NON-MENDELIAN INHERITANCE

BLY 303 1

INTRODUCTION

Non-Mendelian inheritance is any pattern of inheritance in which traits do not segregate in accordance with Mendel's laws.

These laws describe the inheritance of traits linked to single genes on chromosomes in the nucleus.

Mendelian inheritance patterns involve genes that directly influence the outcome of an organism’s traits and obey Mendel’s laws.

In Mendelian inheritance, each parent contributes one of two possible alleles for a trait.

If the genotypes of both parents in a genetic cross are known, Mendel's laws can be used to determine the distribution of phenotypes expected for the population of offspring.

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INTRODUCTION cont’d

There are several situations in which the proportions of phenotypes observed in the progeny do not match the predicted values.

Most genes in eukaryotic species follow a Mendelian pattern of inheritance.

However, there are many that do not.

Non-Mendelian inheritance plays a role in several disease processes.

There are different types of Non-Mendelian inheritance which includes:

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INCOMPLETE DOMINANCE

This is a pattern of heredity in which one allele is not completely dominant over another.

Two alleles may produce an intermediate phenotype when both are present, rather than one fully determining the phenotype.

With incomplete dominance, both traits are expressed at the same time but the traits are blended together rather than occurring in discrete patches.

It thus refers to the condition in heterozygotes where the phenotype is intermediate between the two homozygotes.

Example is found in some plants (Four-o’clock plants (Mirabilis jalapa) or snapdragons (Antirrhinum majus) where the cross of red and white produces pink-flowered progeny

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INCOMPLETE DOMINANCE cont’d 5

CO-DOMINANCE

It is a pattern of heredity in which both alleles are simultaneously expressed in the heterozygote.

Co-dominance is believed to be a violation of the Law of Dominance.

When the alleles for a particular trait are co-dominant, they are both expressed equally rather than a dominant allele taking complete control over a recessive allele.

This means that when an organism has two different alleles (i.e., is a heterozygote), it will express both at the same time.

Example is found in speckled flowers that has patches of colour from both parents. A cross between a black chicken and a white chicken will result in chicken with both black and white feathers (Erminette chicken).

Codominance can also be seen in human blood types: the AB blood type is a result of both the IA allele and the IB allele being codominant.

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CO-DOMINANCE cont’d 7

POLYGENIC INHERITANCE

These are traits controlled by multiple genes (2 or more).

Some characteristics such as the height, IQ, skin pigmentation, eye colour etc. are polygenic.

Often the genes are large in quantity but small in effect.

In polygenic inheritance, traits often form a phenotypic spectrum rather than falling into clear-cut categories.

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MULTIPLES ALLELES

A gene that is controlled by more than two alleles.

Mendel studied just two alleles of his pea genes, but real populations often have multiple alleles of a given gene.

In nature, such genes often exist in several different forms and are therefore said to have multiple alleles.

An individual, of course, usually has only two copies of each gene, but many different alleles are often found within a population.

An example is ABO blood type in humans.

There are three common alleles for the gene that controls this characteristic:

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MULTIPLES ALLELES cont’d

The alleles IA and IB are dominant over i.

A person who is homozygous recessive ii has type O blood.

Homozygous dominant IAIA or heterozygous dominant IAi have type A blood, and homozygous dominant IBIB or heterozygous dominant IBi have type B blood.

IAIB people have type AB blood, because the A and B alleles are co-dominant.

Type A and type B parents can have a type AB child. Type A and type B parents can also have a child with Type O blood, if they are both heterozygous (IBi, IAi).

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MULTIPLES ALLELES cont’d

Another example is the gene for coat colour in rabbits (the C gene) which comes in four common alleles:

1. CC (black or brown fur), 2. cchcch (chinchilla coloration i.e. grayish fur), 3. chch (Himalayan (colour-point) patterning, with a white

body and dark ears, face, feet, and tail), and 4. cc (albino, with a pure white coat).

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PLEIOTROPY

Pleiotropy occurs when some genes affect many different characteristics, not just a single characteristic.

An example of this is Marfan syndrome, which results in several symptoms (unusually tall height, thin fingers and toes, lens dislocation, and heart problems).

These symptoms don’t seem directly related, but as it turns out, they can all be traced back to the mutation of a single gene.

Another example of pleiotropy occurs with sickle cell anemia.

This recessive genetic disorder occurs when there is a mutation in the gene that normally encodes the red blood cell protein called haemoglobin.

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PLEIOTROPY cont’d

People with the disorder have two alleles for sickle-cell haemoglobin so named for the sickle shape that their red blood cells take on under certain conditions such as physical exertion.

The sickle-shaped red blood cells clog small blood vessels, causing multiple phenotypic effects, including stunting of physical growth, certain bone deformities, kidney failure, and strokes.

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ENVIRONMENTAL EFFECTS

Many traits are affected by the environment as well as by genes. This may be especially true for polygenic traits.

For example, adult height might be negatively impacted by poor diet or illness during childhood.

Skin colour is another polygenic trait.

There is a wide range of skin colours in people worldwide. In addition to differences in skin colour genes, differences in exposure to ultraviolet (UV) light cause some of the variations.

Another example is the hydrangea flower. Hydrangea of the same genetic variety may vary in colour from blue to pink depending on the pH of the soil they are in.

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ENVIRONMENTAL EFFECTS cont’d

Skin on the lower part of the arm is much darker in colour than protected skin near the top due to the effects of UV radiation.

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