Multiple Allele and Dihybrid Crosses

Multiple Allele and Dihybrid Crosses

Multiple Allele Crosses

e.g. 3 blood types:

  1. IA = Blood group A (dominant)
  2. IB = Blood group B (dominant)
  3. Io = Blood group O (Recessive)

Dihybrid Crosses

  • The inheritance of two characteristics which are controlled by different genes
  • Each of the two genes will have different alleles
  • Dihybrid crosses can be used to determine the likelihood of offspring inheriting certain combinations of the two characteristics from particular parents
  • A dihybrid cross for two heterozygous parents leads to a 9:3:3:1 phenotypic ration
Monohybrid Inheritance

Monohybrid Inheritance

  • The inheritance of a characteristic controlled by a single gene

Monohybrid Crosses

  • Monohybrid Crosses show the likelihood of the different alleles of that gene (And therefore the different versions of the characteristics) being inherited by offspring of certain parents

e.g. The dominant allele (H) codes for ‘tall’ whereas the recessive allele (h) codes of “short”

Phenotypic Ratios

  • The ratio of different phenotypes in the offspring
  • Genetic diagrams can be used to predict this ration in F1 and F2 offspring
  • Commonly, two heterozygous parents give a 3:1 ratio of dominant: recessive characteristic in the offspring

Monohybrid Inheritance and Codominant Allele

  • In a codominant allele both genes are expressed
  • With two heterozygous parents involving codominant alleles it will commonly give a 1:2:1 ratio in the offspring#





Inheritance of Sex-linked characteristics

  • Sex linkage refers to an allele that code for a characteristic which is located on a sex chromosome
  • As the Y chromosome is smaller than the X, fewer genes are carried on it
  • Therefore, most sex-linked genes are found on the X chromosome (x-linked genes)
  • As males only have one X chromosome they therefore often only have one allele for sex-linked genes, and so there is a higher chance of a recessive gene being expressed
  • Ultimately, males are more likely to express a recessive gene relative to females
  • Genetic disorders caused by a faulty allele located on a sex chromosome include:
    • Colour blindness
    • Haemophilia

e.g. As haemophilia is sex linked, it is therefore on the X chromosome. Therefore, females would require 2 recessive alleles for expression, whereas males would only require one.

As the Y chromosome does not have either gene it is just represented as Y

  XH Xh

If a female carrier and a male non-carrier have offspring there is a 2:1:1 ratio (female without colour blindness : male without colour blindness : male with colour blindness). Overall there is a 50% chance of a male offspring to be colour blind. 

Linkage of autosomal Genes

  • Autosome is any chromosome that is not a sex chromosome
  • Autosomal genes are genes found on the autosomes
  • Genes found on the same autosome are linked (as they stay together during independent segregation of chromosomes in meiosis 1 and the allele will be passed to the daughter cell)
  • The will not occur is crossing over splits them linked genes first
  • The closer the two genes are on the autosome (the closer they are linked due to the decreased chance of crossing over)
Hardy-Weinberg Principle

Hardy-Weinberg Principle

Species: a group of similar organisms that can reproduce to give fertile offspring

Population: A group of organisms of the same species living in a particular area at a particular time

Gene Pool: Complete range of alleles present in a population

Allele Frequency: The frequency of an allele in a population (given as a %)

Hardy-Weinberg Principle

  • Used to predict the frequencies of alleles/genotype/phenotype within a population
  • It can also be test whether or not the Hardy-Weinberg principles applies to a particular allele in a particular population
    • E.g. if frequencies do change between generations in a large population then there’s an influence of some kind
  • The principle will work providing:
    • Large Population size
    • No immigration/Emigration
    • No natural selection
    • Random mating (so all possible genotypes can bread with all others)

Allele Frequency

p + q = 1

p = Frequency of one allele (usually dominant)

q = frequency of one different allele (usually recessive)

  • the frequencies of both p and q must equal to 1 (100%)

Genotype Frequency

p2 + 2pq + q2 = 1

p2 Frequency of homozygous dominant genotype

2pq = Frequency of heterozygous genotype

q2 = Frequency of homozygous recessive genotype

  • Total frequency of all genotypes within a population = 1 (100%)
  • Phenotype can also be determine providing that the relation between phenotype and genotype is known
  • Principle can also be used if two alleles are codominant or if whether the allele is recessive and which is dominant is unknown
    • By making p represent one allele and q represent the other


  • Evolution is the change in an allele frequency over time
  • Natural selection is one explanation of how evolution functions
  • Selection Pressure refers to factors such as food, predators, diseases and competition on a species which results in the frequency of an allele to alter within a gene pool
  • Gene Pool refers to the total number of all alleles within a population at a given time.

Types of Competition:

  1. Intraspecific Competition: Competition between the same species
  2. Interspecific Competition: Competition between different species

Over Production

  • Over production is the result of a high birth rate resulting in intraspecific pressures
  • Those with the allele which is advantageous provide them the limited resources to survive
  • Therefore the allele is passed on


  • The interaction of genes that are not alleles, in particular the suppression of a gene by a different gene.
  • E.g. windows peak:
    • Allele results in a “V shaped” hair
    • Another allele expressed results in baldness
    • Therefore, if carrier of both, as the person is bald the allele for widow’s peak is unclear

Phenotypic ratios for Epistatic Genes

  • Recessive Epistatic Alleles
    • If the epistatic allele is recessive then 2 copies of it will mask the expression of the other gene
    • Homozygous parent + homozygous parent = 9:3:4 phenotypic ratio

(dominant both: dominant epistatic/recessive: recessive epistatic (in F­2))

  • Dominant Epistatic Alleles
    • If the epistatic allele is dominant then 1 copies of it will mask the expression of the other gene
    • Homozygous parent + homozygous parent = 12:3:1 phenotypic ratio

(dominant epistatic: recessive epistatic, dominant other: recessive (in F­2))



  • Where both alleles are equally dominant
  • Therefore, both alleles are expressed in the phenotype

e.g. Snapdragon plant has one allele codes for enzyme which catalyses formation of red pigment and other allele codes for enzyme that lacks this catalytic activity

If alleles showed the usual pattern of 1 dominant and 1 recessive the flower would be either red and white

But as they are co-dominant three colours of flower are found:

Homozygous (1st allele): Both alleles code forthe enzyme and so plant is

Homozygous (2nd allele): No enzyme is produced and so plant is white

Heterozygous: Their single allele for the functional enzyme produces some red and so plant is pink

CR = Red pigment

CW= White pigment

e.g. CRC + CW CW


Phenotypic Ratio: 1

   e.g. CRCW + CRCW


Phenotypic ratio: 2:1:1

 Pink: Red: white

Factors Influencing Jury Decision-Making

6.1.8 – Factors influencing jury decision-making, including characteristics of the defendant and pre-trial publicity, including studies in this area.

Twelve adults serve on the jury for a criminal case. They must follow strict rules to ensure that the case is handled fairly. The judge relies on them to make the decision.

There are a number of factors which have been shown to affect jury decision-making.



The way in which the case is portrayed in the media can affect the court process. This could lead to perceptions, which are hard to change after the fact, being formed about the defendant or other parties involved.

Steblay et al (1999) – Jurors exposed to negative pretrial publicit more likely to find defendant guilty.


Jurors are expected to understand technical legal information. This can affect the trial.
Foster Lee et al (1993) – Giving instructions to jury before, instead of after presenting them with technical information increased their ability to focus on relevant information to the trial. Instructions let them filter out irrelevent information and make sense of the evidence.
Leverett and Kovera (2003) – jururs find it difficult to know what scientific data is inaccurate.
Severance and Loftus (1982) – when the jurors are explained legal terminology, the tend to understand the ones they’ve heard before and not new ones.

During the Trial

Defendant characteristics


White jurors in mock trials demonstrate negative bias to black defendants during sentence decisions.
More black defendents are found guilty than white.
Pfeifer and Ogloff (1991) found that white university students were more likely to rate black defendants
as more guilty than white defendants.
Eberhardt et al – The more stereotypically black a defendant appears, the more likely they are to receive the death penalty.


Generally the more attractive the defendent, the more likely they are to receive a non-guilty verdict. The one exception to this is crimes in which they may have used their good looks to their advantage, such as fraud, sees more attractive defendants more likely to be found guilty.
Michelini and Snodgrass (1980) – Attractive defendants were more likely to be acquitted.
Sigall and Ostrove (1975) – Attractiveness affected length of sentence which pp’s felt was appropriate. For crimes such as fraud, she was sentenced to longer when there was a picture, whereas burglary led to shorter sentences. Not generalisable to real trials because jurors don’t choose how long the sentence is.


Dixon and Mahoney – Birmingham accent seen as more guilty. Repeated and found same results.

Expert Witness Testimony

Despite warning jurors about inaccuracy of EWT, they tend to believe it anyway.
Cutler et al (1989) – use of simple language by the expert witness led to more guilty verdicts. Language can influence jury decision-making.

Story Models

The order in which the story is told can effect jury decision-making.
Pennington and Hastie (1990)  – Easy to understand order increases guilty verdicts.



When an individual gives up their personal views due to group pressure.
Normative conformity  – To avoid rejection by the group.
Informational conformity – They don’t know and look to the group for guidance.

Asch (1951) – Most influential group size to gain conformity was 7:1.

Minority Influence

One person or a small minority can influence the majority’s opinion.

Moscovici(1976) – If they’re consistent, committed to their opinions, acting on principle (not self-gain) and not unreasonable, they can influence the majority.

Social Loafing

Reduction in individual effort in a group task. Unmotivated individuals may leave the decision to the group,

Foreperson Influence

The person who is selected to deliver the verdict to the judge may influence the group as they are seen as a leader and therefore their opinion may be more valued.

Factors Influencing Eye-Witness Testimony (EWT)

6.1.7 – Factors influencing eye-witness testimony, including consideration of reliability (including post-event information and weapon focus).

Stress (Arousal)

Yerkes-Dodson Law

Yerkes-Dodson Law


When witnessing a crime, eye-witnesses are often under a lot of stress. The Yerkes-Dodson Law (1908) states that we reach an optimal point of stress in terms of recalling information. No stress means we won’t remember well, but too much stress will have the same effect. Often when witnessing a crime, we are under too much stress which reduces recall.


(+) Valentine and Mesout (2009)
(-) Recall in articifial situations can also be accurate, despite the low arousal.
(-) There is contradicting evidence to suggest that it applies in field studies of eye-witness testimony of realistic events. (Yuille and Cutshall (1986) – the greater the arousal, the more accurate the recall)

Post-Event Information


Experiences after the event can impact the memory of the event. This is common because they often have lots of time between witnessing the event and recalling it in an interview or in a trial. For example, they may see information on the news, from other people, etc.

Reconstructive memory states that when we recall information, we mix our knowledge of what happened and our expectations of the event (Schemas built from cultural norms, for example). Therefore, we usually recall the main facts of the situation but the rest of the recall is influenced by our reconstruction.

Lots of the evidence for this comes from Loftus. It has been found that exposure to misleading post-event information leads to inaccurate recall (accuracy often falling below levels of chance (Loftus et al., 1978)).

Leading questions are a source of post-event information and can affect accuracy of recall. They can be asked at police interviews and trials. They may be unintentional or intentional, as they can be used to influence the answer and may be used by lawyers in an attempt to get a non-guilty verdict for their defendent.


(+) Loftus & Palmer (1974) – Leading questions do affect eye witness testimony.
(-) Yuille and Cutshall (1986) – Leading question do not affect eye witness testimony.

  • However, only a small sample was used, so participant variables could have had an impact.
  • Field study –> higher ecological validity.

(-) Most of the research focuses on specific facts and very little exists with open ended questions, which is a more ecologically valid way of researching this.
(-) Most of the research is lab-based and therefore lacks ecological validity. This means that it doesn’t consider stress levels whch seem to have a large impact.

Weapon Focus


If there is a weapon present during the crime, this tends to decrease the accuracy of eye-witness testimony. This could be explained in two ways:


The presence of a weapon increases the stress of the situation. This could lead to them reaching the optimum stress level for recall (Yerkes-Dodson Law). This could, however, lead to them remembering a lot about the weapon. If the weapon causes them to stress too much, it could lead them to not remember much about the situation


In most parts of the world (The notable exception being some parts of the USA), seeing a weapon is unusual. This means you will draw your attention and focus on the weapon more than the surroundings. This can lead to less accurate recall because you might not focus as much on the perpetrators’ face, for example.


(+) Loftus et al (1987) – Weapon focus occurs because it draws the witness’ attention away from other important details.
(+) Pickel (1998) – It’s the unusualness of the gun which causes the decrease in accuracy, as it makes you pay more attention to it. It’s not the threat.
(+) Pickel (2006) – With correct training, they found that it is possible to overcome weapon focus.
(-) Wagstaff et al. (2003) – The presence of a weapon had no effect on accuracy of recall.

Psychological Formulations

6.1.4 – The use of psychological formulations to understand the function of offending behaviour in the individual.

Understanding the offender


Psychological formulations are defined as a way of looking back into a person’s history of relationships, biology, social circumstances, life events and how they have interpreted all of this.
It draws upon all available psychological theories.
Any psychological treatment is based on a formulation.
Forumations are not standardised, they can vary depending on the case and who is doing them. However, the BPS have released guidelines and the HCPC approve of these guidelines.
By looking into all of the factors leading up to the event, we can better understand its cause and this can be the first step towards prevention.


(+) Especially when in diagram form, they can help reduce complex information into an easy-to-understand format. This can greatly help with the decision making of the consequence for the individual, including the danger they pose.
(+) Very useful way of explaining exactly why someone commited an offense, which can help the person understand how to prevent similar situations in the future.
(-) It can be very difficult to gain all of the relevent information about the person. It relies on them remembering lots of information and also being willing to speak about it all.
(-) A formulation can include known medical illnesses, but if the person has an unknown illness which is influencing their behaviour, it wouldn’t be included.
(-) When using psychological formulations, we run the risk of being reductionist if we focus too much on one particular aspect, such as family history. We need to make sure to be holistic and consider all factors, (+) which a psychological formulation encourages.