Meiosis:

Georgia Performance Standards:

• Using Mendel’s laws, explain the role of meiosis in reproductive variability.

Essential Questions:• How does meiosis generate variation in offspring?

Forever Linked?

• Some genes appear to be inherited together, or “linked.”

• If two genes are found on the same chromosome, does it mean they are linked forever?

• Study the diagram, which shows four genes labeled A–E and a–e, and then answer the questions on the next slide.

Section 11-5

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Linked Genes Warm-up

1. In how many places can crossing over result in genes A and b being on the same chromosome?

2. In how many places can crossing over result in genes A and c being on the same chromosome? Genes A and e?

3. How does the distance between two genes on a chromosome affect the chances that crossing over will recombine those genes?

Linked Genes:

• It’s easy to see how genes located on different chromosomes assort independently, but what about genes located on the same chromosome?

• Wouldn’t they generally be inherited together?

Linked Genes

• First, each chromosome is actually a group of linked genes.

• Second, Mendel’s principle of independent assortment still holds true.    

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

Genes and Linkage:

• If two genes are found on the same chromosome, does this mean that they are linked forever? Not at all.

• Crossing-over separates genes that had been on the same chromosome onto homologous chromosomes.

• Crossover events

– separate and exchange linked genes

– produce new combinations of alleles

– it helps to generate genetic diversity.

Gene Maps

• A gene map shows the relative locations of each known gene on a chromosomes

• Recombination rates

– measure the frequencies of crossing-over between genes

– used to construct genetic maps

• Ex: human genome map

Earth

Country

State

City

People

Cell

Chromosome

Chromosome fragment

Gene

Nucleotide base pairs

Section 11-5

Comparative Scale of a Gene Map

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Mapping of Earth’s Features

Mapping of Cells, Chromosomes, and Genes

Exact location on chromosomes Chromosome 2

Section 11-5Figure 11-19 Gene of the Fruit Fly

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Checkpoint Questions:1. How does the principle of independent assortment

apply to chromosomes?

2. What are gene maps, and how are they produced?

3. How does crossing-over make gene mapping possible?

4. If two genes are on the same chromosome but usually assort independently, what does that tell you about how close together they are?