Linked genes: sex linkage and pedigrees

Linked GenesGene MappingSex Linked Inheritance◦ X linked recessive◦ X linked dominant◦ Y linked

Pedigrees◦ Autosomal Recessive◦ Autosomal Dominant◦ X linked Recessive◦ X linked DominantVariation

Linkage and recombinationSome genes do not assort independent of each other, but rather are inherited together.We call these genes linked. They are on the same chromosome and are generally inherited together.However, because of crossing over this linkage is never quite complete.

Linkage and gene mappingIt is possible to measure the relative position of various linked genes by comparing the ratio of the parental genotypes to recombined genotypes.

A special type of monohybrid inheritance is called sex linkage. Genes carried on either of the chromosomes responsible for sex (X or Y) are said to be sex-linked. A trait can be said to b X- linked inheritance involving genes carried on the X chromosome or Y-linked inheritance involving genes carried on the Y chromosome.One common example in humans is the inheritance of Colour blindness. Colourblindness is a recessive trait linked to the X chromosome and is more common in males than females. If we cross a Colour blind male with a heterozygous normal for colour vision female then we would have the following Punnett Square and outcomes.This would result in one normal female and one colour blind female and one normal male and one colour blind male.

XB Xb

Xb XBXb XbXb

Y XBY XbY

X linked dominantIn the case of X linked dominant traits the trait is more common in females than X linked recessive traits.A father will always pass such a trait to his daughters.A heterozygous mother will have a 50% chance of passing the trait to all her children regardless of sex.

Y linked inheritanceThe Y chromosome carries a small number of genes concerned with male fertility and sex determination.Traits carried on these chromosomes pass from father to son through the generations.

Pedigrees are used to show the inheritance of a trait or disease in a family and are often used in combination with Punnett Squares to investigate how a certain trait has been passed on or could be passed on.

Things to remember:◦ For an individual to

express an autosomalrecessive trait both copies of the allele must be present

◦ If both parents affected then all offspring will also be affected.

◦ Recessive traits tend to skip generations with few individuals affected.

Things to remember:◦ For an autosomal

dominant trait to be expressed only one copy of the allele is required.

◦ Usually present in each generation, many affected individuals.

◦ If the homozygous dominant condition is lethal the phenotypic and genotypic ratios will be different – good exam trick!

Things to remember:◦ Males only require one

allele to express the trait, therefore more males than females will show the trait.◦ Females require both

alleles to show the trait.◦ Heterozygous females

are described as carriers of the trait

Things to remember:◦ Any individual with the

trait must have a parent with the trait◦ Females may be

heterozygous and show the trait◦ This condition is much

harder to detect, so again a good exam trick question!

Variation is the degree of difference that exists between members of a population. The greater the degree of variation, the more genetically healthy is a population, ie more able to respond to change in the environment.

If there is only one gene with two alleles for a particular trait, you either express the trait or you don’t. Example: the allele for tongue rolling is dominant to the allele for not being able to roll your tongue. That means within a population there is very little variation, you either can roll your tongue or you can’t. This is known as discrete variation.

If there is more than one gene for a particular trait and if each gene has multiple alleles, such as those that control height, or skin colour, there is much greater variation within the population and it usually follows a normal distribution pattern. This is known as continuous variation.

DiscreteVariation

ContinuousVariation

Multiple Alleles and Poly Genes – as discussed above the more alleles present for a particular gene and the more genes involved in a certain trait, then the greater the source of variation.

Crossing Over in Meiosis – during meiosis it is possible for chromatids to become entangle and to break and reform differently causing variation by rearranging the normal groups of alleles present in the gametes

Environment – has an influence over the phenotypic expression of the genotype, thus increasing variation within a species, eg. Diet and nutrition affect the size of individuals.

Independent Assortment – normal fertilization sees the alleles being assorted independently – thus sexual reproduction assists with the increase in variation.

Random Fertilisation – plants are more able to hybridise by mixing gametes between like species and so further variation occurs.

Mutations