Genetic carrier Pedigree of Recessive Traits

Genetic carrier Pedigree of Recessive Traits

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A human male and female couple with normal colored ears discover that, in both of their families, their fathers (who have normal ears) each had siblings with red ears. Red ears is a rare autosomal recessive disorder. None of their grandparents had red ears. Assume that the red ears in both families were caused by mutations in the same gene, that there were no new mutations within the pedigrees, and that the mothers of the couple were not carriers of the disease gene.

A) based on the information, what is the probability that the male in the couple is a carrier of the red ear disease gene?

B) if the couple had 1 child, what would be the probability that child would have red ears?

I was discussing this problem over with a friend and we got different answers.

A)I think the answer is 25%. I multiplied 1/2 and 1/2. B)6.25%. I multiplied 1/4 and 1/4.

Let's take Question A:

Both fathers have siblings with red ears, and red ears are an autosomal recessive trait. The grandparents did not have red ears, we know then that they were carriers of the recessive allele. Each grandparent was Nr (for Normal and red alleles respectively).

The fathers have normal ears, so they could be NN (probability 0.33) or Nr (probability 0.67).

The answer then is that each father independently has a 2/3 chance (0.67 probability) of carrying the red ears gene.

The male's mother, being RR, makes the male a carrier only if the male's father is himself a carrier. To put it mathematically, he has a 1/2 chance of being a carrier with 2/3 probability that he was a carrier to begin with. (2/3)*(1/2) = 1/3.

Therefore the male can be carrier with 1/3 chance.

Now Question B:

Short answer:

Since the male and female have symmetrical pedigrees, and we just solved the chance of the male being a carrier with 1/3 chance, then the chance of them having a child with red ears is (1/3)*(1/3) = 1/9.

Long answer:

The male's and female's fathers may each be heterozygous or homozygous normal, given the information, with both mothers being homozygous normal (NN).

If Father A is NN (crossed with Mother A, also NN), there is a 0 probability of passing on the red ears allele.

If Father A is Nr, then Person A has a 0.5 probability of carrying the red ear allele.

Person A can now be NN (with 0.25 probability) or Nr (with 0.5 probability).

Likewise with Person B.

Now lets consider all 4 combinations of genotypes that Person A and B can have when they have a child. I will write a table with one Person on each side of the table, with the probability of their genotype written in brackets beside. Each intersection will represent the frequency of carrying the red ears allele.

Person B (1/3) (2/3) Person A NN Nr (1/3) NN 0/4 2/4 (2/3) Nr 2/4 2/4

Carrying the red ear gene: (A more interesting example) There are a total of 6 outcomes in which the child carries the red ears allele, but these must be weighted by the probability that each parent has the associated genotype.

Probability of carrying 'r' = [ (1/3)(1/3)(0/4) + (1/3)(2/3)(2/4) + (2/3)(1/3)(2/4) + (2/3)(2/3)(2/4) ] / [ (1/3)(1/3) + (1/3)(2/3) + (2/3)(1/3) + (2/3)(2/3) ] = 0.25

Having a child with red ears: Probability of having red ears = [ (1/3)(1/3)(0/4) + (1/3)(2/3)(0/4) + (2/3)(1/3)(0/4) + (2/3)(2/3)(1/4) ] / [ (1/3)(1/3) + (1/3)(2/3) + (2/3)(1/3) + (2/3)(2/3) ] = 1/9 or about 0.11

It's left as an exercise to the student to derive the remaining probabilites. Nr 4/9 NN 4/9 rr 1/9

3. X-linked recessive traits

How does it work?

X-linked recessive traits are carried on the X chromosome. Because male offspring receive only one copy of the X chromosome, the trait is expressed phenotypically in all men with the X-linked recessive allele. Female offspring can also express an X-linked recessive trait although only if they inherit two X-linked chromosomes (one from each parent) containing the recessive allele.

What phenotypic ratios appear in the offspring?

What does an X-linked recessive pedigree look like?

As you can see above, this trait is more likely to appear in males than females so the pedigree may contain more affected males than females

This trait is never passed from father to son, because a father carries the allele on his X chromosome, but always passes his Y chromosome on to any sons.

Here is a sample X-linked recessive pedigree:

Above are the three most common inheritance patterns that will appear in an introduction to inheritance patterns. Below is a shorter overview of two, more unusual, inheritance patterns that you may come across in your studies:

Autosomal Dominant Disorders

In autosomal dominant disorders, the normal allele is recessive and the abnormal allele is dominant. It might seem paradoxical that a rare disorder can be dominant, but remember that dominance and recessiveness are simply reflections of how alleles act and are not defined in terms of predominance in the population. An example of a rare autosomal dominant phenotype is achondroplasia, a type of dwarfism (see Figure 4-21). In this case, people with normal stature are genotypically d/d, and the dwarf phenotype in principle could be D/d or D/D. However, it is believed that in D/D individuals the two 𠇍oses” of the D allele produce such a severe effect that this genotype is lethal. If true, all achondroplastics are heterozygotes.

Figure 4-21

The human achondroplasia pheno-type, illustrated by a family of five sisters and two brothers. The pheno-type is determined by a dominant allele, which we can call D, that interferes with bone growth during development. Most members of the human population (more. )

In pedigree analysis, the main clues for identifying an autosomal dominant disorder are that the phenotype tends to appear in every generation of the pedigree and that affected fathers and mothers transmit the phenotype to both sons and daughters. Again, the representation of both sexes among the affected offspring argues against X-linked inheritance. The phenotype appears in every generation because generally the abnormal allele carried by an individual must have come from a parent in the previous generation. (Abnormal alleles can arise de novo by mutation. This is relatively rare, but must be kept in mind as a possibility.) A typical pedigree for a dominant disorder is shown in Figure 4-22. Once again, notice that Mendelian ratios are not necessarily observed in families. As with recessive disorders, individuals bearing one copy of the rare allele (A/a) are much more common than those bearing two copies (A/A), so most affected people are heterozygotes, and virtually all matings involving dominant disorders are A/a × a/a. Therefore, when the progeny of such matings are totaled, a 1:1 ratio is expected of unaffected (a/a) to affected individuals (A/a).

Figure 4-22

Pedigree of a dominant phenotype determined by a dominant allele A. In this pedigree, all the genotypes have been deduced.

Huntington’s disease is an example of an autosomal dominant disorder. The phenotype is one of neural degeneration, leading to convulsions and premature death. However, it is a late-onset disease, the symptoms generally not appearing until after the person has begun to have children. Each child of a carrier of the abnormal allele stands a 50 percent chance of inheriting the allele and the associated disease. This tragic pattern has led to a drive to find ways of identifying people who carry the abnormal allele before they experience the onset of the disease. The discovery of the molecular nature of the mutant allele, and of neutral DNA mutations that act as “markers” close to the affected allele on the chromosome, has revolutionized this sort of diagnosis.


Pedigrees of autosomal dominant disorders show affected males and females in each generation and also show affected men and women transmitting the condition to equal proportions of their sons and daughters.

In human populations there are many examples of polymorphisms (generally dimorphisms) in which the alternative phenotypes of the character are determined by alleles of a single gene, for example, the dimorphisms for chin dimple versus none, attached earlobes versus unattached, widow’s peak versus none, and so on. The interpretation of pedigrees for dimorphisms is somewhat different from those for rare disorders, because by definition the morphs in a dimorphism are common. Let’s look at a pedigree for an interesting human dimorphism. Most human populations are dimorphic for the ability to taste the chemical phenylthiocarbamide (PTC): people can either detect it as a foul, bitter taste or—to the great surprise and disbelief of tasters�nnot taste it at all. From the pedigree in Figure 4-23, we can see that two tasters sometimes produce nontaster children. This makes it clear that the allele for ability to taste is dominant and that the allele for nontasting is recessive. Notice, however, that almost all people who marry into this family carry the recessive allele either in heterozygous or in homozygous condition. Such a pedigree thus differs from those of rare recessive disorders, for which it is conventional to assume that all who marry into a family are homozygous normal. As both PTC alleles are common, it is not surprising that all but one of the family members in this pedigree married individuals with at least one copy of the recessive allele.

Figure 4-23

Pedigree for the ability to taste the chemical PTC.


In a polymorphism the contrasting morphs are often determined by alleles of a single autosomal gene.

Disease Inheritance

Next we will examine three diseases caused by deleterious recessive alleles: cystic fibrosis, phenylketonuria, and sickle-cell disease. Phenotypically normal parents must both be carriers (heterozygous) in order for the disease to be observed in their offspring. Remember, these are recessively inherited traits an individual must inherit one allele from each carrier parent to exhibit the phenotype. Each time two carriers conceive a child, there is a 25% chance that the child will exhibit the phenotype, a 50% chance that the child will be a carrier, and a 25% chance that the child will be a non-carrier.

Cystic Fibrosis

Cystic fibrosis (CF) is one of the most common genetic diseases that affects people of Caucasian ancestry. In a room of 20-30 such persons, approximately one is a carrier. The deleterious allele that causes this disease encodes a protein that is involved in chloride ion transport. As a result, individuals with homozygous alleles for this gene have extreme problems with salt balance in cells (particularly those cells that line the lungs and intestines). This salt imbalance causes the mucous coating of certain cells to become unusually thick, causing affected individuals to show an extreme buildup of mucous.

How does a salt imbalance lead to thick mucous? The answer lies in understanding osmosis. Affected individuals accumulate salt in their epithelial cells, the cells which line body cavities. As a result, the cells become hypertonic, with more dissolved solutes inside the cell than outside the cell, so water is drawn into the cell. The mucous that lies outside the cell (which normally is relatively thin and watery) becomes thickened. This viscous mucous does not clear as efficiently as normal mucous. Cystic fibrosis is pleiotropic, and a number of symptoms can result (e.g., lung infections, sterility in males).

To avoid lung infections, many treatments have been developed to either reduce sputum viscosity or help dislodge it from the lungs. These include various aerosolized drugs, as well as various mechanical methods. In the past, physical therapists would manually pound the patient’s chest to help dislodge sputum now there are special vests which use air pressure to produce the same effect. Click here to view a video from the manufacturer detailing how these vests work:

Why is this harmful allele so prevalent if it is so bad? Molecular evolutionary analyses of this allele indicate that it first appeared about 52,000 years ago (about the time when nearby eastern human populations were invading Europe to displace the Neanderthals). The prevalence of this gene in modern populations, along with its age, suggests that there probably was some selective advantage for the heterozygous state. What was this advantage? No one knows for sure. Go to the following Web site and read a short article that presents one possibility. Be prepared to answer a question on the tutorial quiz dealing with one explanation for the Heterozygote Advantage?

For Students & Teachers

For Teachers Only

Naturally occurring diversity among and between components within biological systems affects interactions with the environment.

Explain how chromosomal inheritance generates genetic variation in sexual reproduction.

Segregation, independent assortment of chromosomes, and fertilization result in genetic variation in populations.

The chromosomal basis of inheritance provides an understanding of the pattern of transmission of genes from parent to offspring.

Certain human genetic disorders can be attributed to the inheritance of a single affected or mutated allele or specific chromosomal changes, such as nondisjunction.

Recessive Inheritance

When a trait is recessive, an individual must have two copies of a recessive allele to express the trait. Recessive alleles are denoted by a lowercase letter (a versus A). Only individuals with an aa genotype will express a recessive trait therefore, offspring must receive one recessive allele from each parent to exhibit a recessive trait.

One example of a recessive inherited trait is a smooth chin, as opposed to a dominant cleft chin. Let (S) represent the dominant allele, and (s) represent the recessive allele. Only (ss) individuals will express a smooth chin. To determine the probability of inheritance of a smooth chin (or any other recessive trait), the genotypes of the parents must be considered. If one parent is heterozygous (Ss) and the other is homozygous recessive (ss), then half of their offspring will have a smooth chin. In other words, any of the offspring will have a fifty percent chance of having a smooth chin. A Punnett square can be used to determine all possible genotypic combinations in the parents.

This is a pedigree depicting recessive inheritance. Unaffected parents can produce affected offspring if both parents are carriers (heterozygous) for the trait being tracked in the pedigree. Recessive traits are typically not expressed in every generation. Lastly, males and females are equally likely to express a recessively inherited trait. In this pedigree, carriers (heterozygotes) are indicated with half-shaded circles or squares while homozygotes are completely shaded.

CLICK HERE to learn more about patterns of inheritance
CLICK HERE to learn more about dominant inheritance
CLICK HERE to learn more about X-linked inheritance

Feature: My Human Body

Are you color blind or think you might be? If you inherited this X-linked recessive disorder, a world without clear differences between certain colors seems normal to you. It's all you have ever known. That's why some people who are color blind are not even aware of it. Simple tests have been devised to determine whether a person is color blind and the degree of this visual deficit. An example of such a test is pictured below. What do you see when you look at this circle? Can you clearly perceive the number 74? If so, you probably have normal red-green color vision. If you cannot see the number, you may have red-green color blindness.

Figure (PageIndex<11>): This circle of colors containing the number 74 is part of the Ishihara color blindness test.

Being color blind may cause a number of problems. These may range from minor frustrations to outright dangers. For example:

  • If you are color blind, it may be difficult to color-coordinate clothing and furnishings. You may end up wearing color combinations that people with normal color vision think are odd or clashing.
  • Many LED indicator lights are red or green. For example, power strips and electronic devices may have indicator lights to show whether they are on (green) or off (red).
  • Test strips for pH, hard water, swimming pool chemicals, and other common tests are also often color coded. Litmus paper for testing pH, for example, turns red in the presence of an acid, but if you are color blind, you may not be able to read the test result.
  • Do you like your steak well done? If you are color blind, you may not be able to tell if the meat is still undercooked (red) or grilled just right. You also may not be able to distinguish ripe (red) from unripe (green) fruits and vegetables such as tomatoes. And some foods, such as dark green spinach, may look more like mud than food and be totally unappetizing.
  • Weather maps often are color coded. Is that rain (green) in your forecast or a wintry mix of sleet and freezing rain (pink or red)? If you can't tell the difference, you may go out on the roads when you shouldn't and put yourself in danger.
  • Being able to distinguish red from green traffic lights may be a matter of life or death. This can be very difficult for someone with red-green color blindness. That's why in some countries, people with this vision defect are not allowed to drive.

Being aware of conditions such as colorblindness is also important for anyone creating content online. Developing webpages that are legible to all users is an important skill for a variety of jobs. You can use online tools (such as the Toptal Color Blind Filter) to ensure that the content you create is usable by all of your customers.

Watch the video: How to solve pedigree probability problems (October 2022).