Non-Mendelian Genetics (College Board AP® Biology): Study Guide

Gene linkage

• Some patterns of inheritance do not follow Mendel's laws

• Their observed phenotypic ratios among the offspring differ significantly from the predicted ratios

• Patterns of inheritance of traits that do not follow ratios predicted by Mendel’s laws and can be identified by quantitative analysis

• One example is gene linkage where some genes are not inherited independently from one another

Autosomal linkage

• Loci (singular: locus) refers to the specific linear positions on the chromosome that genes occupy

• Linked genes located on chromosomes 1-22 (not sex chromosomes) are called autosomes and are examples of autosomal linkage

• Dihybrid crosses and their predictions rely on the assumption that the genes being investigated behave independently of one another during meiosis

• However, not all genes assort independently during meiosis

• Some genes which are located on the same chromosome display autosomal linkage and stay together in the original parental combination

• Linkage between genes affects how parental alleles are passed onto offspring through the gametes

• The distance between linked genes on a chromosome can be mapped using the probability that the linked genes will be inherited together

Identifying autosomal linkage from phenotypic ratios

• In the following theoretical example, a dihybrid cross is used to predict the inheritance of two different characteristics in a species of newt

  • The genes are for tail length and scale color

• The gene for tail length has two alleles:

  • Dominant allele T produces a normal length tail

  • Recessive allele t produces a shorter length tail

• The gene for scale color has two alleles:

  • Dominant allele G produces green scales

  • Recessive allele g produces white scales

Without linkage

• Normal Mendelian ratios would result if there is no linkage

• The outcomes for this dihybrid cross if the genes are unlinked are as follows

• Predicted ratio of phenotypes in offspring =

  • 1 normal tail, green scales : 1 normal tail, white scales : 1 short tail, green scales : 1 short tail, white scales

• Predicted ratio of genotypes in offspring =

  • 1 TtGg : 1 Ttgg : 1 ttGg : 1 ttgg

With linkage

• However, if the same dihybrid cross is carried out but this time the genes are linked, we get a different phenotypic ratio

  • There would be a 1 : 1 phenotypic ratio (1 normal tail, green scales : 1 short tail, white scales)

  • This change in the phenotypic ratio occurs because the genes are located on the same chromosome

  • The unexpected phenotypic ratio, therefore, shows us that the genes are linked

• Parental phenotypes: normal tail, green scales x short tail, white scales

  Parental genotypes:       TG   tg             tg   tg

  Parental gametes:       (TG) or (tg)              (tg)

• Predicted ratio of genotypes in offspring =

  • 1 (TG)(tg) : 1 (tg)(tg)

• Predicted ratio of phenotypes in offspring =

  • 1 normal tail, green scales : 1 short tail, white scales

Sex linkage

• Some genetic diseases in humans are sex-linked

• Inheritance of these diseases is different in males and females

  • Sex-linked genes are only present on one sex chromosome and not the other

  • This means the sex of an individual affects what alleles they pass on to their offspring through their gametes

• If the gene is on the X chromosome males (XY) will only have one copy of the gene, whereas females (XX) will have two

  • This occurs in mammals and flies

• If the gene is on the X chromosome, males (XY) will only have one copy of the gene, whereas females (XX) will have two

• There are three phenotypes for females:

  • normal

  • carrier

  • has the disease,

• Males have only two phenotypes

  • normal

  • has the disease

• In certain other species, the chromosomal basis of sex determination is not based on X and Y chromosomes

  • This occurs in birds which have sex chromosomes called Z and W; these determine the sex of the bird

  • Bees have a genetic system called haplodiploidy that determines the sex of offspring; female bees are diploid and develop from fertilized eggs, while males are haploid and develop from unfertilized eggs

Multiple genes

• Multiple genes and physiological processes determine many traits and therefore do not follow Mendelian patterns

• Traits from multiple genes do not follow predictable ratios like 3:1 or 9:3:3:1

• Examples of such traits include height, intelligence and skin color, and result from multiple genes working together

• These are called polygenic traits and show a range of phenotypes (continuous variation), rather than simple dominant or recessive patterns

• Genes can also interact with each other; one gene affects or masks the expression of another, influencing the phenotype in complex ways: this is called epistasis

• The influence of the environment affects the expression of traits due to the interaction with genes which may further complicate inheritance patterns

Non-nuclear inheritance

• Not all characteristics are determined by genes carried on chromosomes in the nucleus in eukaryotes

• Some genes are located within organelles elsewhere in the cell, away from the nucleus

• Mitochondria, chloroplasts (and other plastids) carry DNA

  • This non-nuclear DNA is thought to date back to the origins of eukaryotic life and gives further supporting evidence to the theory of endosymbiosis

• This DNA is not passed onto future generations in the same ways as nuclear DNA, so the traits it carries are inherited in non-Mendelian ways

  • The genes they carry are randomly assorted to daughter cells

• In animals, mitochondria are transmitted by the female egg cell and not by sperm cells

  • Therefore, characteristics coded for in the mitochondrial DNA are inherited through the maternal line

• Similarly, in plants, mitochondria and chloroplasts are passed on in the ovule and not by the pollen grains

  • Therefore, characteristics coded for in the mitochondrial and chloroplast DNA are inherited through the maternal line in plants also