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

• Genes located on the same chromosome are referred to as being genetically linked

  • Because they sit on the same DNA molecule, they tend to be inherited together more often than Mendel’s law of independent assortment would predict

• 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

• 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 - map distance or map unit

  • This calculation is called gene or genetic mapping

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

Codominance

• Codominance occurs when the phenotypes from both alleles in a heterozygous individual are fully and simultaneously expressed in the phenotype

  • The individual's phenotype is distinct from both the homozygous individuals and is not a blend

• Genotype notation uses a capital letter to represent the gene (e.g. I) and superscript letters for the alleles (e.g. Iᴬ, Iᴮ, Iᴼ)

Example 1 of codominance

• A good example of codominance can be seen in human blood types

• The gene for blood types is represented in the genotype by I and the three alleles for human blood types are represented by A, B and O

• Gene controlling blood type = I

• Three alleles: Iᴬ, Iᴮ (codominant), Iᴼ (recessive)

• Genotypes and resulting phenotypes:

  • IᴬIᴬ or IᴬIᴼ → Blood group A

  • IᴮIᴮ or IᴮIᴼ → Blood group B

  • IᴬIᴮ → Blood group AB (codominance)

  • IᴼIᴼ → Blood group O

Example 2 of codominance

• Chickens can have different alleles for the gene that determines the colour of their feathers

  • The gene for colour is represented by the capital letter C

• The two alleles for this gene are white for white feather colour, and black for black feather colour

  • The two alleles are represented by superscript letters, C^W and C^B

• A chicken with the genotype C^WC^W has white feathers as their phenotype

• A chicken with the genotype C^BC^B has black feathers as their phenotype

• A chicken with the genotype C^WC^B has a combination of both feather colours, they are called speckled colour chickens

• Because both alleles are expressed in the phenotype this is called codominance

Incomplete dominance

• Incomplete dominance is similar to codominance because two alleles are expressed together instead of just one dominant allele being expressed

• However, instead of both alleles being expressed, both alleles are partially expressed leading to a phenotype which is a blend of both phenotypes or an intermediate phenotype between the two

Example of incomplete dominance

• The Marvel of Peru flower (Mirabilis jalapa) can have different alleles for the gene that determines the colour of their flowers

  • The gene for colour is represented by the capital letter C

• The two alleles for this gene are white for white flower colour, and red for red flower colour

  • The two alleles are represented using superscript letters, C^W and C^R

• A plant with the genotype C^WC^W has white flowers as their phenotype

  • A plant with the genotype C^RC^R has red flowers as their phenotype

  • A plant with the genotype C^WC^R has a blend of both colours, which is expressed in the phenotype as a pink flower colour

  • Because the flowers are neither white or red, but an intermediate between this two, this is incomplete dominance

Sex linkage

• Some genetic diseases in humans are sex-linked (X- or Y-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

  • Sex-linked traits are inherited at higher rates in males (XY) individuals than they are in females (XX) individuals.

• 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

Pleiotropy

• Pleiotropy is a phenomenon in which the expression of a single gene results in multiple traits or effects

  • A single allele change can produce several effects in different tissues, so those traits co-occur

  • These traits, therefore, do not segregate independently

• Pleiotropy occurs during

  • sickle cell anemia: the gene (HBB) results in many traits, including anemia, pain crises, spleen damage, plus malaria resistance

  • cystic fibrosis: the gene (CTRF) causes multiple traits, including thick lung mucus, pancreatic insufficiency and salty sweat

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