Classical Genetics

Gregor Mendel Showed that parents pass heritable factors (genes) to offspring Mendel’s experiments Used large numbers of pea plants Developed true-breeding strains When self-fertilized, the offspring were identical to the parents Preformed controlled matings (crosses) Took sperm (pollen) and placed it on the egg producing structure (carpel) observed characteristics with only two forms Student Misconceptions and Concerns 1. The authors note that Mendel’s work was published in 1866, seven years after Darwin published Origin of Species. Consider challenging your students to consider whether Mendel’s findings supported Darwin’s ideas. Some scientists have noted that Darwin often discussed the evolution of traits by matters of degree. Yet, Mendel’s selection of pea plant traits typically showed complete dominance, rather than the possibility for such gradual inheritance. Teaching Tips 1. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress.

What Mendel concluded: There are alternative versions of genes -ex: purple color flowers vs. white color -alternative versions are alleles 2. For each characteristic, an organism inherits two alleles, one from each parent A homozygous individual has 2 identical alleles A heterozygous individual has two different alleles There is an allele for purple flower color and a different allele for white flower color. Student Misconceptions and Concerns 1. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling will reflect expected ratios. Teaching Tips 1. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress. 2. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP pp and (b) Pp pp. 3. For students struggling with basic terminology, an analogy between a genetic trait and a pair of shoes might be helpful. A person might wear a pair of shoes in which both shoes match (homozygous), or less likely, a person might wear shoes that do not match (heterozygous). 4. Another analogy that might help struggling students is a pair of people trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. This (heterozygous) couple eats at home (the dominant allele “wins”). 5. Figure 9.4 can be of great benefit when introducing genetic terminology of genes. For students struggling to think abstractly, such a visual aid may be essential when describing these features in lecture.

If the alleles differ (individual is heterozygous), the dominant allele determines the organism’s appearance The recessive allele is the one not seen in the heterozygote The phenotype is the appearance or expression of a trait The genotype is the combination of alleles found in an individual Can be represented by letters P= purple allele p= white allele In reference to hypothesis 3, in the F1 generation, plants received a purple allele from one parent and a white allele from the other. The purple allele is dominant since all the F1 plants have purple petals. In reference to hypothesis 4, in general, a dominant allele provides instructions for producing a functional protein. A recessive allele may lead to a nonfunctional protein or the complete absence of the protein. Mendel needed to explain: Why one trait seemed to disappear in the F1 generation—This is explained by dominance of the purple allele over the white allele. Why that trait reappeared in one fourth of the F2 offspring—This is explained by the segregation of alleles, the generation of all possible combinations of gametes, and the dominance of the purple allele. Student Misconceptions and Concerns 1. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling will reflect expected ratios. Teaching Tips 1. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress. 2. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP pp and (b) Pp pp. 3. For students struggling with basic terminology, an analogy between a genetic trait and a pair of shoes might be helpful. A person might wear a pair of shoes in which both shoes match (homozygous), or less likely, a person might wear shoes that do not match (heterozygous). 4. Another analogy that might help struggling students is a pair of people trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. This (heterozygous) couple eats at home (the dominant allele “wins”). 5. Figure 9.4 can be of great benefit when introducing genetic terminology of genes. For students struggling to think abstractly, such a visual aid may be essential when describing these features in lecture. Copyright © 2009 Pearson Education, Inc.

Mendel’s Law of Segregation Allele pairs separate (segregate) from each other during gamete formation.

B=black allele

The mouse is BLACK B B or BB the mouse is homozygous (purebred) Phenotype The mouse is BLACK BB the mouse is homozygous (purebred) Genotype What alleles can he put in his sperm? (he will only give one) B B or

bb She’s white Phenotype Genotype She has 2 white alleles. We will use to represent the white allele b What alleles can she give to her eggs? bb Genotype b b or

X ?

B b B B b b b B b b B B BB bb BB bb Set up a punnett square to predict offspring from a particular cross BB bb X BB B B b B B b b bb b B b b B

X ?

B B B B B b b B b b b b ratios percentages Bb Bb 3 Black:1 white X ¾ or 75% are black ¼ or 25% are white

Cross a heterozygous black mouse with a white mouse

Hypothesized (not actually seen) Actual results (support hypothesis) Fig. 9-5a Hypothesis: Dependent assortment Hypothesis: Independent assortment P generation RRYY rryy RRYY rryy Gametes RY ry Gametes RY ry F1 generation RrYy RrYy Sperm Sperm 1 – 4 RY 1 4 – rY 1 4 – Ry 1 4 – ry 1 – 2 RY 1 – 2 ry F2 generation 1 – 4 RY 1 – 2 RY RRYY RrYY RRYy RrYy Eggs 1 4 – rY 1 2 – ry RrYY rrYY RrYy rrYy Eggs Yellow round 1 – 4 Ry 16 –– 9 RRYy RrYy RRyy Rryy Green round 16 –– 3 Hypothesized (not actually seen) 1 – 4 ry Yellow wrinkled RrYy rrYy Rryy rryy 16 –– 3 Actual results (support hypothesis) Green wrinkled 1 16 ––

Mendel’s law of independent assortment Each pair of alleles segregates independently of the other pairs off alleles during gamete formation

-tall is dominant, short is recessive A heterozygous tall, heterozygous yellow pea plant is crossed with a heterozygous tall, heterozygous yellow pea plant -tall is dominant, short is recessive -yellow is dominant, green is recessive Give the phenotypic ratios What is the chance of getting a short, yellow pea plant Seed color Seed shape Round Wrinkled A short, heterozygous yellow pea plant is crossed with a heterozygous tall, green pea plant 1) give the phenotypic ratios 2) what is the chance of having the genotype Ttyy? Pod shape Inflated Constricted Figure 9.2D The seven pea characters studied by Mendel. Mendel studied seven characteristics for pea plants. Later studies have shown that pea plants have seven pairs of chromosomes, and each of these characteristics is on a different chromosome. This explains why Mendel’s results were not affected by genetic recombination.

Parents (P) Offspring (F1) White 1 Removed stamens from purple flower Stamens Carpel 2 Transferred pollen from stamens of white flower to carpel of purple flower Parents (P) Purple 3 Pollinated carpel matured into pod 4 Planted seeds from pod Figure 9.2C Mendel’s technique for cross-fertilization of pea plants. This slide demonstrates how Mendel could control matings between pea plants. He removed the pollen-bearing structures from one parental plant and transferred pollen from another parental plant. The true-breeding parental plants are called the P generation, their offspring are the F1 generation, and offspring of an F1  F1 cross are the F2 generation. Offspring (F1)

P generation (true-breeding parents) Purple flowers White flowers Purple flowers White flowers Figure 9.3A Crosses tracking one character (flower color).

Gene loci Dominant allele P a B P a b Recessive allele Genotype: PP aa Gene loci Dominant allele P a B P a b Recessive allele Figure 9.4 Matching gene loci on homologous chromosomes. Genotype: PP aa Bb Homozygous for the dominant allele Homozygous for the recessive allele Heterozygous

P generation (true-breeding parents) Purple flowers White flowers Purple flowers White flowers F1 generation All plants have purple flowers Figure 9.3A Crosses tracking one character (flower color).

of plants have purple flowers of plants have white flowers P generation (true-breeding parents) Purple flowers White flowers F1 generation X All plants have purple flowers Fertilization among F1 plants (F1 X F1) Figure 9.3A Crosses tracking one character (flower color). F2 generation of plants have purple flowers 3 – 4 of plants have white flowers 1 – 4