Inheritance Genes And Cell Division

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Notes

Chromosomes, Genes & Proteins

**Chromosomes** are thread-like structures in the nucleus made of **DNA**.
A **gene** is a short length of DNA on a chromosome that codes for a specific protein (e.g., collagen, enzyme, hormone).
**Alleles** are alternative forms of a gene (e.g., blue, green, brown alleles for eye colour).
Genes determine characteristics by coding for proteins.

The Inheritance of Sex

Sex is determined by **sex chromosomes**: females are **XX**, males are **XY**.
Only males can pass on a **Y** chromosome, determining the child's sex.
Half of sperm carry X, half carry Y; if X fertilises egg (X), offspring is female (XX); if Y fertilises egg, offspring is male (XY).
Punnett square for sex inheritance shows 50% probability of each sex.

Control of Gene Expression (Extended)

**Diploid** cells have two sets of chromosomes (e.g., human body cells: 46 chromosomes).
**Haploid** cells have one set of chromosomes (e.g., human gametes: 23 chromosomes).
Gametes (sperm and egg) are haploid; fertilisation restores diploid number.

Mitosis (Extended)

**Mitosis** is nuclear division producing **genetically identical** diploid cells.
Chromosomes replicate before division, ensuring each daughter cell has the same number of chromosomes.
Mitosis is used for **growth**, **repair** of damaged tissues, **replacement** of cells, and **asexual reproduction**.

Meiosis (Extended)

**Meiosis** is a **reduction division** that halves chromosome number from diploid to haploid.
It produces **four genetically different** haploid cells (gametes).
Meiosis ensures genetic variation and maintains constant chromosome number after fertilisation.

Monohybrid Inheritance

**Inheritance** is the transmission of genetic information from parents to offspring.
**Genotype** is the combination of alleles; **phenotype** is the observable characteristic.
**Dominant** allele masks a **recessive** allele; recessive only shows when homozygous.
**Homozygous** $= two$ identical alleles; **heterozygous** $= two$ different alleles.
**Monohybrid inheritance** involves a single gene; Punnett squares predict offspring ratios.
Example: tall (T) dominant over short (t); cross $TT \times tt$ gives all Tt (tall); $Tt \times Tt$ gives 3:1 ratio tall:short.
**Pedigree charts** trace inheritance patterns through families, using squares (males) and circles (females).

DNA double helix showing sugar–phosphate backbones and complementary base pairs (A–T, C–G).

Monohybrid cross of two heterozygous tall plants (Tt × Tt) showing 3:1 phenotypic ratio.

Animal cell highlighting the nucleus where chromosomes are located.

Practice questions

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1.Where are chromosomes located in a cell?
Easy
- AIn the cytoplasm
- BIn the nucleus
- COn the cell membrane
- DIn the mitochondria
2.What is a short length of DNA that codes for a specific protein called?
Easy
3.Biological females have the sex chromosomes XY.
Easy
True or false?
4.Complete the sentence about cell division.
Medium
Mitosis produces genetically ____ cells.
5.How many chromosomes are in a human diploid cell?
Medium
- A46
- B41
- C51
- D56
6.Match each cell division type to its description.
Hard
- Mitosis
- Meiosis
- Produces genetically identical cells
- Produces genetically different cells
- Reduces chromosome number by half
7.Arrange the following events in the correct order for meiosis:
Hard
- Chromosomes replicate
- Homologous chromosomes separate
- Cell divides twice
- Four haploid cells are formed
8.In a monohybrid cross between two heterozygous tall plants (Tt x Tt), what is the expected phenotypic ratio?
Medium
- A1:2:1
- B3:1
- C1:1
- D9:3:3:1

Slides

Chromosomes, Genes & Alleles

Chromosomes are thread-like structures made of DNA found in the nucleus.
A gene is a short length of DNA that codes for a specific protein.
Alleles are different versions of the same gene.
For example, the gene for eye colour has alleles for blue, green, or brown eyes.

What is a gene?

A short length of DNA that codes for a protein
A whole chromosome
A type of protein
An allele

Sex Determination

Sex is determined by the X and Y chromosomes.
Females have XX; males have XY.
Only males can pass on a Y chromosome, so the father determines the sex of the child.
Half of sperm carry X and half carry Y; if an X sperm fertilises the egg, the child is female (XX); if a Y sperm fertilises the egg, the child is male (XY).

Which statements about sex inheritance are true?

Females have XY chromosomes.
The father determines the sex of the child.
All sperm carry the Y chromosome.
An XX zygote develops into a female.

Haploid & Diploid Cells

Diploid cells have two sets of chromosomes (46 in humans).
Haploid cells have one set of chromosomes (23 in humans).
Body cells are diploid; gametes (sperm and egg) are haploid.
Haploid means 'half' the normal number of chromosomes.

How many chromosomes are in a human diploid cell?

Mitosis – Cell Division for Growth & Repair

Mitosis produces two genetically identical diploid daughter cells.
It is used for growth, repair of damaged tissues, replacement of cells, and asexual reproduction.
Chromosomes are replicated before division so each new cell gets the same number.

Mitosis produces two genetically ____ daughter cells that $are ____$ . (same/different, haploid/diploid)

Meiosis – Cell Division for Gametes

Meiosis is a reduction division that produces four genetically different haploid cells.
It is used to produce gametes (sperm and egg cells).
The chromosome number is halved from diploid to haploid.
This ensures that when gametes fuse at fertilisation, the diploid number is restored.

Which are true about meiosis?

Meiosis produces genetically identical cells.
Meiosis halves the chromosome number.
Meiosis produces four cells.
Meiosis is used for growth and repair.

Key Definitions in Inheritance

Genotype: the combination of alleles an individual has (e.g., Tt).
Phenotype: the observable characteristics (e.g., tall or short).
Homozygous: two identical alleles (TT or tt).
Heterozygous: two different alleles (Tt).
Dominant allele: expressed even if only one copy is present (capital letter).
Recessive allele: only expressed when two copies are present (lowercase letter).

If an organism has the genotype Tt, what is its phenotype when T is dominant for tall and t is recessive for short?

Tall
Short
Medium
Cannot be determined

Monohybrid Crosses & Punnett Squares

Monohybrid inheritance involves a single gene.
A Punnett square shows the possible allele combinations in offspring.
Steps: 1. Determine parental genotypes. 2. Choose letters (dominant capital, recessive lowercase). 3. Split alleles around the square. 4. Fill in the four boxes.
The ratio of phenotypes can be predicted (e.g., 3:1 tall:short).

In a monohybrid cross between two heterozygous parents (Tt x Tt), the expected phenotypic ratio $is ____$ tall : ____ short.

Pedigree Charts

Pedigree charts trace the inheritance of a characteristic through a family.
Males are shown as squares, females as circles.
Affected individuals are often shaded.
Horizontal lines connect parents; children are shown below.

In a pedigree chart, what shape represents a male?

Circle
Square
Triangle
Diamond

Summary: Mitosis vs Meiosis

Mitosis: 2 identical diploid daughter cells; used for growth, repair, asexual reproduction.
Meiosis: 4 genetically different haploid cells; used to produce gametes.
Mitosis maintains chromosome number; meiosis halves it.

Which statements correctly compare mitosis and meiosis?

Mitosis produces genetically identical cells; meiosis produces genetically different cells.
Both mitosis and meiosis produce diploid cells.
Meiosis is involved in growth and repair.
Mitosis produces two cells; meiosis produces four cells.

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