Gene Linkage
Patterns of Inheritance Are Not Always Predictable
- Some patterns of inheritance do not follow Mendel's laws
- Their observed phenotypic ratios among the offspring differ significantly from the predicted ratios
- Gene linkage - means that some genes are not inherited independently of one another
- Many traits are determined by multiple genes and physiological processes and therefore do not follow Mendelian patterns
Gene loci are said to be linked if they are on the same chromosome
- Loci (singular: locus) refers to the specific linear positions on the chromosome that genes occupy
- If genes are on the sex chromosome, they are said to be sex-linked
- Sex-linked genes have characteristics that generally affect only one gender of a species
- These genes are usually on the X chromosome because the Y chromosome contains very few genes
- In humans, color blindness and hemophilia are notable examples of genetic conditions that only affect males
- Linked genes located on the chromosomes 1-22, or any chromosome that is not a sex chromosome (called autosomes) are said to be examples of autosomal linkage
- The likelihood of genes being inherited together, or the extent to which they are linked, is measured in units called centimorgans, in honor of Thomas Hunt Morgan, the geneticist that discovered the concept of linkage
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 probabilities that the linked genes will be inherited together
Identifying autosomal linkage from phenotypic ratios in crossing studies
- 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
Dihybrid Cross without Linkage Punnett Square Table
- 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
- The explanation for this new phenotypic ratio is given in the worked example below:
Worked example
Worked example: Explaining autosomal linkage
- In reality, the genes for tail length and scale color in this particular species of newt show autosomal linkage
Parental phenotypes: normal tail, green scales x short tail, white scales
Parental genotypes: TG tg tg tg
Parental gametes: (TG) or (tg) (tg)
Dihybrid Cross with Linkage Punnett Square Table
- 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
Exam Tip
When you are working through different genetics questions you may notice that test crosses involving autosomal linkage predict solely parental type offspring (offspring that have the same combination of characteristics as their parents). However in reality recombinant offspring (offspring that have a different combination of characteristics to their parents) are often produced. This is due to the crossing over that occurs during meiosis. The crossing over and exchanging of genetic material breaks the linkage between the genes and recombines the characteristics of the parents. So if a question asks you why recombinant offspring are present you now know why!