4.3 Theoretical genetics

Transcription

1 4.3 Theoretical genetics Define genotype, phenotype, dominant allele, recessive allele, codominant alleles, locus, homozygous, heterozygous, carrier and test cross [ Genotype: the alleles of an organism Phenotype: the characteristics of an organism Dominant allele: an allele that has the same effect on the phenotype whether it is present in the homozygous or heterozygous state. Recessive allele: an allele that only has an effect on the phenotype wen present in the homozygous state. Codominant alleles: pairs of alleles that both affect the phenotype when present in a heterozygote ( the terms incomplete and partial dominance are no longer used.) Locus: the particular position on homologous chromosomes of a gene.

2 Homozygous: having two identical alleles of a gene. Heterozygous: having two different alleles of a gene. Carrier: an individual that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for this allele. Test cross: testing a suspected heterozygote by crossing it with a known homozygous recessive Determine the genotypes and phenotypes of the offspring of a monohybrid cross using a Punnett grid. [ The grid should be labeled to include parental genotypes, gametes, and both offspring genotype and phenotype.] Gregor Mendel self crossed peas until he obtained pure strains of 7 contrasting traits. This was his P generation ( homozygous)

3 He then crossed pure round seeds with pure wrinkled seeds getting all round seeds. This was his F 1 generation ( heterozygous) Finally, he self-crossed the F 1 with itself, getting the classic 3:1 ratio of round to wrinkled seeds This was his F 2 generation ( 1:2:1 genotype ratio, and 3:1 phenotype ratio)

4 4.3.3 State that some genes have more than two alleles ( multiple alleles) Describe ABO blood groups as an example of codominance and multiple alleles The classic example of multiple alleles is human blood type: We have 3 different alleles A,B and O It is codominant because both A and B are expressed if present. Phenotype Genotype O ii A I A I A or I A I B I B I B or I B I AB I A I B Explain how the sex chromosomes control gender by referring to the inheritance of X and Y chromosomes in humans.

5 XX is female XY is male polyploidy does occur State that some genes are present on the X chromosome and absent from the shorter Y chromosome in humans. The Y chromosome has lost genes over evolution and now is small and contains almost exclusively genes for male traits. The X chromosome has many genes that have nothing to do with gender, such as color vision, and blood clotting factors Define sex linkage A recessive gene on the X is not masked by the other chromosome (Y) Disorders are linked to every other generation of males, females can be carriers but rarely victims.

6 4.3.8 Describe the inheritance of color blindness and hemophilia as examples of sex linkage. [Both color blindness and hemphilia are produced by a recessive sex-linked allele on the X chromosome X b and X h is the notation for the alleles concerned. The corresponding dominant alleles are X B and X H ] Mr. Evans is red/green colorblind, The classic example of hemophilia is the royal families of Europe, Queen Victoria of England was a carrier, and passed it to many of the royals of Europe. The last known royal hemophiliac was Alex Romanov, who was assassinated by Lenin in the Russian revolution.

7 4.3.9 State that a human female can be homozygous or heterozygous with respect to sex-linked genes. If she is heterozygous, she is a carrier of the trait, but does not express the trait Explain that female carriers are heterozygous for X-linked recessive alleles Predict the genotypic and phenotypic ratios of offspring of monohybrid crosses

8 involving any of the above patterns of inheritance Deduce the genotypes and phenotypes of individuals in pedigree charts [ For dominant and recessive alleles, upper-case and lower-case letters, respectively, should be used. Letters representing alleles should be chosen with care to avoid confusion between upper and lower case. For codominance, the main letter should relate to the gene and the suffix to the allele, both upper case, For example, red and white codominant flower colors should be represented as C R and C W, respectively. For sickle-cell anemia, Hb A is normal and Hb S is sickle-cell