Genotype, Phenotype, and Allele Combinations

In GCSE Biology inheritance, it's essential to understand the difference between genotype and phenotype:

Definition: Genotype is the combination of alleles that causes a characteristic, while phenotype is the expressed characteristic.

For example, in mice:

• Genotype BB or Bb results in a phenotype of black fur
• Genotype bb results in a phenotype of brown fur

This concept is crucial for solving GCSE Biology inheritance exam questions.

Allele combinations can be classified as:

• Homozygous: when both alleles are the same (e.g., BB or bb)
• Heterozygous: when the alleles are different (e.g., Bb)

Highlight: Understanding homozygous and heterozygous combinations is key to predicting inheritance patterns.

An example of genetic inheritance is red-green color blindness, which is a recessive characteristic. The alleles are represented as R (dominant, not color blind) and r (recessive, color blind).

Example: A person with unaffected vision could have the genotype RR or Rr, while a color-blind individual would have rr.

These concepts are frequently tested in genetics GCSE Biology Edexcel and other exam boards.

Genetic Crosses and Probability

This final section focuses on genetic cross diagram techniques and probability calculations in inheritance.

Example: A monohybrid cross between mice with genotypes Bb shows:

• 50% probability of black fur
• 50% probability of brown fur

Highlight: Using Punnett squares helps predict offspring genotypes and phenotypes.

Example: Dimple inheritance problem demonstrates practical application:

• D = dimples (dominant)
• d = no dimples (recessive)
• Cross between Dd × dd shows probability of offspring inheriting dimples

Understanding Chromosomes and Alleles

The nucleus of a cell contains DNA organized into chromosomes. In humans, these chromosomes are arranged in 23 pairs, totaling 46. This pairing is crucial for the process of meiosis, where gametes (sex cells) are produced with half the number of chromosomes.

Definition: Chromosomes are structures made of DNA that carry genetic information.

Each person inherits one set of chromosomes from their mother and one from their father, resulting in two copies of each gene. These different versions of genes are called alleles.

Vocabulary: An allele is a variant form of a gene.

Some characteristics, like fur color in mice, are determined by a single gene. Alleles can be either dominant or recessive:

• Dominant alleles are represented by capital letters (e.g., B) and only need one copy to be expressed.
• Recessive alleles are represented by lowercase letters (e.g., b) and require two copies to be expressed.

Example: In mice, B represents the dominant allele for black fur, while b represents the recessive allele for brown fur.

Understanding these concepts is crucial for genetic inheritance GCSE Biology and forms the foundation for more complex topics in genetics revision notes.