Inheritance

Genetics and Inheritance

Genes and chromosomes:

  • Genes are short sections of DNA that contain instructions for specific characteristics.
  • Genes are found in chromosomes in the nucleus of every cell.
  • Human body cells each contain 23 pairs of chromosomes.

Alleles:

  • Alleles are different versions of the same gene. For example, the gene for eye colour has an allele for blue eye colour and another for brown eye colour.
  • Each person has two alleles for each gene – one from each parent.

Dominant and recessive:

  • An allele can be dominant or recessive.
  • Dominant alleles always express their characteristic, even if only one copy is present.
  • Recessive alleles only express their characteristic if two copies are present.

Genotype and Phenotype:

  • The genotype is the combination of alleles that a person has.
  • The phenotype is the physical characteristics that result from the genotype.

Variation and Evolution

Variation:

  • Variation is the differences between individuals of the same species.
  • Variation can be genetic (controlled by genes) or environmental (resulting from the conditions in which an organism lives) or a combination of both.

Natural selection:

  • Natural selection is the process by which species adapt and change in response to their environment.
  • Organisms with characteristics that enable them to survive and reproduce are more likely to pass on their genes.

Evolution:

  • Over time, natural selection results in the alleles for beneficial characteristics becoming more common in a population. This is evolution.
  • Evidence for evolution can be found in the fossil record, showing how organisms have changed over time.

Genetic Disorders

Sexual reproduction and variation:

  • Sexual reproduction introduces variation into a population because it involves the combination of genetic information from two parents.
  • However, sometimes new combinations of alleles can result in genetic disorders.

Genetic Disorders:

  • Some genetic disorders, such as cystic fibrosis and sickle cell anaemia, are caused by recessive alleles.
  • Polydactyly, in which a person has extra fingers or toes, is caused by a dominant allele.
  • Genetic testing can be used to detect these disorders before birth.

Chromosomal disorders:

  • Some disorders are caused by issues with the chromosomes, not just specific genes.
  • For example, Down’s syndrome is caused by having an extra copy of chromosome 21.