3.7.1 Inheritance

• Genotype

  The genetic constitution of an organism
• Phenotype

  The expression of an organisms genetic constitution, combined with its interaction with the environment
• Allele

  Different forms of a particular gene, found at the same locus on a chromosome. A single gene could have many alleles
• How many alleles per gene do diploid organisms carry? 


  Two
• Dominant allele

  An allele whose characteristic will always appear in the phenotype, whether one or two are present
• Recessive allele

  An allele whose characteristic only appears in the phenotype if no dominant allele is present, meaning two must be present
• Codominant alleles

  Two dominant alleles that both contribute to the phenotype, either by showing a blend of both characteristics, or the characteristics appearing together
• Homozygous

  Both alleles are dominant or both alleles are recessive
• Heterozygous

  One allele is dominant, the other is recessive
• Monohybrid inheritance

  Where one phenotypic characteristic is controlled by a single gene
• Draw a Punnett square to show a monohybrid cross with parental genotypes of GG and gg. What percentage of these offspring will express the characteristic determined by allele G?

  100% of all offspring genotypes are Gg, meaning that none of them will expresses the recessive allele

  
• Draw a Punnett square to show a monohybrid cross with parental genotypes of Bb and Bb. What percentage of these offspring will express the characteristic determined by allele B?

  75% - only one of the combinations results in the expression of the recessive allele

  
• Dihybrid inheritance

  Where two phenotypic characteristics are determined by two different genes present on two different chromosomes at the same time
• Draw a genetic diagram with parent genotypes of Bb and Bb. What is the ratio of their offspring?

  3:1

  
• Draw a Punnett square to show a dihybrid cross with parental genotypes of RYry and ryry. How many of each phenotype will there be?

  Each genotype is expressed 1/4 of the time, so each phenotype should be expressed 4 times

  
• Sex-linkage

  Where an allele is located on one of the sex chromosomes, meaning its expression depends on the sex of the individual
• Why are males more likely to express a recessive sex-linked allele?

  Most sex-linked alleles are located on the X chromosome. Therefore males only get one copy of the allele, so it will express this characteristic even if its recessive. Since females get two alleles it is less likely
• Which parent do males inherit sex-linked characteristics from?

  Their mother, since the Y chromosome can only come from their father. Therefore if the mother is heterozygous for sex-linked alleles, she is a carrier and may pass on the trait
• Draw a Punnett square to show a sex-linked cross with parental genotypes of xN Y and xN xn . What are the four possible phenotypes of these offspring?

  Normal female, carrier female, normal male, affected male

  
• Autosomal linkage

  Where two or more genes are located on the same (non-sex) chromosome. In this case, only one homologous pair is needed for all four alleles to be present. For genes that aren't linked, two homologous pairs are needed
• Epistasis

  Where two non-linked genes interact, with one gene either masking or suppressing the other gene
• Recessive epistasis

  Where two homozygous recessive alleles mask expression of another allele
• Dominant epistasis

  Where one dominant allele masks expression of multiple other alleles
• Draw a Punnett square to show an epistasis cross with parental genotypes of AaBb and AaBb

  12:3:1

  A) 12

  B) 3

  C) 1