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Course: Biology library > Unit 21
Lesson 2: More about genetics- Alleles and genes
- Fertilization terminology: gametes, zygotes, haploid, diploid
- Mendelian genetics
- Aneuploidy & chromosomal rearrangements
- Variation in a species
- Chromosomal inheritance
- Pedigrees
- Pedigree for determining probability of exhibiting sex linked recessive trait
- Pedigrees review
- Extranuclear inheritance 1
- Non-Mendelian genetics
- Gene environment interaction
- Phenotype plasticity
- Polygenic inheritance and environmental effects
- Environmental effects on phenotype
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Pedigrees review
Key terms
| Term | Meaning |
|---|---|
| Pedigree | Chart that shows the presence or absence of a trait within a family across generations |
| Genotype | The genetic makeup of an organism (ex: TT) |
| Phenotype | The physical characteristics of an organism (ex: tall) |
| Dominant allele | Allele that is phenotypically expressed over another allele |
| Recessive allele | Allele that is only expressed in absence of a dominant allele |
| Autosomal trait | Trait that is located on an autosome (non-sex chromosome) |
| Sex-linked trait | Trait that is located on one of the two sex chromosomes |
| Homozygous | Having two identical alleles for a particular gene |
| Heterozygous | Having two different alleles for a particular gene |
Pedigrees
Pedigrees are used to analyze the pattern of inheritance of a particular trait throughout a family. Pedigrees show the presence or absence of a trait as it relates to the relationship among parents, offspring, and siblings.
Reading a pedigree
Pedigrees represent family members and relationships using standardized symbols.
By analyzing a pedigree, we can determine genotypes, identify phenotypes, and predict how a trait will be passed on in the future. The information from a pedigree makes it possible to determine how certain alleles are inherited: whether they are dominant, recessive, autosomal, or sex-linked.
To start reading a pedigree:
- Determine whether the trait is dominant or recessive. If the trait is dominant, one of the parents must have the trait. Dominant traits will not skip a generation. If the trait is recessive, neither parent is required to have the trait since they can be heterozygous.
- Determine if the chart shows an autosomal or sex-linked (usually X-linked) trait. For example, in X-linked recessive traits, males are much more commonly affected than females. In autosomal traits, both males and females are equally likely to be affected (usually in equal proportions).
Example: Autosomal dominant trait
The diagram shows the inheritance of freckles in a family. The allele for freckles (F) is dominant to the allele for no freckles (f).
At the top of the pedigree is a grandmother (individual I-2) who has freckles. Two of her three children have the trait (individuals II-3 and II-5) and three of her grandchildren have the trait (individuals III-3, III-4, and III-5).
Example: X-linked recessive trait
The diagram shows the inheritance of colorblindness in a family. Colorblindness is a recessive and X-linked trait
In generation I, neither parent has the trait, but one of their children (II-3) is colorblind. Because there are unaffected parents that have affected offspring, it can be assumed that the trait is recessive. In addition, the trait appears to affect males more than females (in this case, exclusively males are affected), suggesting that the trait may be X-linked.
Common mistakes and misconceptions
- The presence of many affected individuals in a family does not always mean that the trait is dominant. The terms dominant and recessive refer to the way that a trait is expressed, not by how often it shows up in a family. In fact, although it is uncommon, a trait may be recessive but still show up in all generations of a pedigree.
- You may not always be able to determine the genotype of an individual based on a pedigree. Sometimes an individual can either be homozygous dominant or heterozygous for a trait. Often, we can use the relationships between an individual and their parents, siblings, and offspring to determine genotypes. However, not all carriers are always explicitly indicated in a pedigree, and it may not be possible to determine based on the information provided.
Want to join the conversation?
How can you tell if someone is a carrier?(33 votes)
For Autosomal dominant and X-linked dominant, there is no question of a carrier since the affected allele will be dominant.
For X-linked recessive, the carrier will always be the mother/female. Since she has 2 X-chromosomes, one can compensate for the function of the other non-functional chromosome. The father/male can never be the carrier because he has only one X-chromosome, which, if affected, will be indicated on the pedigree chart.
For Autosomal recessive cases, it is a little tough to figure out the carrier without sufficient information. We can only know that the affected individual will be homozygous recessive, but we have no way to know if the unaffected individual is a dominant homozygote or a heterozygote (carrier). The only exception to this will be when one parent is homozygous recessive (affected). Then we can say that the offspring will definitely be carriers of the condition.
Hope that is helpful!(87 votes)
- If a recessive trait shows up in all the generations, how can we figure out that the trait is recessive?(23 votes)
For autosomal recessive where there is no skipped generation, the other parent must be heterozygous for the trait, which would mean the children have a chance of having the recessive trait through inheriting the recessive chromosome from the carrier parent.
For X-linked recessive, the same is true, with one parent being a carrier. Ways to prove X-linked recessive are that if the trait is recessive, there will be mother-son and/or daughter-father links. This means that if the mother is affected, the son must be as he must inherit one of her affected X chromosomes. The same is true with daughter-father links. If the daughter is affected then the father must also be as she will inherit one of her X chromosomes from him.
Hope this helps someone.(20 votes)
Can you have a male who is a carrier of an x linked recessive trait but doesn't express it?(3 votes)
No that is not possible. Since males only get one x, it is impossible for them to be only carriers of a trait and not express them. Females, on the other hand, have two x chromosomes, which means they can be a carrier for an x-linked trait.
Hope this helps!(24 votes)
whats the difference between x-linked and sex linked?(4 votes)
Genes that are carried by either sex chromosomes are said to be sex linked.
Men usually have a combination of X and Y sex chromosome, while women have two X’s. since men inherit Y chromosomes, they are the only ones to inherit Y-linked traits.
Hope that helped 👍(12 votes)
- So if a parent is a carrier but doesn't show, it has to be female as a male would have to express it.
Am I correct?
thank you for correcting me if it is required.(5 votes)
for sex-linked genetic disorders/diseases, you are correct. This is because the alleles expressing the disorder/disease are usually only present in the x chromosome, although some (few) genes are expressed in both x and y chromosome. This is due to the fact that the y chromosome is smaller.
If you are female, you have xx chromosomes, while you have xy chromosomes if you are a male.
As mentioned earlier, genetic disorders/diseases are usually found only in the x chromosome. Hence, if the allele for the disorder/disease is recessive, to be a carrier, the person must be heterozygous.
Because females have two x chromosomes, they can either get the disease (homozygous recessive), be a carrier (heterozygous), or just not have it at all (homozygous dominant).
But for men, as they only have one x chromosome, only one chromosome can express it, so they can either have it (one recessive allele), or not have it at all (one dominant allele).(8 votes)
- how dow we know if it is autosomal?(6 votes)
Equal number of males and females affected i think(4 votes)
- how to understand whether a trait going within a pedigree chart is recessive or dominant(3 votes)
If both parents do not have the trait and the child does, it is recessive. If one parent has the trait and the child does or does not, it is dominant.(9 votes)
On a pedigree chart, how do you show that a person is a heterozygous carrier for a particular recessive allele that is not expressed in the phenotype? For example, how do you visually show that someone is an UNAFFECTED carrier for a genetic disorder?(3 votes)
I've seen carriers denoted by squares or circles that are half shaded, sort of like a full moon.
Edit: What? Not a full moon, a half moon(7 votes)
- Why is it important that we include BOTH genotypes for some individuals on the pedigree?(6 votes)
- what are twin main clues seen in this pedigree that determines that this is mostly likely a recessive(5 votes)
It skips generations, which would mean if you look at the pedigree chart, one generation wouldn't be colored in black, it would just all be white.(2 votes)
