Parents can produce many types of offspring

Families will have resemblances, but no two are exactly alike.Why is that?

Meiosis and Genetic Linkage

Objectives

Recognize the significance of meiosis to sexual reproduction (TEKS 6G)

• Explain the difference between the chromosome number of body (somatic) cells and gametes.

• Summarize the events of meiosis

• Examine the differences of mitosis and meiosis

• Discuss the significance of meiosis and genetic variation in sexual reproduction

• Describe genetic linkage and the structures that actually assort independently

Remember…• Every cell has a nucleus

• Every nucleus has chromosomes

• Genes are located on chromosomes o genes control the TRAITS of the

individual

• Chromosomes are simply DNA wound up into a threadlike structure found in the nucleus.

• The number of chromosomes depends on the specieso Ex.  Humans have 46

Homologous Chromosomes

Organisms which reproduce sexually must inherit half its genetic material from one parent (aka dad) and half its genetic material from the other parent (aka mom)

These two sets of chromosomes (which contain the genetic material) are homologous to each other; called homologous chromosomes

For example:

Chromosome 11 in this karyotype has two chromosomes. one from “mom” andone from “dad”They are each their own chromosome, and together they are homologous chromosomes

Homologous Chromosomes

Homologous chromosomes contain the same genes, however, the alleles for those genes can be different.

For example: • Chromosome 11 contains the gene for

albinism. • Albinism occurs when two recessive

copies of the gene are combined and causes a complete lack of pigmentation in the skin, hair, and eyes.

Chromosome Number

A cell that contains both sets of homologous chromosomes is said to be diploid.

Somatic (body) cells are diploid.

The number of chromosomes in a diploid cell is sometimes represented by the symbol 2N.

For Example:Humans have a 46 chromosomes in their somatic cells, so the diploid number is 46. This can be written as 2N = 46

Chromosome Number

The gametes of sexually reproducing organisms contain only a single set of chromosomes, and therefore only a single set of genes.

These cells are haploid. Haploid cells are represented by the symbol N.

• For humans, the haploid number is 23, which can be written as N=23.

• Sperm contains 23 chromosomes and eggs also contain 23 chromosomes

Objectives

Recognize the significance of meiosis to sexual reproduction (TEKS 6G)

• Explain the difference between the chromosome number of body (somatic) cells and gametes.

• Summarize the events of meiosis

• Examine the differences of mitosis and meiosis

• Discuss the significance of meiosis and genetic variation in sexual reproduction

• Describe genetic linkage and the structures that actually assort independently

Meiosis

Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.

• Meiosis involves two divisions, meiosis I and meiosis II.

• By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells.

Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis

Interphase I

Meiosis I

Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.

There are 4 chromatids in a tetrad.

Prophase I

When homologous chromosomes form tetrads in meiosis I, they exchange portions of their chromatids in a process called crossing over.

Crossing-over produces new combinations of alleles.

Spindle fibers attach to the chromosomes.

The homologous chromosomes meet in the middle for metaphase I

Metaphase I

The fibers pull the homologous chromosomes apart toward opposite ends of the cell.

Anaphase I

Telophase I and Cytokinesis

• Nuclear membranes form. • The cell separates into two cells.• Each new daughter cell is now haploid

(N). o It only has one chromosome from

one parent, although that chromosome has been duplicated.

o This is why meiosis must undergo a second division to separate the two sister chromatids into their own cell (sperm or egg)

The two cells produced by meiosis I have chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I.

Telophase II and Cytokinesis

Prophase II Metaphase II Anaphase IITelophase I and Cytokinesis I

Meiosis II

In male animals, meiosis results in four equal-sized gametes called sperm.

In many female animals, only one egg results from meiosis. The other three cells, called polar bodies,

are usually not involved in reproduction.

Objectives

Recognize the significance of meiosis to sexual reproduction (TEKS 6G)

• Explain the difference between the chromosome number of body (somatic) cells and gametes.

• Summarize the events of meiosis

• Examine the differences of mitosis and meiosis

• Discuss the significance of meiosis and genetic variation in sexual reproduction

• Describe genetic linkage and the structures that actually assort independently

Mitosis vs MeiosisMitosis results in the production of two genetically identical diploid cells. Meiosis produces four genetically different haploid cells.

Mitosis• Cells produced by mitosis have the

same number of chromosomes and alleles as the original cell.

• Mitosis allows an organism to grow and replace cells.

• Some organisms reproduce asexually by mitosis.

Meiosis• Cells produced by meiosis have half the

number of chromosomes as the parent cell.

• These cells are genetically different from the diploid cell and from each other.

• Meiosis is how sexually-reproducing organisms produce gametes.

Objectives

Recognize the significance of meiosis to sexual reproduction (TEKS 6G)

• Explain the difference between the chromosome number of body (somatic) cells and gametes.

• Summarize the events of meiosis

• Examine the differences of mitosis and meiosis

• Discuss the significance of meiosis and genetic variation in sexual reproduction

• Describe genetic linkage and the structures that actually assort independently

Meiosis is significant to the genetic variation in offspring for 3 main reasons.

1. Independent assortment of chromosomes

2. Crossing over3. Random fertilization

Objectives

Recognize the significance of meiosis to sexual reproduction (TEKS 6G)

• Explain the difference between the chromosome number of body (somatic) cells and gametes.

• Summarize the events of meiosis

• Examine the differences of mitosis and meiosis

• Discuss the significance of meiosis and genetic variation in sexual reproduction

• Describe genetic linkage and the structures that actually assort independently

Gene Linkage

One problem arises when discussing independent assortment:Two genes will independently (randomly) assort during meiosis if they’re on different chromosomes, but what if they’re on the same chromosome?

Genes that are linked on the same chromosome do NOT segregate independently.

It is the chromosomes that assort independently, not individual genes.

Gene Maps

Thomas Hunt Morgan (the guy who made Drosophila, aka common fruit fly, genetically famous) discovered that many genes seemed to be “linked” together.

Remember Mendel’s peas and his dihybrid cross that demonstrated independent assortment?Here he saw a 9:3:3:1 phenotypic ratio

Linked genes do NOT show this ratio. The phenotypes that are different from the parents are much more rare.

These recombinants are only formed during crossing over

How rare are they?

Gene Maps

• Recombination (crossing over) depends on how far apart the two genes are on the chromosome.

• The farther apart they are on a chromosome, the more likely they are to cross over; producing the recombinant gametes in a higher frequency.

• The closer the genes are on a chromosome, the less likely they are to cross over. This makes those recombinant gametes more rare.