Topic 4 of 18Pearson EdExcel

Genetics & Inheritance

Explores how DNA's code dictates traits via genes, alleles, and predictable inheritance patterns.

### Section 1: The Chemical Basis of Inheritance

At the heart of genetics is DNA (deoxyribonucleic acid), the molecule that carries the genetic instructions for the development, functioning, growth, and reproduction of all known organisms. Its structure is a double helix, resembling a twisted ladder.

Structure of DNA: Each strand of the helix is a polymer of nucleotides. A nucleotide consists of three parts: a phosphate group, a deoxyribose sugar, and a nitrogenous base.

Base Pairing Rule: There are four bases: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). The structure's stability comes from specific pairing between the bases on opposite strands, known as complementary base pairing: Adenine always pairs with Thymine (A-T), and Guanine always pairs with Cytosine (G-C).

Within the cell's nucleus, this DNA is tightly coiled and packaged into structures called chromosomes. Humans have 23 pairs of chromosomes in most body cells (46 total): 22 pairs of autosomes and one pair of sex chromosomes.

A gene is a specific section of DNA on a chromosome that contains the instructions to make a specific protein. It is the sequence of bases (A, T, C, G) within a gene that determines the sequence of amino acids in a protein, which in turn determines the protein's function and, ultimately, a specific trait (e.g., eye colour, production of an enzyme).

### Section 2: Key Genetic Terminology

Understanding inheritance requires a precise vocabulary:

Allele: An alternative form of a gene. For example, the gene for seed shape in pea plants has two alleles: one for round (R) and one for wrinkled (r).

Genotype: The genetic makeup of an organism concerning a particular trait, represented by the combination of alleles it possesses (e.g., RR, Rr, or rr).

Phenotype: The observable physical or biochemical characteristic of an organism, resulting from its genotype and environmental factors (e.g., round seeds, brown eyes).

Homozygous: Having two identical alleles for a particular gene (e.g., RR or rr). Also known as 'pure-breeding'.

Heterozygous: Having two different alleles for a particular gene (e.g., Rr). Individuals who are heterozygous for a recessive condition are often called carriers.

Dominant Allele: An allele that is always expressed in the phenotype, even if only one copy is present. It is represented by a capital letter (e.g., R for round seeds).

Recessive Allele: An allele that is only expressed in the phenotype if two copies are present (i.e., the organism is homozygous recessive). It is represented by a lowercase letter (e.g., r for wrinkled seeds).

### Section 3: Monohybrid Inheritance & Punnett Squares

Monohybrid inheritance is the study of the inheritance of a single characteristic controlled by a single gene. We can predict the outcomes of genetic crosses using a Punnett square.

Step-by-step guide to using a Punnett Square:

Determine Parental Genotypes: Identify the genotypes of the two parents being crossed.

Determine Gametes: List the possible alleles each parent can pass on in their gametes (sperm or egg cells).

Draw the Grid: Draw a square and divide it into four smaller squares.

Label Axes: Write the alleles from one parent across the top and the alleles from the other parent down the side.

Fill the Grid: Combine the alleles from the top and side in each box to find the possible genotypes of the offspring.

Calculate Ratios: Determine the genotypic ratio (e.g., 1 BB : 2 Bb : 1 bb) and the phenotypic ratio (e.g., 3 dominant trait : 1 recessive trait) of the offspring.

Example Cross: A heterozygous tall pea plant (Tt) is crossed with another heterozygous tall pea plant (Tt).

Parental Phenotype: Tall × Tall

Parental Genotype: Tt × Tt

Gametes: (T, t) × (T, t)

| | T | t |

|---|---|---|

| T | TT | Tt |

| t | Tt | tt |

Genotypic Ratio: 1 TT : 2 Tt : 1 tt

Phenotypic Ratio: 3 Tall : 1 short

Exam Trap: Remember that these ratios represent probability. For any single offspring, the chance of being short is 1 in 4 (25%). It does not guarantee that in a group of four offspring, exactly one will be short.

### Section 4: Sex Determination

In humans, sex is determined by the 23rd pair of chromosomes, the sex chromosomes.

Females have two X chromosomes (XX).

Males have one X and one Y chromosome (XY).

During meiosis, a female produces eggs that all contain a single X chromosome. A male produces sperm, of which half contain an X chromosome and half contain a Y chromosome. The sex of the child is therefore determined by which sperm fertilises the egg.

| | X (from mother) | X (from mother) |

|---|---|---|

| X (from father) | XX (Female) | XX (Female) |

| Y (from father) | XY (Male) | XY (Male) |

This results in an approximate 50% probability of having a male child and a 50% probability of having a female child.

### Section 5: Codominance

Codominance is a pattern of inheritance where both alleles in a heterozygous individual are fully expressed in the phenotype. Neither allele is dominant or recessive to the other.

Practical Application: ABO Blood Groups

The gene for blood type has three alleles: Iᴬ, Iᴮ, and i.

Iᴬ and Iᴮ are codominant with each other.

Both Iᴬ and Iᴮ are dominant over the allele i.

This leads to four possible phenotypes (blood groups):

Genotype IᴬIᴬ or Iᴬi → Phenotype Blood Group A

Genotype IᴮIᴮ or Iᴮi → Phenotype Blood Group B

Genotype IᴬIᴮ → Phenotype Blood Group AB (codominance)

Genotype ii → Phenotype Blood Group O

Key Points to Remember

1DNA: double helix, bases A-T, C-G pairing
2Dominant allele expressed over recessive
3Punnett square shows offspring probabilities
4Cystic fibrosis: recessive; Huntington's: dominant

Pakistan Example

Thalassaemia in Pakistan — Genetics with Real Impact

Pakistan has one of the world's highest rates of beta-thalassaemia — a recessive genetic disorder. Carriers (Tt) appear healthy but can pass the allele on. When two carriers marry, there is a 25% chance each child will have the disease (tt). The National Thalassaemia Programme now encourages genetic testing before marriage. This is EdExcel genetics applied to a critical Pakistani public health issue.

Quick Revision Infographic

Biology — Quick Revision