Edexcel IGCSE Biology 复习笔记 3.2.4 Predicting Genetic Inheritance

• 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

   

• 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

Pea plants

• 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

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. It shows that all offspring will be have the tall phenotype.

Crossing the offspring from the first cross

A genetic cross diagram (F2 generation). It shows a ratio of 3 tall : 1 short  for any offspring.

• 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 the second 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

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

Constructing 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 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

Calculating probabilites from Punnett squares

• 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%

   

Polygenic 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