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Mendel's Laws of Genetics

By Levi Clancy for Student Reader on

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Fertilization (the union of two haploid gametes) results in a zygote (fertilized egg) that is diploid. If the zygote is heterozygous, then it carries two different alleles for a trait. If a zygote is homozygous, then it carries the same two alleles for a trait. Mendellian genetics studies the flow of these alleles through families, and allows geneticists to determine the likelihood that an offspring will inherit a particular trait or disease. Mendellian genetics rely on two principles:

SegregationThe Principle of Segregation states that segregation of any paired allele is independent of segregation of other alleles. This pertains to gametogenesis, when chromosome pairs (and, accordingly, alleles on those chromosomes) separate so that each gamete (a sperm or egg) receives a single chromosome (and a single copy of every allele). In other words, The Principle of Segregation states that a gamete is equally likely to receive an allele from either chromosome.
Independent AssortmentThe Principle of Independent Assortment states that the allele passed on for one trait has no effect on the allele passed on for another trait. In other words, The Principle of Independent Assortment states that the allele passed on for one trait has no impact on the probability a particular allele is passed on for another trait.

3:1 inheritance (classical dominance)

Requirements of Classical Mendelian Inheritance are
  1. Complete dominance

  2. Two alleles per trait

  3. No gene-to-gene interaction

  4. No linkage

  5. No environmental effect

AutosomalAn autosomal gene is present on any chromosome except the sex chromosomes X and Y. To determine if an allele is autosomal, use the process of elimination to first verify that it is not present on the X chromosome (X-linked) nor the Y chromosome (Y-linked).
X-LinkedAn X-linked disease allele is located on the X chromosome. Thus, a maximum of one copy can be present in a male, and a maximum of two copies can be present in a female. An X-linked recessive allele: will never be expressed in children of an affected male; and will be expressed in all sons and no daughters of an affected female. An X-linked dominant allele (very rare) is inherited very similar to an autosomal dominant allele, except that more females are affected than males. If you are looking at genetic markers, then a major tip-off of an autosomal dominant disorder is that all males carry only marker while females can carry more than one.
Y-LinkedA Y-linked allele is located on the Y chromosome. An affected male will pass on the allele (and thus the phenotype) to all his sons. A disease that is Y-linked is neither dominant nor recessive, since only one copy is ever present in an individual.

Up until this point we have only looked at traits that follow classical Mendelian inheritance. However, not every trait exhibits a classical dominant/recessive relationship. Therefore, the expended 3:1 Mendelian ratio does not always occur.

Extensions to 3:1 inheritance (non-classical dominance)

Incomplete dominanceTwo alleles are expressed together -- for example, being HAIRredHAIRblonde and winding up with orange hair.
CodominanceTwo alleles are fully expressed simultaneously -- for example, being HAIRredHAIRblonde and winding up with red and blonde streaks of hair.
EpistasisDominance between genes, not alleles -- for example, being HAIRredHAIRblonde but also having a HEADbald allele that makes you bald and is dominant to the alleles encoding your lovely hair
Multiple Alleles
Lethal alleles

Incomplete dominance

The phenotype of the heterzygote is different from the phenotype of either of the homozygote, and the heterzygote's phenotype lies somewhere in the range between the phenotypes of the homozygotes.
  • Four O'Clock flower

  • Red x White → Pink

  • Pink x Pink → 1:2:1 Red:Pink:White

  • Instead of seeing 3:1 ratio, we observe a 1:2:1 phenotypic ratio.

  • Note that the phenotypic ratio is the same as the genotypic ratio.


Codominance is the condition under which the heterozygote manifests phenotypically the features of two different alleles, rather than an intermediate phenotype between the homozygous dominant and homozygous recessive phenotypes.

Although there is 1:2:1 phenotypic ratio, as in incomplete dominance, the heterozygote does not represent an intermediate phenotype, but rather a phenotype in which the traits from the two different alleles coexist.

  • ABO blood types. IA, IB, and IO are the alleles for a gene that produces blood antigen:

  • IA and IB alleles encode for antigen A and B, respectively.

  • IO alleles does not encode for any antigen.

  • An AB individual has both A and B antigen. So allele IA and IB are codominant.

  • The L and M antigens are similar to the ABO system. The ability to produce M and N antigens is determined by alleles of single gene.

  • Homozygotes for M allele LMLM produce only M antigens.

  • Homozygotes for N allele LNLN produce only N antigens.

  • Heterozygotes LMLN produce both M and N antigens.

  • If two AB blood type individuals mate, what type of blood will the offspring have?

  • There will be a 1 :2: 1 ratio, in which 1/4 of the offspring will be type A, Y2 will be type AB, and 1/4 will be type B.

  • AB x AB ⇒ 1A : 2AB : 1B

Multiple alleles

They are often multiple forms of the same gene present in a population. As a diploid organism, only two of the alleles are expressed in the organism.
  • Many genes have more than two alleles: for example: blood groups (discussed above) have 3 (A, B, O).

  • Example: Multiple alleles determine hair color in rabbits.

  • cc genotype - Albino (white hair allover the body)

  • Ch Ch genotype - Himalayan (Black hairs on the extremities and white hair on the body)

  • Cch Cch genotype - Chinchilla (White hair with black tips over the entire body CC colored hair (wild type))

  • Cch and Ch show codominance: the genotype Cch Ch has white hair with black tips on the body but black hair at the extremities.

  • Cch and c show incomplete dominance: Crossing a chinchilla with an albino produces a rabbit with white hair with gray tips (light chinchilla). Its genotype is Cch c.

Lethal alleles

Some alleles turn out to be lethal if carried by individuals. Most of these lethal alleles are only lethal when homozygous.

Such alleles may distort the expected 3:1 ratio a 2:1 ratio may be observed because the individuals that are homozygous for the lethal allele die.

  • Example: coat color in mice.

  • Yellow coat color (Y) is dominant over brown coat color (y). When two heterozygous (Yy) mice mate, 2/3 of the offspring are yellow (Yy) and 1/3 are brown (yy). The 2:1 ratio results because the homozygous yellow (YY) mice die.

  • Yy x Yy → 1 YY (dead!) : 2 Yy (yellow) : 1 yy (brown)

  • Question: What happens if the lethal allele is sex-linked?

  • Answer: The offspring will skew toward having more females than males.

Mendellian 9:3:3:1 inheritance

CrossResulting Genotypes
AaBb x AaBb9: A_ B_
3: A_ bb
3: aa B_
1: aa bb

The 3:1 ratio deals with one gene. The 9:3:3:1 ratio deals with two genes. However, there are many extensions to this ratio. The classical 9:3:3:1 ratio is produced when we cross two double heterozygote individuals exhibiting traits with complete dominance. The table below explains patterns which result via extensions of the 9:3:3:1 ratio.

  • Epistasis

  • Complementation

  • Duplication

  • Suppression


The genotype at one locus affects the phenotypic expression of the genotype at another locus. Usually the interaction is such that the genotype at one locus "masks" the genotype of another locus by changing its phenotype.


9:3:3:1No EffectThe expression of gene A has no effect on the expression of gene B. Interaction between gene A and B will lead to the variation of this ratio. There are 9 possible variations to this ratio. The most common ones result from epistasis, duplication, complementation, and suppression, and are listed below.

Complementation implicates different genes needed for a particular trait. If two recessive mutations are alleles of the same gene, then the phenotype of an organism containing both mutations is mutant; if they are alleles of different genes, then the phenotype of an organism containing both mutations is wild type (non mutant).

The flower color in sweet peas. If you cross two mutant white flowers and end up with purple wildtype F1 progeny, complementation occurs.

Homozygous mutant ppxHomozygous mutant cc
-----C-----p------ -----c-----P------

At each pair of loci in the F1 heterozygous progeny, there is at least one dominant allele present. Therefore, all of these flowers will be purple. Would you expect to see complementation if the mutations were in the same genes?

If the mutations are on the same gene, crossing two mutations will not restore the wildtype phenotype. In the other words, of the two mutations don't complement. (Complementation is a commonly used test in genetics to examine whether the mutations are on the same gene).

12:3:1Dominant epistasis
9:4:3Recessive Epistasis

Recessive epistasis, a condition where gene A masks the effect of gene B when A is homozygous aa. For example, the production of pigment (i.e. Albinism) in a species of snake. We are looking at two genes (P and G) which affect coloration in these snakes:

P=pigment producing
p=no pigment produced

Cross two heterozygotes (green snakes): PpGg x PpGg

9 P_ G_green snakes
3 P_ ggorange snakes
3 pp G_albino
1 pp ggalbino

The phenotypic ratio observed here is 9:3:4. (9 green: 3 orange: 4 albino).

In this case, the P gene determines whether pigment will be produced (P) or not produced (p). The G gene determines if the color produced will be green or orange. If the genotype is pp, then no pigment is produced, and the allele of the color (G) allele is irrelevant.


Suppression, where One genotype suppresses the expression of a second genotype. This usually results in only two phenotypes observed.

Sometimes one gene suppresses the effect of an abnormal (mutant) allele of another gene, resulting in the normal phenotype. The wild type phenotype is therefore restored.

15:1Duplicate Genes

Duplicate genes, where only one dominant allele of either gene is needed in order to produce a given phenotype (two genes share the same phenotype).

This results in a 15: 1 ratio, where only the homozygous recessive are different from the rest of the offspring.

Question: The plant shepard's purse has 2 different fruits (heart shaped or narrow). A cross between 2 heart-shaped heterozygotes produces offspring with heart-shaped fruits and narrow fruits in the ratio of 15: 1.

How might you explain this? Using an example, discuss why duplication is an important precursor to evolution.

Answer: The observation that there are only two phenotypes might make you think that there is only 1 gene involved, which isn't the case since you should see a 3:1 ratio in a monohybrid cross. However, if that gene becomes duplicated and you cross two individuals that are heterozygous for both copies of the gene, only l out of 16 will not have a single dominant allele. The dominant phenotype appears if a dominant allele is present for either copy of the gene. This is true because the genes have identical actions.

Al a1 A2 a2 x Al al A2 a2
9 Al_ A2_
3 Al_ a2a2 15 plant with heart-shaped fruits
3 alal A2_
1 alal a2a2 1 plant with long, narrow fruits

If there is a dominant allele for either Al or A2 a heart shaped fruit is produced. If a mutation occurs in one of the copies and causes production of an enzyme slightly different from the original, its carrier has not lost the gene's function.

Moreover, if the duplicated gene mutates again, causing it to produce a novel gene product that is valuable the individual may actually be at an advantage relative to other individuals. Duplication is an important precursor to evolution because with two identical genes one is free to evolve into something better.


The following questions refer to offspring ratios after the mating of individuals heterozygous for the suppressor (gene A) and heterozygous for the gene it suppresses (gene B), e.g. AaBb x AaBb

Question 1: What ratio of offspring phenotypes would you expect if suppression occurs when "gene A" possesses a dominant allele and the target gene produces a mutant phenotype when "gene B" is homozygous recessive?

A = suppressor gene: suppresses mutant pigment when genotype is AA or Aa
B = pigment gene: produces mutant pigment when genotype is bb
9 A_B_ normal
3 A_bb normal (mutant pigment is suppressed)
3 aaB_ normal
1 aabb mutant (not suppressed)

Ratio is 15 : 1 (normal : mutant).

Question 2: What ratio of offspring phenotypes would you expect if suppression occurs when "gene A" is homozygous recessive and the target gene produces a mutant phenotype when "gene B" possesses a dominant allele?

A= suppressor gene: suppresses the mutant pigment when genotype is aa
B= pigment gene: produces a mutant pigment when genotype is BB or Bb
9 A_B_ Mutant (not suppressed)
3 A_bb normal
3 aaB_ normal (mutant pigment is suppressed)
1 aabb normal
Ratio is 7 : 9 (normal : mutant).