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Chapter 10

Biology - Chapter 10.docx

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Biological Sciences
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 Chapter 10- Mendel, Genes and Inheritance  -Sickle cell anaemia: autosomal recessive gene; symptoms: weakness, dizziness, shortness of breath and pain  10.1 The Beginning of Genetics: Mendel’s Garden Peas: - Until about 1900 scientists and the general public believed in the blending theory of inheritance which suggested that hereditary traits blend evenly in offspring through mixing of the parents’ blood - Mendel studied a variety of heritable characteristics called characters (such as flower colour etc.)  -A variation in a character is called a trait  -Mendel established that characters are passed to offspring in the form of discrete hereditary factors which are now known as genes  -The inheritance patterns Mendel observed are the results of segregation of chromosomes, on which the genes are located, to gametes in meiosis  -Mendel chose the garden pea (Pisum sativum) because the plant could grow easily in the monastery garden  -The male gametes are sperm nuclei contained in the pollen, produced in the anthers of the flower  -Female gametes are egg cells produced in the carpel  -Mendel prevented self-fertilization of the plants by cutting off the anthers and using pollen from a different plant (called cross-pollination)  -True –breeding: when these plants are self-fertilized they passed traits without change from one generation to the next  -REFER TO PAGE 216 FIGURE 10.5  -first generation of offspring from the cross- 1 generation(F stands for filial)  -plants used in the initial cross are called the parental or P generation  -Mendel then allowed the F g1neration plants to self-produce to make the F 2 generation  -In all his cases he observed a uniform F generation in which only one of the two 1 traits was present  -In the F2generation, the missing trait reappeared, and both traits were present in the F1generation was present in definite, predictable proportion among the F 2 offspring  -Mendel’s first hypothesis: the adult plants carry a pair of factors that govern the inheritance of each character  -In modern terminology Mendel’s factors are called genes which are located on chromosomes  -The different versions of a gene that produce different traits of character are called alleles  -Mendel deduced that the trait that had seemed to disappear in the F 1eneration was actually present but masked in some way by the “stronger” allele  -Mendel called the masking effect dominance  -Mendel’s second hypothesis: if an individual’s pair of genes consist of different alleles, one allele is dominant over the other, recessive, allele  -When an organism carries two different alleles the dominant allele is simply the one that determines the appearance of the organism  -Mendel’s third hypothesis (Principle of Segregation): the pairs of alleles that control a character segregate (separate) as gametes are formed; half the gametes carry one allele, and the other half carry the other allele  -During fertilization, fusion of the haploid maternal and paternal gametes produces a diploid nucleus called the zygote nucleus  -The zygote nucleus receives one allele for the character from the male gamete and one allele for the same character from the female gamete  -Homozygous: same alleles for a gene, heterozygous: different alleles for a gene  -An F 1eterozygote produced from a cross that involves a single character is called a monohybrid  -Monohybrid cross a cross between two individuals that are each heterozygous for the same pair of alleles  -Genotype: genetic constitution of an organism  -Phenotype: outward appearance of an organism (physical appearance)  -Mendel’s three hypotheses (summarized): - The genes that govern genetic characters are present in two copies in individuals - If different alleles are present in an individual’s pair of genes, one allele is dominant over the other - The two alleles of a gene segregate and enter gametes singly - Genetic probability: the possibility that an outcome will occur if it is a matter of chance, as in the random fertilization of an egg by a sperm cell that contains one allele or another  -REFER TO FIGURE 10.6 PAGE 217  -Product Rule when two or more event are independent, the probability that they will both occur is calculated by multiplying the individual probabilities  -Sum Rulewhen several different events all give the same outcome; that is, the probability that either event A or event B or event C will occur equals the probability of even A plus the probability of event B plus the probability of event C. The sum rule
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