Genetics Review. Alleles. The Punnett Square. Genotype and Phenotype. Codominance. Incomplete Dominance

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1 Genetics Review Alleles These two different versions of gene A create a condition known as heterozygous. Only the dominant allele (A) will be expressed. When both chromosomes have identical copies of the dominant allele for a gene, the organism is said to be homozygous dominant for that gene. When both chromosomes have identical copies of the recessive allele for a gene, the organism is said to be homozygous recessive for that gene. Genes occupying the same locus or position on a chromosome code for the same trait and are said to be alleles. Maternal chromosome that originated from the egg of this person's mother. Paternal chromosome that originated from the sperm of this person's father. The Punnett Square While the concept of dominance came from Mendel s experiments, it was a man named Punnett who devised a diagram to predict the outcome of a particular cross. Remember Punnet Squares are Probability NOT Actuality 3:1 Ratio of Tall:Short Genotype Genotype and Phenotype The genotype of an organism refers to its genetic make-up. The phenotype of an organism refers to its observable features or traits. There can be more than one genotype for a given phenotype because a recessive allele will not be expressed if a dominant allele is present that masks its effect. By convention, dominant alleles are labeled with a capital letter and recessive ones with a lower case letter. A heterozygote will be a carrier for a recessive allele. Carriers may be unaware that they carry a recessive allele until they have children. PP (homozygous) Pp (heterozygous) Pp (heterozygous) pp (homozygous) Phenotype White Grouping the second generation offspring from a monohybrid cross for flower color reveals two phenotypes, but three genotypes Incomplete Dominance In cases of incomplete dominance, neither allele dominates and the heterozygote is intermediate in phenotype between the two homozygotes. In crosses involving incomplete dominance, the genotype and phenotype ratios are identical. Codominance In cases of codominance, both alleles are independently and equally expressed in the heterozygote. 1

2 Multiple Alleles Many genes have more than two alleles and are therefore said to have multiple alleles. This does not mean that individuals can have more than two alleles. It only means that more than two alleles exist in a population. In humans, genes for blood type and eye color are expressed through multiple alleles. In most cases multiple alleles exhibit codominance. Polygenic Traits Many traits are produced by the interaction of several genes. Traits controlled by two or more genes are said to be polygenic traits meaning having many genes. Polygenic traits often show a wide range of phenotypes. Skin color in humans exhibits a wide range of phenotypes partly because there are 5 or more genes that control this trait. Lethal Alleles Lethal alleles are gene mutations that result in a gene product which is not only non-functional, but affects organism survival. Some lethal alleles are fully dominant and are therefore lethal in the heterozygote. Dominant lethal alleles are usually eliminated rapidly, because their expression is fatal (except in cases like Huntington disease). In other cases, the allele mutation results in a viable heterozygote with a recognizable phenotype. Recessive lethal alleles are fatal only in the homozygote since their effect is masked in the heterozygote carrier. Diploid v. Haploid A cell that contains both sets of homologous chromosomes is said to be diploid meaning two sets. This number of chromosomes is often written 2N. The sex cells, or gametes, of sexually reproducing organisms are said to be haploid meaning one set. This number of chromosomes is often written N. Meiosis I The key to understanding meiosis is that while the first half of the process is very similar to mitosis, chromosomes pair to form a structure called a tetrad. There are 4 chromatids in a tetrad (versus the 2 paired in mitosis). During this pairing a process known as crossing over can occur. Crossing over results in the exchange of alleles between homologous chromosomes and produces new combinations of alleles. Meiosis II The two cells produced by meiosis I then enter a second meiotic division. However, unlike the first division no replication occurs during this process leading to the separation of alleles and yielding a haploid number of chromosomes. 2

3 Comparison: Meiosis and Mitosis Goal Divisions Outcomes Chromosomes Mitosis Identical Cells Divides Once 2 Identical Cells All 46 chromosomes are present (2N) Meiosis Sex Cells (egg or sperm) Divides Twice 4 sex cells Only 23 chromosomes are present (N) Crossing Over Higher organisms such as plants and animals reproduce through sexual reproduction. Offspring possess a combination of characteristics from maternal and paternal chromosomes. The maternal and paternal chromosomes pair at meiosis and genetic material is exchanged between them by a process called crossing over. Crossing over can only occur when homologous chromosomes synapse (come together side-by-side) during the early stages of meiosis. The overlap point is called the chiasma and can lead to swapping genetic information. Genes Maternal chromosome Centromere Paternal chromosome Y Linked Genes Pedigree Analysis Genes located on the same chromosome are said to be linked (e.g. genes A and B). Linked genes tend to be inherited together. Linkage results in fewer genetic combinations of alleles in offspring (compared to genes on separate chromosomes). The inheritance patterns involving linked genes do not follow expected Mendelian ratios. Pedigree analysis is a way of illustrating inheritance patterns. It is a good way to follow the inheritance of genetic disorders through generations. Symbols are used to represent males, females etc. For traits of interest, symbols can be shaded to indicate individuals carrying the trait. Individuals are designated by their generation number and then their order number in that generation. Pedigree Analysis Sex Linkage Normal female Sex unknown Affected female Nonidentical twins Carrier (heterozygote) I, II, III Generations Affected male Died in infancy Normal male Identical twins Sex linkage refers to the phenotypic expression of an allele that is dependent on the sex of the individual and is directly tied to the sex chromosomes. Most sex linked genes are present on the chromosome (-linkage) and have no corresponding allele on the smaller male chromosome. Fathers pass sex-linked alleles to all their daughters but not to their sons. Mothers can pass sex-linked alleles to both sons and daughters. In females, sex-linked recessive traits will be expressed only in the homozygous condition. In contrast, any male receiving the recessive allele from his mother will express the trait. 1, 2, 3 Children (in birth order) 3

4 Autosomal & Sex Linked Genes Autosomal Genes Sex-Linked Genes 1. All individuals carry two 1. Males carry only one allele of alleles of each gene each gene (hemizygous) 2. Dominance operates in 2. Dominance operates in both males and females females only. 3. Reciprocal crosses 3. Reciprocal crosses produce produce the same results different results. 4. Alleles passed equally to 4. Criss-cross inheritance male and female offspring pattern: father to daughter to grandson, etc Colorblind female Normal male Reciprocal cross Normal female Colorblind male Genomic Imprinting The phenotypic effects of some mammalian genes depend on whether they were inherited from the mother or the father. This phenomenon, called genomic imprinting, is part of epigenetics, which looks at the heritable changes in gene function that occur without involving nuclear DNA. Just as cells inherit genes, they also inherit instructions that tell the genes when to become active, in which tissue, and to what extent. Epigenetic phenomena are important because they regulate when and at what level genes are expressed. Imprinted Genes How are imprinted genes different? In some cases, an imprinted gene is activated only if it is inherited from the father; in other cases, only if it comes from the mother. The corresponding allele is inactivated. Evidence of this is seen in two human genetic disorders. Both are caused by the same mutation: a specific deletion of chromosome 15. If the mutation is inherited from the father, the result is Prader- Willi syndrome. If the mutation is inherited from the mother, the result is Angelman syndrome. Paternal chromosome Prader-Willi syndrome Mental retardation, obesity, short stature, unusually small hands and feet Deletion mutation Maternal chromosome Angelman syndrome Uncontrollable laughter, jerky movements, motor and mental abnormalities Chromosome Mapping Generally, the further apart any two genes are on the same chromosome, the greater the incidence of crossing over between them. By determining crossover frequencies between two genes, their sequence and genetic distance on a chromosome can be established. A 26.8% B Calculation: Using the crossover frequency (COV) between the two genes, the relative position of genes A and B can be represented on a genetic map: 10 map units 22 map units By providing cross over frequencies for other combinations of genes, it is possible to create a chromosome map. A map for the following cross over frequencies: A C = 10%, A B = 22%, B C = 12% 12 map units Identifying Defective Genes Sex Determination Test Carrier screening Preimplantation genetic diagnosis Prenatal diagnosis Presymptomatic testing Explanation Identifying unaffected individuals who carry one copy of a gene for a disease that requires two copies for expression of the disease. Screens for genetic flaws in embryos used for in-vitro fertilization. Tests for chromosomal abnormalities such as Down syndrome. Testing before symptoms appear to determine the risk of developing adultonset disorders, e.g. Huntington s disease or cancer. Y (Y determines maleness ) Mammals (including humans) Fruit fly Drosophila Some dioecious (separate male and female) plants such as kiwifruit. WZ (W determines femaleness ) Birds Butterflies and moths Some fish O (female offspring when egg fertilized) Grasshoppers Aphids Honey bees Hemiptera (bugs) 4

5 The normal condition for a body cell is for chromosomes to be present as homologous pairs. Aneuploidy is a condition where one or more chromosomes are missing from or added to the normal somatic cell chromosome number. Examples: Aneuploidy Nullisomy Monosomy Trisomy Tetrasomy 0 homologues 1 homologue 3 homologues 4 homologues Aneuploidies usually result from non-disjunction, failure to separate properly, during anaphase of meiosis. 5