Cell Division and Reproduction
2. Sexual Reproduction
Learning outcomes
-
I can define sexual reproduction.
- I can explain the role of gametes in sexual reproduction.
- I can describe how genetic information is inherited from two parents.
- I can compare sexual and asexual reproduction.
- I can explain how sexual reproduction increases genetic variation.
What Is Sexual Reproduction?
Sexual reproduction is a form of reproduction involving the fusion of:
male and female gametes.
A gamete is a reproductive cell.
In animals, the gametes are usually:
Sperm cell = male gamete
Egg cell = female gamete
When the two gametes join, their nuclei fuse in a process called:
fertilization.
Fertilization produces a new cell called a:
zygote.
The zygote contains genetic information from:
both parents.
The Main Idea
Sexual reproduction can be summarized as:
Male gamete + Female gamete → Fertilization → Zygote → New organism
Each gamete contributes genetic information to the:
offspring.
As a result, sexually produced offspring are genetically different from:
their parents and one another.
The Role of Gametes
Gametes are specialized cells that carry:
genetic information from one generation to the next.
In humans:
Sperm cells carry 23 chromosomes.
Egg cells carry 23 chromosomes.
During fertilization:
23 + 23 = 46 chromosomes
The resulting zygote has:
46 chromosomes.
It therefore receives approximately half of its nuclear genetic information from each:
parent.
Why Do Gametes Have Half the Chromosome Number?
Most body cells contain chromosomes in:
pairs.
These cells are described as:
diploid.
Gametes contain only one chromosome from each pair.
They are described as:
haploid.
This is important because fertilization combines:
two haploid gametes.
The normal diploid chromosome number is then:
restored.
Haploid and Diploid
We can represent the chromosome number using:
n and 2n.
Haploid = n
Diploid = 2n
Therefore:
n + n → 2n
For humans:
23 + 23 → 46
The sperm and egg are:
haploid.
The zygote is:
diploid.
Why Chromosome Number Must Be Reduced
Imagine if gametes contained the full number of chromosomes.
In humans:
46 + 46 = 92
The chromosome number would double every:
generation.
Instead, gametes contain half the normal chromosome number so that fertilization restores the correct:
diploid number.
How Are Gametes Produced?
Gametes are produced by a special type of cell division called:
meiosis.
Meiosis reduces the chromosome number from:
diploid to haploid.
It also produces genetic differences among:
gametes.
This is one of the reasons sexual reproduction creates:
genetic variation.
Mitosis vs Meiosis
Do not confuse:
mitosis
with:
meiosis.
Mitosis usually produces cells used for:
growth, repair, and asexual reproduction.
Meiosis produces:
gametes for sexual reproduction.
A simplified comparison is:
| Feature | Mitosis | Meiosis |
|---|---|---|
| Main role | Growth and repair | Gamete production |
| Chromosome number | Maintained | Halved |
| Genetic similarity | Usually very similar | Genetically varied |
| Sexual reproduction | Not directly | Yes |
Fertilization
Fertilization occurs when the nuclei of male and female gametes:
fuse.
The genetic material from the two gametes is:
combined.
This produces a:
zygote.
The zygote is the first cell of the new:
organism.
From Zygote to Organism
After fertilization, the zygote begins dividing by:
mitosis.
One cell becomes:
2 cells
then:
4 cells
then:
8 cells
and so on.
As development continues, cells become specialized and form:
tissues and organs.
Eventually, a complete organism develops.
Genetic Information from Two Parents
DNA contains the genetic instructions used in the development and functioning of an:
organism.
During sexual reproduction, offspring inherit DNA from:
two parents.
One set of chromosomes comes from the:
male gamete.
Another set comes from the:
female gamete.
Therefore, the offspring contains a unique combination of:
genetic information.
Chromosome Pairs
In humans, body cells normally contain:
23 pairs of chromosomes.
For each pair:
one chromosome was inherited from the biological mother
and:
one chromosome was inherited from the biological father.
This means that genes are commonly present in:
pairs of versions.
These different versions of genes are called:
alleles.
Genes and Alleles
A gene is a section of DNA that contributes to a particular biological characteristic or function.
Different versions of the same gene are called:
alleles.
An offspring may inherit:
one allele from one parent
and:
another allele from the other parent.
The combination contributes to the offspring's:
genotype and characteristics.
Why Siblings Are Different
Brothers and sisters with the same biological parents usually do not have exactly the same:
genetic information.
This is because meiosis produces genetically different:
gametes.
Also, which sperm fertilizes which egg is largely:
random.
Each fertilization therefore creates a different combination of:
alleles.
Genetic Variation
Genetic variation means differences in genetic information among individuals.
Sexual reproduction is an important source of genetic variation because it:
- combines DNA from two parents
- produces genetically different gametes
- involves random combinations of gametes during fertilization
The offspring therefore receive:
new combinations of alleles.
Meiosis Creates Variation
During meiosis, chromosomes are distributed into gametes in different:
combinations.
Chromosomes can also exchange sections of DNA through:
crossing over.
As a result, the gametes produced by one individual are not normally:
genetically identical.
This greatly increases the number of possible genetic combinations in:
offspring.
Independent Assortment
Chromosome pairs are distributed independently during:
meiosis.
This process is called:
independent assortment.
Different gametes therefore receive different combinations of:
maternal and paternal chromosomes.
Independent assortment is one important source of:
genetic variation.
Crossing Over
During meiosis, matching chromosome pairs can exchange sections of:
DNA.
This process is called:
crossing over.
Crossing over creates chromosomes containing new combinations of:
alleles.
This further increases variation among:
gametes.
Random Fertilization
Variation increases even further because fertilization is:
random.
One sperm out of many possible sperm fertilizes:
one particular egg.
Each sperm and egg may contain a different combination of:
alleles.
Therefore, fertilization creates an enormous number of possible:
genetic combinations.
Three Major Sources of Variation
Sexual reproduction generates genetic variation through:
1. Independent assortment during meiosis
Chromosomes are distributed into gametes in different combinations.
2. Crossing over during meiosis
Chromosomes exchange sections of DNA.
3. Random fertilization
Different combinations of sperm and egg can unite.
Together, these processes produce:
genetically unique offspring.
Sexual Reproduction in Animals
In animals, sexual reproduction usually involves:
sperm and egg cells.
Sperm cells are typically small and adapted for reaching the:
egg.
Egg cells are generally larger and contain resources that support early:
development.
Fertilization may occur:
inside or outside the body.
Internal Fertilization
In internal fertilization, sperm and egg unite inside the:
female reproductive system.
This occurs in many:
- mammals
- reptiles
- birds
- insects
Internal fertilization can help protect gametes and developing embryos from:
environmental conditions.
External Fertilization
In external fertilization, gametes are released into the:
environment.
Fertilization then occurs outside the parents' bodies.
This is common in some:
- fish
- amphibians
- aquatic invertebrates
Large numbers of gametes may be released because many will:
not successfully meet or survive.
Sexual Reproduction in Plants
Plants also reproduce:
sexually.
In flowering plants:
pollen contains the male gametes
and:
ovules contain the female gametes.
Pollination transfers pollen to the:
stigma.
A pollen tube can then grow toward the ovule.
Eventually, fertilization occurs when male and female gametes:
fuse.
Fertilization in Flowering Plants
A simplified sequence is:
Pollination
↓
Pollen lands on stigma
↓
Pollen tube grows
↓
Male gamete travels toward ovule
↓
Fertilization
↓
Zygote forms
↓
Seed develops
The seed contains an embryo produced through:
sexual reproduction.
Sexual Reproduction in Different Organisms
Sexual reproduction occurs in many:
animals, plants, fungi, and other eukaryotes.
The details vary greatly.
However, the central principle remains:
genetic material from different gametes is combined to produce offspring.
Sexual vs Asexual Reproduction
Sexual and asexual reproduction both produce:
new organisms.
However, they differ in important ways.
| Feature | Sexual Reproduction | Asexual Reproduction |
|---|---|---|
| Gametes | Involved | Usually not involved |
| Fertilization | Occurs | Does not occur |
| Genetic sources | Usually two gametes | Usually one parent |
| Genetic variation | High | Usually low |
| Speed | Often slower | Often faster |
| Mate required | Often | No |
| Offspring | Genetically varied | Usually genetically very similar |
Similarities Between Sexual and Asexual Reproduction
The two forms of reproduction also have important:
similarities.
Both:
- produce new organisms
- pass DNA to offspring
- involve cell division
- allow populations to continue
- transfer genetic information between generations
The major difference concerns:
how the genetic information is combined.
Advantages of Sexual Reproduction
The major advantage of sexual reproduction is:
genetic variation.
Individuals within a population have different combinations of:
alleles.
This variation can be especially important when environmental conditions:
change.
Variation and Environmental Change
Imagine a population experiencing a new:
disease.
If individuals are genetically different, some may have characteristics that make them more:
resistant.
These individuals may be more likely to survive and:
reproduce.
Genetic variation therefore provides the raw material for:
natural selection and evolution.
Variation Does Not Guarantee Survival
Genetic variation does not mean that every individual will:
survive.
It means that individuals differ genetically.
Some variations may be:
- beneficial
- neutral
- harmful
Their effects often depend on the:
environment.
Advantages in Changing Environments
Sexual reproduction can be particularly valuable when:
environmental conditions change.
Genetic variation means that individuals may respond differently to:
- disease
- temperature changes
- drought
- predators
- competition
- changes in food availability
A diverse population may therefore have a greater range of characteristics on which:
natural selection can act.
Disadvantages of Sexual Reproduction
Sexual reproduction also has:
costs.
In many species, individuals must:
find a mate.
This can require:
- time
- energy
- competition
- courtship
- travel
Reproduction may therefore be slower than:
asexual reproduction.
Fewer Offspring in the Same Time
Some asexually reproducing organisms can produce large numbers of offspring:
very quickly.
Sexual reproduction often requires more:
time and energy.
As a result, population growth may sometimes be:
slower.
Successful Gene Combinations Are Reshuffled
Asexual reproduction can preserve a successful genotype almost:
unchanged.
Sexual reproduction continually reshuffles:
alleles.
This produces variation, but it also means that a particularly successful combination of alleles is not necessarily passed to offspring:
unchanged.
When Might Sexual Reproduction Be Advantageous?
Sexual reproduction can be especially useful when:
- environmental conditions change
- diseases are present
- populations face new challenges
- genetic diversity improves the chance that some individuals survive
Its major strength is:
variation.
When Might Asexual Reproduction Be Advantageous?
Asexual reproduction can be especially useful when:
- conditions are stable
- rapid reproduction is beneficial
- mates are difficult to find
- an organism is already well adapted
- preserving a successful genotype is useful
Its major strengths are:
speed and efficiency.
Neither Method Is Always Better
Sexual and asexual reproduction are different:
reproductive strategies.
Asexual reproduction can be extremely successful under:
stable conditions.
Sexual reproduction can provide important advantages when environments:
change.
Some organisms can even use:
both methods.
Organisms That Use Both Strategies
Some organisms reproduce sexually under some conditions and asexually under:
others.
For example, some plants can reproduce through:
seeds
and through:
vegetative propagation.
This gives them access to advantages from:
both reproductive strategies.
Example: Strawberry Plants
Strawberry plants can reproduce asexually using:
runners.
This allows them to spread rapidly and produce:
genetically similar plants.
They can also reproduce sexually through:
flowers and seeds.
Sexual reproduction introduces new:
genetic combinations.
Example: Genetic Variation in a Family
Consider two biological parents with several children.
The children may share characteristics because they inherited genes from the:
same parents.
However, they are usually not genetically identical.
Each child receives a different combination of:
alleles.
This explains why siblings can resemble one another while still being:
different individuals.
Identical Twins: An Important Exception
Identical twins develop when one fertilized egg splits into:
two embryos.
Because both embryos developed from the same zygote, they have nearly identical:
genetic information.
They are an exception to the usual pattern of genetically different offspring from:
sexual reproduction.
Environmental influences and later mutations can still create some differences between them.
Sexual Reproduction and Evolution
Sexual reproduction contributes to evolution by generating:
genetic variation.
Natural selection acts on differences among:
individuals.
If certain inherited characteristics improve reproductive success in a particular environment, the alleles associated with those characteristics may become more common over:
generations.
Sexual reproduction therefore contributes to the genetic diversity upon which:
evolutionary processes operate.
Sexual Reproduction and Adaptation
An adaptation is an inherited characteristic that increases reproductive success in a particular:
environment.
Sexual reproduction does not deliberately produce organisms with the adaptations they:
need.
Instead, it generates:
variation.
Natural selection can then favour inherited variations that happen to be advantageous under particular:
conditions.
A Common Misconception
It is incorrect to say:
Organisms reproduce sexually because they need to create useful adaptations.
Sexual reproduction does not predict what characteristics will be:
useful.
Instead:
variation occurs first
and:
selection acts on that variation.
Worked Example: Chromosome Number
A species has:
20 chromosomes in its body cells.
What chromosome number should its gametes contain?
Body cells are:
diploid = 2n = 20
Therefore:
n = 10
Each gamete contains:
10 chromosomes.
At fertilization:
10 + 10 = 20
The diploid chromosome number is:
restored.
Worked Example: Identifying the Process
A cell contains half the chromosome number of ordinary body cells and is able to fuse with another reproductive cell.
This cell is a:
gamete.
When two gametes fuse, the process is:
fertilization.
The cell produced is called a:
zygote.
Worked Example: Comparing Reproduction
Population A reproduces rapidly from one parent and produces genetically similar offspring.
This is most likely:
asexual reproduction.
Population B produces offspring by combining genetic information from gametes.
This is:
sexual reproduction.
Population B is likely to show greater:
genetic variation.
The Sexual Reproduction Cycle
A simplified cycle is:
Diploid adult
↓
Meiosis
↓
Haploid gametes
↓
Fertilization
↓
Diploid zygote
↓
Mitosis and development
↓
Diploid adult
This cycle maintains the chromosome number from:
generation to generation.
Common Mistake: Sexual Reproduction Always Means Two Parents
The defining feature of sexual reproduction is the:
fusion of gametes.
In many familiar organisms those gametes come from two separate individuals.
However, some organisms can produce both types of gametes.
Therefore, the most precise definition focuses on:
gamete fusion and genetic recombination.
Common Mistake: Fertilization and Pollination Are the Same
In flowering plants:
pollination is the transfer of pollen to a stigma.
fertilization is the fusion of male and female gametes.
Pollination usually occurs:
before fertilization.
They are related but:
different processes.
Common Mistake: Gametes Are Produced by Mitosis
In animals and many other organisms, gametes are produced through:
meiosis.
Meiosis halves the chromosome number and contributes to:
genetic variation.
Mitosis maintains chromosome number and is primarily associated with:
growth and repair.
Common Mistake: Sexual Reproduction Creates Mutations
Sexual reproduction mainly increases variation by creating:
new combinations of existing alleles.
Mutations are changes in:
DNA.
Mutations create new genetic variants, while sexual reproduction:
reshuffles genetic information.
Both contribute to genetic diversity, but they are:
different processes.
Common Mistake: Variation Means Every Trait Is Different
Sexually produced offspring still inherit many genes from their:
parents.
They may strongly resemble parents and siblings.
Genetic variation means that their overall combinations of alleles are:
not identical.
Check Your Understanding
1. Define sexual reproduction.
2. What is a gamete?
3. Name the male and female gametes in humans.
4. What is fertilization?
5. What is a zygote?
6. What type of cell division produces gametes?
7. Why must gametes contain half the normal chromosome number?
8. Define haploid.
9. Define diploid.
10. How many chromosomes are found in a human sperm cell?
11. How many chromosomes are found in a human egg cell?
12. How many chromosomes are normally found in the resulting human zygote?
13. Explain how an offspring receives genetic information from two parents.
14. Why are siblings usually genetically different?
15. Explain how meiosis contributes to genetic variation.
16. What is independent assortment?
17. What is crossing over?
18. Explain how random fertilization increases genetic variation.
19. Give three differences between sexual and asexual reproduction.
20. Give two similarities between sexual and asexual reproduction.
21. Why does sexual reproduction generally produce greater genetic variation?
22. Explain why genetic variation may be advantageous when environmental conditions change.
23. Explain why variation may help a population respond to disease.
24. Give one disadvantage of sexual reproduction.
25. Why can sexual reproduction require more energy than asexual reproduction?
26. Explain the difference between pollination and fertilization.
27. Describe sexual reproduction in flowering plants.
28. A species has 32 chromosomes in its body cells. How many chromosomes should each gamete contain?
29. Explain how fertilization restores the diploid chromosome number.
30. Explain why sexual reproduction is important for genetic diversity and evolution.
Key Terms
- Sexual reproduction: Reproduction involving the fusion of gametes and the combination of genetic information.
- Gamete: Haploid reproductive cell.
- Sperm: Male gamete in animals.
- Egg: Female gamete in animals.
- Fertilization: Fusion of male and female gametes or their nuclei.
- Zygote: Diploid cell formed during fertilization.
- Meiosis: Cell division that produces haploid cells and contributes to genetic variation.
- Mitosis: Cell division producing genetically similar daughter cells while normally maintaining chromosome number.
- Haploid: Having one set of chromosomes.
- Diploid: Having two sets of chromosomes.
- Chromosome: DNA-containing structure carrying genes.
- Gene: Section of DNA containing genetic information.
- Allele: Alternative version of a gene.
- Genetic variation: Genetic differences among individuals.
- Independent assortment: Distribution of chromosome pairs into different combinations during meiosis.
- Crossing over: Exchange of DNA between homologous chromosomes during meiosis.
- Random fertilization: Random combination of male and female gametes.
- Genotype: Genetic makeup of an organism.
- Pollination: Transfer of pollen to the stigma of a flower.
- Adaptation: Inherited characteristic that increases reproductive success in a particular environment.
- Natural selection: Process in which inherited differences influence survival and reproductive success.
- Genetic diversity: Variety of genetic information within a population.
Key Takeaways
- Sexual reproduction involves the fusion of gametes.
- Gametes carry genetic information from one generation to the next.
- Sperm and egg cells are examples of gametes.
- Gametes are haploid, meaning they contain one set of chromosomes.
- The zygote formed during fertilization is diploid.
- Human gametes normally contain 23 chromosomes, while a human zygote normally contains 46.
- Gametes are produced through meiosis.
- Meiosis halves the chromosome number.
- Fertilization restores the diploid chromosome number.
- Offspring inherit genetic information through chromosomes contributed by the gametes.
- Sexual reproduction creates new combinations of alleles.
- Meiosis produces genetically different gametes.
- Independent assortment contributes to genetic variation.
- Crossing over creates new combinations of alleles.
- Random fertilization further increases genetic variation.
- Sexually produced siblings are usually genetically different from one another.
- Sexual reproduction generally produces more genetic variation than asexual reproduction.
- Asexual reproduction usually produces genetically similar offspring more rapidly.
- Sexual reproduction may require more time and energy and often requires finding a mate.
- Genetic variation can be particularly important when environments change.
- Variation can mean that individuals respond differently to disease and other environmental pressures.
- Genetic variation provides material on which natural selection can act.
- Sexual reproduction does not deliberately create useful adaptations.
- Mutations create new genetic variants, while sexual reproduction primarily reshuffles existing genetic information.
- Plants as well as animals reproduce sexually.
- Pollination and fertilization are different processes.
- Some organisms can reproduce both sexually and asexually.
- Neither reproductive strategy is universally superior; each has advantages under different environmental conditions.
- The central sequence is: meiosis → gametes → fertilization → zygote → growth and development.