Genotype: combination of alleles

Phenotype: observable features of an organism

Gene: length of DNA for a characteristic

Chromosome: long DNA molecule

Locus: position of a gene on a chromosome

Allele: alternative form of a gene Homozygous: both alleles are the same (both dom, both rec)

Heterozygous: alleles are different (dominant & recessive)

Dominant and recessive characteristics

This video shows a description of Mendel's work with pea plants to demonstrate dominant and recessive inheritance. Between 1856 and 1863, Gregor Mendel bred over 30,000 different plants, crossbreeding those with distinctive characteristics. Crossing pink- and white-flowered pea plants produced all pink flowers in the first generation. In the second generation the ratio of pink to white flowers was 3:1, indicating that producing white flowers was a recessive characteristic masked by the dominant characteristic of having pink flowers.


Sex-linked Inheritance

Occurs when the gene occurs on the sex chromosomes

Males cannot be carriers, if the faulty allele is on the X chromosome, as they will only have one copy of the gene.

Men tend to be much more affected, as women must have both faulty alleles, whereas men only need have one

  • - red/green colour blindness: recessive on X chromosome
  • - pattern baldness: dominant on Y chromosome
  • - haemophilia: recessive on X chromosome
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