Lethal alleles Multiple Alleles

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1 Sponge Questions Extensions and Exceptions to Mendel s Laws Genetics Notes Is it possible for a type O child to be born from a mating between a type-a parent and a type-b parent? Between a type AB parent and a type O parent? What are the chances that a woman who s Rh- will have an Rh+ child if her mate is heterozygous? In the famous Charlie Chaplin paternity case in the 1940 s, Chaplin was accused of fathering an illegitimate child. The baby s blood was B, the mother s A and Chaplin s O. If you had been the judge, how would you have decided the case? Exceptions to Mendel s Law Mendel chose traits in peas that showed two distinct forms. Not all genes exhibit such simple inheritance. Alleles interact Gene interaction Non-nuclear genes Segregation of genes on same chromosome Lethal alleles Some allele combinations are lethal. Mexican hairless dogs result from a mutation in a gene that shows lethality hh hairy the wildtype trait Hh hairless one mutation present creates a visible phenotype HH dies two mutation are lethal Lethal alleles Multiple Alleles A gene may exist in more than two allelic forms in a population. Genes can mutate in many ways at any nucleotide in their DNA sequence. 1

2 Multiple alleles: coat color in rabbits Incomplete dominance Grey CC or Cc ch or Cc h or Cc Chinchilla c ch c ch The heterozygous phenotype is typically intermediate to the homozygous phenotype Himalayan c h c or c h c h Light grey c ch c h or c ch c Albino cc Codominant alleles are observed simultaneously Codominant Alleles The ABO gene encodes a cell surface protein. Allele A makes A protein Allele B makes B protein Allele O makes no protein Alleles A and B can be present on the cell surface at the same time. Alleles A and B are codominant. Allele O is recessive to both A and B alleles Blood Types Example Epistasis when one gene affects the expression of a second Epistasis gene. - Condition results when one gene masks another - H gene is epistatic to the ABO gene. - H protein attaches the A or B protein to the cell surface. - hh genotype = no H protein - All ABO genotypes appear as type O 2

3 Variable expressivity - Expressivity of a phenotype is the severity, degree and/or extent of the expression of a trait FF or Ff Incomplete penetrance all show mild, moderate or profound deafness - Penetrance refers to the all or none expression of a geneotype - Occurs when the disease phenotype is not always observed among individuals carrying the diseaseassociated genotype. DD or Dd 80% polydactyly DD or Dd 20% no polydactyly Pleiotropy One gene controls or influences the expression of many symptoms in a disorder. These symptoms may be variably expressed Occurs when a single protein affects different parts of the body or participates in the different biochemical processes King George III - Recessive Inheritance Photo North Wind Picture Archives of porphyria variegata Phenocopy A trait caused by the environment that mimics an inherited condition Exposure to teratogens Thalidomide causes limb defects akin to rare inherited phocomelia A birth defect in which the upper portion of a limb is absent or poorly developed, so that the hand or foot attaches to the body by a short, flipperlike stump Infection Rubella in pregnant mothers causes deafness mimicking inherited forms of deafness Genetic Heterogeneity Individuals with identical phenotypes may reflect different genetic causes. Deafness Albinism Cleft palate Poor blood clotting Mitochondrion Organelle providing cellular energy Contains small circular DNA No crossing over or DNA repair Many copies of the mitochondrial genome per cell 37 genes, no histones, no introns Maternal inheritance Mitochondrial Inheritance Mitochondria and their genome are transmitted from a mother to all of her offspring. 3

4 There are many copies of the mitochondrial DNA Heteroplasmy is the condition in which mitochondrial DNA sequence is not the same in all copies Heteroplasmy Linkage Is the term indicating that two genes are not transmitted independently. Why? Two genes physically near each other on a chromosome will not assort randomly in meiosis. Linkage Unlinked : 4 type of gametes PL, Pl, pl, pl Tightly linked: 2 types of gametes PL and pl NOTE for LINKAGE which two types are observed (PL and pl OR Pl and pl) depends on which alleles are on the same chromosome in the parent! Recombination When chromosomes recombine new combinations of alleles are created Parental chromosomes have the alleles present in the original configuration Recombinant chromosomes have new combinations of alleles Recombination The frequency of recombination between two genes is directly related to the physical distance between the genes 4

5 Inheritance of linked genes The genes for Rh factor (R) and anemia (E) are linked, but some recombination occurs between the two genes A linkage map is a diagram indicating the relative distance between genes. 1% recombination = 1 map unit = 1 centimorgan (cm) Map distances are additive. Linkage Disequilibrium The non-random association between alleles at two locations on a chromosome is called linkage disequilibrium Two genes, A and B, exist in a population. If the frequency of chromosomes with AB=Ab=aB=ab then the genes are in equilibrium If the frequency of one allele of gene A is seen more frequently with a particular allele of gene B, then the genes are in linkage disequilibrium Haplotype Karyotype A haplotype is the set of alleles inherited on one chromosome 5