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Reproduction - Sexual Reproduction

Grade 9CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Sexual reproduction involves the fusion of two specialized cells called gametes (germ cells), one from each parent, to form a single cell called a zygote.

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Gametes are haploid, meaning they contain half the number of chromosomes (nn) compared to normal body cells (somatic cells).

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The fusion of male and female gametes restores the diploid number of chromosomes (2n2n) in the offspring, ensuring the stability of the species' DNA.

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In flowering plants, the reproductive parts are located in the flower. The stamen is the male reproductive part (producing pollen grains), and the carpel/pistil is the female reproductive part (containing the ovary, style, and stigma).

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Pollination is the transfer of pollen grains from the anther to the stigma. It can be self-pollination (within the same flower/plant) or cross-pollination (between different plants).

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In humans, puberty marks the period where reproductive tissues begin to mature. In males, testes produce sperms and testosterone; in females, ovaries produce eggs (ova), estrogen, and progesterone.

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The placenta is a specialized tissue that connects the fetus to the uterine wall, facilitating the exchange of nutrients (O2O_2 and glucose) and waste (CO2CO_2 and urea).

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The gestation period in humans is approximately 99 months or 38−4038-40 weeks.

📐Formulae

n+n=2nn + n = 2n

Ploidy of Zygote=Ploidy of Male Gamete+Ploidy of Female Gamete\text{Ploidy of Zygote} = \text{Ploidy of Male Gamete} + \text{Ploidy of Female Gamete}

Number of Chromosomes in Gamete=Number of Chromosomes in Somatic Cell2\text{Number of Chromosomes in Gamete} = \frac{\text{Number of Chromosomes in Somatic Cell}}{2}

💡Examples

Problem 1:

In humans, a sperm cell contains 2323 chromosomes. Calculate the total number of chromosomes present in the zygote after fertilization.

Solution:

The number of chromosomes in the zygote is calculated by adding the chromosomes from the male and female gametes: 23+2346\begin{array}{r} 23 \\ + 23 \\ \hline 46 \end{array}

Explanation:

Since both the sperm and the egg are haploid (n=23n = 23), their fusion results in a diploid zygote with 2n=462n = 46 chromosomes.

Problem 2:

If a plant's root cell has 1414 chromosomes, how many chromosomes will be present in its pollen grain?

Solution:

Root cells are somatic (2n=142n = 14). Pollen grains are gametes (nn). n=142=7n = \frac{14}{2} = 7

Explanation:

Gametes are formed through the process of meiosis, which reduces the chromosome count by half to maintain species-specific chromosome numbers across generations.

Problem 3:

A researcher observes that a specific organism has a diploid count of 2n=802n = 80. During a subtraction analysis to find the difference between somatic and germ cell chromosome counts, what is the result?

Solution:

80−4040\begin{array}{r} 80 \\ - 40 \\ \hline 40 \end{array}

Explanation:

The somatic cell has 8080 chromosomes and the germ cell (gamete) has half of that, which is 4040. The difference is 80−40=4080 - 40 = 40.