How do organisms reproduce, and how does a fertilized egg develop into a new individual?
Compare sexual and asexual reproduction, explain fertilization and early development from zygote to embryo, and describe the role of reproductive structures in humans (NYSSLS LS1 and LS3, patterns; systems and system models).
A NYSSLS-level answer on reproduction for the New York Life Science: Biology Regents: sexual versus asexual reproduction, fertilization and the zygote, early development into an embryo, and the role of human reproductive structures.
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What this topic is asking
NYSSLS asks you to compare the two kinds of reproduction and to explain how a fertilized egg develops into a new individual. On the Life Science: Biology Regents this is often set in a real context (a gardener, a population, a developing embryo) and tested with the crosscutting concepts of patterns and systems and system models.
Sexual versus asexual reproduction
The key contrast is variation. Sexual reproduction shuffles alleles (through meiosis and fertilization, see meiosis and sexual reproduction), so offspring differ from their parents and one another. Asexual reproduction copies the parent's DNA by mitosis, so offspring are identical.
Advantages of each
Neither is universally better; the advantage depends on the environment. A changing environment favors the variation of sexual reproduction, while a stable one can favor the speed and reliability of asexual reproduction. The exam often asks you to weigh one advantage of each.
Fertilization and the zygote
In sexual reproduction, fertilization is the fusion of a sperm and an egg (the gametes) to form a single cell, the zygote. Because each gamete carries half the chromosome number (23 in humans), the zygote has the full number (46). The zygote is the first cell of the new individual and is genetically unique, carrying a combination of alleles from both parents.
From zygote to embryo
The zygote divides repeatedly by mitosis, producing a ball of identical cells. These cells then differentiate, switching on different genes to become specialized cell types (nerve, muscle, blood and so on), forming an embryo and eventually all the tissues and organs of the body. The crucial point is that every body cell carries the same DNA; the cells differ because they express different genes (see cell differentiation and gene expression). In humans the embryo develops in the uterus, supported by the placenta, which exchanges materials between parent and embryo.
Try this
Q1. State one advantage of sexual reproduction and one advantage of asexual reproduction. [2]
- Cue. Sexual: genetic variation (adaptability, disease resistance). Asexual: fast, needs one parent, offspring reliably identical to a successful parent.
Q2. Explain why all the cells of a human body contain the same DNA even though they are very different. [2]
- Cue. The body develops from one zygote by mitosis (which copies the DNA exactly); the cells differ because they express different genes, not because their DNA differs.
Exam-style practice questions
Practice questions written in the style of NYSED exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Regents (Life Science sample, 2024)3 marksA gardener can grow new strawberry plants either from seeds (sexual reproduction) or from runners that produce clones (asexual reproduction). (a) State one advantage of the sexual method for the population. (b) State one advantage of the asexual method for the gardener. (c) Explain why the clones are genetically identical to the parent plant.Show worked answer →
A 3-point constructed-response item assessing patterns and cause and effect.
(a) 1 point: sexual reproduction produces genetic variation, so the population is better able to adapt to changing conditions or resist disease.
(b) 1 point: the asexual method reliably produces offspring identical to a parent with desired traits, and can be faster with one parent.
(c) 1 point: clones are produced by mitosis from the parent's cells, so they receive identical copies of the parent's DNA and are genetically identical.
Markers reward variation (sexual) versus reliable identical offspring (asexual) and mitosis explaining the clones.
Regents (Life Science CR, 2025)2 marksIn humans, fertilization produces a zygote that develops into an embryo. (a) Define fertilization. (b) Explain how a single zygote can give rise to the many different cell types in the body.Show worked answer →
A 2-point item on fertilization and early development.
(a) 1 point: fertilization is the fusion of a sperm and an egg (gametes) to form a zygote.
(b) 1 point: the zygote divides repeatedly by mitosis to make many cells; these cells then differentiate (express different genes) to become specialized cell types, all carrying the same DNA.
Markers reward "fusion of gametes" and "mitosis then differentiation" for the variety of cells.
Related dot points
- Explain how cells with the same DNA become specialized through differential gene expression, and describe the role of stem cells in development and repair (NYSSLS LS1, structure and function; cause and effect).
A NYSSLS-level answer on differentiation for the New York Life Science: Biology Regents: how cells with identical DNA specialize by expressing different genes, what stem cells are, and how this builds and maintains a multicellular body.
- Explain how meiosis produces gametes with half the chromosome number and generates genetic variation through crossing over and independent assortment, and how fertilization restores the chromosome number (NYSSLS LS3, patterns; cause and effect).
A NYSSLS-level answer on meiosis for the New York Life Science: Biology Regents: how meiosis halves the chromosome number to make gametes, how crossing over and independent assortment create variation, and how fertilization restores the chromosome number.
- Explain how mitosis and the cell cycle produce two genetically identical cells, describe its role in growth, repair and asexual reproduction, and explain how uncontrolled division leads to cancer (NYSSLS LS1 and LS3, stability and change; cause and effect).
A NYSSLS-level answer on mitosis for the New York Life Science: Biology Regents: the cell cycle, how mitosis produces two identical cells, its role in growth, repair and asexual reproduction, and what happens when division is not controlled.
- Explain how the nervous system (neurons and signals) and the endocrine system (hormones) coordinate responses and maintain homeostasis, comparing the speed and duration of their effects (NYSSLS LS1, systems and system models; stability and change).
A NYSSLS-level answer on coordination for the New York Life Science: Biology Regents: how neurons carry nerve signals, how hormones act more slowly and widely, how the two systems compare, and how they maintain homeostasis.
- Use the rules of inheritance (dominant and recessive alleles, genotype and phenotype) and Punnett squares to predict the outcomes of genetic crosses, and interpret pedigrees (NYSSLS LS3, patterns; using mathematics).
A NYSSLS-level answer on inheritance for the New York Life Science: Biology Regents: alleles, genotype and phenotype, dominant and recessive traits, using Punnett squares to predict ratios and probabilities, and reading pedigrees.
Sources & how we know this
- New York State P-12 Science Learning Standards (Life Science) — New York State Education Department (2016)
- Educator Guide to the Regents Examination in Life Science: Biology — New York State Education Department (2025)