Inheritance

• allele 


  a version of a gene
• autosomal linkage 


  when two or more genes are positioned in the same autosome. they are unlikely to be separated by crossing over during meiosis so are often inherited together
• autosome


  a chromosome that is not an X or Y chromosome
• carcinogen


  a type of mutagen that causes cancer
• chi squared $x^2$


  a statistical test used to determine whether a pattern of inheritance is statistically significant
• chromosome mutation 


  a change in the structure or number of chromosomes. this affects many genes
• codominance


  when both alleles for a gene in a heterozygous organism equally contribute to the phenotype
• degrees of freedom 


  the number of categories minus one
• dihybrid mendelian inheritance 


  the determination if a trait but the inheritance of two genes
• dominant


  describes an allele is always expressed. represented by a capital letter
• down’s syndrome 


  a genetic disorder characterised by delayed development and learning disabilities. due to non-disjunction, an affected individual possesses three copes of chromosome 21
• duchenne muscular dystrophy (DMD)


  an X-linked recessive condition characterised by muscle degeneration and weakness. it is more common among males because they only inherit one X chromosome
• epigenetics 


  the study of changes in gene expression that are not due to alterations in the nucleotide base sequence of DNA
• F1 generation 


  the first generation of offspring resulting from the cross of two individuals in the parental generation
• F2 generation


  the second generation of offspring resulting fro, the cross of two individuals in the F1 generation
• gene


  a length of DNA on a chromosome that codes for the production of one or more polypeptide chains and functional RNA
• gene mutation 


  a change to at least one nucleotide base in DNA or the arrangement of bases. gene mutations can occur spontaneously during DNA replication and may be beneficial, damaging or neutral
• genotype


  an organisms genetic composition. describes all alleles
• haemophilia 


  an X-linked recessive condition that results in excessive bleeding and blood that is slow to clot. it is more common among males because they only inherit one X chromosome
• heterozygous 


  when someone has two different alleles of a gene, e.g. Ff
• histone modification 


  the alteration of histones by the addition of methyl, acetyl or phosphate groups. this can increase or decrease gene expression by making the histone more less accessible to transcription factors
• homozygous 


  when so one has two identical alleles of a gene, e.g. ff or FF
• locus


  the position of a gene on a chromosome
• methylation 


  the transfer of methyl groups to cytosine bases of DNA. methylation inhibits transcription by making the DNA less accessible to transcriptional factors from binding. this deactivated the gene
• monohybrid mendelian inheritance


  the determination of a trait by the inheritance of a single gene
• mutagen 


  a chemical, biological or physical agent that increases the rate of gene mutations above normal level
• non disjunction 


  the failure of homologous chromosomes to spear are in anaphase I or sister chromatids to separate in anaphase II, resulting in a change in the diploid number of chromosome
• oncogene


  mutations of proto-oncogenes that are activated continuousl, resulting in uncontrolled cell division
• phenotype


  an organisms observable characteristics. due to interactions of the genotype and the environment
• recessive


  describes an allele that is only expressed in the absence of a dominant allele. represented by a small letter
• sex chromosome 


  a chromosome that determines the sex of an organism, e.g. X and Y chromosomes in human and other mammals
• sex linkage 


  genes which are usually inherited together on the same sex cheomsome (on either the. or Y chromsome in animals) and so expression is sex dependent. this is often due to many gene loci on the X chromosome having no counterpart on the Y chromosome
• sickle cell anaemia 


  a recessive genetic disorder caused by substitution mutation on chromosome 11. this results in abnormal haemoglobin which distorts red blood cells