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Genetics: The Study of Heredity

Science, Grade 6

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Table Of Contents: Genetics: The Study of Heredity

1. How Traits Are Inherited

2.1. What Is Heredity?
Heredity is the passing of traits or characteristics from one generation to the next.
2.2. Fertilization
When an egg and sperm come together at fertilization, the embryo receives a copy of each parent's chromosomes.
2.3. Chromosomes Inherited from Each Parent
Humans have 46 chromosomes that are present in 23 pairs. One chromosome in each pair is inherited from the mother, and the other from the father.
2.4. Two Copies of Each Gene
The inherited genes on these chromosomes contain DNA sequences that are related to particular traits. A person inherits two copies of each gene.
2.5. Alternate Gene Forms
Genes exist in alternate forms called alleles. For example, the gene that determines eyebrow shape has two alleles—one codes for bushy eyebrows and the other fine eyebrows.
2.6. Homozygous and Heterozygous Alleles
A person can inherit two of the same alleles, called homozygous. Or a person can inherit two different alleles, called heterozygous.

2. Gregor Mendel

3.1. Significance of Mendel's Experiments
In the mid 1800s Gregor Mendel performed pea plant experiments to learn about heredity and traits. These experiments laid a foundation for our modern understanding of genetics.
3.2. Why Use the Pea Plant?
Mendel used the garden pea plant for breeding experiments because it had easy to recognize traits and reproduced quickly. The pea plant could also produce offspring using self-pollination or cross-pollination.
3.3. Two Forms of Pea Plant Characteristics
Mendel initially focused on the inheritance of single characteristics that only occurred in two different forms. For example, pea plants were either tall or short.

3. Mendel's Experiments

4.1. True-Breeding Plants
For his experiments, Mendel used true-breeding plants, called purebreds. The offspring of purebreds always had the same trait as the parent.
4.2. P Generation Cross
Mendel tracked a characteristic over three generations. First he crossed two purebreds that had different forms of a characteristic. This is called the parental, or P generation.
4.3. F1 Offspring
All the first generation (F1) offspring looked the same, displaying only one of the parental forms.
4.4. F1 Generation Cross
The first generation (F1) plants were crossed and the offspring displayed both forms of the characteristic.
4.5. F2 Generation
Seventy-five percent (75%) of the F2 generation plants displayed the same form as the F1 parents, and 25% were the other form. These results were consistent when he repeated the crosses for each characteristic.
4.6. Mendel's Hypothesis
Mendel hypothesized that for each characteristic, plants inherit a unit of information from each parent. These units of information control the form of the characteristic in the offspring.
4.7. Modern Understanding of Mendel's Hypothesis
We now understand that these units of information are genes and they exist in pairs called alleles. The form of the characteristic is known as a trait.

4. Dominant and Recessive

5.1. What Is an Allele?
Different forms of the same gene are called alleles. An organism inherits two alleles for each gene.
5.2. Notation of Alleles
The alleles for rabbit fur color can be dominant or recessive. A capital letter is used to represent a dominant allele and a lowercase letter is used for a recessive allele.
5.3. Alleles and Traits
When both a dominant and recessive allele are present, the dominant trait is always seen.
5.4. Alleles Determine Traits
The dominant allele masks the presence of the recessive allele. The recessive trait is seen when there are two copies of the recessive allele.

5. Using a Punnett Square

6.1. What Is a Punnett Square?
A Punnett square is a chart that shows the allele combinations from a genetic cross. It helps predict the probability of a parental trait showing up in the offspring.
6.2. Information for a Punnett Square
To make a Punnett square, you need to know the genetic information from each parent. Each parent has two alleles, symbolized with letters.
6.3. Drawing a Punnett Square
When you make a Punnett square, the first step is to draw a square and divide it into four boxes.
6.4. Parental Alleles on a Punnett Square
Then, write one parent's alleles above the two boxes on the top and the other parent's alleles along the left side.
6.5. Filling in a Punnett Square
Copy the parent's alleles into the boxes, moving across the rows. Then copy the other parent's alleles into the boxes moving down the columns.
6.6. Completed Punnett Square
Each box now has two alleles, one from each parent. These boxes represent the genetic combinations that can occur in the offspring.
6.7. Allele Combinations and the Punnett Square
In this cross, there are three possible allele combinations for the offspring.
6.8. Probability and the Punnett Square
The probability of these combinations showing up in the offspring can also be determined.

6. Pause and Interact

7.1. Review
Use the whiteboard tools to complete the activity.
7.2. Understanding a Punnett Square
Follow the onscreen directions to complete the activity.

7. Phenotypes and Genotypes

8.1. What Are Genotypes and Phenotypes?
A genotype refers to the genetic makeup, or allele combinations, of an organism. A phenotype is the physical appearance of an organism.
8.2. Dominant and Recessive Phenotypes
If a dominant allele is present, then the phenotype will always be the dominant trait. A phenotype will be the recessive trait when two copies of the recessive allele are present.
8.3. BB x bb Cross
In this Punnett square, the parental cross results in offspring with only one allele combination. All the offspring would have the same genotype and phenotype.
8.4. BB x Bb Cross
The offspring in this Punnett square have two different genotypes, but all have the same phenotype.
8.5. Bb x Bb Cross
For this cross there are three possible genotypes and two possible phenotypes.
8.6. Probability for a Bb x Bb Cross
You could predict that 75% of the offspring would have black fur and 25% white fur. This is the same percentage that Mendel observed when crossing the F1 generations of pea plants.

8. Making a Pedigree

9.1. What Is a Pedigree?
A pedigree is a diagram that shows how a genetic trait is inherited through several generations of a family. It can help us predict how phenotypes and genotypes may be passed on to future generations.
9.2. Parts of a Pedigree
This pedigree shows the inheritance of attached earlobes within a family. Roll over the image to learn more about how to create a pedigree diagram.

9. Codominance

10.1. Mendel's Pea Plant Characteristics
All the pea plant characteristics that Mendel studied had dominant and recessive alleles.
10.2. What Are Codominant Alleles?
Sometimes traits are controlled by codominant alleles that are neither dominant nor recessive. Both alleles are expressed in the offspring. Chicken feather color is an example of codominance.
10.3. Expression of Codominant Alleles
When homozygous black and homozygous white chickens are crossed, the offspring inherit one allele for each color. Both alleles are expressed, resulting in a black and white feather pattern.
10.4. Symbols for Codominant Alleles
Codominant alleles are symbolized using capital letters with superscripts.

10. Pause and Interact

11.1. Review
Use the whiteboard tools to complete the activity.

11. Vocabulary Review

12.1. Genetics Vocabulary Matching
All the pea plant characteristics that Mendel studied had dominant and recessive alleles.

12. Virtual Investigation

13.1. Mendel's Pea Plant Experiments
In this virtual investigation you will perform many of the same genetic crosses as Gregor Mendel. You will study the heredity of four pea plant characteristics by doing parental (P) and first generation (F1) crosses. In this activity, you should assume that the parental crosses are true-breeding plants.

13. Assessment

14.1. Genetics
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