INHERITANCE
3.7 GENETICS, POPULATIONS, EVOLUTION & ECOSYSTEMS
Inheritance MISSESTRUCH 2021
Within this topic, there are many technical terms that you could be asked to define, typically for 1
mark. Below is a table of these key terms, the ones which are new at A-level, compared to GCSE, are
in bold.
Genetic crosses
In addition to defining key terms for inheritance, you could also be asked to predict or interpret
results from genetic diagrams for each type of inheritance. I will go through an example for each,
but first, you need to know the coding systems to use to represent an allele or gene for each type
of inheritance.
1 MISSESTRUCH 2021
,INHERITANCE
3.7 GENETICS, POPULATIONS, EVOLUTION & ECOSYSTEMS
Monohybrid inheritance MISSESTRUCH 2021
This type of inheritance only involves one gene. A common example for this is cystic fibrosis, which is
caused by a recessive allele of one gene. Below is an example of a common exam question.
QUESTION:
Cystic fibrosis is caused by a recessive allele. If two carriers reproduce, what is the probability they
will have a child with cystic fibrosis? What is the probability they will have a girl with
cystic fibrosis? (F = dominant allele, f= recessive allele)
In all genetic crosses you must
include the following
information to gain full marks:
1. The parents' genotypes
2. The gametes
3. The offspring genotype
4. The offspring
phenotypes
5. The probability of the
offspring having the
condition/feature
Codominance example
QUESTION
Cows can be red, white or roan in colour. Red and white are both
dominant. If two roan cows reproduce, what is the probability they
will produce red offspring?
2 MISSESTRUCH 2021
, INHERITANCE
3.7 GENETICS, POPULATIONS, EVOLUTION & ECOSYSTEMS
Multiple alleles example MISSESTRUCH 2021
Human blood groups is a common example to asses multiple alleles and codominance at the same
time. You do not need to remember the information on blood groups, as this would be provided for
you. However, you are expected to determine the genotypes or phenotypes from genetic crosses.
QUESTION
Parents with blood group AB and blood group O reproduce. I^A and I^B are codominant and I^O is
recessive. What is the probability that they will have an offspring with blood group A?
Sex-linkage
Question
Colour blindness is caused by a recessive allele found only on the X chromosome. If a non-colour
blind male reproduces with a female carrier of the allele, what is the probability their children will
be colour-blind?
3 MISSESTRUCH 2021
3.7 GENETICS, POPULATIONS, EVOLUTION & ECOSYSTEMS
Inheritance MISSESTRUCH 2021
Within this topic, there are many technical terms that you could be asked to define, typically for 1
mark. Below is a table of these key terms, the ones which are new at A-level, compared to GCSE, are
in bold.
Genetic crosses
In addition to defining key terms for inheritance, you could also be asked to predict or interpret
results from genetic diagrams for each type of inheritance. I will go through an example for each,
but first, you need to know the coding systems to use to represent an allele or gene for each type
of inheritance.
1 MISSESTRUCH 2021
,INHERITANCE
3.7 GENETICS, POPULATIONS, EVOLUTION & ECOSYSTEMS
Monohybrid inheritance MISSESTRUCH 2021
This type of inheritance only involves one gene. A common example for this is cystic fibrosis, which is
caused by a recessive allele of one gene. Below is an example of a common exam question.
QUESTION:
Cystic fibrosis is caused by a recessive allele. If two carriers reproduce, what is the probability they
will have a child with cystic fibrosis? What is the probability they will have a girl with
cystic fibrosis? (F = dominant allele, f= recessive allele)
In all genetic crosses you must
include the following
information to gain full marks:
1. The parents' genotypes
2. The gametes
3. The offspring genotype
4. The offspring
phenotypes
5. The probability of the
offspring having the
condition/feature
Codominance example
QUESTION
Cows can be red, white or roan in colour. Red and white are both
dominant. If two roan cows reproduce, what is the probability they
will produce red offspring?
2 MISSESTRUCH 2021
, INHERITANCE
3.7 GENETICS, POPULATIONS, EVOLUTION & ECOSYSTEMS
Multiple alleles example MISSESTRUCH 2021
Human blood groups is a common example to asses multiple alleles and codominance at the same
time. You do not need to remember the information on blood groups, as this would be provided for
you. However, you are expected to determine the genotypes or phenotypes from genetic crosses.
QUESTION
Parents with blood group AB and blood group O reproduce. I^A and I^B are codominant and I^O is
recessive. What is the probability that they will have an offspring with blood group A?
Sex-linkage
Question
Colour blindness is caused by a recessive allele found only on the X chromosome. If a non-colour
blind male reproduces with a female carrier of the allele, what is the probability their children will
be colour-blind?
3 MISSESTRUCH 2021
