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Grade 11 University Biology - Unit 2 Genetics Inheritance of Linked Genes Chapter 6.2 – Pages 251-259 Mendel’s Law of Independent Assortment states that the alleles for a gene segregate independently of the alleles for the other genes during gamete formation. That is, the inheritance of one trait has no influence on the inheritance of another trait. Let’s review this idea... Using Mendel’s work with peas with two characteristics: seed shape and seed colour. The dominant allele for seed shape is round (R) while the recessive allele is wrinkled (r) The dominant allele for seed colour is yellow (Y) while the recessive allele is green (y) Mendel crossed two homozygous parents for the two traits (RRYY and rryy) to produce two pairs of heterozygous alleles (Rr and Yy). This is called a DIHYBRID CROSS. The Punnett square below illustrates the cross. RY RY ry RrYy RrYy ry RrYy RrYy A heterozygous individual for two characteristics (RrYy) produces four possible gametes: RY, Ry, rY and ry. NOTE: There is no gamete labelled Rr or Yy....rather, the alleles (R and r; Y and y) separate independently during the formation of the gametes. This is the Law of Independent Assortment (e.g., alleles R and r separate). Given your understanding so far, why does this make practical and logical sense? Yet, this is not always so. Walter Sutton showed that alleles on the same chromosome do NOT assort independently. In other words, some genes are inherited together, and as such, some traits are often inherited together (i.e., some traits are “linked”). Linked Genes Genes that are on the same chromosome and that tend to be inherited together When a pair or set of genes are on the same chromosome, they are usually inherited together or as a single unit. For example, in fruit flies the genes for eye color and the genes for wing length are on the same chromosome of fruit flies. As such, these traits are inherited together...or linked. Crossing Over and Inheritance of Linked Genes A single chromosome may contain a few thousand genes All the genes on one chromosome are called a LINKAGE GROUP. Why? The traits tend to be inherited together. Yet, linked genes do NOT always stay together. Why? Crossing Over during Prophase 1 of meiosis whereby non-sister chromosomes exchange pieces of chromosomes See Figure 6.11 on Page 252 and illustration above NOTE: Crossing Over is a random event and it occurs infrequently Using Gene Linkage for Chromosome Mapping It is known that alleles for a given pair of linked genes separate with a predictable frequency AND the frequency is different for different pairs of linked genes The frequency of crossing over depends on how close the alleles of the linked genes are positioned on a chromosome. That is, crossing over occurs more frequently between alleles that are FAR APART on a chromosome. Predicting the rate of crossing over frequency is called chromosome mapping Sex-Linked Inheritance A sex-linked trait is a trait controlled by genes on the X or Y chromosome. Recall, female is XX and male is XY Humans have 23 pairs of chromosomes. One set is the sex chromosomes, while the other 22 pairs are autosomes (non-sex chromosomes). If an allele is found on an autosome, it is called autosomal inheritance (...inheritance of alleles located on non-sex chromosomes). Thomas Hunt Morgan crossed fruit flies (Drosophila melanogaster). The cross between whiteeyed males and red-eyed females produced fruit flies with only red eyes in Generation F 1; yet, when two F1 parents were crossed, the Generation F2 ratio was 100% females with red eyes, 50% males with red eyes and 50% males with white eyes. Try to explain why using a Punnett square. NOTE: Assume white eye is homozygous recessive and red eye is homozygous dominant. P Generation R R r Rr Rr r Rr Rr Outcome is the F1 generation is all flies with dominant heterozygous red eyes F1 Generation R r R RR Rr r Rr rr Outcome is the F2 generation is the anticipated 1:2:1 ratio of phenotypes expressions. Yet, this does not explain the SEX difference observed. There must be a gender link. That is, the gene for eye colour must be present on the X sex chromosome. This is called a SEX-LINKED TRAIT A trait controlled by genes on the X chromosome or Y chromosome, and trait is influenced by different rates as they appear on males and females The X and Y chromosomes had very little homologous DNA. The X chromosome contains over 2000 chromosomes while the Y chromosome has about 100 chromosomes. As well, all other genes on the X chromosome in the female consist of two copies (Recall XX). This results in alleles on the X and Y chromosomes not always being paired equally. As such, an allele for a particular trait may be located on only one sex chromosome (...usually the X chromosome). The difference in expression of traits on X chromosome is called X-Linked Genes. Likewise, difference on Y chromosome is Y-Linked Genes. The X chromosome carries hundreds of genes but few, if any, of these have anything to do directly with sex. However, the inheritance of these genes follows special rules due to the following facts: males have only a single X chromosome almost all the genes on the X have no counterpart on the Y any gene on the X, even if recessive in females, will be expressed in males. Repeat the experiment above, but assign the genotype to the X or Y chromosome. X RX R Female, red eyes (homozygous dominant) X RX r Female, red eyes (heterozygous dominant) X rX r Female, white eyes (homozygous dominant) X RY Male, red eyes X rY Male, white eyes P Generation XR XR Xr X RX r X RX r Y X RY X RY XR Xr XR X RX R X RX r Y X RY X rY F1 Generation Also see Figure 6.13 on Page 254. Outcome explains the observed sex differences with 100% red-eyed females, 50% red-eyed males and 50% white-eyed males in the F2 Generation. Sex-Linked Traits in Humans Many disorders are sex-linked (see Table 6.2 on Page 255) Disorder is X-linked dominant Affected male passes allele to only females with 100% chance of disorder Disorder is X-linked dominant Affected female passes allele to son or daughter with 100% chance of inheritance Male only need to inherit one allele to inherit the disorder. The female needs to inherit both alleles to inherit the disorder. Thus, more males will be affected. Disorder is X-linked recessive Male may inherit the disorder with 25% chance of inheritance Female may inherit the disorder (25% chance), but she is a carrier (i.e., no phenotype expression) Also see Figure 6.14 on Page 255 Hemophilia is X-linked recessive. It is also called the Royal Disease. Barr Bodies As you know, human females inherit two copies of every gene on the X chromosome, whereas males inherit only one (...with, of course, some exceptions). You also know, that there is an inactivation of one of the chromosomes during fertilization. What if the inactivation chromosome remains? In this case, the cell is called a Barr body or sex chromatin. Females usually have one Barr body; males usually have none. Why? A Barr body is an inactive, condensed X chromosome in the female cell. Inactivation is random. The expression of the Barr body produces different phenotypes (see Figure 6.16 on Page 258). The human abnormalities called Kleinefelter's syndrome and Turner's syndrome both result from an unnatural presence or absence of a Barr body. In the former, the male possesses a Barr body that it would normally not have (XXY and, as such, 47 total chromosomes), and in the latter, the Barr body is absent (X or less than 46 total chromosomes). The chart opposite is Turner’s syndrome...note the missing X chromosome (circled) Questions Conduct Internet research of the Royal Disease. That is, “Why did it pass through royal families of Europe?” Read the story reported in Aronova-Tiuntseva, Y. and C.F. Herreid. 2003. Hemophilia: The Royal Disease” and follow the pedigree to explore the inheritance of the disorder online at http://www.sciencecases.org/hemo/hemo.asp Do Sample Problem on Pages 256-257 Answer Questions 11-20 on Page 258 Answer Questions 2, 4-5, 7-10 and 15 on Page 259 Test your understanding In watermelons, the green colour gene (G) is dominant over the striped colour gene (g). As well, the round shape (R) is dominant over the long shape (r). A heterozygous round green colour watermelon (GgRr) is crossed with another heterozygous round green colour watermelon (GgRr). Calculate the expected phenotype ratio of the F1 generation. Try solving the problem prior to reading the solution, Solution 1. Determine possible gametes from each parent In this case – GR Gr gR gr 2. Draw a Punnett square of the dihybrid cross and execute the cross. NOTE: Since there are four possible gametes from each parent, make a 4 X 4 grid as shown GR Gr gR gr GR GGRR GGRr GgRR GgRr Gr GGRr GGrr GgRr Ggrr gR GgRR GgRr ggRR ggRr gr GgRr Ggrr ggRr ggrr 3. Determine which plants have the same phenotype. That is, the plants that express the same traits (e.g., GGRR, GrRR and GgRr all are green and round shaped). 9 = green and round (G_R_) 3 = green and long (G_rr) 3- striped and round (ggR_) 1 = striped and long (ggrr) Therefore, the phenotype ratio is 9:3:3:1