Class 9 BiologyChapter 11NCERT 11.2 and 11.2.1
Sexual reproduction and variation
Same two parents, never the same child twice. Shuffle the beads and see why.
Think it over
- You and your brother or sister have the same parents. Why do you not look exactly alike?
- Each of your parents has 46 chromosomes in a body cell, and so do you. How is that possible if you got chromosomes from both?
- Why might it help a species if its members are all slightly different from one another?
In one minute
- Sexual reproduction: the young get a share of genetic material from each of two parents.
- Full-set gametes would double the chromosome number every generation. Meiosis prevents that by halving it: 46 in human body cells, 23 in gametes.
- Gametes: sperm and eggs in animals; in flowering plants the pollen grain carries male gametes to the ovule, which holds the egg.
- In meiosis each chromosome pair separates at random, so gametes carry different mixes. Three pairs already give 8 combinations (Activity 11.4).
- The shuffling makes every individual unique, and those differences, variation, help species cope with change and, over long periods, evolve.
Two parents and a counting problem
In sexual reproduction a new individual is made from two parents, and each parent hands on part of its genetic material. That raises a puzzle of simple arithmetic.
Suppose each parent gave its whole set of chromosomes. The child would carry twice as many as either parent, its children twice as many again, and so on without end. The fix is meiosis, a cell division unlike mitosis. Compare the two cases below.
If each gamete carried the full set, a human child would get 46 + 46 = 92 chromosomes, the next generation 184, and so on, doubling every time.
- 1Without meiosis
If each gamete carried the full set, a human child would get 46 + 46 = 92 chromosomes, the next generation 184, and so on, doubling every time.
- 2With meiosis
Meiosis halves the number, so each gamete carries 23. At fertilisation 23 + 23 = 46, the same as each parent, generation after generation.
Quick check
Without meiosis, what would happen to the chromosome number from one generation to the next?
Show the answer
B. It would double every generation Two full sets joined at fertilisation would give twice the parent's number each time. Meiosis halves the number in gametes to prevent this.
Meiosis makes gametes
Inside every nucleus are chromosomes, thread-like carriers of genetic information. Their number is fixed for each species. Human body cells hold 23 pairs, 46 in all, and of each pair one chromosome came from each parent.
Meiosis starts from a cell with the full, paired set (diploid) and ends with cells that have just half (haploid). The body uses these half-set cells for one thing only: they are the gametes, the cells of reproduction. The two members of every chromosome pair are pulled apart during meiosis, so a gamete receives one from each pair. That makes 23 chromosomes in each human gamete.
Animals make sperm (male) and eggs (female). In a flowering plant the male gametes travel inside a pollen grain, which delivers them to an ovule; the ovule holds the egg.
- Diploid
- Having chromosomes in pairs, the full number (46 in human body cells).
- Haploid
- Having one chromosome from each pair, half the number (23 in human gametes).
- Gamete
- A haploid cell made for reproduction: sperm or egg.
Quick check
A human egg cell carries how many chromosomes?
Show the answer
B. 23 Eggs and sperm are made by meiosis and are haploid: one chromosome from each of the 23 pairs.
The bead game: how many kinds of gamete?
Activity 11.4 models meiosis with three pairs of beads. Each pair stands for two contrasting forms of a character sitting on different chromatids of one chromosome:
Pair 1, green (chromosome 1): light green for blonde hair, dark green for black hair. Pair 2, blue (chromosome 2): light blue for straight hair, dark blue for curly hair. Pair 3, red (chromosome 3): light red for brown eyes, dark red for black eyes.
To make a gamete, take one bead from every pair without looking. Tap Make a gamete again and again and fill the grid. Then try the slider: what if there were more pairs?
Parent cell: three pairs
- Pair 1, greenLight: blonde hair. Dark: black hair.
- Pair 2, blueLight: straight hair. Dark: curly hair.
- Pair 3, redLight: brown eyes. Dark: black eyes.
Your gamete
Tap the button to pick one bead from each pair.
All possible gametes 0 of 8 found, 0 made
- blonde hair, straight hair, brown eyes 0
- blonde hair, straight hair, black eyes 0
- blonde hair, curly hair, brown eyes 0
- blonde hair, curly hair, black eyes 0
- black hair, straight hair, brown eyes 0
- black hair, straight hair, black eyes 0
- black hair, curly hair, brown eyes 0
- black hair, curly hair, black eyes 0
Written the book's way, the first is "light green, light blue, light red". Each pair gives 2 choices, so 2 × 2 × 2 = 8.
Shuffle and pair: three siblings
Both parents carry the same three bead pairs. Each child gets one gamete from each parent, so each child has a new pair of beads for every chromosome.
Now imagine more pairs
8 different gametes possible
3 pairs give 8 kinds of gamete; with the 23 pairs of a human cell, one parent alone could make 83,88,608 kinds, before counting the many characters each chromosome carries.
At 23 pairs, as in human gametes, the count passes 83 lakh, from one parent alone.
With three pairs you can make 8 different gametes, because each pair offers 2 choices: 2 × 2 × 2 = 8. A human gamete gets one chromosome from each of 23 pairs, and each chromosome carries information for many characters, so the number of possible mixes is enormous.
This random mixing is why children differ genetically from their parents and from each other. Each of us carries a combination of chromosomes that nobody else has.
Quick check
With four pairs of beads instead of three, how many different gametes could you make?
Show the answer
C. 16 Each pair gives 2 choices, so 2 × 2 × 2 × 2 = 16.
Why variation matters
Small differences between individuals are called variation. They are not just interesting; they help a species survive. When the environment changes, some individuals may happen to carry a mix that suits the new conditions better, and they are more likely to survive and reproduce.
People show this too. Some cope well with the thin air of high mountains, where oxygen is low; others can still digest milk when grown up. Over many generations, differences like these add up to evolution, and now and then to new kinds of species.
Quick check
Why is variation useful for a species?
Show the answer
B. It helps some individuals adapt when the environment changes Different individuals cope differently with change; those that suit new conditions survive and pass on their characters.