Reproduction

• Sexual reproduction


  Cell division through meiosis, joining (fusion) of male and female sex cells (gametes) - sperm and egg in animals, pollen and ovule in plants, produces non-identical offspring that are genetically different to parents, results in wide variation within offspring and species
• Asexual reproduction


  Cell division through mitosis, no fusion of gametes, produces offspring that are genetically identical to parent (clones), no mixing of genetic information, results in wide variation
• Meiosis


  Halves the number of chromosomes in gametes, fertilisation (joining of two gametes) restores the full number of chromosomes, the fertilised cell divides by mitosis, producing more cells, as the embryo develops, the cells differentiate
• DNA


  Genetic material in the nucleus of a cell, made up of two strands forming a double helix
• Chromosome


  Structures that contain DNA
• Gene


  Small section of DNA on a chromosome that codes for a specific sequence of amino acids to produce a specific protein
• Genome


  The entire genetic material of an organism
• The whole human genome has been studied, and has allowed scientists to search for genes linked to different diseases, understand and treat inherited disorders, and trace human migration patterns from the past
• Dominant allele


  Allele that only needs one copy present to be expressed (it is always expressed)
• Recessive allele


  Allele that needs two copies present to be expressed
• Homozygous


  When an individual carries two copies of the same allele for a trait
• Heterozygous


  When an individual carries two different alleles for a trait
• Genotype


  Combination of alleles an individual has
• Phenotype


  Physical expression of the genotype - the characteristic shown
• Genetic cross


  Considering the offspring that might result from two known parents, Punnett squares can be used to predict the outcome of a genetic cross, for both the genotypes the offspring might have and their phenotypes
• Normal human body cells contain 23 pairs of chromosomes - one of these pairs determines the sex of the offspring
• In human females the sex chromosomes are the same (XX, homozygous), and in males they are different (XY, heterozygous)
• The probability of offspring being male or female is always 50% in humans as there are two XX outcomes and two XY outcomes