Genetic Inheritance

• Gamete - Gametes are sex cells (in animals: sperm and ovum;

  in plants pollen nucleus and ovum).
• Chromosome - Chromosomes are thread-Like structures of DNA, carrying genetic information in the form of genes. They are located in the nucleus of cells.
• Gene - Genes are short Lengths of DNA found on chromosomes. They code for specific proteins.
• Allele - Alleles are different versions of a particular gene.
• Dominant - A dominant allele is always expressed, even if only one copy is present.
• Recessive - A recessive allele is only expressed if two copies are present (therefore no dominant allele present).
• Homozygous - If the two alleles of a gene are the same, we describe the individual as being homozygous (homo = same).
• Heterozygous - If the two alleles of a gene are different, we describe the individual as being heterozygous (hetero = different).
• Genotype - The combination of alleles that control each characteristic is called the genotype.
• Phenotype - The observable characteristics of an organism (seen just by looking-Like eye colour, or found through testing-like blood type) is called the phenotype.
• Monohybrid Inheritance 1

  • Some characteristics are controlled by a single gene, such as fur colour in mice; and red-green colour blindness in humans
  • The inheritance of these single genes is called monohybrid inheritance (mono = one)
  • As 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
• Monohybrid Inheritance 2

  • For example, 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
  • The observable characteristics of an organism (seen just by looking – like eye colour; or found – like blood type) is called the phenotype
  • The combination of alleles that control each characteristic is called the genotype
• • 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
• • If the two alleles of a gene are the same, we describe the individual as being homozygous (homo = same)
  • An individual could be homozygous dominant (having two copies of the dominant allele), or homozygous recessive (having two copies of the recessive allele)
• • If the two alleles of a gene are different, we describe the individual as being heterozygous (hetero = 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
• Multiple Gene Inheritance

  • Most characteristics are a result of multiple genes interacting, rather than a single gene
  • Characteristics that are controlled by more than one gene are described as being polygenic
• • Polygenic characteristics have phenotypes that can show a wide range of combinations in features
  • The inheritance of these polygenic characteristics is called polygenic inheritance (poly = many/more than one)
  • Polygenic inheritance is difficult to show using genetic diagrams because of the wide range of combinations
• • An example of polygenic inheritance is eye colour – while it is true that brown eyes are dominant to blue eyes, it is not as simple as this as eye colour is controlled by several genes
  • This means that there are several different phenotypes beyond brown and blue; green and hazel being two examples
• Predicting Inheritance

  • Monohybrid inheritance is the inheritance of characteristics controlled by a single gene
  • This can be determined using a genetic diagram known as a Punnett square
  • A Punnett square diagram shows the possible combinations of alleles that could be produced in the offspring
  • From this, the ratio of these combinations can be worked out
  • Remember the dominant allele is shown using a capital letter and the recessive allele is shown using the same letter but lower case
• Predicting Inheritance

  Example:

  • The height of pea plants is controlled by a single gene that has two alleles: tall and short
  • The tall allele is dominant and is shown as T
  • The small allele is recessive and is shown as t

  ‘Show the possible allele combinations of the offspring produced when a pure breeding short plant is bred with a pure breeding tall plant’

  • The term ‘pure breeding’ indicates that the individual is homozygous for that characteristic
  • This shows that all the offspring will be tall

  
• ‘Show the possible allele combinations of the offspring produced when two of the offspring from the first cross are bred together’

  • All of the offspring of the first cross have the same genotype, Tt (heterozygous), so the possible combinations of offspring bred from these are: TT (tall), Tt (tall), tt (short)
  • There is more variation in this cross, with a 3:1 ratio of tall : short
  • The F2 generation is produced when the offspring of the F1 generation (pure-breeding parents) are allowed to interbreed

  
• ‘Show the results of crossing a heterozygous plant with a short plant’

  • The heterozygous plant will be tall with the genotype Tt
  • The short plant is showing the recessive phenotype and so must be homozygous recessive – tt
  • The results of this cross are as follows:
  • In this cross, there is a 1:1 ratio of tall to short

  
• How to construct Punnett squares 1

  • Determine the parental genotypes
  • Select a letter that has a clearly different lower case, for example, Aa, Bb, Dd
  • Split the alleles for each parent and add them to the Punnett square around the outside
  • Fill in the middle four squares of the Punnett square to work out the possible genetic combinations in the offspring
• How to construct Punnett squares 2

  • You may be asked to comment on the ratio of different allele combinations in the offspring, calculate percentage chances of offspring showing a specific characteristic or just determine the phenotypes of the offspring
  • Completing a Punnett square allows you to predict the probability of different outcomes from monohybrid crosses
• Family Trees

  • Family tree diagrams are usually used to trace the pattern of inheritance of a specific characteristic (usually a disease) through generations of a family
  • This can be used to work out the probability that someone in the family will inherit the genetic disorder
• • Males are indicated by the square shape and females are represented by circles
  • Affected individuals are red and unaffected are blue
  • Horizontal lines between males and females show that they have produced children (which are shown underneath each couple)
  • The family pedigree above shows:
  • Both males and females are affected
  • Every generation has affected individuals
  • There is one family group that has no affected parents or children
  • The other two families have one affected parent and affected children as well

  
• Exam Tip

  You should always write the dominant allele first, followed by the recessive allele.If you are asked to use your own letters to represent the alleles in a Punnett square, try to choose a letter that is obviously different as a capital than the lower case so the examiner is not left in any doubt as to which is dominant and which is recessive.For example, C and c are not very different from each other, whereas A and a are!
• • Predicting Probability
  • A Punnett square diagram shows the possible combinations of alleles that could be produced in the offspring
  • From this, the ratio of these combinations can be worked out
  • However, you can also make predictions of the offsprings’ characteristics by calculating the probabilities of the different phenotypes that could occur
• • For example, in the second genetic cross (F2 generation) that was given earlier (see above), two plants with the genotype Tt (heterozygous) were bred together
  • The possible combinations of offspring bred from these two parent plants are: TT (tall), Tt (tall), tt (short
  • The offspring genotypes showed a 3:1 ratio of tall : short
  • Using this ratio, we can calculate the probabilities of the offspring phenotypes
  • The probability of an offspring being tall is 75%
  • The probability of an offspring being short is 25%