Chapter 12

Mendel and Heredity

Pre-class Question 3/30

How do plants reproduce, sexually or asexually? Depends

HomeworkChapter 12 Vocabulary due Thursday 4/2“In the Glass House” article, due tomorrow

Species Comparison

Character and Trait

A character is a recognizable inherited feature or characteristic of an organism

Example: eye color

A trait is one or two or more forms of a character.

Example: blue, brown, or hazel eye color

Genetic Trait Activity

Earlobe Attachment

Free

Attached

Hitchhiker’s Thumb Normal

Mid Digital Hair

More Genetic Traits

Cleft Chin

Widow’s Peak

Pre-class Question 3/31/09

What type of plant did Mendel work with?

Pea Plant (Pisum)

Homework (due tomorrow) – Active Reading, “Mendel’s Theory”Reminder – Chapter 12 Vocabulary due Thursday

Pea Plant Characteristics and Traits

Characterisitics

Traits

Variation of Traits

Some traits are controlled by a single gene, which gives rise to few varieties.

Some traits are controlled by more than one gene (polygenic) which gives rise to many varieties.

“In the Glasshouse” Homework

1. What were the seven characteristics of pea plants that Mendel examined in his experiments?

Mendel chose the common garden pea, of the genus Pisum, to experiment on. First he chose seven characteristics to look at: the position of the flowers on the stem, the stem length, the color of the unripe pod, the shape of the ripe seed, the color of the seed coat, the shape of the ripe pod, and the color of the ripe seed. Then he planted his first generation of peas and watched them grow. For two years he kept planting the seeds to make sure they were pure breeding, that is, they always produced offspring that had the same characteristics as their parent plants. Then he began his experiments.

2. What percentage of seeds that Mendel produced from crossing round seeds and wrinkled seeds in his first experiment were round?

The first of the experiments that he conducted crossed plants with round seeds and plants with wrinkled seeds. The resulting plants produced all round seeds! The next year he grew plants from these round seeds and allowed them to self-fertilize and produce more seeds. This time there were 5474 round seeds and 1850 wrinkled seeds, making a 3:1 ratio of round to wrinkled seeds.

3. What seed type reappeared in the 2nd generation plants that had seemed to disappear in the 1st generation?

Mendel was fascinated! It seemed that the wrinkled trait, which seemed to disappear in the first generation of offspring, reappeared in the second generation. He called the round trait "dominating" and the wrinkled trait "recessive."

4. What combination of letters represents a pure dominant trait, a hybrid trait and a recessive trait?

Mendel was the first person to use a single letter to represent each trait, with a capital letter (A) to represent the dominant trait and a lowercase letter (a) to represent the recessive trait. When two pure-breeding parent plants were crossed, their hybrid offspring were represented as Aa. For every four peas produced in the second generation of offspring (when the hybrids, represented by Aa, were allowed to self-fertilize), one pea could be called AA (pure dominant round), two peas could be called Aa (round but also carrying the potential to produce wrinkled peas), and one pea could be called aa (pure recessive wrinkled).

5. What ratio of seed traits did Mendel observed for each seed characteristic he studied?

Mendel repeated the experiment with the other six characteristics and got the same 3:1 ratios. He also tried crosses considering two different characteristics at the same time. For example, he crossed plants producing round yellow peas with plants producing green wrinkled peas. Finally, he did backcrosses, in which first generation hybrids were crossed with either pure-breeding dominants or pure-breeding recessives.

Mendel’s First Experiments (3 Steps)

Mendel performed monohybrid crosses to study one pair of contrasting traits.

Step 1 (Parental Generation)Mendel allowed plants to self pollinate to create true-breeding plants (ones that produced the same offspring over and over again.

Step 2 (F1 Generation)Mendel crossed true-breeding plants that had contrasting traits.

Step 3 (F2 Generation)Mendel allowed the F1 generation plants to self-pollinate.

Mendel’s Results

3:1

Pre-class Question 4/1

What did the Punnett Square wear to school?

His Jeans

Using Punnett Squares to Explain Mendel’s Results

P P

p

p

F1 Generation

PpPp

Pp

F2 Generation

Pp

P p

p

P

Pp

PpPP

p p

Mendel’s Theory of Simple Inheritance -Alleles

Alternate versions of genes

The alleles for flower color in pea plants are P and p

P is the dominant allele and p is the recessive allele

Whenever P is present, the flower color is purple

The only way a flower can be white is if both alleles are p (pp)

Genotype vs. Phenotype

Genotype is the actual combination of genes

PP Pp pp

Phenotype is the expression of those genes or trait

Homozygous vs. Heterozygous

Homozygous = Same

Homozygous Dominant = PP

Homozygous Recessive = pp

Heterozygous = Different

Heterozygous = Pp

Law of Segregation of Alleles

When an organism produces gametes, each pair of alleles is separated and each gamete has equal chance of receiving one of the alleles.

Mendel’s Second Experiments

Dihybrid Crosses (involves two traits)

Law of Independent Assortment

During gamete formation, the alleles of each gene segregate independently. The inheritance of one trait is not affected by the inheritance of another trait.

Genes Linked on Chromosomes

Genes that are located close to one another on a chromosome do not separate independently. Some genes are linked to together.

The genes that Mendel studies were far apart from each other.

Pre-Class Question 4/2

Characteristic = SpotsTraits S=spots s= no spots

Perform the following cross: Ss x ss

Pre-class Question 4/13

Genes for Hitch hikers thumbN=normal thumb n=hitch hikers thumb

Genotypes

Homozygous Dominant Heterozygous Homozygous Recessive

NN Nn nn

Phenotypes

Pre-class Question 4/14/09

What do you think the term “polygenic character” means?

A character that is controlled by more than one gene

HomeworkThe Relationship Between Genotype and Phenotype (Picture and Announcement Due Thursday)

Vocabulary Quiz Friday

Pre-class Question 4/15/09

What is a pedigree?A diagram that shows the occurrence of a genetic trait in several generations of a family

HomeworkWorksheet, Baby Picture and Announcement Due TomorrowVocabulary Quiz Friday

Pre-class question 4/20If you were to perform a cross between two individuals with linked genes using a punnett square, how many different combinations of genes could you create.

HomeworkBikini Bottom Genetics 2 worksheet, due WednesdayChapter 12 Test Friday

GgHh x HHgg

Pre-class Question 4/22

A cross between a red-flowered plant and a white-flowered plant results in offspring that have pink flowers. This is an example of _____________.

A. Polygenic inheritance

B. Incomplete Dominance

C. Multiple Alleles

D. Codominance

Chapter 12 Test Friday