parents can produce many types of offspring · recognize the significance of meiosis to sexual...

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 living cell has DNA • Chromosomes are DNA • Genes are located on chromosomes

o genes control the TRAITS of the individual

• The number of chromosomes depends on

the species o 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 7 in this karyotype has two chromosomes. one from “mom” and one 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 7 contains the gene for cystic fibrosis

(CF). • CF occurs when two recessive copies of the gene are

combined and causes those people to secrete abnormal body fluids. This includes unusual sweat and a thick mucus that prevents the body from properly cleansing the lungs. The mucus interrupts the function of vital organs and leads to chronic infections.

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 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







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.

1 2

Meiosis

• Just prior to meiosis I, the cell undergoes a round of chromosome replication during interphase (S phase).

• As in mitosis, each replicated chromosome consists of two identical chromatids joined at the center.

Meiosis

• These identical chromatids are called sister chromatids. They are genetically identical to one another. • Homologous chromosomes are

NOT genetically identical. They simply carry the same set of genes (but the alleles for those genes can be different).

• Sister chromatids are joined

together at their centromere and aren’t pulled apart until meiosis II

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 (sister chromatid).

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 II Telophase 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







Mitosis vs Meiosis Mitosis 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







Meiosis is significant to the genetic variation in offspring for 3 main reasons. 1. Independent assortment of

chromosomes 2. Crossing over 3. Random fertilization

Objectives







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 P generation 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.

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