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Part 2: Genetics and molecular biology Chapter 9: Inheritance Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-1 Inheritance of a single gene • Blending inheritance was the popular theory in the late 1800s as nothing was known of the molecular nature of genes Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-2 Gregor Mendel • Mendel studied pure-breeding lines of pea plants, in which all progeny are the same as the parent plants • His question was: ‘If the traits of the two parents differ, what do the offspring look like?’ Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-3 Monohybrid cross • Mendel studied seven traits of pea plants, each of which had two alternative forms (see Fig. 9.2) • Traits could be studied one at a time • When pure-breeding lines with each trait were crossed, only one form was present in the offspring • The offspring are called the F1 (first filial) generation • The F1 form was always the same, regardless of the strain source of pollen or egg Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-4 Fig. 9.2 (top): Seven traits of garden peas studied by Mendel Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-5 Fig. 9.2 (bottom): Seven traits of garden peas studied by Mendel Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-6 Monohybrid cross (cont.) • Plants with yellow seeds crossed with greenseeded plants always had progeny producing yellow seeds • To determine the fate of the green trait, the yellow F1 plants were crossed together to produce an F2 generation • In this generation the green trait reappeared in a proportion of the plants, having been masked in the F1 Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-7 Fig. 9.3: The results of Mendel’s first type of experiment Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-8 Mendel’s conclusions • Each genetic trait must be determined by two factors—these factors are now known as genes • The two copies of each gene may differ from one another—copies are known as alleles • Where alleles are the same, the organism is homozygous for that gene • Where alleles are different, the organism is heterozygous for that gene Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-9 Mendel’s conclusions (cont.) • Alleles do not blend, but remain as discrete units of inheritance • Where alleles for a single gene are different, only one is expressed in the phenotype • This allele is said to be dominant over the nonexpressed recessive allele • Because the alleles do not blend, the recessive allele becomes visible in the F2 generation Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-10 Mendel’s conclusions (cont.) • When a trait is produced by a single gene having two alleles, and one allele is dominant – the ratio between the dominant and recessive phenotypes will be 3:1 in the F2 generation – the ratio of genotypes in the F2 generation is 1:2:1 for the homozygous dominant, heterozygote and homozygous recessive respectively – this ratio was consistent for all the pairs of traits Mendel studied Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-11 Mendel’s conclusions (cont.) Principle of segregation • Individuals carry pairs of genes, termed alleles, that influence particular inherited traits. The alleles segregate during gamete formation such that any individual gamete contains only one of each pair of alleles Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-12 Fig. 9.4: Mendel’s breeding program following the inheritance of seed colour in peas over two generations Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-13 Dihybrid cross • Mendel also crossed together pure-breeding strains differing in two unrelated traits e.g. seed colour and shape • In each case the F1 generation showed the dominant phenotype of each allele pair: yellow and round • In the F2 generation the following occurred – new combinations of traits not present in the parents – the ratios of different phenotypes were specific and consistent Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-14 Fig. 9.6: Mendel’s breeding program following the inheritance of both seed colour and seed shape in peas simultaneously Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-15 Dihybrid cross (cont.) • Independent assortment is shown in the F2 generation by the presence of every combination of alleles in equal numbers • There are only four different phenotypes possible • The ratio between double dominant homozygote: heterozygote (gene 1): heterozygote (gene 2): double recessive homozygote is 9:3:3:1 Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-16 Principle of independent assortment • Alleles of a gene controlling one trait assort into gametes independently of alleles of another gene controlling a different trait • Independent assortment of genes is possible when the two genes considered are located on different chromosomes • The F2 generation phenotype ratio of 9:3:3:1 requires independent assortment Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-17 Multiple effects of single genes • Often a single gene affects more than one trait • The gene allele producing purple pigment in flowers also produces colour in other parts of the plant, such as stems • A coat-colour allele in mammals causes not only yellow fur but abnormal cartilage development • This phenomenon is called pleiotropy, where more than one trait is influenced by a single gene Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-18 Codominance and blood groups • Mendel’s analysis required two alleles for each gene and one to be dominant in the phenotype • Many genes have more than two alleles in a population • Some alleles are coexpressed in the phenotype rather than being dominant or recessive • The ABO blood group system illustrates these points Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-19 Table 9.2: Characteristics of the human ABO blood group system Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-20 Codominance and blood groups (cont.) • The ABO proteins are antigens on the surface of red blood cells • A single gene has three alleles, IA, IB and i, of which each individual has only two • Allele IA produces antigen A, IB produces antigen B and i has no product (or no antigens)—called group O when homozygous Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-21 Codominance and blood groups (cont.) • A and B antigens are separate molecules; when both are present the blood group is AB since they are codominant • Either A or B, when present with allele i, will determine the blood group, so each is dominant over O Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-22 Question 1: Jill and Tom are concerned because they have blood types A and B respectively, but their new daughter, Amanda, has blood type O. Does this mean that Jill or Tom might not be Amanda’s parents? a) Only Jill can be Amanda’s Mother b) Only Tom can be Amanda’s Father c) Neither Jill nor Tom can be Amanda’s parents d) Both Tom and Jill can be Amanda’s parents Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-23 Backcrosses and testcrosses • A backcross is a cross between the heterozygous F1 progeny and either homozygous parent • A cross with the homozygous recessive organism is called a testcross • Since only the dominant alleles are visible in the heterozygote, the genotype cannot be distinguished from homozygous dominant for those alleles • A testcross reveals the presence of recessive alleles in the heterozygote Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-24 Mendelian inheritance in humans • Many human traits are inherited by Mendelian principles • Of particular interest in human genetics are disease-causing alleles • The inheritance of traits in families is followed using pedigrees, where people are assigned symbols depending on their genotype and phenotype for a particular trait Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-25 Fig. 9.7a: Pattern of inheritance of a genetic disease: cystic fibrosis Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-26 Fig. 9.7b: Pattern of inheritance of a genetic disease: Huntington disease Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-27 Patterns of disease inheritance • Defined by the pattern of expression of the disease-causing allele of the gene relative to the normal one • Based on the expression of the disease gene in the phenotype • Also determined by the location of the disease gene on an autosome or sex chromosome (predominantly X) Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-28 Sex determination and linkage • In many species, from insects such as Drosophila melanogaster through to humans, sex is determined by chromosomes • These are called sex chromosomes • In each case, one sex will have two sex chromosomes of the same type (homogametic) and the other will have two different sex chromosomes (heterogametic) Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-29 Fig. 9.8: Pattern of inheritance of sex chromosomes in humans Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-30 Sex determination and linkage (cont.) • In humans and Drosophila melanogaster, females have two X chromosomes but males only have one X and a Y • Males cannot be homozygous or heterozygous for the alleles on the X—rather they are said be hemizygous • For sex-linked inheritance the sex of the offspring matters – males inherit their X chromosome only from their mother – females inherit X chromosomes from both parents Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-31 Fig. 9.9: Sex linkage and chromosome inheritance in Drosophila melanogaster Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-32 X-linked traits in humans Fig. 9.10a: A pedigree showing inheritance of colour-blindness Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-33 X-linked traits in humans (cont.) Fig. 9.10b: A test plate used for detecting colourblindness Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-34 Question 2: Red–green colour-blindness is an X-linked recessive disorder. What is the probability that a female child who has a colour-blind father, and a normal sighted mother (whose father was colourblind), would also be colour-blind? a) ¼ b) ½ c) ¾ d) 0 Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-35 Linkage on autosomes • When genes are located on the same chromosome, they are obliged to travel together during meiosis—this is called linkage • During prophase 1 of meiosis, chromatids of homologous chromosomes exchange information • These crossing-over events are called chiasmata • Since the homologous chromosomes will be heterozygous for some genes, alleles will be recombined Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-36 Recombination • To test for independent assortment a testcross is done between a double heterozygote and the double recessive homozygote • If the genes are assorting independently, the four possible phenotypes should be present in the ratio 1:1:1:1 • Any deviation from that ratio in the progeny indicates that the genes are not assorting independently and may be linked Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-37 Fig. 9.11a: The wild-type Australian sheep blowfly, Lucilia cuprina Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-38 Fig. 9.11b: A mutant white (w) fly Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-39 Fig. 9.11c: Bristles on a mutant crooked bristles fly Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-40 Fig. 9.11d: Genotypes Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-41 Recombination (cont.) • The allele combination present on the original chromosomes is called the ‘parental’ genotype • New combinations generated by chiasmata are called ‘recombinant’ genotypes • The presence in the progeny of recombinant allele combinations indicates that genes concerned are linked (i.e. on the same chromosome) Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-42 Linkage and recombination • The number of the progeny that have recombinant genotypes is proportional to the distance between the genes • Analysis of allele recombination is the basis for genetic mapping • Genes are ‘located’ relative to one another by a series of crosses and measurement of recombination frequencies between the loci Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-43 Linkage and recombination (cont.) • The distances are nominal, rather than actual physical units of distance • The unit is the centimorgan (cM): the number of recombinant progeny/total progeny x 100 • Relative positions of genes have been extensively mapped by this process Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-44 Fig. 9.12a: Chromosome 1 of Drosophila melanogaster Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-45 Fig. 9.12b: Human chromosome 1 Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-46 More variations • Incomplete dominance – where expression of both alleles leads to an intermediate phenotype, such as in snapdragon flower colour • Gene interactions – recombined alleles of different genes may interact to produce new phenotypes Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-47 Fig. 9.13: Eye colour phenotypes of (a) wild-type, and two mutants (b) brown and (c) scarlet of Drosophila melanogaster. (d) A different eye colour phenotype, white. (a) (c) (b) (d) Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-48 More variations (cont.) • Gene expression may be conditional, requiring certain environmental conditions to become visible – an example is the c coat colour allele in Siamese cats, where the allele is only active at low temperatures Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-49 More variations (cont.) • Not all genes are fully expressed in an individual (expressivity) or in a population (penetrance) • Polygenic traits—influenced by the combined expression of a number of genes e.g. height and skin colour in humans Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-50 Epigenetic regulation • X chromosome inactivation – in eutherian mammal females, one X chromosome is inactivated randomly in each cell to equalise the expression of genes in both sexes Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-51 Epigenetic regulation (cont.) • Imprinting – the parental origin of some chromosomes determines the expression pattern of the genes – in marsupials the paternal X chromosome is always inactivated – an allele on human chromosome 15 can cause different diseases depending on the parental origin (see Box 9.2 in the textbook) Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-52 Summary • Genotype is the genetic constitution of an organism • Phenotype is an organism’s observable traits, which depend on both genotype and environment • Generally, individuals carry two alleles for each gene, which separate (segregate) into gametes • Independent assortment: the segregation of alleles of one gene into gametes has no influence on the segregation of the alleles of another gene • Phenotypes may be dominant, codominant or recessive Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-53 Summary (cont.) • Absence of independent assortment indicates gene linkage through location on same chromosome • Linked genes can be separated if crossing over occurs, resulting in recombination. The frequency of recombination is related to the distance between the two loci • Polygenetic traits: many genes, one trait • Epigenetic regulation: activity of some genes can be modified Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 9-54