CIE IGCSE Biology: 复习笔记：17.1.8 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

• 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

• 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

• 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)

• 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

• 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

• 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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