BSCI 106 Dr. Via

Spring 2000

Lecture 4. Patterns of Inheritance: Mitosis, Meiosis, Mendel

1. What is transmission genetics and why do we care??

a. "Transmission genetics" is the set of mechanisms by which genes are transmitted between dividing cells and from parents to offspring.

b. These genetic rules govern not only how traits are inherited, but determine genetic variation in populations

c. This genetic variation is the raw material for evolution!

2. Terminology (see back)

a. Chromosome, gene locus allele

b. Homologous, homolog

c. Homozygous, heterozygous

d. Recessive allele, dominant allele

e. Genotype, phenotype

3. Differences between mitosis and meiosis (YOU MUST KNOW THIS COLD, see handouts and your text)

a. Mitosis occurs during cell division of somatic (body) cells for growth and repair.

b. Meiosis occurs in germ cells during the production of gametes.

c. Mitosis leads to 2 diploid (2N) cells identical to the original cell.

d. Meiosis leads to 4 haploid (N) gametes, each with one copy of every chromosome. None of these chromosomes are necessarily identical with any in the original cell due to recombination (much more later)

e. Mitosis -- one cell division. Meiosis-- two cell divisions.

f. A key mechanistic difference between mitosis and meiosis: during the first division (Metaphase in mitosis, Metaphase I in meiosis), chromosomes line up differently!!

g. Follow individual alleles through mitosis and meiosis, without and with crossing over (recombination) during Prophase I of meiosis. Be sure that you can do this!

4. Mendelian inheritance: how genes are transmitted and how Mendel figured it out

a. Who was Gregor Mendel (1822-1884) -- Darwin published 'On the Origin of Species'?? (1876)

b. Controlled breeding experiments with garden pea

c. True breeding line-- followed inheritance of 7 different traits

d. used 3 generations: P, F1, F2

6. How genes are transmitted: One locus

a. Monohybrid cross = cross involving one character followed over 3 generations

i. With dominance, trait not seen in F1, "reappears" in F2 (fig. 16.1)

ii. Mendel explained results with a "particulate" theory -- genes

iii. physical appearance (phenotype) a result of genetic constitution (genotype)

b. Punnett Square -- a technique to figure out genotype and phenotype ratios

***when figuring out offspring from a cross, frequencies = 0.5, BUT, this will NOT be true when we do gene frequencies in POPULATIONS)

c. Mendel's Law of Segregation (1st Law): Each individual has 2 alleles for each gene. When gametes are formed, those alleles segregate and pass into different gametes.

d. Gametes combine during mating at random-- thus Mendelian ratios are averages not absolutes!

Next time: Part II: Partial dominance, using probability instead of Punnett Squares, two loci, beyond Mendelian Genetics!

KEY DIFFERENCES BETWEEN MITOSIS AND MEIOSIS:

	MITOSIS	MEIOSIS
Importance to Organism	growth and repair of body tissue (somatic cells)	production of gametes (germ cells)
# of Resulting Cells	2	4
Genetic Composition of Resulting Cells	diploid (2n); identical to original cell	haploid (n); not identical to original cell
# of Divisions	1	2 (but DNA replication only at first division)

TERMINOLOGY:

Chromosome: a structure along which genes are located

Gene: a portion of a chromosome that codes for some product such as a protein, tRNA or rRNA

Locus: a particlar location on a chromosome (occupied by a specific gene)

Allele: alternate forms of a gene at a locus

homozygous: having identical alleles of a given gene on both homologous chromosomes

heterozygous: having different alleles of a given gene on homologous chromosomes

recessive allele: an allele whose phenotypic effect is masked in heterozygotes by the presence of dominant allele; recessive phenotype expressed only in homozygous individuals

dominant allele: allele whose phenotype is expressed ina heterozygote