Alleles: Their Expression and Inheritance For Pedigree Analysis: Visit Physical Anthropology Thursday, October 28, 2010
What is a genome? A genome is all of the chromosomes and DNA that an organism can possess. The study of genomes is called genomics. Genome size does not reflect complexity. The number of genes varies between species. table here Thursday, October 28, 2010
Principle of Segregation The principle of segregation is founded on the generation of gametes by meiosis. It assumes that * Alleles occur in pairs because chromosomes occur in pairs * During gamete production, members of each gene pair separate so each gamete contains one member of each pair * During fertilization, the full number of chromosomes is restored and members of a gene are reunited Thursday, October 28, 2010
Genotypes and Phenotypes A gene that controls one function can exist in different forms called alleles . A genotype is the genetic composition of an individual, especially in terms of alleles for particular genes. We might say that the flower shown here is The expression of the genotype is called the displaying incomplete, or phenotype . This expression might be partial, dominance - it’s physical, chemical or physiological. phenotype is a mixture of the traits coded by its two Each characteristic that makes up a different colour alleles. phenotype is called a trait . Thursday, October 28, 2010
Alleles Control Traits- ABO Blood Groups Genotype Trait Relationship Phenotype Combination Phenotype for allele A Homozygous AA displays dominance over or A or the phenotype for allele o Heterozygous Ao AA Ao Phenotype for allele B Homozygous BB or displays dominance over B or the phenotype for allele o Heterozygous Bo BB Bo Phenotype for allele B displays codominance with the AB Heterozygous AB phenotype for allele B AB Phenotype for allele o is only observed in the phenotype of individuals O Homozygous oo homozygous for the allele. It is a recessive oo phenotype. Thursday, October 28, 2010
ABO Blood Groups The correct notation for blood group alleles is I A , I B , and i . Answer the following questions using this system. 1. If two people who have blood type A (both with the I A i genotype) have children, what proportion of their children would be expected to have the O blood type? Why? 2. Can these parents have a child with AB blood? Why or why not? 3. Mary (AB) marries Barry. Barry’s mother was type O and his father type A. Mary and Barry have a son with type B blood. Can Barry’s blood type and genotype be determined? Thursday, October 28, 2010
Genetics Terminology Checkpoint Genotype: The genetic makeup of an individual; the alleles at a particular locus Phenotype: The observable or detectable physical characteristics of an organism Heterozygous: Having different alleles at the same locus on members of a chromosome pair Homozygous: Having the same allele at the same locus on both members of chromosome pair Recessive traits will only be expressed where there are two copies of the allele Dominant traits prevent the expression of recessive traits in heterozygotes Codominance occurs where two traits are expressed together in a heterozygous genotype. Incomplete dominance occurs where the phenotype is intermediate of two traits. Thursday, October 28, 2010
Pedigree Analysis Pedigree analysis is a way of determining the pattern of inheritance of particular traits. You just applied pedigree analysis of an autosomal trait to the previous questions on ABO blood groups. Pedigree analysis is often used in genetic counseling to determine the probability that offspring might carry particular traits or diseases. A Simple Pedigree Key Normal male Normal female Affected female Affected male 1. How many children and of what sex? A pedigree chart shows us phenotypes . 2. Which child is affected? Often we can also use a pedigree to determine genotypes . 3. Is the trait dominant or recessive? Thursday, October 28, 2010
Pedigree Charts Thursday, October 28, 2010
Pedigree Charts Carrier (autosomal recessive) If you see symbols like these around the place this is what they mean Carrier female (sex linked recessive) Thursday, October 28, 2010
Examples Huntington’s Disease Widow’s peak Long Eyelashes Hair on the back of your hand! * Both males and females can be affected * All affected individuals will have at least one affected parent * A heterozygote will show the trait * In large samples equal numbers of males and females will be affected * Once the trait disappears from a branch of the pedigree it does not reappear If individual II-9 married an unaffected woman what would be the chances of their first child being affected? Thursday, October 28, 2010
Examples Cystic Fibrosis Phenylketonuria Sickle-Cell disease * Both males and females may be affected * Two unaffected parents can have an affected child * The trait may skip generations * The trait is only expressed in a homozygous state If individual II-12 married an affected male what is the likelihood their first child will be affected? Thursday, October 28, 2010
Examples Very uncommon Rett Syndrome * The male will pass on the trait to all his daughter but not his sons. * A female will pass the trait to both her daughter and her sons. * Every affected person has at least one parent with the trait. If individuals III-8 and III-9 had a son (ew!) what is the probability he would show the dominant trait? Thursday, October 28, 2010
Examples Duchenne Muscular Dystrophy Haemophilia A & B Red-Green Colour Blindness * All sons of a female with the trait will be affected * All the daughters of an affected male will be carriers of the trait * In a large sample more males than females show the trait. If individual III-9 had a daughter with II-6 (ew!) what is the probability she would be affected? Thursday, October 28, 2010
Pedigree Analysis 1: Video here Thursday, October 28, 2010
Pedigree Analysis 2: Video Thursday, October 28, 2010
Pedigree Analysis 3: Video Thursday, October 28, 2010
Mendelian Inheritance: Independent Assortment For Mono & Dihybrid Crosses: Visit Mendelian Genetics Thursday, October 28, 2010
Mendelian Inheritance Mendel established an early model of inheritance through a study of pea plants. Mendel had no knowledge of meiosis but observed that the inheritance of many traits followed the same patterns and ratios throughout subsequent generations. His work, published in 1866, was ignored by biologists for thirty years. At this time much of the scientific community was focused on Darwin’s Origin of Species and the differences between species- rather than those differences that might exist within a species. In 1900 Mendel’s model of inheritance was rediscovered and accepted. You have already covered the basic foundations of Mendelian genetics by conducting pedigree analyses. The emphasis now is on ratio and experimental design. Gregor Mendel (1822-1884) Thursday, October 28, 2010
Mendelian Inheritance Mendel studied the inheritance of a number of traits in pea plants. Why pea plants? Mendel took purebred plants bearing different traits and bred them. This type of experiment is called a cross and Mendel conducted two major versions. * Monohybrid cross : A study of the inheritance of one trait in each generation. * Dihybrid cross : Two traits are studied simultaneously in each generation. When we study test crosses we use the following terms to refer to the different generations. P Parental generation F 1 First filial generation F 2 Second filial generation Thursday, October 28, 2010
Monohybrid Cross Mendel fertilised over a thousand pea plants in one of his most famous monohybrid crosses and these were his results. Can you 1. Determine which alleles are dominant and recessive? 2. Explain why all the F 1 generation are tall? 3. Explain why a 3:1 ratio was observed for the population of tall:short plants in the F 2 generation? Thursday, October 28, 2010
Monohybrid Cross- The Punnet Square The punnet square is a useful tool for predicting the approximate phenotypic ratio of our test crosses. P generation T T gametes t Tt Tt t Tt Tt All the F 1 generation will be heterozygous and tall. F 1 generation T t gametes T TT Tt t Tt tt We can expect 3 / 4 of the F 2 generation to be tall and 1 / 4 to be short. When answering questions about crosses always state your answers as a probability Thursday, October 28, 2010
Dihybrid cross The dihybrid cross studies the inheritance of two traits in an experiment. So what do we observe if we watch for the inheritance of both these traits? RR ; yy P generation gametes First, lets cross two purebred parents and YY ; rr look at the F1 generation. Try this yourself: What would our F1 generation look like if we crossed purebreds YYRR and yyrr? Heredity Thursday, October 28, 2010
Dihybrid cross What will be the pattern among our F2 generation? Rr ; Yy F1 generation gametes Rr ; Yy How many of each pea do we end up with? Express this as a ratio. Heredity Thursday, October 28, 2010
Dihybrid cross The dihybrid cross adds another trait into the experiment. Thursday, October 28, 2010
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