Studying inheritance

When populations are geographically isolated, there is no interbreeding between members of each population and so there genes are also isolated. In time, the genes within each population will change and this lead to the formation of new species.

Genotype sets the limits within which the characteristics of an individual may vary. e.g. if a baby can grow to be 1.8m tall, but the actual height is determined by other factors such as diet.

Any change to the genotype as a result of a change to the DNA is called a mutation and it may be inherited if it occurs in the formation of gametes.

The environment can alter an organism's appearance. Any change to the phenotype that does not affect the genotype is not inherited and is called a modification

These polypeptides make up the enzymes that are required in the biochemical pathway that leads to the production of a characteristic e.g. a gene could code for brown pigment in the iris of the eye.

The position of a gene on a chromosome is known as the locus.

All individuals of the same species have the same genes, but not necessarily the same alleles of these genes.

Sexually reproducing organisms have chromosomes that occur in pairs called homologous chromosomes.

If the 2 alleles are different, then the organism is said to be heterozygous for the characteristic.

The alleles of the heterozygote that expresses itself in the phenotype is said to be dominant, whilst the one that is not expressed is said to be recessive.

The effect of a recessive allele is only apparent in the phenotype of a diploid organism when it occurs in the presence of another identical allele i.e. when it is the homozygous state.

In this situation when both alleles occur together, the phenotype is either a blend of both features ( e.g. snapdragons with pink flowers resulting from an allele for red and white) or both features are represented ( e.g. the presence of both A and B antigens in blood group AB)

Sometimes a gene has more than 2 allelic forms. In this case, the organism is said to have multiple alleles for the character. However, as there are only 2 chromosomes in a homologous pair, it follows that only 2 of the 3 or more alleles can be present in a single organism.

Monohybrid inheritance is the inheritance of a single gene.

When the heterozygous plants of the first filial generation are crossed with one another, the offspring ( the second filial generation) are always in a n approximate ratio of 3 plants with green pods to each 1 plant with yellow pods.

In diploid organisms, characteristics are determined by alleles that occur in pairs. Only 1 of each pair of alleles can be present in a single gamete.

In human females, the 2 sex chromosomes appear the same and are called x chromosomes. In the human male there is a single x chromosome like that in the female, but the second one is the smaller in size and shaped differently. This is the y chromosome.

Females have 2 x chromosomes, all the gametes contain an x chromosome.

Any gene that is carried on either the x or y chromosome is said to be sex-linked.

Those characteristics that are controlled by a recessive allele on the non-homologous portion of the x chromosome will appear more frequently in the male. This is because there is no homologous portion on the y chromosome that might have the dominant allele.

There has been selective removal of this gene from the population, so its occurrence is relatively rare.

One of a number of causes of haemophilia is a recessive allele with altered DNA nucleotides that therefore does not code for the required protein. This results in the individual being unable to produce a protein that is required in the clotting process.

When showing sex linkage, you show the allele linked to the appropriate chromosome: i.e. XH Xh or XH Y. If the allele is sex linked to the x chromosome, you do not draw an allele present on the y chromosome. There is no equivalent allele on the y chromosome as it does carry the gene for the clotting protein.

If their mother does not suffer from the disease, she may be heterozygous for the character ( XH Xh). Such females are called carriers because they carry the allele without showing any signs of the character in their phenotype.

Pedigree charts: A useful way to trace inheritance of sex-linked characters, such as haemophilia.

co-dominance: in which both alleles are equally dominant.

When both alleles are dominant i.e.co-dominant both alleles are expressed in the phenotype.

There are 3 alleles associated with the gene I ( immunoglobulin gene) which lead to the production of different antigens on the surface membrane of red blood cells.

Although there are 3 alleles, only 2 can be present in an individual at any one time, as there are only 2 homologous chromosomes and therefore only 2 gene loci.

Possible crosses:
Blood group A = IA IA or IA IO
Blood group B = IB IB or IB IO
Blood group AB = IA IB
Blood group O = IO IO

When certain individuals of blood group A are crossed with certain individuals of blood group B, there children could be of any of the 4 blood groups.

Gene pool = All the alleles of all the genes of all the individuals in a population at any one time/ All the alleles of one gene of all the individuals in a population at any one time.

If there are 10000 people in a population, there will be twice as many alleles ( 20000) in the gene pool of this gene.

If all individuals are homozygous dominant ( GG) the frequency of dominant alleles in the gene pool is 1.0

If all heterozygous ( Gg) the frequency of dominant alleles = 0.5 and recessive alleles = 0.5

The principle predicts that the proportion of dominant and recessive alleles of any gene in a population remains the same from one generation to the next provided that 5 conditions are met:

You can use the hardy-Weinberg equation to:
predict allele frequency
predict genotype frequency
predict % of next generation with each genotype
show if external factors affect allele frequency

p2 + 2pq + q2 = 1
2 pq = heterozygous frequency