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Name: ______________________ Genetics Lab Learning Outcomes · The student will demonstrate practical knowledge of Mendelian genetics by performing and analyzing monohybrid and dihybrid crosses. · The student will demonstrate an understanding of modern genetics by performing and analyzing crosses that illustrate incomplete dominance, co-dominance, and sex-linkage. You should be able to define the following terms: GeneHomozygousGenotype AlleleHeterozygousPhenotype Dominant Punnett squareMultiple allele systems Recessive DominanceSex-linked trait Co-dominanceIncomplete dominance Introduction A gene corresponds to a specific location on a chromosome that carries the genetic information specific to a particular characteristic (such as flower color, height, etc.). Our autosomal (non-sex) chromosomes have at least one gene pair (two genes) for every characteristic because, as you learned in the lecture and lab work relating to meiosis, we receive one chromosome of a homologous pair from each of our parents. Both genes correspond to a single type of characteristic (e.g. flower color), but the kinds of information they carry may be different (e.g. purple or white). These different possibilities for traits (such as flower color) are alternate forms of the same gene, known as alleles. An allele is considered to be the dominant allele if it can mask the effect of the other allele, whereas the allele whose expression is hidden is called the recessive allele. Dominant alleles are expressed regardless of the other nature of the other allele. For recessive traits to be expressed the recessive allele must be present on both chromosomes OR the dominant allele must be absent. If two recessive alleles are present (one on each of the homologous chromosomes), the condition is said to be homozygous recessive. If the dominant allele is present on both homologous chromosomes, it is termed homozygous dominant. If the dominant allele is found on one homologous chromosome and the recessive allele on the other, then this situation is termed heterozygous. The genotype describes the genetic content of the cell or organism. That is the allele composition. Capital letters indicate the dominant allele and lower case letters the recessive allele. Homozygous dominant: AA Heterozygous : Aa Homozygous recessive : aa Heterozygous individuals are often referred to as “carriers” of the recessive condition. The observable expression of an individual’s genotype is known as phenotype. That is what the trait of the individual is, what it looks like. Part 1: Monohybrid (One Factor) Crosses A cross between individuals, dealing with only a single characteristic is called a one factor, or monohybrid, cross. Crosses of this type are easily demonstrated using a Punnett square. The possible gametes that can be produced by the female parent are listed along the top of the square and the possible gametes that can be produced by the male parent are listed along the side. 1. A homozygous dominant male mates with a homozygous recessive female. Using the letter B to represent the gamete, complete the Punnett square in the space below. b b Genotype of the male: _____________ Genotype of the female: _____________ B State the genotype of the resulting offspring B using letter symbols and the word used to describe it. ______________________________________ 2. Albinism is a lack of pigment coloration in the skin, hair and eyes. It is caused by a recessive allele (a). An albino man, both of whose parents were normally pigmented, marries a normally pigmented woman. They have one child, an albino daughter. List the genotypes of all five persons: the man, man’s father, man’s mother, wife and daughter. Individual Genotype Man Man’s father Man’s mother Man’s wife Daughter Could this couple produce normally pigmented offspring? How do you know? Part 2: Sex-linked Characteristics The Y chromosome is shorter and lacks most of the genes present on the X chromosome. Therefore, many genes on the X chromosome do not have corresponding alleles on the Y chromosome. Those genes on the X chromosome that are missing on the Y chromosome are genes that control seX-linked characteristics in humans. A recessive gene on the X chromosome in males will therefore cause the expression of the trait. SeX-linked characteristics such as color blindness and hemophilia are determined by recessive alleles carried on the X chromosome. Females may be “carriers” of the trait, but they rarely express them relative to men. Use the Punnett Square below to answer the question below. 1. From whom will a man inherit an X-linked characteristic, his father or his mother? Why? Part 3: Multiple Allele Systems and Co-Dominance Any individual can carry only two alleles for a specific gene at any given time, but there are genes for which more the two alleles can exist within an entire population. These are multiple allele systems. Human blood types are a good example of a multiple allele system. There are three possible alleles (A, B, O) for the gene that determines blood type. Which two alleles you carry depends upon which alleles were inherited from your parents. The allele specifies the type of sugar that is added to the surface of your red blood cells. This sugar, called an antigen, acts as a specific cell surface marker (glycolipid). Alleles A and B are dominant when paired with O, but are co-dominant when they occur together. Allele O is a recessive allele. Circulating in the blood are protein antibodies that normally protect the body by reacting with foreign antigens. Individuals with blood type A produce antibodies against B, those with blood type B produce antibodies against A, those with O have produce antibodies against A and B, and individuals with blood type AB don’t produce any antibodies against other blood types. If incompatible blood types are mixed, clumping of the blood cells occurs and the vessels become clogged. These relationships are summarized below. Blood Type (phenotype) Genotype Antigen(s) Present on RBC Antibodies Produced Can receive blood from which types? A AA or AO A Anti-B A, O B BB or BO B Anti-A B, O AB AB A and B None A, B, AB, O O OO None Anti-A and Anti-B O For the next two problems, assume that you have type A blood and are heterozygous for this characteristic (AO). List all potential genotypes and phenotypes in the population, even if the % is zero. 1. Your partner has type O blood. What is the probability of having a type O child? What is the probability of having a child with your blood type? What possible blood types could your children have? 2. Your partner has type AB blood. What is the probability of having a type O child? What is the probability of having a type A child? What is the probability of having a type B child? 90 89