modifications and deviations from mendelian inheritance

incomplete dominance

when the phenotype of a heterozygote is distinct from and often intermediate between the phenotypes of the two homozyogtes

co dominance

Co-dominance occurs when both alleles contribute to the phenotype AND the effects of both alleles are seen to their full extent– not an intermediate ‘blended’ phenotype as seen with incomplete dominance

ABO Blood grouping in humans. A and B alleles cause modifications to glycoproproteins on RBC surface. A and B are co-dominant to each other (both modifications seen if Type AB). A and B are dominant to O, which is recessiveEpistasis

epistasis

the phenotype caused by one gene is influenced by one or more other genes

sex linked inheritance

• normally assumed that the individual has 2 copies of each locus

• this is only true of the genes on the autosomes however

• animals have pairs of sex chromosomes which may be the same of different depending on the sex of the animal

• genes located on sex chromosomes do not follow mendelian patterns of inheritance

X linked dominant inheritance

• as with autosomal dominant inhertance only only dominant allele is required for the phenotype to be displayed

• with X linked dominant disorders the sex of the offpsring doesnt make much difference to the phenotype outcome

• the individual will display th ephenoytype if they inherit the dominant allele, whatever their sex.

X linked recessive inheritance

• in htis instance the sex of offspring doesn not matter

• as females are xx they require 2 recessive alleles to show phe otype

• as males have only one x they show si inherit x bearing the recessive allel

• therefore 100 percent of male offpsirng of a female with an x linked recessive phentype will shwo the phenotype

Y linked inheritance

• only definite trait inherited via the y chromosome is maleness- SRY sex determining gene

genetic linkage

Law of independent assortment states that genes segregate independently of each other

❑ However, we now know that (10s to 1000s of) genes are arranged on long DNA strands which form chromosomes

❑ Genes on the same piece of DNA are therefore physically linked

❑ These genes may not always be able to segregate independently of each other ❑ Genes that are linked to each other by being on the same chromosome are said to belong to the same linkage group

recombination

consequence of linkage

Genes that remain linked together will segregate in groups during gamete formation.

❑Consequently offspring will have a higher than expected proportion of parental genotypes and less recombinant genotypes

❑Think about the dihybrid cross in the last lecture, we had parents which were:

❑rug hair/no horns x short hair/horns

❑And after crossing their offspring, we got offspring that were:

❑rug hair/no horns, short hair/no horns, rug hair/horns, short hair/horns

❑These new combinations could not occur if the genes were completely linked – all offspring would be rug hair/no horns or short hair/horns