• 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

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• 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
• 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

Alleles of a gene can carry the same instructions or different instructions. You can only inherit two alleles for each gene, and they can be the same or different

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• 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
• 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?- as such,?heterozygous individuals are not pure breeding

• Monohybrid inheritance?is the inheritance of characteristics controlled by a single gene (mono = one)
• 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

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

A pure-breeding genetic cross in pea plants

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• This shows that there is a 100% chance 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’

A genetic cross diagram (F2 Generation)

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• All of the offspring of the first cross have the same genotype,?Tt?(heterozygous), so the possible combinations of offspring bred from these are:
• There is more variation in this cross, with a?3:1?ratio of tall : short, meaning each offspring has a 75% chance of being tall and a 25% chance of being 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:

A cross between a heterozygous plant with a short plant

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• In this cross, there is a?1:1 ratio of tall to short, meaning a?50%?chance of the offspring being tall and a?50%?chance of the offspring being short

How to construct Punnett squares

• 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
• You may be asked to comment on the ratio of different allele combinations in the offspring, calculate a 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

Pedigree Charts

• Family pedigree 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

A family pedigree chart

• 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
  • That 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

   

• Breeders can use a?test cross?to find out the genotype of an organism showing the dominant phenotype
• This involves crossing the unknown individual with an individual showing the recessive phenotype - if the individual is showing the recessive phenotype, then its genotype must be homozygous recessive
• By looking at the?ratio of phenotypes in the offspring, we can tell whether the unknown individual is homozygous dominant or heterozygous

‘A plant breeder has a tall plant of unknown genotype. How can they find out whether it is homozygous dominant or heterozygous?’

• The short plant is showing the recessive phenotype and so must be homozygous recessive -?tt

Determining genotypes from offspring

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• If the tall plant is homozygous dominant,?all offspring produced will be tall
• If the tall plant is heterozygous,?half the offspring will be tall and the other half will be short

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