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Pseudo-dominance – Recessive Allele Mimicking Dominant Pattern

Pseudo-dominance – Recessive Allele Mimicking Dominant Pattern

Pseudo = false. Here it is pseudodominance that means it is false dominance.

Pseudodominance: is a condition where a recessive allele mimics the pattern of a dominant allele. 

To understand pseudodominance let’s start with the concept of dominant and recessive allele. In humans, since we are diploid organism we have two copies of every gene i.e. we have two alleles for every gene. And these alleles can be dominant or recessive. For a dominant allele to be expressed only one copy is enough. A dominant allele gets expressed even in heterozygous condition. But for a recessive allele to be expressed, there has to be two copies present i.e. it has to be in homozygous recessive condition for a recessive allele to be expressed.

But in pseudodominace, only one copy of recessive allele would get expressed. That’s why we say recessive allele mimics the dominant pattern.

Examples:

Sex Chromosome:

Females have two X chromosomes (XX) but males have only one X chromosome (XY). That means females would have two alleles of every gene present on X chromosome whereas males would have only one allele. So for a female to express any recessive allele on X chromosome it has to be in homozygous condition. But in case of male whatever allele is present on their single X chromosome that would get expressed. It can be either dominant allele or recessive allele. What I mean is, for males even one recessive allele is enough on X chromosome to express it because it doesn’t have homologous X chromosome. Therefore in case of males recessive allele on X chromosome mimics the dominant pattern and thus it is an example of pseudodominance.

Autosomal Chromosome:

Pseudodominance can be seen in autosomal chromosomes when in a homologous pair of chromosome, one allele is deleted and the other allele is recessive. So here that one recessive allele will get expressed because the other allele is deleted.

Why because of pseudodominance males are more commonly and severely affected with X-linked recessive genetic disorders?

X-linked genetic disorders such as hemophilia and color blindness are recessive genetic disorders which mean they require two copies of recessive alleles in order have the disorder. So for females there has to be homozygous recessive condition for this disorder but for males only one recessive allele would result in these disorders. Therefore, males are more susceptible than females.

I hope this helps 🙂

Watch a video on this topic here.

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Heterochromatin and Euchromatin – What is the Difference?

As the term says heterochromatin and euchromatin, both are different forms of chromatin. Chromatin is DNA wrapped around histone proteins. Heterochromatin and euchromatin are different levels of condensation of chromatin.

After mitosis when two daughter cells are separated, the highly condensed chromosome will return back to chromatin phase which has two outcomes. One is the loose interphase condition which is called euchromatin and around 10% remains condensed throughout the interphase which is called heterochromatin. So when we observe the nucleus we see the darkly stained heterochromatin near the nuclear envelope which is highly condensed and thus transcriptionally inactive. And in the center we observe lightly stained euchromatin which is transcriptionally active.

Euchromatin: ‘Eu’ = well. Euchromatin is less condensed form and thus it is available for transcription. Therefore, all the genes needed to be expressed are present in euchromatin region. It is gene rich.

Heterochromatin: ‘Hetero’ = different. Heterochromatin is highly condensed form and thus it is transcriptionally inactive. It is gene desert. As the term says hetero or different meaning there are two different forms of heterochromatin.

  • Constitutive Heterochromatin:

Constitutive = continuously present. That means it is the type of heterochromatin which is always or continuously in condensed form.

It remains in condensed state in all the cells at all the times. It is always transcriptionally inactive. Therefore, it represents the DNA that is permanently silenced.

It has lot of repeated sequences and has low genes (gene desert).

Examples: Centromere and Telomere

  • Facultative Heterochromatin:

Facultative = capable of but not restricted to a particular function or mode. That means it is the type of heterochromatin which can also exist in euchromatin phase.

It is specifically inactivated and it can revert back to active euchromatin phase.

It is silenced by histone deacetylation and RNAi.

Example: X-chromosome inactivation (Barr body) where one of the X-chromosome is specifically inactivated (heterochromatin phase). But during meiosis this inactivated X-chromosome need to go back to its active euchromatin stage otherwise half of the daughter cells would get inactivated X-chromosome.

Why heterochromatin is located near the nuclear envelope and euchromatin in the center?

Since the transcription is very low or not present near the nuclear envelope and heterochromatin is transcriptionally inactive we find heterochromatin near the nuclear envelope. Whereas the center of a nucleus has high levels of transcription taking place so in center we will find euchromatin because it is transcriptionally active form of chromatin.

I hope this helps 🙂

Watch a video on this topic here.