Genetics: Help needed?

Question

What is single gene inheritance?can you please differentiate between autosomal dominant inheritance and autosomal recessive inheritance with examples.?i will be glad if you give me a detailed account of this.

Answer 1

SINGLE GENE INHERITANCE is where the characteristic is determined only at one gene locus.  Despite what many of us learned in middle/high school, traits like hair color and eye color are actually controlled by multiple genes (polygenic inheritance) - that's why there's such variation in hair and eye color - not just brown and blonde, or brown and blue.  There are 6-7 genes controlling these traits.  Remember, that if the organisms is diploid (has 2 sets of chromosomes) there will still be two copies of this gene, which may or may not be alike (they could have different alleles, or be heterozygous for the same trait).  What's important here is that of all the chromosomes in a set, there is only ONE place on ONE chromosome that controls the trait.

Don't confuse this with multiple alleles, though - in multiple alleles (like blood type), there are three possible alleles (A, B, O), but only one gene.  So you can only (ever) have 2 of the three allele types.

AUTOSOMAL DOMINANT INHERITANCE - Okay, a definitions first:  autosomes are the 22 non-s*x chromosomes  So this doesn't cover s*x-linked traits where you also have to follow the gender as well as the trait to determine if/how a characteristic is expressed.  This gets back to the stuff you learn in school where a trait either is expressed or is masked.  If either of the alleles in a diploid organism is dominant, the dominant trait will be expressed, regardless of what the other allele codes for.  To fall back on the eye color with which most people are familiar, brown (B)is dominant to blue (b).  So whether you have the genotype BB or Bb, your eyes would be brown.  It's only when you have both alleles for the recessive trait (bb) that you'd have blue eyes.  So if you were tracking an autosomal dominant trait on a family tree (pedigee chart), it would show up in just about all generations (rows).

AUTOSOMAL RECESSIVE TRAIT - an example of this would be the blue eyes from above.  Since the trait is recessive, it only shows up when the person has both alleles for the trait.  If even one of the allels is for the dominant trait, the recessive trait is masked.  If the recessive gene is for a genetic disorder (say sickle-cell anemia), a person who is heterozygous is called a "carrier" - the dominant gene keeps them from having/showing the disabling effects of the disorder, but they can still pass the gene for the disorder along to their children.  Since an autosomal recessive trait needs both genes for the trait to be expressed, and is masked by the presence of a dominant gene, it often skips generations in a pedigree, or it might be shown in symbols that are 1/2 colored in to show which people are carriers of the trait.

Answer 2

Genetic conditions caused by a mutation in a single gene follow predictable patterns of inheritance within families. Single gene inheritance is also referred to as Mendelian inheritance as they follow transmission patterns he observed in his research on peas. There are four types of Mendelian inheritance patterns:

Autosomal: the gene responsible for the phenotype is located on one of the 22 pairs of autosomes (non-s*x determining chromosomes).  

X-linked: the gene that encodes for the trait is located on the X chromosome.

Dominant: conditions that are manifest in heterozygotes (individuals with just one copy of the mutant allele).

Recessive: conditions are only manifest in individuals who have two copies of the mutant allele (are homozygous).

  

Dominant conditions are expressed in individuals who have just one copy of the mutant allele. The pedigree on the right illustrates the transmission of an autosomal dominant trait. Affected males and females have an equal probability of passing on the trait to offspring. Affected individual's have one normal copy of the gene and one mutant copy of the gene, thus each offspring has a 50% chance on inheriting the mutant allele. As shown in this pedigree, approximately half of the children of affected parents inherit the condition and half do not.

Recessive conditions are clinically manifest only when an individual has two copies of the mutant allele. When just one copy of the mutant allele is present, an individual is a carrier of the mutation, but does not develop the condition. Females and males are affected equally by traits transmitted by autosomal recessive inheritance. When two carriers mate, each child has a 25% chance of being homozygous wild-type (unaffected); a 25% chance of being homozygous mutant (affected); or a 50% chance of being heterozygous (unaffected carrier).



I hope this helps. Genetics is complicated.

Answer 3

Mendelian genetics makes a very simple statement - calling one gene's allele dominant and another allele recessive - that can be difficult to relate to actual genetics and the proteins that function, or function differently, in complex metabolic paths.

Autosomal refers only to the chromosomes that are in matching pairs, while allosomes are the mismatched pair, the X & Y only.  So anything inherited on an autosomal chromosome pair should be inherited in two copies, ideally BOTH functioning copies. It is like an assembly line with two people working to make the parts.  To be fully functional two people or two copies are required. If one copy is broken there is a single gene left functioning, making this a single gene inheritance.

While genes are inherited in pairs they do not have to be exact copies hence we call them alleles meaning DNA sequence is similar but slightly different. There are many ways this difference can happen. It can alter the coding sequences or the regulatory sequences.

The alleles differences can be in the protein coding sequence. That can lead to an extreme difference in function between the two proteins encoded by the two alleles OR next to no observable difference.

Another way two alleles can differ is in the DNA sequences outside the gene that regulate it. The protein expression is altered, as in the amount of protein produced.  No protein might be made from this copy, or way too much, or even made at the wrong times. The effect of a sequence error either inside or outside of the gene coding sequence is as if a person inherited a single copy of the gene since only one of the copies has the functioning shape or amount of protein at the right time. When one allele's product malfunctions the entire job is carried out by the only gene producing a correctly functioning protein. If this gene encodes a critical protein, meaning it does a job that many other steps rely on, it can affect an entire metabolic pathway when one allele's protein malfunctions.

Dominant genes simply have more impact. In a disease state one protein function is drastically impaired or altered to such an extent that the remaining correctly functioning gene cannot do the entire job alone. When this altered gene is inherited, even in one copy, the disease manifests itself. Huntington's Chorea (HC) is an example.  The gene's protein slowly disrupts normal nerve function. The disease may not manifest until after the person has had children and passed on the disease to the next generation. A famous case is Woody Guthrie. He died from this disease but no one knew if his son Arlo Guthrie had it until Arlo passed the age it normally appears. Today it can be tested for.

http://evolution.berkeley.edu/evolibrary...

Recessive genes have less impact. When inherited as heterozygous alleles the correctly functioning copy is able to carry out the job alone, or at least most of the job. These recessive alleles only produce a disease when inherited in two copies or homozygously. Sickle cell anemia (SCA) is the most used example.

SCA has a single base error, or a point mutation, in one allele for the gene. This single error causes blood cells to sickle, meaning they cannot transport oxygen at normal levels.  If someone is heterozygous half of their red blood cells supply oxygen properly and the individual can live a fairly normal life, but probably not an athletic life. If a person inherits two copies of the SCA allele they will show the full disease state.

http://www.nlm.nih.gov/medlineplus/ency/...

Many alleles can exist for a single gene. MC1R has seven definitely known and it is thought that as many as 30 could be found eventually. This gene's protein processes our hair/skin's pigment molecule from phomelanin (red/yellow) into eumelanin (black/brown). The full conversion of the pigment into eumelanin is dominant.  MC1R's other alleles do not convert all the phomelanin. Some alleles convert most and some convert very little or none. These latter alleles leave a person with very white skin and red hair if two copies are inherited. This is one of the ways we get so many subtle differences in skin and hair color. We inherit two MC1R copies that control the ratio of phomelanin to eumelanin. Two copies that convert all the pigment means much darker skin and hair.

Another, as yet unnamed, gene controls how much or little phomelanin ever gets made. We know this gene will also have many alleles because there are so many pigment variations in Europeans. These two genes appear to be linked.

When a lot of pigment is made and fully converted it gives black hair and really pigmented skin. SOME pigment made and converted gives a brunette, if less is converted auburn is the result. BUT when little pigment is made, though all is converted, we get a blonde, if a bit less is converted the result is strawberry blonde.  

All this from two genes that Mendelian genetics calls dominant or recessive. But we now know that the presence of more pigment, converted to the final form (eumelanin), is able to color hair and skin even when only one dominant copy is inherited. This is the effect of one ALLELE masking another ALLELE – both are fully functional with no disease state, but one is “dominant” in the final phenotype.

Answer 4

see the following link

Alternative Names  

Genetics - autosomal recessive; Inheritance - autosomal recessive

Definition    Return to top

Autosomal recessive is one of several ways that a trait, disorder, or disease can be passed down through families.

An autosomal recessive disorder means two copies of an abnormal gene must be present in order for the disease or trait to develop.

Information    Return to top

Inheritance of a specific disease, condition, or trait depends on the type of chromosome affected (autosomal or s*x chromosome) whether the trait is is dominant or recessive.

A mutation in a gene on one of the first 22 non-s*x chromosomes can lead to an autosomal disorder.

Genes come in pairs. Recessive inheritance means BOTH genes in a pair must be defective to cause disease. If a person only has one defective gene in the pair, they are considered a carrier. However, they can pass the abnormal gene to their children.

CHANCES OF INHERITING A TRAIT

For example, if you are born to parents who both carry the autosomal recessive gene, you have a one in four chance of getting the genes from both parents and developing the disease. You have a 50% chance of inheriting one abnormal gene, which makes you a carrier.

In other words, if four children are born to a couple who both carry the gene (but do not have signs of disease), the STATISTICAL expectation is as follows:

    * 1 child is born with 2 normal chromosomes (normal)

    * 2 children are born with 1 normal and 1 abnormal chromosome (carriers, without disease)

    * 1 child is born with 2 abnormal chromosomes (has the disease)

Note: This does not mean that children WILL necessarily be affected.

See also:

    * Autosomal dominant

    * s*x-linked dominant

    * s*x-linked recessive

    * Genetic counseling and prenatal diagnosis

    * Heredity and disease