Genes and Patterns of Inheritance

Diploid organisms possess pairs of homologous chromosomes that carry alleles of the same gene at the same locus. If the alleles are the same the individual is homozygous; if the alleles are different, they are heterozygous.

There are different patterns of inheritance:

• dominance - dominant alleles are always expressed; recessive alleles are only expressed when homozygous
• codominance - two alleles are both expressed so that the heterozygote has a distinctive phenotype
• lethality - one homozygote may inherit a lethal allelic combination, so that two heterozygotes produce a 2:1 ratio
• multiple alleles - there are more than two alleles of a gene, so there is more than one type of heterozygote
• sex-linkage - alleles are located on a chromosome that determines gender (e.e. an X chromosome in mammals)

A test cross determines whether an organism showing a dominant feature is homozygous or heterozygous - the organism is bred with one that shows the recessive trait

Dihybrid inheritance involves two separate genetic loci. If both parents are heterozygous for the two genes and, at each locus, an allele exhibits dominance, then a 9:3:3:1 phenotypic ratio results

Polygenic inheritance occurs when the alleles of many different genes contribute to a characteristic

Epistasis occurs when an allelic combination at one locus determines/influences whether the alleles at another locus are expressed

Mendel's laws:

• the law of segregation - only one allele is inherited via a gamete
• the law of independent assortment - an allele of one gene may be inherited with either allele of another gene