Monohybrid Crosses

What is the result of a monohybrid cross where there is a dominant and recessive allele for the same characteristic

two heterozygous individuals will produce offspring with a phenotypic ratio of 3 dominant:1 recessive

What is a monohybrid backcross (test cross)

• method used in genetics to determine if an individual is homozygous or heterozygous for a dominant characteristic

  • eg tall pea plant could be homozygous dominant (TT) or heterozygous dominant (Tt)

• backcross is a ‘cross’ between an individual with the dominant phenotype but unknown genotype and a known ‘recessive’ genotype

  • recessive phenotype has known genotype because only one allele combination can produce it

What is co-dominant inheritance

• sone characteristics controlled by genes with alleles that aren’t completely dominant or recessive

• eg:

  • alleles that express themselves equally in phenotype

  • heterozygote generally shows a phenotype intermediate between those of the two homozygotes

What is sex linked inheritance

• inheritance of genes found on sex chromosomes

  • non-sex chromosomes known as autosomes

  • men XY women XX

• Y chromosome shorter than X so some genes only present on X chromosome

• In male, recessive alleles carried on X chromosome will be expressed in phenotype as are unpaired so no dominant allele present

  • called sex-linkage

• male carrying sex-linked allele on X chromosome can’t pass it on to sons as they must receive Y chromosome to become male (they would get the X from mum Y from dad)

• Daughters of male with sex-linked allele on X chromosome must receive the recessive allele from him

• Females that are heterozygous for sex-linked recessive traits are known as carriers so have 50% chance of passing recessive allele to sons