GENETICS Chapter 15 Memorise Understand Importance * Define: phenotype, genotype, gene, locus, * Importance of meiosis; compare/contrast with High level: 10% of GAMSAT Biology allele (single and multiple) mitosis * Homo/heterozygosity, wild type, * Segregation of genes, assortment, linkage, questions released by ACER are related to recessiveness, complete/co-dominance recombination content in this chapter (in our estimation). * Incomplete dominance, gene pool * Single/double crossovers * Note that approximately 75% of the * Sex-linked characteristics, sex determination * Hardy-Weinberg Principle questions in GAMSAT Biology are related * Test cross: back cross, concepts of parental, to just 7 chapters: 1, 2, 3, 4, 7, 12, and 15. F1 and F2 generations; evaluating a pedigree Introduction Genetics is the study of heredity and variation in organisms. The observations of Gregor Mendel in the mid- nineteenth century gave birth to the science which would reveal the physical basis for his conclusions – DNA – about 100 years later. This is the last of the 3 ‘high-level importance’ chapters. Approximately 45% of GAMSAT Biology questions among ACER’s official practice materials contain content related to 3 chapters: 1, 7 and 15, this chapter. Also, be forewarned: despite the fact that a dihybrid cross (BIO 15.3) is time consuming, it appears in 3 separate ACER GAMSAT practice booklets. Multimedia Resources at GAMSAT-Prep.com Open Discussion Boards Foundational Online Videos Flashcards Special Guest THE BIOLOGICAL SCIENCES BIO-267 15.1 Background Information Genetics is a branch of biology which environment. Consider a heterozygote that deals with the principles and mechanics of expressed one gene (dark hair) but not the heredity; in other words, the means by which other (light hair). The expressed gene would traits are passed from parents to offspring. be called dominant while the other unex- To begin, we will first examine some relevant pressed allele would be called recessive. The definitions - a few of which we have already individual would have dark hair as their phe- discussed. notype, yet their genotype would be hetero- zygous for that trait. The dominant allele is Chromosomes are a complex of DNA expressed in the phenotype. This is known as and proteins (incl. histones; BIO 1.2.2). A Mendel’s Law of Dominance. High-level ImportanceHigh-level gene is that sequence of DNA that codes for a protein or polypeptide. A locus is the posi- It is common to symbolise dominant tion of the gene on the DNA molecule. Recall genes with capital letters (A) and recessive that humans inherit 46 chromosomes - 23 genes with small letters (a). From the pre- from maternal origin and 23 from paternal ceding paragraphs, we can conclude that origin (BIO 14.2). A given chromosome from with two alleles, three genotypes are pos- maternal origin has a counterpart from pater- sible: homozygous dominant (AA), heterozy- nal origin which codes for the same products. gous (Aa), and homozygous recessive (aa). This is called a homologous pair of chromo- Note that this only results in two phenotypes somes. since both AA and Aa express the dominant gene, while only aa expresses the recessive Any homologous pair of chromosomes gene. have a pair of genes which codes for the same product (i.e. hair colour). Such pairs of Each individual carries two alleles genes are called alleles. Thus for one gene while populations may have many or mul- product, a nucleus contains one allele from tiple alleles. Sometimes these genes are maternal origin and one allele from paternal not strictly dominant or recessive. There origin. If both alleles are identical (i.e. they may be degrees of blending (= incomplete code for the same hair colour), then the indi- dominance) or sometimes two alleles may vidual is called homozygous for that trait. If be equally dominant (= codominance). ABO the two alleles differ (i.e. one codes for dark blood types are an important example of mul- hair while the other codes for light hair), then tiple alleles with codominance. the individual is called heterozygous for that trait. Incomplete dominance occurs when the phenotype of the heterozygote is an inter- The set of genes possessed by a particu- mediate of the phenotypes of the homozy- lar organism is its genotype. The appearance gotes. A classic example is flower colour in or phenotype of an individual is expressed snapdragon: the snapdragon flower colour is as a consequence of the genotype and the red for homozygous dominant and white for BIO-268 CHAPTER 15: GENETICS GAMSAT MASTERS SERIES homozygous recessive. When the red homo- snapdragon flower. The pink snapdragon is zygous flower is crossed with the white homo- the result of the combined effect of both domi- zygous flower, the result yields a 100% pink nant and recessive genes. ImportanceHigh-level Figure IV.A.15.1: Genotype vs. phenotype; dominant allele W, recessive allele w, in a fly. (Courtesy: National Human Genome Research Institute) 15.2 ABO Blood Types Codominance occurs when multiple is A; if they have only antigen B, then the blood alleles exist for a particular gene and more type is B; if they have both antigens, the blood than one is dominant. When a dominant allele type is AB; if neither antigen is present, the is combined with a recessive allele, the phe- blood type is O. There are three allelic genes notype of the recessive allele is completely in the population (IA, IB, iO). Two are codominant masked. But when two dominant alleles are (IA, IB) and one is recessive (iO). Thus in a given present, the contributions of both alleles do population, there are six possible genotypes not overpower each other and the phenotype which result in four possible phenotypes: is the result of the expression of both alleles. A classic example of codominance is the ABO blood type in humans. Genotype Phenotype IAIA, IAiO blood type A Red blood cells can have various antigens B B B O I I , I i blood type B (BIO 8.1) or agglutinogens on their plasma A B membranes which aid in blood typing. The I I blood type AB O O important two are antigens A and B. If the red i i blood type O blood cells have only antigen A, the blood type THE BIOLOGICAL SCIENCES BIO-269 High-level ImportanceHigh-level Figure IV.A.15.2: Diagram of the ABO blood groups, the antigens and the antibodies (IgM; BIO 8.2) present in each. Blood typing is critical before doing a The only other antigens which have blood transfusion. This is because people some importance are the Rh factors which are with blood type A have anti-B antibodies, coded by different genes at different loci from those with type B have anti-A, those with type the A and B antigens. Rh factors are either AB have neither antibody, while type O has there (Rh+) or they are not there (Rh-). 85% of both anti-A and anti-B antibodies. If a person the population are Rh+. The problem occurs with type O blood is given types A, B, or AB, when a woman is Rh- and has been exposed the clumping of the red blood cells will occur to Rh+ blood and then forms anti-Rh+ antibod- (= agglutination). Though type O can only ies (note: unlike the previous case, exposure receive from type O, it can give to the other is necessary to produce these antibodies). If blood types since its red blood cells have no this woman is pregnant with an Rh+ fetus her antigens {type O = universal donor}. Type AB antibodies may cross the placenta and cause has neither antibody to react against A or B the fetus’ red blood cells to agglutinate (eryth- antigens so it can receive blood from all blood roblastosis fetalis). This condition is fatal if left types {type AB = universal recipient}. untreated. BIO-270 CHAPTER 15: GENETICS GAMSAT MASTERS SERIES 15.3 Mendelian Genetics High-level ImportanceHigh-level Recall that in gametogenesis homolo- predict that the results of a genetic cross gous chromosomes separate during the first should be the same regardless of which par- meiotic division. Thus alleles that code for the ent introduces the allele. However, it can be same trait are segregated: this is Mendel's shown that some traits follow the inheritance First Law of Segregation. Mendel's Sec- of the sex chromosomes. Humans have one ond Law of Independent Assortment states pair of sex chromosomes (XX = female, XY = that different chromosomes (or factors which male), and the remaining 22 pairs of homolo- carry different traits) separate independently gous chromosomes are called autosomes. of each other. For example, consider a pri- mary spermatocyte (2N) undergoing its first Since females have two X chromosomes meiotic division. It is not the case that all 23 and males have only one, a single recessive chromosomes of paternal origin will end up in allele carried on an X chromosome could be one secondary spermatocyte while the other expressed in a male since there is no sec- 23 chromosomes of maternal origin ends up ond allele present to mask it. When males in the other. Rather, each chromosome in a inherit one copy of the recessive allele from homologous pair separates independently of an X chromosome, they will express the trait. any other chromosome in other homologous In contrast, females must inherit two copies pairs. to express the trait. Therefore, an X-linked recessive phenotype is much more frequently However, it has been noted experi- found in males than females. In fact, a typi- mentally that sometimes traits on the same cal pattern of sex linkage is when a mother chromosome assort independently! This passes her phenotype to all her sons but none non-Mendelian concept is a result of cross- of her daughters.