Heredity Inheritance and Variation

LEARNING ACTIVITY SHEET Science 9 (Q1 –Lesson 3) Heredity: Inheritance and Variation I. Introduction Genetics is the study of inheritance in which the traits from the parents are transferred to their offspring. In Grade 8, you have learned that one of the processes of cell division called Meiosis produces genetic variations in Mendelian patterns of heredity. You have learned how Gregor Mendel described the passing of traits from one generation to the next. His principles laid the foundation for the study of inheritance. The potential combinations of offspring from two parents of known genotype can now be determined. Traits can more easily be predicted. In Grade 9, we will focus on non-Mendelian patterns of heredity and describe the molecular structure of the DNA. The location and role of DNA in the transmission of traits from the parents to the offspring. This Learning Activity Sheets will help you discover more about inheritance and variations. Enjoy learning! II. Learning Competency This Learning Activity Sheets was designed and written with you in mind. It is here to help you explain the different patterns of non-Mendelian inheritance. (S9LT-Id-29). III. Objectives After going through this Learning Activity Sheets, you are expected to: 1. Explain the different patterns of Non-Mendelian genetics; 2. Describe the genotype and phenotype in incomplete dominance, codominance and multiple alleles; 3. Solve genetic problems involving non-Mendelian genetics; 4. Describe the location of genes in chromosome; and 5. Identify the components of DNA molecule. IV.Discussion Non-Mendelian Pattern of Inheritance Gregor Mendel was able to lay the foundation of genetics even before the discovery of genes. His principles form the bases for understanding heredity (similarities) and variation (differences). It follows the principle that a dominant trait will always be the one that is observable in the phenotype of the offspring when combined with a recessive trait. However, he was not able to discuss the factors in his laws of inheritance which lead other scientists to further investigate the science of heredity. As a result, a new pattern called non-Mendelian genetics was born which include incomplete dominance, codominance and multiple alleles. Incomplete Dominance In this pattern of inheritance, the resulting phenotype of the heterozygous offspring is in between the phenotypes of the parents. In other words, although both alleles are dominant, neither of the alleles of the parents will be dominant over the other. Hence, the traits of both parents will not be visible, instead a third phenotype will be observed on the offspring. For example a homozygous red (RR) flower plant is crossed with a homozygous white (WW) flower plant, following the principle of incomplete dominance, the resulting color of the heterozygous offspring will not be red or white but a third phenotype which is pink. This is because no allele from the parents is dominant over the other. Figure 1: A Punnett Square showing the possible genotypes and phenotypes in incomplete dominance. Codominance In this pattern of inheritance, the contrasting alleles are equally expressed in the heterozygous offspring. It means that the characteristics of both of the father and mother will be visible on their offspring. For instance, a homozygous red (RR) flower plant is crossed with a homozygous white (WW) flower plant, following the principle of codominance, the resulting phenotype (color) of the heterozygous offspring will be red and white. Again both traits of the parents will be expressed equally since neither of them are dominant over the other. Figure 2: A Punnett Square showing the possible genotypes and phenotypes in codominance. Multiple Alleles In certain human traits, inheritance is controlled by a single gene but sometimes several or multiple alleles may exist. A typical example of this pattern of inheritance called multiple alleles is the human blood type. The blood type is determined by the presence of antigen on the surface of the red blood cells (RBCs). As previously discussed, persons with type A blood have an antigen A on the surface of their RBCs. Those with blood type B possess antigen B on the surface of their RBC’s. People with type AB blood have both antigens A and B on the surface of their RBC’s. When there is NO antigen A or B, then the person has a type O blood Figure 3: Some antigens found in the human blood which controls the blood type. Figure 4: The possible genotypes and phenotypes of human blood. Type A blood contains Antigen A. Type B blood contains Antigen B. Type AB blood contains both Antigen A and B while Type O blood does NOT contain any antigen. Sex Determination In humans, there are a total of 23 pairs of chromosomes. One pair of these chromosome is called sex chromosomes which determines the gender of an individual. One of these chromosomes comes from the father and one from the mother. Females have a pair of XX chromosomes while males have a pair of XY chromosomes. An egg cell will always have an X-bearing cell while a sperm cell may be an X-bearing cell or Y-bearing cell. Limited, Sex Linked, and Sex-Influenced Figure 9: A pair of XY chromosomes results to a male offspring while a pair of XX sex chromosomes produces a female child. (Photo from www.funscience.in) Some traits are carried on the sex chromosomes, X and Y. Most traits carried are present on only the X-chromosome. The Y-chromosome is smaller, and so, very few genes are located on this chromosome. Sex traits can be categorized into three types of inheritance: sexlimited, sex-linked, and sex-influenced.