Inheritance Patterns

InHERITAnCE PATTERnS Monohybrid Crosses Sex Linkage A monohybrid cross determines the allele combinations for potential offspring for one gene only • Crosses can be represented via the use of Punnett grids Sex linkage refers to when a gene is on a sex chromosome • I.e. X or Y (all other chromosomes are autosomal) Monohybrid crosses are calculated via the following steps: • Designate letters to represent alleles (e.g. A, a) • Identify genotype / phenotype of parents (P generation) • Determine genotype of gametes (haploid) • Work out gamete combinations with a Punnett grid • Identify ratios of offspring (F1 generation) • Y chromosome is short and has few genes (<100) Sex chromosomes (X/Y) • X chromosome is large with many genes (~2000) X Y Sex-Linked Traits Bb P Gametes B Bb b B B b BB Bb Sex-linked traits have altered inheritance patterns: • Males have a higher rate of X-linked recessive conditions as they cannot mask the recessive allele (are hemizygous) • Females can be carriers for X-linked recessive conditions (heterozygotes can carry the allele but not express it) b B Bb b For X-linked conditions: • Recessive: Affected mothers must have affected sons • Dominant: Affected fathers must have affected daughters bb BB F1 B Bb B B bb Bb b B b b b Examples of X-linked recessive traits include: • Haemophilia (cannot clot blood properly) • Red-green colour blindness Modes of Inheritance A pedigree is a chart of genetic history over several generations AUTOSOMAL DOMINANT In a typical pedigree chart: • Males are represented as squares, while females as circles • Shaded symbols denote individual has a specified condition • A horizontal line between man and woman represents mating • Offspring numbered from left to right according to age Autosomal Dominance: • If both parents are affected by a trait and any offspring is not, the trait must be dominant (parents must be heterozygous) PROOF Not recessive as two affected parents could not have an unaffected offspring AUTOSOMAL RECESSIVE Autosomal Recessive: • If neither parents is affected by a trait but any offspring is, the trait must be recessive (parents must be heterozygous) Sex-Linked Traits: • No way to conclusively prove sex-linkage with a pedigree chart, but certain patterns may suggest the possibility PROOF Not dominant as two unaffected parents could not have an affected offspring