Asexual reproduction involves one parent and produces offspring that are genetically identical to each other and to the parent. Sexual reproduction involves two parents and produces offspring that is genetically unique. The sequence of stages an organism goes through from one generation to the next is its life cycle.
- CA.9–12.IE.1.d, CA.9–12.LS.1.c; CA.9–12.LS.2.a, b, d, e
- NSES.9–12.A.1.4, 6; NSES.9–12.C.1.6; NSES.9–12.C.2.2
- AAAS.9–12.5.B.2, 3; AAAS.9–12.5.C.4, 5
- Compare and contrast asexual and sexual reproduction.
- Give an overview of sexual reproduction, and outline the phases of meiosis.
- Explain why sexual reproduction leads to variation in offspring.
- Define life cycle, and identify different types of sexual life cycles.
asexual reproduction: reproduction that involves a single parent and results in offspring that are all genetically identical to the parent
crossing-over: exchange of genetic material between homologous chromosomes when they are closely paired during meiosis I
diploid: having two of each type of chromosome
egg: female gamete
fertilization: union of two gametes that produces a diploid zygote
gamete: reproductive cell produced during meiosis that has the haploid number of chromosomes
gametogenesis: development of haploid cells into gametes such as sperm and egg
haploid: having only one chromosome of each type
independent assortment: independent segregation of chromosomes to gametes during meiosis
life cycle: series of stages a sexually reproducing organism goes through from one generation to the next
meiosis: type of cell division in which the number of chromosomes is reduced by half and four haploid cells result
sexual reproduction: type of reproduction that involves the fertilization of gametes produced by two parents and produces genetically variable offspring
sperm: male gamete
zygote: diploid cell that forms when two haploid gametes unite during fertilization
Check Your Understanding
In the previous lesson, students were taught about mitosis. Before launching into the material in this lesson, have a classroom discussion about mitosis. Ask students to recall what they learned about mitosis, and if there are any steps of the process that are unclear to them. Discuss how mitosis and meiosis will share some similar mechanisms.
Introducing the Lesson
Sketch a simple diagram of human fertilization on the board or an overhead. A sample diagram is shown below. Point out that a zygote, like other human cells, has 23 pairs of chromosomes, or 46 chromosomes altogether. Ask students to explain how two cells (the egg and sperm) can combine and produce a cell (the zygote) with the normal number of chromosomes. (Students should infer that the egg and sperm each contain just 23 chromosomes, or one of each pair.) Tell the class they will learn in this lesson how egg and sperm cells form with half the normal number of chromosomes.
Build Science Skills
Review the pros and cons of asexual vs. sexual reproduction. Challenge students to predict conditions under which each type of reproduction might be advantageous. Then assign Problem 5 (Asexual vs. sexual reproduction) at the URL below.
Pair English language learners with native English speakers. Ask pairs to make a table comparing and contrasting mitosis and meiosis. Suggest they add the table to their science notebook. ELL
Ask a few students to make diagrams illustrating how crossing-over and independent assortment occur and how they increase genetic variation in gametes. Students might use different colors to distinguish chromosomes and parts of chromosomes. Sample diagrams of crossing-over and independent assortment are shown below. Have students explain their diagrams to the class.
Present the following scenario to students: During metaphase II of meiosis, one duplicated pair of chromosomes fails to attach to a spindle. Despite this error, meiosis and cytokinesis progress and two daughter cells result.
- Ask: What chromosomes will each daughter cell have? (One of the daughter cells will have an extra copy of the chromosome that failed to attach to the spindle; the other daughter cell will have no copies of that chromosome.)
Correct the following common student misconceptions about sexual and asexual reproduction. Explain why each statement is false.
- Sexual reproduction always involves mating. (False. Sexual reproduction also occurs in other ways, such as conjugation in bacteria and cross-pollination in plants.)
- Plants do not reproduce sexually. (False. Many plants reproduce sexually. In flowering plants, pollen is the male gamete and an egg is the female gamete.)
- Asexual reproduction occurs only in microorganisms. (False. Plants and fungi may also reproduce asexually.)
Reinforce and Review
Copy and distribute the lesson worksheets in the CK-12 Biology Workbook. Ask students to complete the worksheets alone or in pairs as a review of lesson content.
Have students answer the Review Questions listed at the end of the lesson in the FlexBook®.
Points to Consider
In sexually reproducing organisms, parents pass a copy of each type of chromosome to their offspring by producing gametes. When gametes are fertilized and form offspring, each has a unique combination of chromosomes and genes from both parents. The inherited gene combination determines the characteristics of the offspring.
- Is it possible to predict possible gene combinations in offspring from the genes of their parents?
- Can the characteristics of offspring be predicted from the characteristics of their parents?