Deviation from mendelian inheritance.docx

Mendelian inheritance describes inheritance patterns that’s obeys: Law of segregation Law of independent assortment. Genetic diseases are caused by mutant alleles. Most are caused by recessive alleles. This causes a change in protein: Missing protein Altered functions. Effects enzyme activity. If re...

Mendelian inheritance describes inheritance patterns that’s obeys: Law of segregation Law of independent assortment. Genetic diseases are caused by mutant alleles. Most are caused by recessive alleles. This causes a change in protein: Missing protein Altered functions. Effects enzyme activity. If recessive allele does not cause disease, they are carriers of the disease. Carriers (heterozygotes) 50% of the normal protein is enough to accomplish protein’s cellular function. A heterozygote may produce more than 50% of the functional protein. Incomplete dominance The phenotype is intermediate e.g. a red and a white parent produces a pink offspring. 1:2:1 genotype and phenotype. Example of incomplete dominance= flower colour in snap dragon plants. RR= Red flowers WW= white flowers RW= pink flowers as it’s a blend of white and red. Codominance The phenotype of the heterozygote is not an intermediate between phenotypes of the homozygotes. The heterozygote expresses the phenotypes of both homozygotes. e.g. Coat colour for certain cattle breed R- red coat colour W- white coat colour Heterozygous expresses both red and white patches so both R and W alleles are expressed. Certain circumstances alter the expression of phenotypes although Mendel’s law still apply. Lethal alleles Essential genes required for survival. Absence of their protein results in lethal phenotype. Lethal allele has the potential to cause death of an organism. Typically, a result of mutations in essential genes Inherited in recessive manner. Some lethal alleles interfere with cell division therefore killing the organism. Conditional lethal alleles may kill an organism only when certain environmental conditions prevail. Such as temperature sensitive. Semi-lethal alleles that kill some in the population but not all of them. Combinations of alleles cause death. e.g. genetic disorder in humans called Tay Sachs disease. Rare inherited disorder causes by gene called HEXA gene. It effects the brain and spinal cord due to accumulation of toxic GM2 ganglioside. Sex influenced traits. Alleles behave differently in male and females. Where allele is dominant in one sex and recessive in opposite sex. Caused by autosomal gene. E.g. presence of horn or hornless in cattle breeds In bulls (male)- presence of one dominant allele leads to development of horns- not effected by hormonal factors In cows (female)- expression of the trait can be influenced by hormonal factors e.g. testosterone. Sex-limited traits Traits observed in one of the two sexes. Determined by sex hormones. e.g. egg production in birds. Ability to produce eggs is limited to females. Trait influenced by sex hormones by females e.g. oestrogen. Multiple alleles 2 alleles for each gene- one on each chromosome homologue May be different mutations in gene to give hundreds of potential combinations with different phenotypes. e.g. coat colour in rabbits There are multiple alleles at the C gene locus that determines coat colour Penetrance Whether a genotype is expressed as a phenotype. Proportion of organisms whose phenotype matches their genotype for a given character is called penetrance of the genotype. Expressivity Genes that are expressed to different degrees in different organisms- variable expressivity. The same mutant gene can result in a severe form of a disease in one person but a mild form in another. e.g. polydactyl Pleiotropy Single gene (monogenic) disorder. Many symptoms A single gene has a lot of different phenotypes. e.g. sickle cell disease. A single mutation in HBB gene can lead to multiple across all body systems. Opposite to Pleiotropy is genetic heterogeneity. Different genes that produce the same phenotype Phenocopies An environmentally caused trait that appears to be inherited as a phenocopy. e.g. individual who develop melanoma which is a type of skin cancer caused due to UV.