Sexual reproduction produces genetic diversity among offspring, which increases the chance that some offspring survive in changing environments.
Question #1
Which two organisms are genetically the most similar?
Pair A
Pair B
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ANSWER
Which two organisms are genetically the most similar?
Pair A
Pair B
Pair A are identical to each other
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Karyotype
During cell division is when chromosomes are the most compacted and visible. The division process is stopped and stains are added. A photograph is taken and chromosome pairs are arranged.
This image is called a karyotype and can tell a lot about chromosome structure in a body cell.
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Karyotype
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Diploid Cells Contain Two Homologous Sets of Chromosomes
Section 9.2 Figure 9.3
Most cells in a sexually reproducing organism have two sets of chromosomes: one from the father and one from the mother.
Diploid Cells Contain Two Homologous Sets of Chromosomes
Section 9.2 Figure 9.3
The chromosomes denoted by a letter are the sex chromosomes. In humans, an individual with two X chromosomes is female, and an individual with one X and one Y chromosome is male.
Diploid Cells Contain Two Homologous Sets of Chromosomes
Section 9.2 Figure 9.4
The chromosomes of a homologous pair look alike and carry the same sequence of genes for the same traits.
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Diploid Cells Contain Two Homologous Sets of Chromosomes
Section 9.2 Figure 9.4
However, the members of a homologous pair may carry different alleles, which are alternative versions of the same gene.
Question #2
The photo below shows two chromosomes from one cell. Which statement is true?
A. Both came from theperson’s mother.B. The 1‐2 pair is identicalto the 3‐4 pair.C. 1 is identical to 2.D. The chromosomes havenot been replicated yet.
1 2 3 4
ANSWER
The photo below shows two chromosomes from one cell. Which statement is true?
A. Both came from theperson’s mother.B. The 1‐2 pair is identicalto the 3‐4 pair.C. 1 is identical to 2.D. The chromosomes havenot been replicated yet.
During crossing over, two homologous chromosomes exchange genetic material during prophase I.
Figure 10.3b
Exchange of DNA during prophase I increases genetic variability. Chromatids are no longer exact duplicates.
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Crossing‐Over
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Meiosis Generates Variability
Section 9.5 Figure 9.9
In independent assortment, chromosome pairs align randomly during metaphase I.
Meiosis Generates Variability
Section 9.5 Figure 9.9
A germ cell with three pairs of chromosomes has four possible arrangements during metaphase I, yielding eight possible gametes (23 = 8).
During metaphase, chromosomes line up in PAIRS, but they line up randomly. This picture shows all the different possible arrangements for an organism with 6 chromosomes.
This is called INDEPENDENT ASSORTMENT
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Check for understanding
1. What phase directly follows metaphase I?
2. How many cells are present at the end of meiosis I ?
3. A cell has a diploid number of 60, what is the organism's haploid number?
4. Meiosis occurs in what type of cells?
5. In what phase do homologous chromosomes pair up and crossing-over can occur?
6. In what phase do the CHROMATIDS separate?
anaphase I
2
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gametes
prophase 1
anaphase 2
7. Identify the phase: 8. Identify the phase:
anaphase 1 anaphase 2
Meiosis Generates Variability
Section 9.5 Figure 9.10
If meiosis generates such great variability among gametes, how can siblings be identical?
Many sets of triplets consist of a “pair and a spare;” that is, two identical babies and one non‐identical baby. Producing this arrangement requires ___ sperm and ___ eggs.
A. 2 … 2B. 1 … 3C. 3 … 1D. 2 … 3
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ANSWER
Many sets of triplets consist of a “pair and a spare;” that is, two identical babies and one non‐identical baby. Producing this arrangement requires ___ sperm and ___ eggs.
