Patterns of InheritancePatterns of Inheritance

Chapter 14: Mendel and the Gene IdeaChapter 14: Mendel and the Gene Idea

Patterns of InheritancePatterns of Inheritance

Parents and offspring often share observable traits.

Grandparents and grandchildren may share traits not seen in parents.

Why do traits disappear in one generation and reappear in another?

Parents and offspring often share observable traits.

Grandparents and grandchildren may share traits not seen in parents.

Why do traits disappear in one generation and reappear in another?

Possible HypothesesPossible Hypotheses The “blending” hypothesis

states that

• Example: Blue and yellow paint blend to make green

The “particulate” hypothesis states that

• These heritable units are

that can be passed on to the next generation

Gregor Mendel documented a through his experiments with

The “blending” hypothesis states that

• Example: Blue and yellow paint blend to make green

The “particulate” hypothesis states that

• These heritable units are

that can be passed on to the next generation

Gregor Mendel documented a through his experiments with

Gregor MendelGregor Mendel

Austrian monkAnalyzed

• Asked why• Tested his theories

• Worked with

Austrian monkAnalyzed

• Asked why• Tested his theories

• Worked with

Pea plantsPisum sativum

Pea plantsPisum sativum

Advantages of pea plants for genetic study:• Many varieties with distinct

heritable features, or

(such as flower color) Character variants (such as

purple or white flowers) are called

•

• Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels)

•

Advantages of pea plants for genetic study:• Many varieties with distinct

heritable features, or

(such as flower color) Character variants (such as

purple or white flowers) are called

•

• Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels)

•

Traits Mendel usedTraits Mendel used

True breeding plantsTrue breeding plants

Began with true-breeding varieties (pure-bred)• •

Crossed with other true-breeding variety• Offspring called

Began with true-breeding varieties (pure-bred)• •

Crossed with other true-breeding variety• Offspring called

GenerationsGenerations

Mendel mated two contrasting, true-breeding varieties• Process called

The true-breeding parents are the

The hybrid offspring of the P generation are called the

• Referred to as hybrids When F1 individuals self-

pollinate, the is produced

Mendel mated two contrasting, true-breeding varieties• Process called

The true-breeding parents are the

The hybrid offspring of the P generation are called the

• Referred to as hybrids When F1 individuals self-

pollinate, the is produced

Crosses Crosses Had lots of varieties• 7 traits with two forms of each trait

Crossing a tall plant with a short plant is a

• A monohybrid cross is

Trait - • Two variations -

Generations • P - parental (true-breeding): • F1 - first filial or son (children): • F2 - next generation (grandkids):

What happened? How did the short plants reappear?

Had lots of varieties• 7 traits with two forms of each trait

Crossing a tall plant with a short plant is a

• A monohybrid cross is

Trait - • Two variations -

Generations • P - parental (true-breeding): • F1 - first filial or son (children): • F2 - next generation (grandkids):

What happened? How did the short plants reappear?

Mendel’s ConclusionsMendel’s Conclusions

1. Alternative versions of genes account for

For example, the gene for flower color in pea plants exists in two versions: purple flowers and white flowers

These alternative versions of a gene are now called Each gene resides at a specific

Therefore, we distinguish between an organism’s• •

1. Alternative versions of genes account for

For example, the gene for flower color in pea plants exists in two versions: purple flowers and white flowers

These alternative versions of a gene are now called Each gene resides at a specific

Therefore, we distinguish between an organism’s• •

AllelesAlleles

Genotype TerminologyGenotype Terminology If alleles are identical = • If both alleles are recessive =

Genotype is • If both alleles are dominant =

Genotype is If both alleles are different = • • One dominant allele and one recessive

allele ( )An organism’s traits do not always reveal

If alleles are identical = • If both alleles are recessive =

Genotype is • If both alleles are dominant =

Genotype is If both alleles are different = • • One dominant allele and one recessive

allele ( )An organism’s traits do not always reveal

Mendel’s ConclusionsMendel’s Conclusions2. For each character, an organism inherits

Factors (genes) that determine traits can be Alleles may be

• traits expressed in the F1 generation• traits not expressed in the F1 generation

2. Mendel observed the same pattern of inheritance in 7 pea plant characters, each represented by two traits

3. What Mendel called a “heritable factor” is what we now call a

2. For each character, an organism inherits

Factors (genes) that determine traits can be Alleles may be

• traits expressed in the F1 generation• traits not expressed in the F1 generation

2. Mendel observed the same pattern of inheritance in 7 pea plant characters, each represented by two traits

3. What Mendel called a “heritable factor” is what we now call a

Mendel’s ConclusionsMendel’s Conclusions3. If two alleles at a locus differ, then one

(dominant allele) determines the organism’s appearance,

When Mendel crossed contrasting, true-breeding white and purple flowered pea plants,

When Mendel crossed the F1 hybrids,

Mendel discovered a ratio of about

3. If two alleles at a locus differ, then one (dominant allele) determines the organism’s appearance,

When Mendel crossed contrasting, true-breeding white and purple flowered pea plants,

When Mendel crossed the F1 hybrids,

Mendel discovered a ratio of about

Mendel’s Law of Segregation

Mendel’s Law of Segregation

4. The law of segregation states that the two alleles for a heritable character

Thus, an egg or a sperm gets only

This segregation of alleles corresponds to the distribution of

4. The law of segregation states that the two alleles for a heritable character

Thus, an egg or a sperm gets only

This segregation of alleles corresponds to the distribution of

Principle of SegregationPrinciple of Segregation

1.Two copies of each trait (gene)• Fully expressed gene -

•Other gene - 2. Gametes only have

3. Fertilization restores

1.Two copies of each trait (gene)• Fully expressed gene -

•Other gene - 2. Gametes only have

3. Fertilization restores

Monohybrid & Dihybrid Crosses

Monohybrid & Dihybrid Crosses

Mendel derived the law of segregation by following

Mendel identified his second law of inheritance by following

Crosses involving two traits are called

A dihybrid cross can determine whether two characters are transmitted to offspring

Using a dihybrid cross, Mendel developed the

Mendel derived the law of segregation by following

Mendel identified his second law of inheritance by following

Crosses involving two traits are called

A dihybrid cross can determine whether two characters are transmitted to offspring

Using a dihybrid cross, Mendel developed the

Law of Independent

Assortment Law of Independent

Assortment The law of independent assortment states that each

pair of alleles

This law applies only to genes on

Genes located near each other on the same chromosome

The law of independent assortment states that each pair of alleles

This law applies only to genes on

Genes located near each other on the same chromosome

Probability RulesProbability Rules Mendel’s laws reflect

When tossing a coin, the outcome of one toss has on the

outcome of the next toss In the same way, the alleles of

one gene

The multiplication rule states that the

Example: probability of 2 coins landing heads up is

Mendel’s laws reflect

When tossing a coin, the outcome of one toss has on the

outcome of the next toss In the same way, the alleles of

one gene

The multiplication rule states that the

Example: probability of 2 coins landing heads up is

Probability RulesProbability Rules Each gamete has a chance of

carrying the and a chance of carrying the

• Similar to heads and tails Another rule is needed to figure

out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous

The rule of addition states that the probability that any one of two or more exclusive events will occur is calculated

Example: probability of one heads & one tails is

Each gamete has a chance of carrying the and a chance of carrying the

• Similar to heads and tails Another rule is needed to figure

out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous

The rule of addition states that the probability that any one of two or more exclusive events will occur is calculated

Example: probability of one heads & one tails is

Probability RulesProbability Rules These rules can be used to predict the outcome of

crosses involving A dihybrid or other multicharacter crosses are

equivalent to two or more

In calculating the chances for various genotypes, each character is

These rules can be used to predict the outcome of crosses involving

A dihybrid or other multicharacter crosses are equivalent to two or more

In calculating the chances for various genotypes, each character is

Punnett squaresPunnett squares Probability can be depicted

through the use of a

• A diagram for predicting the results of a genetic cross between

Predicts for all possible gametes with

Same letter used for trait •

•

Probability can be depicted through the use of a

• A diagram for predicting the results of a genetic cross between

Predicts for all possible gametes with

Same letter used for trait •

•

Setting up a Punnett Square

Setting up a Punnett Square

Step 1. Designate letters which will represent the genes/traits. • T = tall t = short

Step 2. Write down the genotypes (genes) of each parent. These are often given to you or are possible to determine.• TT (tall) X tt (short) - both homozygous or purebred

Step 3. List the genes that each parent can contribute. Parent 1 Parent 2

Step 1. Designate letters which will represent the genes/traits. • T = tall t = short

Step 2. Write down the genotypes (genes) of each parent. These are often given to you or are possible to determine.• TT (tall) X tt (short) - both homozygous or purebred

Step 3. List the genes that each parent can contribute. Parent 1 Parent 2

Setting up a Punnett Square

Setting up a Punnett Square Step 4. Draw a Punnett square and write the possible

gene(s) of one parent across the top and of the other parent along the side.

Step 5. Fill in each box of the Punnett square by transferring the letter above and in front of each box into each appropriate box. As a general rule, the capital letter goes first and a lowercase letter follows.

Step 4. Draw a Punnett square and write the possible gene(s) of one parent across the top and of the other parent along the side.

Step 5. Fill in each box of the Punnett square by transferring the letter above and in front of each box into each appropriate box. As a general rule, the capital letter goes first and a lowercase letter follows.

Step 6. List the possible genotypes and phenotypes of the offspring for this cross.

Genotypic Ratio:

Phenotypic Ratio:

Practice!Practice!

1. Cross a homozygous tall plant with a short plant. What are the genotypic and phenotypic ratios?

2. Cross a heterozygous tall plant with a homozygous tall plant. What are the genotypic and phenotypic ratios?

3. Cross a heterozygous tall plant with a short plant. What are the genotypic and phenotypic ratios?

1. Cross a homozygous tall plant with a short plant. What are the genotypic and phenotypic ratios?

2. Cross a heterozygous tall plant with a homozygous tall plant. What are the genotypic and phenotypic ratios?

3. Cross a heterozygous tall plant with a short plant. What are the genotypic and phenotypic ratios?

Testcross Testcross Used to determine • Individual expressing

• Could be

Cross with Make prediction with Punnett

square If homozygous dominant:• TT x tt•

If heterozygous:• Tt x tt•

Used to determine • Individual expressing

• Could be

Cross with Make prediction with Punnett

square If homozygous dominant:• TT x tt•

If heterozygous:• Tt x tt•

More Practice!!!More Practice!!!1. In rabbits, the allele for black fur (B) is dominant over the allele for brown fur (b). If a heterozygous male mates with a heterozygous female, what are the chances that the offspring will have black fur?

2. In humans, dimples are dominant to no dimples. If a homozygous dominant man reproduces with a heterozygous female, what are the chances of having a child with no dimples?

3. In humans, freckles are dominant over no freckles. A man with freckles reproduces with a woman with freckles, but the children have no freckles. What chance did each child have for freckles?

4. If a man is homozygous for widow’s peak (dominant) reproduces with a woman homozygous for straight hairline (recessive), what are the chances of their children having a widow’s peak? A straight hairline?

5. In humans, pointed eyebrows (B) are dominant over smooth eyebrows (b). Mary’s father has pointed eyebrows, but she and her mother have smooth. What is the genotype of the father?

1. In rabbits, the allele for black fur (B) is dominant over the allele for brown fur (b). If a heterozygous male mates with a heterozygous female, what are the chances that the offspring will have black fur?

2. In humans, dimples are dominant to no dimples. If a homozygous dominant man reproduces with a heterozygous female, what are the chances of having a child with no dimples?

3. In humans, freckles are dominant over no freckles. A man with freckles reproduces with a woman with freckles, but the children have no freckles. What chance did each child have for freckles?

4. If a man is homozygous for widow’s peak (dominant) reproduces with a woman homozygous for straight hairline (recessive), what are the chances of their children having a widow’s peak? A straight hairline?

5. In humans, pointed eyebrows (B) are dominant over smooth eyebrows (b). Mary’s father has pointed eyebrows, but she and her mother have smooth. What is the genotype of the father?

Review QuestionsReview Questions1. Differentiate between the blending and particulate

hypotheses of inheritance.2. Explain the importance of Gregor Mendel’s work with

garden peas. Also, explain why he used garden peas.3. Define the following vocabulary associated with basic

genetics: character, trait, hybrid, gene, allele, locus, genotype, phenotype, dominant, recessive, homozygous, & heterozygous.

4. Differentiate between the P, F1, and F2 generations.5. Differentiate between monohybrid and dihybrid crosses.6. Explain Mendel’s four basic conclusions regarding

inheritance patterns.7. Explain the three parts to the law of segregation.8. Explain the law of independent assortment.9. Properly construct a Punnett square for use in solving a

genetics problem involving probability.10. Explain the idea of a testcross.