The science of heredity

2. Section 1: Mendel’s work
Objectives:
Describe the results of Mendel’s experiments.
Identify what controls the inheritance of traits in organisms.
Key terms:
Heredity
Trait
Genetics
Fertilization
Purebred
Gene
Alleles
Dominant allele
Recessive allele
hybrid

3. Gregor mendel
Mendel was a mid 19 century priest who tended a garden in a central European monastery.
His work revolutionized heredity.
Heredity: the passing of physical characteristics from parents to offspring.
Traits: a characteristic that an organism can pass on to its offspring through its genes.
Genetics: the scientific study of heredity.

4. Mendel’s experiments
Fertilization: the process of joining the genetic materials in an egg and sperm.
Crossing Pea Plants:
Purebred: an organism that is the offspring of many generations that have the same trait.
Mendel selected purebred pea plants with contrasting traits.
He then crossbred these plants with each other and observed the results.
The F1 offspring:
In Mendel’s first experiment, he crossed purebred tall plants with purebred short plants.
This is the parent generation or P generation and they produced the F1 generation.
The F1 generation were all tall plants.
The F2 offspring:
Mendel cross bred the F1 generation with each other and the offspring are the F2 generation.
Three fourths of the F2 generation were tall and one fourth were short (3:1 ration)
Experiments with other traits:
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.

5. Dominant and recessive alleles
One of Mendel’s many conclusions was individual factors, or sets of genetic information must control the inheritance of traits in peas.
The factors that control each trait occur in pairs…one trait from the female and one trait from the male.
One factor can mask the other factor (tall vs short).
Genes and Alleles:
Genes: the scientific word for the factors that control a trait.
Alleles: the different forms of a gene.
An organism’s traits are controlled by the alleles it inherits from its parents.
Dominant allele: traits that always show up in the organism when the allele is present.
Recessive allele: an allele that is hidden when a dominant allele is present.

6. Dominant and recessive alleles
2. Alleles in Mendel’s Crosses:
In Mendel’s P generation, each plant had two alleles for the trait expressed (tall plants had the same two alleles and short plants also had the same two alleles).
Hybrid: an organism that has two different alleles for a trait.
All the F1 generation were tall, but they possessed the short allele.
3. Symbols for Alleles:
Dominant alleles are represented by a capital letter (T for tall).
Recessive alleles is represented by the lowercase version of the letter (t for short).
4). Significance of Mendel’s Contribution
Changed scientists’ ideas about heredity.
Most people thought an organism was a blend of its parents (medium sized plants should have been produced).

7. Section 2: probability and heredity
Objectives:
Define probability and describe how it helps explain the results of genetic crosses.
Explain the results of genetic crosses.
Explain what is meant by genotype and phenotype.
Tell what codominance is.
Key terms:
Probability
Punnett square
Phenotype
Genotype
Homozygous
Heterozygous
codominance

8. Principals of probability
Probability: a number that describes how likely it is that an event will occur.
Mathematics of probability:
Each time you toss a coin, there are two possible ways that the coin can land (heads up or tails up).
The probability that a coin toss will land with heads up is 1 in 2.
The law of probability predict what is likely to occur, not necessarily what will occur.
Independence of events:
When you toss a coin more than once, the results of one toss do not affect the results of another toss.
Each event occurs independently.
If you toss a coin 5 times and all five time land as heads, the next toss is still a 1 in 2 chance it will land as tails.

9. Probability in genetics
Punnett squares: a chart that shows all the possible combinations of alleles.
Using a Punnett square
In a genetic cross, the allele that each parent will pass on to its offspring is based on probability.
Predicting probability.
Punnett squares are very useful in predicting probability. Examples:

10. Phenotypes and genotypes
Phenotype: the organisms physical appearance, or visible traits.
Genotype: the organisms genetic makeup.
Homozygous: an organism that has two identical alleles for a trait.
BB for brown eyes and bb for blue eyes.
Heterozygous: an organism that has two different alleles for a trait.
Brown eyes with Bb alleles.

11. Codominance
In codominance, alleles are neither dominant nor recessive. As a result, both alleles are expressed in the offspring.

12. Section 3: The cell and inheritance
Objectives:
Describe the role chromosomes play in inheritance.
Identify the events that occur during meiosis.
Explain the relationship between chromosomes and genes.
Key term:
Meiosis

13. Chromosomes and inheritance
Walter Sutton had a hypothesis that chromosomes were important in the inheritance of traits.
He found evidence in grasshoppers’ cells.
A grasshoppers body cell has 24 chromosomes, but the grasshoppers’ sex cells only have 12 chromosomes.
Chromosome pairs:
Sutton observed what happened when a sperm cell and an egg cell joined during fertilization.
The fertilized egg that formed had 24 chromosomes.
The 24 chromosomes existed in 12 pairs. One pair from the male and one pair from the female.
Genes on chromosomes:
According to the chromosome theory or inheritance, genes are carried from parents to their offspring on chromosomes.

14. Meiosis
Meiosis: the process by which the number of chromosomes is reduced by half to form sex cells.
What happens during meiosis:
During Meiosis, the chromosome pairs separate and are distributed to different cells.
The resulting sex cells have only half as many chromosomes as the other cells in the organism.
Meiosis and Punnett Squares:

15. Meiosis

16. Lineup of Genes
Chromosomes are made up of many genes joined together like beads on a string.
Although your body contains 23 pairs of chromosomes, your body cells each contain between 20,000 and 25,000 genes.
Each gene controls a trait.
Genes are in the same order, but the alleles are different.
One chromosome came from the female and the other came from the male.

17. Section 4: The dna connection
Objectives:
Understand the forms of the genetic code.
Know the steps of how a cell produces a protein.
Explain how a mutation can affect on organism.
Key terms:
Messenger RNA
Transfer RNA

18. The Genetic Code
The main function of genes is to control the production of proteins.
Proteins help to determine the size, shape, color, and many other traits of an organism.
Genes and DNA:
Chromosomes are composed mostly of DNA.
DNA is made up of four different nitrogen bases (A, T, C, G).
A gene is a section of DNA that contains the info to create code for one specific protein.
Order of the bases:
The order of the nitrogen bases along a gene forms a genetic code that specifies what type of protein will be produced.
Protein are long-chained molecules made of individual amino acids.
Amino Acids are made of three base codes (ATC)

20. How cells make proteins
During protein synthesis, the cell uses information from a gene on a chromosome to produce a specific protein.
The role of RNA:
Before protein synthesis can take place, a messenger must carry the genetic code from DNA to the outside of the nucleus.
RNA (two major differences from DNA)
Single strand
Uracil instead of thymine
Types of RNA:
Messenger RNA (mRNA): this copies the coded message from the DNA molecule inside the nucleus and carries it to the cytoplasm.
Transfer RNA (tRNA): this carries amino acids to the ribosome and adds them to the growing protein.

21. How cells make proteins
3. Translating the code:
DNA unzips between the base pairs and is copied for RNA (or mRNA).
mRNA then leaves the nucleus to the cytoplasm
mRNA attaches to a ribosome and is given the code for a protein.
Molecules of tRNA are attached to the mRNA in a three base pair sequence (amino acid).
The protein molecule grows longer as each tRNA molecule puts the amino acid sequence it is carrying along the growing protein chain.

23. 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.
Types of mutations:
A change in hereditary material.
One base substituted for another.
Chromosomes don’t separate correctly during meiosis.
Effects of mutations:
Genetic variety.
Can be helpful or harmful