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Non-Mendelian Inheritance Complex Patterns of Inheritance • Many things can happen to Mendel’s factors during the process of meiosis – Mutations – cross-over between homologues – non-disjunction. Complex Patterns of Inheritance (cont’d) • There are also exceptions to the postulate that factors occur in pairs and the law of dominance, as well as the law of independent assortment—it is now knows many genes are linked on the same chromosome. • Linked traits are genes that are located on the same chromosome. Increasing Genetic Variability Recall: Cross-over in Meiosis • During Meiosis I, the chromatids in a tetrad pair are so tightly aligned together that the non-sister chromatids from homologous chromosomes actually exchange genetic material in a process known as crossing over. • This further shuffles the ancestral genes so that a single chromosome in a gamete may have genes from both the maternal and paternal ancestors. • Crossing over can occur at any location on a chromosome, and it can occur at several locations at the same time. • It is estimated that during meiosis in humans, there is an average of two to three crossovers for each pair of homologous chromosomes. • The chromosome pieces further away from the centromere cross over most frequently; the frequency diminishes as the centromere is approached. • There can also be multiple cross-overs. • They may occur because the genetic code on each section of the chromatid is similar. Sex-Linked Traits (p 318-320) • Some patterns of inheritance seem to weigh more heavily in males – I.e. more males than females have hemophilia, red-green colour blindness, Duchenne muscular dystrophy and others. How this was discovered: • In 1910 T. H. Morgan studied the Drosophila fly and found a mutant male fly, which expressed the trait of white eyes instead of the normal red eyes. • This trait was very unusual in that species and Morgan wanted to see if the trait would be passed on to its offspring. • He experimented to find if this strange trait would be inherited according to Mendel's research. • First he crossed the mutant male fly with a normal female with red eyes, to observe whether the white or red eyes were dominant. • The F1 generation all had red eyes, which made Morgan conclude that red eyes were dominant over white. (See Fig 12.11 p 319) • He continued the steps of Mendel's experiment by crossing two flies from the F1 generation with each other. • Out of 4252 flies in his F2 generation, 782 had white eyes but surprisingly all the flies with white eyes were also male. • This strange observation puzzled Morgan to wonder why there weren't any females with white eyes. • He then crossed flies from the F1 generation with the original male fly with white eyes. • This cross resulted in white-eyed and redeyed males and females, making a 1:1:1:1 ratio. • We see this pattern in humans—in hemophilia, and red-green colourblindness. • Why? How does it specifically affect males more often than females? Sex-Linked Trait • If a gene is found only on the X chromosome and not the Y chromosome, it is said to be a sex-linked trait. • Because the gene controlling the trait is located on the sex chromosome, sex linkage is linked to the gender of the individual. • Usually such genes are found on the X chromosome. The Y chromosome may be missing such genes (See Diagram above.). • The result is that females will have two copies of the sex-linked gene while males will only have one copy of this gene. • If the gene is recessive, then males only need one such recessive gene to have a sex-linked trait rather than the customary two recessive genes for traits that are not sex-linked. • This is why males exhibit some traits more frequently than females. Showing sex-linked Punnet squares, Red-Green Colourblindness: • Because the allele is linked to the X chromosome, we show it as a superscript on the X: Eg. XC Xc normal vision gene colour blind gene • Children who inherit this trait have difficulties with green hues, usually seeing them towards a “red” spectrum. • Reds tend to be seen darker, and in low light colour differentiation is difficult—both appearing black to the person. Eg. In humans, red-green colourblindness is a recessive trait located on the X chromosome. • The Y chromosome does not carry this trait at all. • A normal-vision mother whose father was colour blind has a child with a normalvision man. • What is the probability the couple will have a child with colour-blindness? • What is the probability a son born to them will be colour blind? XCXc XC XC Xc XCY x Y XC XC XCY XC Xc XcY Genotypic Ratios: 1:1:1:1 Phenotypic Ratios: 3:1 • Notice, Y doesn’t have this gene! Hemophilia • Hemophilia is a term that covers a wide variety of clotting disorders— some clotting factor(s) are missing or are defective, resulting sometimes in uncontrolled bleeding. • Hemophilia is a recessive sex-linked trait on the X chromosome. • What are the chances a couple will have a daughter with hemophilia if the mother is a probable carrier and the dad has a form of hemophilia? • What is the chance they would have a child with hemophilia? XHXh x Xh XhY Y XH Xh Genotypic Ratios: Phenotypic Ratios: • Notice, Y doesn’t have this gene! SEX-INFLUENCED TRAIT • These are not true sex-linked because it is not on the X nor Y chromosomes, but because of hormones or other such differences between genders, these traits show up and “look” like sex-linked traits. • For example, in male-pattern-baldness is a dominant trait in males but recessive in females. Male Pattern Baldness • For example, male-pattern-baldness is a dominant trait in males but recessive in females. – Male heterozygotes will go bald. – Female heterozygotes will not go bald. • A female would need to inherit the trait from both parents to lose her hair. • We would do a standard punnett square for a dihybrid cross: • A female with a lot of hair, is a carrier for baldness has children with a man who is losing his hair, yet is a carrier (dad was not bald). • What will be the phenotypes and genotypes of their children? XXBb XB x XYBb Yb XB Xb Genotypic Ratios: Phenotypic Ratios: • REMEMBER the Bb has different meanings depending on gender!!! TYPES OF DOMINANCE: EXCEPTIONS to MENDEL’s Law of Dominance Incomplete Dominance • In Mendel’s law of dominance, heterozygotes exhibited the dominant phenotype. • When two alleles of a gene appear to be blended in the phenotype, the alleles are said to show incomplete dominance – they don’t look like either parent. An example is snapdragon colour • Red snapdragons crossed with white snap dragons yield pink snap dragons—a complete BLENDING of the alleles. • In the example, note how the allele is shown. • Another way the alleles are shown is to use RR for the incompletely dominant red colour, and R’R’ for the white colour. THUS pink flowers would be RR’ • If two pink snap dragons are crossed, what will be the expected phenotypes and genotypes? __________ ? ? ? Genotypic Ratios: Phenotypic Ratios: X _________ ? Curly Hair • In humans, curly hair is incompletely dominant to straight hair. • Children who are heterozygous will have wavy hair. • What will be the phenotypes and genotypes of the children from a female with curly hair and man with wavy hair? • C represents curly hair OR HC • C’ represents straight hair or HS __________ ? ? ? Genotypic Ratios: Phenotypic Ratios: X _________ ? Co-Dominance Inheritance • It may seem as if incomplete dominance and co-dominance are the same, but they are not. • When two alleles of a gene are clearly expressed in the phenotype, the alleles are said to be co-dominant. • This results in two distinct and detectable gene products Blended: At a distance, the cattle appear roan coloured and mottled: • The individual hairs are either red or they are white. • In doing the PUNNET square we use both capital letters for the trait—R for red and W for white. • Eg. A roan bull (RW) mates with a white cow. • What will be the phenotypes and genotypes of their offspring? RW X WW Genotypic Ratios: Phenotypic Ratios: • 50% will be roan and 50% will be white Appaloosa Horses • In horses, gray horses (GG) are codominant to white horses (WW). • The heterozygous horses(GW) is an appaloosa horse (a white horse with gray spots on the rump and loins). • Cross a white horse with an appaloosa horse __________ ? ? ? Genotypic Ratios: Phenotypic Ratios: X _________ ? Blood Typing • Blood is typed according to what type(s) of antigen (a cellular product that induces antibody formation in a foreign host) are found on the surface of the red blood cells. • Blood type is determined by reacting the blood with antibody against the antigens. • Typical blood types are the ABO bloodgroups. (Text P 325 & 978) Type AB Blood • The AB blood type in humans is the result of an individual carrying both the IA and the IB alleles. MN Blood Group system in Humans • In humans, our M and N blood groups are co-dominant. • Our blood cells exhibit both antigens but on separate blood cells. • The MN blood group system is under the control of an autosomal locus found on chromosome 4, with two alleles designated LM and LN. • The blood type is due to a glycoprotein present on the surface of red blood cells. The table below is an old one but it shows the frequencies of MM blood, MN blood, and NN blood. Blood Type and Geographic Location • HowStuffWorks Videos "Why Tell Me Why: Different Blood Types" INCREASING THE GENE-POOL MULTIPLE ALLELE INHERITANCE • Mendel never knew that some traits occur in more than pairs. • This is called multiple allele inheritance because more than two alleles are possible for one trait—but only two alleles are inherited and involved. • In humans, our major blood type system is a classic example. ABO Blood Grouping • We commonly call it the ABO system. • As a multiple allele, we write it like we did for co-dominance—that is because both the A and B are equally strong. • The exception is the type O which is a recessive condition: IA - for type A antigens IB - for type B antigens i - for the recessive O condition (which produces neither the A nor B antigens and does not interfere with type A or B blood). • The following allele combinations are possible: IA IA IB IB IA IB ii IA i type A blood (notice A is dominant to O) IB i type B blood type AB blood—both antigens are present type O blood—no A nor B antigens are present • A woman is homozygous for type B blood (IBIB)has a child with a man who is heterozygous for type A blood. • This means he is IAi. • What will be the genotypes and phenotypes of their children? IAi IA IB IB Genotypic Ratios: Phenotypic Ratios: X IBIB i • What is the probability a couple whose blood types are AB and O will have a child with type A blood? __________ ? ? ? Genotypic Ratios: Phenotypic Ratios: X _________ ? POLYGENIC INHERITANCE • Many characters cannot vary in a population across a continuum (gradient). • For example, skin color in humans is a quantitative character – this means the character is controlled by more than one gene at the same time (polygenic inheritance) – that is, the trait depends on several chromosomal locations at the same time. • This is different from multiple allele inheritance where only TWO alleles are passed on but in the population there are several types of alleles for one trait—try not to confuse these!.