Monohybrid Inheritance

• Inheritance is the transmission of genetic information from one generation to the next generation
• A gene is a short length of DNA found on a chromosome that codes for a particular characteristic

  • expressed by the formation of different proteins
• Alleles are variations of the same gene
• We have two copies of each chromosome, we have two copies of each gene and therefore two alleles for each gene

  • One of the alleles is inherited from the mother and the other from the father
  • This means that the alleles do not have to ‘say’ the same thing
  • e.g. if an individual has two copies of the gene for eye colour but one allele could code for brown eyes and one allele could code for blue eyes
• Phenotype - the observable characteristics of an organism (seen just by looking - like eye colour, or found – like blood type)
• Genotype - the combination of alleles that control each characteristic
• Alleles can be dominant or recessive

  • A dominant allele only needs to be inherited from one parent in order for the characteristic to show up in the phenotype
  • A recessive allele needs to be inherited from both parents in order for the characteristic to show up in the phenotype.
• If there is only one recessive allele, it will remain hidden and the dominant characteristic will show
• Homozygous - if the two alleles of a gene are the same

  • An individual could be homozygous dominant (having two copies of the dominant allele), or homozygous recessive (having two copies of the recessive allele)
• Heterozygous - If the two alleles of a gene are different
• When completing genetic diagrams, alleles are abbreviated to single letters

  • The dominant allele is given a capital letter and the recessive allele is given the same letter, but lower case
• We cannot always tell the genotype of an individual for a particular characteristic just by looking at the phenotype

  • a phenotype associated with a dominant allele will be seen in both a dominant homozygous and a dominant heterozygous genotype
• Different allelles of a gene

  A) homozygous

  B) dominant

  C) homozygous

  D) recessive

  E) heterozygous

  F) diploid

  G) pairs

  
• If two individuals who are both identically homozygous for a particular characteristic are bred together, they will produce offspring with exactly the same genotype and phenotype as the parents

  • we describe them as being ‘pure breeding’ as they will always produce offspring with the same characteristics
• A heterozygous individual can pass on different alleles for the same characteristic each time it is bred with any other individual and can therefore produce offspring with a different genotype and phenotype than the parents

  • heterozygous individuals are not pure breeding
• Humans all have 23 pairs of chromosomes

  • the 23rd pair is the sex chromosomes
• Female - XX
• Male - XY

  • has to have one from the mother and one from the father
• If you inherit the same allele, you are homozygous for this gene
• If you have 2 different alleles, you are heterozygous
• Genotypes - what alleles you have
• Phenotype - what you actually look like and present as
• Brown eye alleles are dominant while blue eye alleles are recessive
• Dominant - only needs 1 allele to be shown in phenotype
• Recessive - will only be shown in phenotype if there is no dominant allele
• One gene controls the inheritance of dimples
• What is the rule for phenotypic ratio?

  Always state the colour in the ratio
• Why is there a 1:1 ratio of male babies to female babies?

  Males carry an X & a Y chromosome and females carry 2 X chromosomes. The males produce half of their sperm carrying the X chromosome and the other half carrying Y. Females produce all eggs/gametes carrying the X chromosome. Fertilisation will produce XY or XX with even chances.
• Define homozygous


  Homozygous is when organisms have two of identical alleles of a gene
• Why might the phenotype of an organism differ to its genotype?


  Phenotype is the observable features WHILE genotype is the alleles present and whether the individuals are heterozygous or homozygous
• How do you test cross?

  1. A parent with unknown genotype is crossed with a parent with homozygous recessive genotype
  2. Offspring may be heterozygous or homozygous recessive which