Chapter 2.4

• Alleles are the variation of genes
• Alleles occupy the same position on homologous chromosomes
• An allele that is dominant is represented as a big letter (Big R)
• A recessive allele is represented by a small letter (small r)
• A double allele (RR or rr) is homozygous
• A mix of alleles is Heterozygous
• Punnett squares show the possible breeding outcomes that an offspring can have
• Heterozygotes have a dominant and recessive allele
• Homozygotes have either 2 dominant or 2 recessive alleles
• Genotypes are the genetic makeup of an organism, while phenotypes are their observable traits.
• All eggs have a X chromosome
• Sperm has either a X or Y chromosome
• If a sperm is an X chromosome, it is a girl. If it's a Y chromosome, it is a boy.
• Genes found on the sex chromosomes= Sex-linked genes
• X chromosomes are longer and carries more genes
• Most sex-linked genes are on the X chromosome
• Male (XY): Males derive the trait from the X chromosome. The Y chromosome has no matching gene= They either have the trait or not: 2 options - X(H)Y  or X(h)Y The H and h are superscripts as they are on the sex chromosomes.
• Female (XX)= Females have 2 copies of X chromosome= 3 possibilities= Have the trait or not plus the potential to be a carrier. Heterozygous form - X(H)X(h)= A carrier but unaffected. X(H)X(H)=  not affected. X(h)X(h)= has Haemophilia= unlikely. The H and h are superscripts as it is on the sex chromosome
• If chromatids fail to separate during meiosis, the child will be born with an extra chromosome or part of a chromosome. e.g. Down syndrome has an extra '21' chromosome. Klinefelter syndrome has an extra X chromosome (XXY)
• A mutation happens when there is a change/mistake when copying DNA
• Mutations can occur spontaneously or due to environmental factors such as radiation.
• Silent Mutation: Change in genetic code that doesn't affect the individual. e.g. A single base is changed on the DNA that doesn't affect the protein
• Missense Mutation: Changes that don't stop the gene from making protein. However, it is a different protein. It may not function properly and cause disease
• Nonsense Mutation: Causes cells to stop reading information on the gene. The protein is incomplete and can't function. e.g Cystic Fibrosis
• Frameshift Mutation: The insertion or deletion of one or more bases. causes information to be jumbled. e.g. Tay-Sachs disease
• Mutagens are agents that cause mutation
• Carcinogens are agents that cause cancer
• 3 bases make up a codon
• Somatic mutations happen in mitosis
• Inheritable mutations happen in meiosis
• Non-ionising radiation include radio waves, microwaves. Does not cause mutations
• Ionising radiation includes UV, X-rays, gamma rays
• UV light causes Thymine to crosslink if they are on top of each other