Inheritance, Variation, and Evolution in GCSE Biology

Principles of Inheritance

Inheritance is the process by which genetic information is passed from parents to offspring. This information is stored in DNA molecules, which are organized into genes and located on chromosomes. Each gene can have different forms called alleles.

When an organism inherits one allele from each parent, they are termed heterozygous. If both alleles are the same, the organism is homozygous. Some alleles are dominant (expressed) while others are recessive (masked).

Punnett squares can be used to predict the inheritance patterns of genetic crosses by modeling the possible combinations of alleles in offspring.

Genetic Variation

Variation arises due to differences in the genetic makeup of individuals within a population. There are two main sources of variation:

• Genetic variation: Caused by mutations, gene reshuffling during meiosis, and random fertilization events.
• Environmental variation: Differences in environmental factors like climate, nutrition, and lifestyle.

Evolution by Natural Selection

Charles Darwin's theory of evolution by natural selection explains how populations change over time through the following process:

1. Variation exists within a population.
2. Some variations are advantageous and increase an organism's chance of survival and reproduction.
3. Organisms with advantageous traits produce more offspring, passing these traits to subsequent generations.
4. Over many generations, the advantageous traits become more common, and the population evolves.

Evidence for evolution includes fossil records, similarities in anatomy and embryology, and the development of antibiotic resistance in bacteria.

Speciation and Applications

As populations evolve and become reproductively isolated, new species can arise through a process called speciation.

Humans have applied principles of inheritance and variation in selective breeding (choosing desired traits) and genetic engineering (directly modifying genes). These techniques have applications in agriculture, medicine, and other fields but also raise ethical concerns.

Worked Example: Punnett Square

Problem: In pea plants, tallness (T) is dominant over shortness (t). If a heterozygous tall plant (Tt) is crossed with a short plant (tt), what are the possible genotypes and phenotypes of the offspring?

Solution:

The Punnett square shows that the offspring will have a 1:1 ratio of Tt (tall) to tt (short) genotypes.