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Science (2. Inheritance and Variation)
- 1. Heredity: Inheritance and Variation Overview Non-Mendelian Patterns of Inheritance Cells divide to produce new cells and meiosis is one of the processes producing genetic variations in Mendelian patterns of inheritance. Gregor Mendel’s principles form the base for the understanding of heredity and variation. Although Mendel’s work failed to discuss thoroughly the ‘factors’ or genes he mentioned in his laws of inheritance, his findings prompted other scientists to probe further into the mystery of heredity. Several researches were conducted after the rediscovery of Mendel’s work. Walter Sutton and Theodore Boveri became popular because they found the best evidence that an inherited trait is determined by chromosomes. Chromosome Theory of Inheritance explained that genes are in the chromosomes. Mendelian laws of inheritance have important exceptions to them. For example, not all genes show simple patterns of dominant and recessive alleles. In the Mendelian patterns of inheritance, the effects of the recessive gene are not observed when the dominant gene is present. In this lesson, you will find out that certain traits do not always follow the Mendelian principles of heredity. 1. Incomplete Dominance In incomplete dominance, a heterozygote shows a phenotype that is intermediate between the two homozygous phenotypes. Neither allele is dominant over the other. An example of incomplete dominance is flower color in four o’clock plant, like those shown in Figure 1. When a pure red-flowered four o’clock plant is crossed with a pure white-flowered four o’clock plant, the offspring will produce neither red nor white flowers. Instead, all flowers will be pink. In incomplete dominance, it is only the phenotype that is intermediate. The red and white alleles remain separate and distinct. Half the gametes of the pink four o’clock carry the allele for red and half carry the allele for white. Therefore, the genotypic ratio also becomes the phenotypic ratio. 2. Codominance (adapted from Grade 8 Learner’s module –since non-Mendelian is not included in the learning competency) Another pattern of inheritance is codominance. This results when one allele is not dominant over the other. The resulting heterozygotes exhibit the traits of both parents. One example of codominance is the MN blood typing in humans. On the surface of our red blood cells are proteins bound to sugar molecules, forming complexes called antigens. One group of antigens are controlled by a pair of alleles, LM and LN. The pairing of these alleles will determine the blood type of an individual, and there are three: M, MN and N. Table 1 summarizes the genotypes and phenotypes of the MN blood typing in humans.
- 2. Key Concepts Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely dominant over the other allele. This results in a third phenotype in which the expressed physical trait is a combination of the dominant and recessive phenotypes Now, that you are familiar with incomplete dominance, let us find out what happens when one allele is not dominant over the other. Table 1 Human MN blood types and their genotypes. Blood Types Genotype M LMLM MN LMLN N LNLN Key Concepts In codominance, both alleles are expressed equally in the phenotype of the heterozygote. For example, red cows crossed with white cows will have offspring that are roan cows. Roan refers to cows with red hair and white blotches. Many genes have multiple alleles. An example is ABO blood type in humans. 1. Multiple Alleles (adapted from Grade 8 Learner’s module –since non-Mendelian is not included in the learning competency) Sometimes, even if only two alleles control a trait, there may actually be more than two types of alleles available. This will also lead to more than two phenotypes expressed. Another blood group system in humans, the ABO system, is an example of a character governed by multiple alleles. Three alleles are responsible for this blood system: IA , IB, and i. The ABO blood type is determined by the presence or absence of two antigens, A and B. Allele i does not code for an antigen. There are four possible blood types as shown in Table 2. Table 2. Human ABO Genotypes blood types and their phenotypes. Blood Types A IA IA , IA i B IB IB , IB i AB IA IB O ii Key Concepts: recessive, two O alleles must be present for a person to have type O blood. The inheritance of some characters does not strictly follow Mendel’s Law of Independent Assortment. There are many traits that are inherited together more frequently. For example, the expression of certain traits depends on whether one is male or female. Apparently, the expression of the traits is determined by or related to one’s sex. Sex Chromosomes and Sex Determination Humans have 46 chromosomes in each cell. Observation of the human body cells shows 23 pairs of chromosomes for both males and females. Twenty- two pairs are somatic chromosomes. The 23rd pair consists of sex chromosomes. Human males and some other male organisms, such as other mammals and fruit flies, have non-identical sex chromosomes (XY). Females have identical (XX) sex chromosomes. If an egg is fertilized by a sperm with a Y chromosome, as shown in Figure 4, the offspring is male. When an egg is fertilized by a sperm carrying an X chromosome, the offspring is female. Note that there is a 50 percent chance of having a male or female offspring. The greater the number of offspring, the greater is the chance of getting the expected 1:1 ratio of male and female. Key Concepts children. Females have 44 body chromosomes and two sex chromosomes, both X. The total number in each cell of an individual is 46. These chromosomes contain the genes, which are the factors of heredity.
- 3. This section discusses three kinds of sex-related inheritance, namely, sex-limited, sex-influenced and sex-linked. Sex-Linked Genes Genes located on the X chromosomes are called X-linked genes. Genes on the Y chromosomes are called Y-linked genes. An example of an X-linked trait in humans is hemophilia. A person suffering from hemophilia could die from loss of blood even from a small wound because the blood either clots very slowly or does not clot at all. Another example of an X-linked trait is color blindness. To illustrate the inheritance of an X-linked trait, we will use color blindness in our discussion. Let us study Table 3. The X chromosome with the gene for color blindness is represented as XC, while the one without is represented as X. Table 3 Genotypes and Phenotype phenotypes of color blindness in humans Genotype 1. X X Normal female 2. X XC Normal female, carrier of the gene 3. XC XC Color- blind female 4. XY Normal male 5. XC Y Color-blind male Can you identify the genotype of the female who is color-blind? Notice that for a female to become color-blind, she must be homozygous (XC XC) for the color-blind genes. The trait is, therefore, recessive in females. If a female has only one X chromosome with the allele for color blindness, she becomes normal but can pass on the trait to her offspring. She is therefore a carrier of the trait. Since males have only one X chromosome, the gene for color blindness when present in the male, will always be expressed because it does not have an allele to hide or prevent its expression. Thus, the male will be color- blind. This is the reason why color blindness is more common in males than in females. Figure 5 is an example of a Y-linked trait, hypertrichosis pinnae auris, a genetic disorder in humans that causes hairy ears. Since the trait is found in the Y chromosome, then only males can have the trait. A father who has the condition will pass it on to all his sons, and they, in turn, will pass it on to their own sons. Key Concepts -linked traits are inherited through the X chromosomes. e trait without being affected if it acts in a recessive manner Key Concepts -linked traits are inherited through the X chromosomes. les have two X chromosomes. Therefore, they can inherit/carry the trait without being affected if it acts in a recessive manner. Have you seen a bald man? What about a bald woman? It appears that gender matters for the other kinds of traits as well. Sex-Limited Traits Sex-limited traits are generally autosomal, which means that they are not found on the X or Y chromosomes. The genes for these traits behave exactly the same way that any autosomal gene behaves. The difference here comes in the expression of the genes in the phenotype of the individual. Sex-limited traits are expressed in only one gender. In cattle, for instance, lactation is expressed in females but never in males. Both male and female cattle however possess a gene pair for lactation. The gene for lactation (L) is dominant over the non-lactating
- 4. gene(l). Table 4 shows the genotypes and phenotypes of the gene for lactation. These genes are carried by both males and females, but it is only expressed in females. noticed that in female cattle, if at least one gene pair is for lactation (L), the female produces milk? In male cattle, it does not matter if they possess one or two genes for lactation. They never produce milk. Expression of Lactation in Female Phenotypes Cattle Female Genotypes XXLL Female lactating XXLl Female lactating XXII Female not lactating Male Genotypes Male Phenotypes XYLL Male not lactating XYLI Male not lactating XYII Male not lactating Key Concepts Sex-limited traits are those that are expressed exclusively in one sex. Sex-influenced traits are expressed in both sexes but more frequently in one than in the other sex. DNA is a macromolecule that forms a double helix structure in the cells (mostly in the nuclei and mitochondria) of living organism. The DNA molecule is considered the genetic material of all living cells. It is present in bacteria, protists, fungi, plants and animals. DNA is responsible for determining the specific characteristics of an organism. DNA: The Genetic Material We’ve talked a lot so far about chromosomes and the inherited trait that genes produce. But what are genes? How do they work? After the discovery of the Chromosome Theory of Inheritance, many questions remained unanswered. what material are genes made? Scientists now know that the genetic material is DNA (Deoxyribonucleic acid). Modern research techniques helped scientists to answer many questions about DNA and heredity. The work of earlier scientists gave Watson and Crick a lot of information about DNA. By the end of the 1940’s, scientists had found that DNA consists of long strands of nucleotides. Each nucleotide contains a pentose sugar called deoxyribose, a phosphate group, and one of the four compounds called nitrogenous bases. Look at Figure 5. What are the components of the nucleotide? DNA is a complex molecule that is found in almost all living organisms. Key Concepts DNA is composed of chains of nucleotides built on a sugar and phosphate backbone and wrapped around each other in the form of a double helix. The backbone supports four bases: guanine, cytosine, adenine, and thymine. Guanine and cytosine are complementary, always appearing opposite each other on the helix, as are adenine and thymine. This is critical in the reproduction of the genetic material, as it allows a strand to divide and copy itself, since it only needs half of the material in the helix to duplicate successfully. Summary complicated by factors such as codominance, incomplete dominance, multiple alleles, and sex-linked traits. Incomplete dominance occurs when the phenotype of the offspring is somewhere in between the phenotypes of both parents; a completely dominant allele does not occur. Codominance occurs when both alleles are expressed equally in the phenotype of the heterozygote. ABO blood type in humans. There are three common alleles for the gene that controls this characteristic. The alleles IA and IB are dominant over i. XX chromosomes determine femaleness and XY determine maleness. sex-linked trait is on the X chromosome. Females have two X chromosomes; they can inherit or carry the trait without being affected if it acts in a recessive manner. Sex-limited traits are those that are expressed exclusively in one sex. Sex-influenced traits are expressed in both sexes but more frequently in one than in the other sex. DNA contains the information needed to form and control the physical make-up and chemical processes of an organism. double-stranded helix made up of repeating units of nucleotides.
- 5. nucleotide is composed of the following: sugar and phosphate molecules, and nitrogeneous bases. The base can either be adenine, guanine, thymine, and cytosine. Glossary Allele – a different form of a gene that controls a certain trait. Codominance – two dominant alleles of a contrasting pair fully expressed at the same time in the heterozygous individual. Incomplete dominance - occurs when the phenotype of the offspring is somewhere in between the phenotypes of both parents; a completely dominant allele does not occur. Multiple Alleles – when more than two alleles control the inheritance of a character. Sex-influenced traits – are expressed in both sexes but more frequently in one sex than in the other. Sex-limited traits that are expressed exclusively in one sex of the species. Sex-linked traits – traits that are controlled by genes located on the same sex chromosome. DNA - deoxyribonucleic acid Punnett square – the method by which one can determine the possible genotypes and phenotypes when two parents are crossed Antigen - A substance that when introduced into the body stimulates the production of an antibody Gamete - are reproductive cells that unite during sexual reproduction to form a new cell called a zygote.