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Unit 3 – Genetic Continuity Crosses with answers- http://mail.esdnl.ca/~patrick_wells/biology/genetics/genetics.html Mendelian Genetics (patterns of inheritance) Genetics is the study of patterns of inheritance. In humans, physical characteristics of a person (phenotype) are due to heredity (genotype) and environmental influences. Mendel studied patterns of inheritance using pea plants. He had two main laws aside from his Law of Dominance. Law of Segregation – alleles split during gamete formation (proved by monohybrid cross 3:1 ratio – do a sample on the side Tt x Tt) Law of Independent Assortment - alleles for different traits split independent of each other during gamete formation (proved by his dihybrid cross 9:3:3:1 ratio). F1 TTBB x ttbb F1 Results? F2 Results? In earlier times, scientists thought that inherited traits were "blended" in the offspring. Mendel discovered traits were passed on for each characteristic with one copy of that trait coming from each parent. For many characteristics there was more than one version of a trait (we now call these alleles). For each gene an organism has two alleles one inherited from each parent. We know about meiosis – this cell division produces monoploid gametes (eg: sperm and egg) and the allele pairs are separated from each other at this time (Mendel’s segregation). Fertilization reunites allele pairs to make the gene complete again - the fertilized egg has one allele for each gene and the sperm has one for each gene. This interaction makes of alleles is what makes you and genetics so interesting! As Mendel discovered alleles are usually DOMINANT or RECESSIVE. A dominant allele is fully expressed when present. A recessive allele is fully masked and is only expressed in homozygous form. Recessive phenotype is also a genetype – this is useful in solving problems! Mendel was able to come to this conclusion through performing monohybrid crosses (looking at a single trait and following its expression through F1 and F2 generations). You should practice your crosses. Also use your memory tricks. Do some crosses: http://mail.esdnl.ca/~patrick_wells/biology/genetics/genetics.html Terms diploid cell: A somatic cell in human. In humans we have 23 maternal and 23 paternal chromsomes giving us a diploid number of 46. (2n=46) Males – 44 autosomes and XY sex chromosomes Females 44 autosomes and XX sex chromosomes monoploid cell: Sex cell in humans. Also called haploid as these cell have only half the normal number of chromosomes (sperms and eggs). A monoploid human egg or sperm has 23 chromosomes. (1n=23) gametes: Sex cells, either eggs or sperm, these cells are always monoploid. Homozygous or Pure means having two identical alleles of a gene. TT or tt Heterozygous or Hybrid is when you have two different alleles of a gene. Tt A carrier is hybrid and has a recessive allele of a gene that does not have an effect on their phenotype. A test cross is testing an unknown dominant individual by crossing it with a known homozygous recessive. If ANY recessive offspring result the unknown must have been heterozygous. The genotype is the genes possessed by an organism. The phenotype is the physical characteristics of an organism. A dominant allele is an allele that has the same effect on the phenotype whether it is present in the homozygous or heterozygous state (upper case letters). A recessive allele is an allele that only has an effect on the phenotype when present in the homozygous state (lower case letters). Incomplete dominant alleles: Alleles that are simultaneously expressed in an altered form – snap dragons hybrids are pink not white or red. (RR – red, WW – white and RW – pink) Codominant alleles are pairs of alleles that both affect the phenotype when present in the heterozygous state. Alleles that are simultaneously fully expressed in the heterozygous condition (AB blood type). These can be symbolized by upper case letters. We use these a a a o symbols I I I I for type A blood. The ABO blood groups are an example of multiple alleles of a single a gene because this gene exists in three allelic forms: I , Ib and Io(or some teachers us i).The Ia and Ib is o dominant to the I allele. Type O will only be expressed in the homozygous form; when combined with A or B alleles it will not be expressed. For example, a person with both the A and B alleles, carries AB type blood. Both blood group A and B are fully expressed. Some genes have more than two alleles (multiple alleles). Blood groups are the best examples of multiple alleles and co-dominance – know how to do these crosses! Genes are on Chromosomes! (Sutton suggested this theory) The chromosome theory states that chromosomes are linear sequences of genes. The unifying theory states that inheritance patterns may be generally explained by assuming that genes are located in specific sites on chromosomes (recall that each gene makes a specific protein). Sutton was the first to point out that chromosomes obey Mendel's rules—the first clear argument for the chromosome theory of heredity. Sutton worked with grasshopper chromosomes, and it was in this paper that he showed that chromosomes occur in distinct pairs, which segregate at meiosis. Linked genes are on the same chromosome – they do not follow the Law of Independent Assortment. The discovery that genes were on chromosomes made the Law of Independent Assortment invalid for genes on the same chromosome. Refers to meiosis – tetrad separation is the reason for independent assortment – but it only works if the genes that are assorting are on different chromosomes Sex in humans is determined by two chromosomes, called X and Y (X is bigger than Y in karyotypes). All males (♂) have one X chromosome and one Y chromosome. Females (♀) have two X chromosomes. In meiosis, therefore, females can only produce gametes with an X chromosome, while males can produce gametes with either an X or a Y chromosome. The male's gametes, then, are those that decide gender: the child can have XX (female) or XY (male) chromosomes depending on what it receives from its father. This is another example of segregation. Color-blindness and hemophilia are probably the most common examples of sex-linked traits in humans. Both are due to a recessive sex-linked allele on the X chromosome. They are more common in males than females. Obviously a recessive X-linked gene will only be expressed in the homozygous form, as this is part of the definition of recessive genes. Therefore, if an X-linked recessive alleles is present in a male, it will always be expressed, as this is the only X gene the male possesses. However, females have two X genes, only one of which is actually expressed. The other is bound up in an inactive structure known as a Barr body. Therefore if the X chromosome is the one bound in the Barr body, its recessive alleles are not expressed, and the female may be a carrier without displaying any effects. Morgan suggests sex linkage in fruit fly eye colour – cross. Polygenic traits have two or more genes. As opposed to monogenic such as blood type – you have a distinct blood type. Skin color and height are polygenic. It is by determined by a number of genes (so we have a range of height in humans not tall and short people depending on how many alleles you have for “tallness”). You must be able to analyze and interpret models of human karyotypes (remember the lab on Karyotypes). If you can’t perform crosses for dominant, sexlinked, codominant and incomplete dominant traits you will have great difficulty with the remainder of this course! Molecular Genetics (Genetics of the DNA molecule) First the history ……….. Levene analyzed the components of the DNA molecule. He found it contained four nitrogenous bases: cytosine, thymine, adenine, and guanine; deoxyribose sugar; and a phosphate group. Frederick Griffith was able to induce a non-pathogenic (harmless) strain of the bacterium Streptococcus pneumoniae to become pathogenic (deadly). Griffith referred to a transforming factor (we know it as DNA) that caused the non-pathogenic bacteria to become pathogenic. Oswald Avery, Colin MacLeod, and Maclyn McCarty revisited Griffith's experiment and concluded the transforming factor was DNA. Their evidence was strong but not totally conclusive. The then-current favourite for the hereditary material was protein; DNA was not considered by many scientists to be a strong candidate. Hershey and Chase sought an answer to the question, “Is it the viral DNA or viral protein coat (capsid) that is the viral genetic code material which gets injected into a host bacterium cell? With the blender experiment they conducted show that DNA was the molecule of heredity. Watson and Crick gathered all available data in an attempt to develop a model of DNA structure. Franklin took X-ray diffraction photomicrographs of crystalline DNA extract, the key to the puzzle. The data known at the time was that DNA was a long molecule, proteins were helically coiled (as determined by the work of Linus Pauling), Chargaff's base data (A to T and G to C ratios), and the x-ray diffraction data of Franklin and Wilkins. Barbara McClintock was the first scientist to predict that transposable elements, mobile pieces of the genetic material (DNA), were present in eukaryotic genomes. Structure and Replication The sides of the ladder of DNA consist of alternating phosphate groups and deoxyribose (a sugar). The two sides are antiparrallel, meaning that the sugar and phosphates are running in opposite directions (anti parallel). DNA is the primary molecule of heredity and controls the production of proteins in all organisms. RNA is a molecule that assists DNA conduct its primary function – making proteins or polypepetides.