Bio 178 Lecture 23Meiosis (Cntd.) and Genetics

J. Elson-Riggins

Reading

• Chapters 12, P 1068 - 1074, & 13

Quiz Material

• Questions on P 240, 1080, & 276-278

• Chapter 12, 50, & 13 Quizzes on Text Website (www.mhhe.com/raven7)

Outline

• Meiosis (cntd.)

• Gametogenesis

• Genetics

Mendelian Patterns of Inheritance

Non-Mendelian Patterns of Inheritance

Mitosis Vs. Meiosis

MITOSIS MEIOSIS

# Divisions

# Daughter Cells

Final Ploidy

Cell Type

Genetic Variation?

Metaphase Alignment

Anaphase Separation

Mitosis Vs. MeiosisMITOSIS MEIOSIS

# Divisions 1 2

# Daughter Cells 2 4

Final Ploidy 2N N

Cell Type Somatic Gametes

Genetic Variation?

No Yes

Metaphase Alignment Chromosomes

I - Homologues

II - Chromosomes

Anaphase Separation Sister chromatids

I - Homologues

II - Sister chromatids

Evolutionary Consequences of Sex

• Random Fertilization

In addition to independent assortment and crossing over, random fertilization generates diversity because 2 gametes (each of which is different from the parent) fuse.

• Importance for Evolution

The diversity generated by sexual reproduction results in individuals that may be fitter than others (not always!) in a particular environment and will therefore be selected by that environment.

Spermatogenesis

Spermatogenesis

McGraw-Hill Video

http://webanatomy.net/histology/reproductive/seminiferous_tubule.jpg

Structure of Seminiferous Tubules

Structure of Sperm

Oogenesis

Prophase I

Metaphase II

(Fertilization)

Mature Follicle

(Metaphase II)

Initial Ideas About Heredity

• Pre-Mendel

Read about classical assumptions (P 242) & the work of Koelreuter & Knight.

• Mendel (1822-1884)

What was different about Mendel’s work?

Mendel’s Experiments• Characteristics of the Study SubjectThe garden pea was a good choice because:

1. Can cross or self-fertilize.

2. Produces large numbers of offspring.

3. Easy to grow.

4. Has a number of varieties with different traits.

5. Short generation times.

6. Produces hybrid varieties.

Mendel’s Experiments• Experimental DesignMendel worked with 7 easily distinguishable traits as follows:

1. Ensured the parental (P) generation was true-breeding.

Eg. White flowered plants White flowered progeny

2. Crossed varieties with alternate traits.

Eg.

Mendel’s Experimental Design (Cntd.)

3. Self-fertilized the hybrid generation (F1)

Mendel’s Results

• F1 Generation

No intermediate progeny - all resembled one of the parents. This was referred to as the dominant trait.

Eg. P: Purple X White

F1: Purple

Mendel’s Results (Cntd.)• F2 GenerationF1: Purple X Purple

F2: 3 Purple :

1 White

Mendel’s Experiments (Cntd.)

• Self-fertilize the F2 Disguised 1:2:1 Ratio

Mendel’s Experiments (Cntd.)

• Self-fertilize the F2 Disguised 1:2:1 Ratio

Mendel’s Conclusions

1. Blending (hybridization) was not occurring.

2. For each alternative trait, one alternative was not expressed in the F1, but reappeared in the F2.

3. Pairs of alternate traits segregated among progeny of a particular cross.

4. These alternate traits were expressed in the F2 generation as 3 dominant : 1 recessive.

Principles of GeneticsMendel’s ideas in modern terms:

• Genes

Information about traits is transmitted from parent to offspring in the form of genes.

• Homologous Chromosomes

A diploid individual receives 2 alleles (forms of a gene) for each trait, one on each of 2 homologous chromosomes.

Homozygous

Heterozygous