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Theoretical genetics Ms. Jackie Important definitions Genotype- the alleles possesed by an organism Phenotype- all the characteristics of an organism. Dominant allele- an allele which has the same effect on the phenotype whether it is present in the homozoygous or heterozygous state Recessive allele- an allele which only has an effect on the phenotype when present in the homozygous state Important definitions Codominant alleles- alleles which only has an effect on the phenotype when present in heterozygous but a greater effect in homozygous individuals. Locus- The particular position of a gene on homologous chromosomes. Homozygous- two identical alleles of a gene Heterozygous- having two different alleles of a gene Carrier- an individual that has a recessive allele of a gene that does not have an effect on the phenotype Test cross- testing a suspected heterozygote by crossing with a known homozygous recessive. Construction of Punnett grids Way of finding he expected ratio of the offspring, giving certain parental phenotypes Punnett grids Example using characteristics used by Mendel He study the size of pea plants and found that tall is dominant over short Lets start with two pure breeding (homozygous) plants with contrasting traits (tall and short) F1 generation will have the dominant phenotype tall but will be heterozygous. Phenotypes Corresponding genotypes Tall TT or Tt Short tt T t t T Tt Tt Tt Tt Punnett grids IF the F1 generation self fertilized we will obtained F2: ¾ tall ¼ short 3:1 1TT:2Tt:1tt T T t t TT Tt Tt tt Lets practice IF a tall garden pea is crossed with a dwarf garden pea, the F1 are all tall. Predict the result of self fertilization of the F1, using a Punnett square. Pedigree chart Used to show the inheritance of certain traits over several generations of humans Example Queen Victoria hemophilic condition Males are normal or affected Females normal or carriers Hemophilia allele is homozygous lethal as well as sex linked since it is carried on the X chromosome Males can be XHY (normal) or XhY (affected) Females can be XHXH (normal) or XHXh (carrier) Queen Victoria and hemophilia Affected male Affected female Normal female Normal male Victoria Victoria Edward Alice George V Duke of Windsor George VI Alfred Helena Albert Louise Arthur Leopold Beatrice 3 Irene Frederick Alexandra NicholasII 3 Elizabeth Margaret Alexis Multiple alleles Some genes have more than two alleles Multiple alleles can be found in blood groups ABO blood groups Based on 4 different phenotypes (A;B;AB and O) Caused by different combinations of 3 different alleles Phenotypes Genotypes A IAIA or IAi B IBIB or IBi AB IAIB 0 ii ABO blood groups Using a Punnett Grid work out the possible combinations of blood groups of a male with blood group B and a female with blood group Using a Punnett Grid explain why a female with blood group O and a male with blood group AB cannot have children with either pf the parents´ blood group. How sex chromosomes determine gender Gender determination Decided by sex chromosomes Females has two X chromosomes Males have one X and one Y chromosome Genes in X and Y chromosomes The X chromosome is relatively large Y chromosome is much smaller Several genes are located on the X chromosome Sex linked examples Colorblindness hemophilia Few genes on the Y Hairy ears Define sex linkage and state two examples Genes carried on the sex chromosomes, most often on the X chromosome. Examples Color blindness Certain colors cannot be distinguished, and is most commonly due to an inherited condition. Red/Green color blindness is by far the most common form, about 99%, and causes problems in distinguishing reds and greens. Another color deficiency Blue/Yellow also exists, but is rare and there is no commonly available test for it. Depending on just which figures you believe, color blindness seems to occur in about 8% - 12% of males of European origin and about one-half of 1% of females. Total color blindness (seeing in only shades of gray) is extremely rare. There is no treatment for color blindness, nor is it usually the cause of any significant disability. Define sex linkage and state two examples Hemophilia Blood condition in which an essential clotting factor is either partly or completely missing. This causes a person with hemophilia to bleed for longer than normal. Cuts and grazes are not great problems as a little pressure and a plaster are usually enough to stop bleeding. The main problem is internal bleeding into joints, muscles and soft tissues. Hemophilia is a lifelong inherited genetic condition, which affects females as carriers and males who inherit the condition. Human females can be homozygous, hetero with respect to sex linked genes Human males cannot be heterozygous since they only have one copy of one allele Example Existing alleles XB for normal Colorblindness vision Xb for colorblindness A female can be XBXB or XBXb XbXb A male can be XBY XbY Human females can be homozygous, hetero with respect to sex linked genes Hemophilia example Existing alleles XH for normal Xh for hemophilia A female can be XHXH or XHXh XhXh (not exist) A male can be XHY XhY •Homozygous lethal •Female carriers are heterozygous for X linked recessive alleles •They will not show the disease but can pass it on to the next generation Explain that female carriers are heterozygous for X-linked recessive alleles Heterozygous females for sex linked traits Are said to be carriers They will not show the disease Can pass the disease to the next generation Calculate and predict the genotypic and phenotypic ratios of monohybrid crosses Monohybrid crosses Involves the cross between two individuals to study the inheritance of one gene Example Character: Tongue Rolling Gene: R Alleles: R, r Genotypes: RR, Rr, rr Phenotypes: rollers, non rollers Cross: Heterozygous X Heterozygous Rr X Rr Gametes R, r X R, r Punnett square R r R RR Rr r Rr Ratio 3:1 rr Calculate and predict the genotypic and phenotypic ratios of monohybrid crosses In your notebooks calculate the genotypic and phenotypic ratios of a cross between a heterozygous tongue roller and a recessive individual for tongue rolling Calculations and codominance Type pf inheritance in which the two alleles of the gene are equally dominant and so they are both expressed in the heterozygous genotype. Alleles R red W white RR red WW white RW- pink Calculations and codominance The two alleles are equally dominant This kind of inheritance produces more variation. Example Phenotypes of parents Red X White Genotypes of parents RR X WW Gametes R and W F1- RW all pink Perform a self cross between the F1 generation Deduce the genotypes or phenotypes of individuals in pedigree charts Go to page 434-435. Answer exercises 2 and 4