Inheritance, Genes & Cell Division

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Notes

Chromosomes, Genes & Proteins

**Inheritance** is the transmission of genetic information from generation to generation.
**Chromosomes** are thread-like structures of **DNA** in the nucleus, carrying genes.
A **gene** is a short length of DNA on a chromosome that codes for a specific protein.
**Alleles** are different versions of the same gene; individuals inherit two alleles (one from each parent).
The sequence of bases in a gene determines the sequence of amino acids in a protein.

The Inheritance of Sex

Sex is determined by the sex chromosomes: **XX** = female, **XY** = male.
The father determines the sex of offspring because he can pass on either an X or a Y chromosome.
Half of sperm carry X, half carry Y; if an X sperm fertilises the egg, the baby is female (XX); if a Y sperm, male (XY).
A Punnett square with X and Y chromosomes shows a 1:1 ratio of male:female offspring.

Protein Synthesis (Extended)

**Transcription**: DNA base code is copied onto **mRNA** in the nucleus.
**Translation**: mRNA attaches to a **ribosome**; the ribosome reads codons (triplets) and assembles amino acids.
**tRNA** brings specific amino acids to the ribosome, matching anticodons to mRNA codons.
The amino acid chain folds to form a functional protein (e.g., enzyme, antibody).

Which Proteins are Synthesised? (Extended)

**Gene expression** controls whether a gene is transcribed/translated in a cell.
Most genes are 'switched off' to save energy; only essential genes are expressed.
All body cells contain the same genes, but different cells express different genes.
DNA controls cell function by directing protein production.

Mitosis

**Mitosis** is nuclear division producing two **genetically identical** daughter cells.
Chromosomes duplicate before division, then line up and are pulled apart.
Used for **growth**, **repair**, **replacement** of cells, and **asexual reproduction**.
Daughter cells are **diploid** (same chromosome number as parent).

Meiosis (Extended)

**Meiosis** is a reduction division producing four **haploid** gametes (genetically different).
Chromosomes duplicate, then the cell divides twice; homologous chromosomes separate in the first division.
Produces **genetic variation** via recombination and independent assortment.
Gametes (sperm, egg) have half the chromosome number of body cells.

Monohybrid Inheritance

**Monohybrid inheritance** involves a single gene with two alleles (dominant/recessive).
**Genotype** = allele combination; **phenotype** = observable characteristic.
**Homozygous** = two identical alleles; **heterozygous** = two different alleles.
A **Punnett square** predicts offspring genotypes and phenotypes; e.g., Tt × tt gives 1:1 tall:short.
**Test cross** (cross with recessive homozygote) reveals if a dominant phenotype is homozygous or heterozygous.

Codominance & Sex-Linked Characteristics (Extended)

**Codominance**: both alleles contribute to phenotype (e.g., ABO blood groups: I^A and I^B are codominant).
**Sex-linked** genes are on the X chromosome; males (XY) are more likely to show recessive conditions (e.g., colour blindness).
Females can be **carriers** (heterozygous) and pass the allele to sons.
Punnett squares for sex-linked traits use X and Y chromosomes with alleles as superscripts.

Monohybrid cross: heterozygous tall (Tt) × short (tt) gives 1:1 ratio tall:short.

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

Practice questions

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1.What does the term haploid mean?
Easy
- AA nucleus containing two sets of chromosomes.
- BA nucleus containing a single set of chromosomes.
- CA person with only one X chromosome.
- DA person with XXY chromosomes.
2.When a somatic (body) cell undergoes cell division, two daughter cells are produced with the same chromosome number as the parent cell. Which type of division has taken place?
Easy
- AMeiosis producing genetically different cells.
- BMitosis producing genetically identical cells.
- CMeiosis producing genetically identical cells.
- DMitosis producing genetically different cells.
3.Which shows the correct size order of structures, starting with the smallest?
Easy
- Agene → DNA strand → chromosomes → nucleus
- BDNA strand → chromosomes → gene → nucleus
- Cchromosomes → gene → DNA strand → nucleus
- Dnucleus → chromosomes → DNA strand → gene
4.Males and females have a unique set of chromosomes. What is the correct combination of male and female chromosomes?
Easy
- AMale XY, Female YY
- BMale YX, Female XY
- CMale XY, Female XX
- DMale XX, Female XY
5.Albinism is a condition caused by a recessive allele. If both parents have albinism, what are the chances of the offspring being an albino child?
Medium
- A0%
- B25%
- C75%
- D100%
6.Which of the following structures in plant cells carry out protein synthesis under the control of the nucleus?
Medium
- ARibosomes
- BChloroplasts
- CMitochondria
- DCell walls
7.The diagram shows the sex chromosomes of a woman and a man. This also shows the alleles for a recessive sex-linked condition. What is the percentage chance that their daughter will show the sex-linked condition? (Assume the woman is a carrier and the man is normal.)
Hard
- A0%
- B25%
- C50%
- D75%
8.Which row in the table shows a comparison between meiosis and mitosis?
Medium
- Amitosis produces 4 daughter cells; meiosis produces 2 daughter cells
- Bmitosis produces genetically identical cells; meiosis produces cloned cells
- Cmitosis daughter cells are diploid; meiosis daughter cells are haploid
- Dmitosis involves two cell divisions; meiosis involves four cell divisions

Slides

Chromosomes, Genes & DNA

Inheritance is the transmission of genetic information from generation to generation.
Chromosomes are thread-like structures of DNA found in the nucleus.
A gene is a short length of DNA on a chromosome that codes for a specific protein.
Alleles are different versions of the same gene.

What is a gene?

A thread-like structure of DNA
A short length of DNA that codes for a protein
A version of a chromosome
A type of protein

Haploid & Diploid Cells

Diploid nuclei have two sets of chromosomes (e.g., 46 in humans).
Haploid nuclei have one set of unpaired chromosomes (e.g., 23 in human gametes).
Gametes (sperm and egg cells) are haploid.

A nucleus with two sets of chromosomes is called ____. A nucleus with one set of unpaired chromosomes is called ____.

Mitosis: Cell Division for Growth & Repair

Mitosis produces two genetically identical daughter cells.
Chromosomes are copied before division, so each daughter cell has the same diploid number.
Used for growth, repair of tissues, and asexual reproduction.

Which of the following is a use of mitosis?

Production of gametes
Growth and repair of tissues
Increasing genetic variation
Halving chromosome number

Meiosis: Cell Division for Gametes

Meiosis produces four genetically different haploid cells.
Chromosome number is halved (diploid → haploid) – a reduction division.
Used to produce gametes (sperm and egg cells).
Increases genetic variation through recombination and independent assortment.

Which of these are true about meiosis?

Produces genetically identical cells
Halves the chromosome number
Produces four daughter cells
Used for growth and repair

Inheritance of Sex

Females have sex chromosomes XX; males have XY.
The father determines the sex of the child because he can pass on either X or Y.
Half of sperm carry X, half carry Y – 50% chance of male or female.

What are the sex chromosomes of a human male?

Monohybrid Inheritance

Monohybrid inheritance involves a single gene with two alleles.
Dominant allele (capital letter) masks recessive allele (lowercase).
Phenotype = observable characteristics; genotype = allele combination.
Homozygous = two identical alleles; heterozygous = two different alleles.

The observable characteristics of an organism are its ____. The combination of alleles is its ____.

Punnett Squares

A Punnett square shows possible offspring genotypes from a cross.
Example: Tt (tall) × tt (short) gives 50% Tt (tall) and 50% tt (short).
Probability can be expressed as a ratio or percentage.

In pea plants, tall (T) is dominant over short (t). What is the probability of a short offspring from a cross Tt × tt?

0%
25%
50%
100%

Codominance & Sex-Linked Inheritance (Extended)

Codominance: both alleles contribute to phenotype (e.g., ABO blood groups).
Sex-linked genes are on the X chromosome; males are more likely to show recessive conditions.
Example: red-green colour blindness is X-linked recessive.

Why are sex-linked recessive conditions more common in males?

Males have two X chromosomes
Males have only one X chromosome
The Y chromosome carries the dominant allele
Females cannot inherit recessive alleles

Protein Synthesis (Extended)

DNA sequence determines amino acid sequence in proteins.
Transcription: DNA code is copied into mRNA in the nucleus.
Translation: ribosome reads mRNA triplets to assemble amino acids.

What is the role of mRNA in protein synthesis?

Carries amino acids to the ribosome
Carries a copy of the gene from nucleus to cytoplasm
Assembles amino acids into proteins
Forms part of the ribosome

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