Mendelian Genetics

Note by , created about 6 years ago

1 Biology (AS91157 Genetic Variation & Change) Note on Mendelian Genetics, created by mckenziedev on 09/10/2013.

Created by mckenziedev about 6 years ago
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When one dominant allele is present in the genotype for example Bb, B is always expressed and shown in the organisms phenotype. A recessive allele can only be expressed and shown in the phenotype when the dominant allele is absent in the genotype for example bb.

EXAMPLEFur colour in rabbitsHomozygous black rabbit crosses with a homozygous white one. The black allele is dominant and represented by B, the white allele is recessive and represented by b.All potential offspring have a heterozygous genotype and due to black being dominant will show a black phenotype.


Codominance is when the effect of both alleles can been seen in the phenotype of a heterozygote. Both alleles are INDEPENDENTLY and FULLY EXPRESSED so therefore both phenotypes can be seen in the heterozygous individual.

EXAMPLECoat colour in cattleWhen a red (RR) bull is mated with a white (WW) cow, the offspring are all roan (RW) which is a red coat with spots of white hairs. Red and white hairs are both present. if two heterozygotes (RWxRW) are crossed, the phenotype ratio will be 1 red:2 roan:1 white.


Incomplete dominance is a single gene inheritance in which NEITHER ALLELE in a HETEROZYGOUS organism is DOMINANT. The individual is visibly affected by both alleles and their phenotype is INTERMEDIATE between the two homozygous phenotypes (one has diluted the colour of the other).

EXAMPLE Snapdragon flower colours Purebreeding individuals can be either red (CrCr) or white (CwCw). Heterozygote individuals (CrCw) are pink in colour. If a plant with red flowers is crossed with a white flowered plant, the resulting offspring would have PINK flowers. If two heterozygotes (CrCw) are crossed, the phenotype ratio will be 1 red:2 pink:1 white.



These are alleles which cause the production of non-functional proteins and may affect the organism's chance of survival.--------> Dominant lethal alleles with true dominant lethal alleles only the homozygous recessive would survive e.g. in humans this is seen in the late onset condition of Huntington's disease. The allele encodes a mutant protein which destroys the nerve tissue. Both HH and Hh die prematurely. Many miscarriages are likely to be a result of a dominant lethal mutation.

EXAMPLEManx CatsMLML (homozygous dominant) do not survive. MLMI (heterozygote cats) are Manx cats - they have no tail, while the homozygous recessive individuals are normal. Where two heterozygote manx cats are mated, the offspring will again present with a 2:1 ratio (because 1/4 of the offspring - MM - will die in the womb, at birth or shortly after.


Multiple alleles occur when more than 2 different alleles can occupy the same gene locus on a chromosome. Only 2 alleles of the gene can occur in any one individual, but in the population there can be more than two different forms of the allele.

EXAMPLEThe ABO blood group system in humansThe 3 possible alleles are Ia, Ib and i. Ia and Ib are co-dominant to each other and i is recessive to both of these. These three different alleles are responsible for creating 4 different blood types:-

NOTE: The expression of this gene is the presence or absence of antigens on surface of the red blood cells. Antigens cause the body to produce an antibody reaction.i.e. Ia or A allele is responsible for the production of type A antigen     Ib or B allele is responsible for the production of type B antigen     i or O allele is responsible for the production of neither antigenThe Ia (A) allele & Ib (B) allele are co-dominant (equally dominant) and are both dominant to the i (O) allele. An individual can only possess two alleles at a time.

Example: Colour and shape of peasIn pea plants, one gene (R) determines the shape of the peas, and another gene (Y) determines the colour of the pea. The two genes are found on different chromosomes.

Question: Complete a punnet square to show the phenotype ratio for the resulting when two round, yellow peas from above fertilise (RrYy x RrYy)

STEP 1: Consider the gametes that can be produced by the parents

STEP 2: Complete a punnett square to show all possible outcomes

Phenotype ratio: 9 round yellow:3 round green:3 wrinkled yellow:1 wrinkled green

If we want to find the unknown genotype of an individual which has a dominant phenotype, we can cross them with a homozygous recessive individual and look at the phenotype ratio of the offspring If the recessive phenotype appears in the offspring, the individual must be heterozygous for the trait and not homozygous dominant If all the offspring show the dominant trait the parent with unknown genotype is likely to be homozygous dominant. To be more certain do further crosses to increase the sample size as the greater the number of offspring the more certain you can be




This is a diagram that shows the phenotypes of members of a family and the relationships between individuals in the family. Pedigrees may be used to work out patterns of inheritance and to calculate the probability of children being born with a particular genetic defect such as haemophilia or cystic fibrosis. 

When interpreting a pedigree diagram you will need to determine the inheritance pattern and decide whether the inheritance pattern is:-(a) dominant or recessive(b) autosomal or sex linked

Understanding Pedigree Charts Summary

Monohybrid Crosses

Dihybrid crosses

Test (Back) Cross

Pedigree Diagrams