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Chapter 14: Genotype, phenotype and crosses Key questions 1 A gene is a series of nucleotides that code for a characteristic or instruction. An allele is a variation of that characteristic. 2 A genotype is the genetic detail (alleles) of an individual’s characteristic(s) being investigated. The phenotype of the individual is the trait that a given genotype expresses. The environment also plays a role in the expression of the phenotype. 3 In humans, hemizygous individuals are male because they only have one X chromosome, i.e. half the number of X chromosomes in females (hemi = half). 4 In some cases the phenotype of an organism is not visible to the naked eye and may require a microscope or further testing. Blood groups, colour blindness and vitamin D resistant rickets require biochemical investigation. 5 Both genetic and environmental factors contribute to the phenotype of individuals. For example, the presence of skin pigment is controlled by genes, but the degree of colour or tan of the skin can also be influenced by the amount of exposure an individual has to the sun. 6 Ee—heterozygous; ee—homozygous; EE—homozygous; eE—heterozygous 7 In blowflies Ww and WW, both individuals will have the same eye colour or phenotype even though they have different genotypes. This results when the expression of one allele is dominant over another. In this case, W refers to red eye colour and w refers to white. If Ww genes are present, the W gene masks the expression of w gene. 8 A recessive trait is only expressed in a homozygote. In an example of seed shape, RR or Rr show round seed phenotypes, while the genotype rr expresses oval seeds. Therefore, the expression of the R allele is dominant to the expression of the r allele. 9 a i A red-eyed individual would have the genotype Rr or RR. ii A white-eyed individual would have the genotype rr. b The phenotype for the heterozygote would be red because only one dominant allele needs to be present to mask the recessive allele for white. 10 Complete dominance occurs when the heterozygote form of a genotype as well as the dominant homozygote genotype is expressed as the same phenotype. In incomplete dominance, the heterozygote genotype results in a blending of the two homozygote traits. An example of incomplete dominance is found in some flower colours. A cross involving pure-breeding red and white snapdragons produces pink flowers. 11 Codominance occurs when both alleles are expressed equally. In the ABO blood grouping, the AB blood group produces both antigen A and antigen B on the red blood cell surface. Incomplete dominance differs from codominance in that no blending occurs in codominance. 12 The ABO blood grouping system in humans is controlled by a single gene, but there are more than two alleles for the gene, which results in more than two phenotypes. The alleles are IA, IB and i. Genotypes IA IA and IAi generate a phenotype of blood type A. Genotypes IBIB and IBi generate a phenotype of blood type B. Genotype IAIB generates a phenotype of blood type AB. Blood type O has the genotype ii. 13 a b An example of a single gene that affects multiple phenotypic characters is found in the vinegar fly. In a mutant variety, a gene called lozenge affects eye structure, sensory bristles on the antennae, and taste receptors. Also affected were claws, a section of the brain, a group of blood cells involved in the immune system and sperm storage in females resulting in infertility. A single gene can affect multiple characteristics if the gene is a regulatory one, as is the lozenge gene. Copyright © Pearson Australia (a division of Pearson Australia Group Pty Ltd) 75 14 Identical twins often have visible differences due to environmental influences such as diet and exercise. 15 Yes, monozygote twins have identical genotypes, as do plants produced by asexual reproduction or by cuttings. 16 Studies of the IQ of identical twins raised in different homes from birth have provided evidence that environmental factors, such as upbringing and education, play a vital role in the phenotypes of the individuals. Twins have also been used to investigate the contribution of genes to behavioural traits such as personality, handedness and alcoholism. 17 Due to the absence of the enzyme phenylalanine hydroxylase, an accumulation of the amino acid phenylalanine in the tissues of individuals with phenylketonuria results in impaired development that leads to mental retardation. Controlling the environmental factor of diet in these individuals by deleting phenylalanine (most protein foods) overcomes the problem of phenylalanine accumulation and normal development occurs. 18 A monohybrid cross is a cross between two individuals with respect to a single gene locus. For example, eye colour in sheep blowfly is controlled by a single gene with two alternative alleles, W: red eye and w: white eye. A cross between two individuals with respect to this single gene is a monohybrid cross. For example: WW ww all offspring have genotype Ww and phenotype red eyes. Ww Ww ¾ offspring have phenotype red eyes (genotype either WW or Ww) and ¼ offspring have phenotype white eyes (genotype ww) 19 a b A pure-breeding blowfly has two identical alleles such as WW (red eye) or ww (white eye). When two pure-breeding strains, such as homozygous red-eyed blowflies, breed (WW WW) the offspring have the same genotype and phenotype as the parents. Geneticists use specific names to identify between a series of crosses involving individuals and offspring of the same family: • parental (P) generation—WW ww • first filial (F1) generation—all offspring are Ww • second filial (F2) generation—Ww Ww (a cross of F1 individuals). c Individuals of the F1 generation are all red-eyed because the genotype Ww is heterozygous red eye. The expression of the W allele is dominant and therefore masks the expression of the allele w. d The phenotypes in the F2 generation occur in the ratio of 3 red eye : 1 white eye because the parents are heterozygous: each carries one red-eye allele and one white-eye allele. When a cross occurs, the probability that a parent will pass on a white allele is 50%. The probability that a parent will pass on a W allele is also 50%. Therefore, the probability that a WW will occur is 25%, that a Ww will occur is 50% and that a ww will occur is 25%. So the total probability that WW or Ww (red) will occur is 75% and that ww (white) will occur is 25%. As red eye is dominant, only one allele needs to be present for the phenotype to be expressed. 20 a parental (P) generation—dark red snapdragons P generation genotype—R1R1 white snapdragons R2R2 P gametes—R1 and R2 F1 generation Gametes R1 R1 R2 R1R2 R1R2 R2 R1R2 R1R2 F1 genotype—all R1R2 Copyright © Pearson Australia (a division of Pearson Australia Group Pty Ltd) 76 F1 phenotype—all pink b The genotypic and phenotypic ratios in the F2 generation are as follows: (F1) P generation—R1R2 R1R2 P gametes—1/2 R1, 1/2 R2 1/2 R1, 1/2 R2 F2 generation Gametes 1/2 R1 1/2 R2 1/2 R1 1/4 R1R1 1/4 R1R2 1/2 R2 1/4 R1R2 1/4 R2R2 F2 genotypes—1/4 R1R1, 1/2 R1R2, 1/4 R2R2 F2 phenotypes—1 dark red : 2 pink : 1 white 21 a A 3:1 ratio in the F2 generation suggests that the variation in the trait is controlled by two alleles of a single autosomal gene. In this case, three-quarters of the progeny would display the dominant trait. b A 1:2:1 ratio in the F2 generation suggests that two alleles are involved and that partial dominance or codominance is the mode of inheritance. 22 A monohybrid cross involves a cross that deals with only one characteristic, such as hairline or, in plants, stem length. A dihybrid cross involves a cross that deals with two characteristics at the same time, such as eye colour and length of bristles. These characteristics can be found on the same chromosome or on separate chromosomes. 23 Independent assortment is the principle of segregation of homologues to opposite poles within the nucleus of the cell. During meiosis the homologues line up at the equator during Metaphase 1. The homologue chromosomes/chromatids containing the gene being investigated can move to either pole of the cell. Each homologous pair of chromosomes in the cell can move to either end of the cell independently of the movement of other homologues. 24 a b Number of different phenotypes Number of different genotypes Parents 2 2 F1 1 1 F2 4 8 9:3:3:1 25 A cross between AABB and aabb individuals results in all offspring with genotype AaBb and the dominant phenotype. F1 individuals can each produce four different kinds of gametes: AB, Ab, aB, ab. A cross between these individuals can be tracked using a Punnett square. AB Ab aB ab AB AABB AABb AaBb Aabb Ab AABb AAbb AaBb Aabb aB AaBB AaBb aaBB aaBb ab AaBb Aabb aaBb aabb 9/16 of the offspring have at least one allele that confers the dominant phenotype for each characteristic; 3/16 have at least one allele that confers the dominant phenotype for the first characteristic combined with homozygous recessive for the second (recessive phenotype); 3/16 are homozygous recessive for the first characteristic (recessive phenotype) and have at least one allele that confers the dominant phenotype for the Copyright © Pearson Australia (a division of Pearson Australia Group Pty Ltd) 77 second characteristic; 1/16 are homozygous recessive at both gene loci, resulting in recessive phenotype for both characteristics. Overall, this represents a 9:3:3:1 ratio. 26 a b Testcrosses are used to determine whether individuals with a dominant phenotype are heterozygous or homozygous for that dominant trait. A backcross is a cross between the F1 and either one of the pure-breeding parental strains. The difference between a testcross and a backcross is that a testcross involves a cross between the individual in question and the parent with the homozygous recessive genotype. 27 The genotypes of the F1 guinea pigs are 1/4 BB, 1/2 Bb, 1/4 bb. The phenotypes of the cross would be 3 black : 1 white. 28 The genotypes and phenotypes of the F1 guinea pigs and a backcross to the pure-breeding black coat colour are 1/2 BB and 1/2 Bb. The phenotypes would be all black. Chapter review questions 1 a F: free lobes; f: attached lobes b Three possible genotypes: FF, Ff, ff. Two phenotypes: free lobes: FF and Ff; attached lobes: ff c Homozygous man with free lobes: FF; Heterozygous woman: Ff F F F FF FF f Ff Ff All of the children will have free lobes, although expect 50% to be homozygous and 50% to be heterozygous. d Two people who both have free lobes could have children with attached lobes if they are heterozygous. F f F FF Ff f Ff ff Expect 3/4 offspring to have genotype that confers dominant phenotype of free lobes (either homozygous dominant or heterozygous) and 1/4 to have attached lobes (homozygous recessive). e 2 The chance of having a child with attached lobes is 25% or 0.25 or 1:3. B: black coat (dominant); b: white coat (recessive) B B B BB Bb b Bb bb Expect a genotypic ratio of BB:Bb:bb of 1:2:1, which equates to a phenotypic ratio of ¾ black offspring: ¼ white offspring, ie. 3:1. 3 a i Cc would represent curly-wing flies and cc would represent wild type. The curly-wing flies must be heterozygotes because the F1 do not all show this dominant trait. ii At 25°C the temperature allows both traits to be expressed without the effect of the environment. iii The difference in results between 25°C and 18°C suggests that low temperatures reduce the effect of the curly-winged allele. Therefore, some individuals with the Cc genotype will express the recessive wild type trait or curly-wing flies will have a lower ability to survive low temperatures, therefore dying before they mature. Copyright © Pearson Australia (a division of Pearson Australia Group Pty Ltd) 78