Mendelian genetics

• Mendelian inheritance is the basis of Population Genetics.
• Dominance in genetics refers to the relationship between different alleles and how the alleles 'show' in the trait.
• Inheritance predictions can be made in mono-hybrid and di-hybrid crosses.
• Mendel's Laws apply to the inheritance of monogenic traits in humans.
• An observable characteristic of an individual is called a 'Trait' or 'Phenotype' such as hair colour, eye colour, blood group, height.
• The DNA (genomic) sequence that controls a trait is a 'Gene' (or 'Locus') such as HBB (sickle cell), CFTR (cystic fibrosis), OCA2 (eye colour).
• A gene may have several variants, which can lead to the differences in the trait.
• Genetic inheritance is how traits are passed down from one generation to the next.
• Genes are inherited during meiosis, the cell division that generates gametes (egg and sperm).
• Humans are diploid organisms, meaning they have two copies of each type of chromosome (2N).
• The alleles in a diploid cell can be the same or different, this is referred to as 'Homozygous' or 'Heterozygous'.
• Human gametes (egg and sperm) are haploid (1 copy of each chromosome).
• One haploid gamete from each parent combine to create a new diploid individual.
• Recessive traits only appear in homozygous recessive individuals.
• In a diploid organism, each genotype has two alleles.
• A dominant allele of a gene (UPPERCASE = D) will ‘show’ in the trait (phenotype) if an individual has at least one dominant allele.
• Gregor Mendel discovered that alleles can be dominant or recessive, meaning the trait (phenotype) is not a ‘blend’ of the two alleles.
• In diploid organisms, haploid gametes from each parent combine to form a new, diploid, offspring.
• Mendel’s First Law: Two members of a gene pair (alleles) segregate from each other in the formation of gametes: half the gametes carry one allele, and the other half carry the other allele.
• All F1 offspring resemble only one of the parents.
• Homozygous dominant (SS) is a genotype and phenotype.
• Huntington Disease is an example of a disorder that follows Mendelian inheritance, with the H (dominant allele) and h (recessive allele) genotypes.
• Single gene (monogenic) disorders follow Mendelian inheritance, such as cystic fibrosis, Huntingtons’ disease, thalassemia, and muscular dystrophy.
• Cystic fibrosis is an example of a disorder where unaffected parents can have affected offspring, and carriers are heterozygous.
• Eye colour is a continuously variable trait influenced by the OCA2 gene (oculocutaneous albinism II), and MC1R, ASIP, and HERC2 genes.
• Most human traits are polygenic, meaning they are influenced by allelic variants at multiple genes/loci.
• Pedigree analysis is a tool for studying inheritance in humans, using multiple generations and predicting inheritance of disease (monogenic traits).
• Pedigree analysis is a tool for mapping the inheritance of monogenic (single gene) disorders in humans.
• The P (parental) cross would result in F1 genotypes all being Ss and F1 phenotypes all being smooth.
• The F1 x F1 cross would result in F2 genotypes being ¼ SS, ½ Ss, ¼ ss and F2 phenotypes being ¾ smooth, ¼ wrinkled seeds.
• Homozygous recessive (ss) is a genotype and phenotype.
• Most common disorders with an inherited component are also polygenic (also multifactorial), such as type 2 diabetes, Crohns’ disease, heart disease, and multiple sclerosis.
• Mendel found that the dominant (smooth) and recessive (wrinkled) traits occur in the F2 generation in a ratio of approx 3:1.
• A RECESSIVE allele (Lowercase = d) will only 'show' in the trait (phenotype) if an individual has two (both) recessive alleles, i.e. dd.