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High school biology
Course: High school biology > Unit 5
Lesson 1: Introduction to heredity- Introduction to heredity
- Alleles and genes
- Worked example: Punnett squares
- Mendel and his peas
- The law of segregation
- The law of independent assortment
- Probabilities in genetics
- Introduction to heredity review
- Introduction to heredity
- Punnett squares and probability
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Introduction to heredity review
Key terms
| Term | Meaning |
|---|---|
| Genetics | The study of biological inheritance |
| Trait | A specific characteristic of an individual |
| Gene | A unit of heredity that is passed from parent to offspring |
| Allele | One of different forms of a gene |
| 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 |
| Homozygous | Having two identical alleles for a particular gene |
| Heterozygous | Having two different alleles for a particular gene |
| Punnett square | Diagram that can be used to predict the genotypes and phenotypes resulting from a genetic cross |
Mendelian inheritance
Gregor Mendel's principles of heredity, observed through patterns of inheritance in pea plants, form the basis of modern genetics.
Mendel proposed that traits were specified by "heritable elements" called genes. Genes come in different versions, or alleles, with dominant alleles being expressed over recessive alleles. Recessive alleles are only expressed when no dominant allele is present.
In most sexually reproducing organisms, each individual has two alleles for each gene (one from each parent). This pair of alleles is called a genotype and determines the organism's appearance, or phenotype.
Mendel's laws
When an organism makes gametes, each gamete receives just one gene copy, which is selected randomly. This is known as the law of segregation.
Mendel's second law is the law of independent assortment, which states that the alleles for one gene sort into gametes independently of the alleles of another gene.
Punnett squares and probability
A Punnett square can be used to predict genotype and phenotypes of offspring from genetic crosses. A single-gene, or monohybrid cross is pictured below.
A test cross can be used to determine whether an organism with a dominant phenotype is homozygous or heterozygous.
Punnett squares can be used for a two-gene crosses, or dihybrid crosses by following the same basic rules as for a monohybrid cross. However, since there are now more gamete types, there must also be more squares in the table.
Probabilities in genetics
The two probability rules that are most relevant to Punnett squares are the product rule and the sum rule.
The product rule states that the probability of two (or more) independent events occurring together can be calculated by multiplying the individual probabilities of the events.
In some genetics problems, you may need to calculate the probability that any one of several events will occur. In this case, you’ll need to apply another rule of probability, the sum rule. According to the sum rule, the probability that any of several mutually exclusive events will occur is equal to the sum of the events’ individual probabilities.
Common mistakes and misconceptions
- Dominant traits are not always the most common. Some people may think that dominant trait is the most likely to be found in the population, but the term "dominant" only refers to the fact that the allele is expressed over another allele. An example of this is Huntington's disease. Even though Huntington's is caused by a dominant allele, it only affects about 30,000 people in the United Statesstart superscript, 1, end superscript.
- Traits are not always the product of a single gene. For example, there are at least 3 different genes that are associated with eye color in humans. In addition, there are sometimes more than two alleles for each gene. For example, there are 3 different alleles of one gene determine coat color of cats.
Want to join the conversation?
How does the dominant allele mask the recessive one at a molecular level? Does the recessive allele not code for proteins? (If, so how does the cell know which genes to code for and which not to code for?)(11 votes)
There isn't a DNA sequence that tells us why or how the dominant allele masks the recessive allele. When we say that a gene is "expressed" and "not expressed" we really mean that it is not seen in the organism's phenotype. Recessive alleles apparently are expressed. However, the proteins which result from the expression would technically "not work," meaning that it would not have a direct effect on the organism's phenotype. For example, let's say that there is a allele A that produces some sort of red pigment in, well, I don't know, alligators. The recessive allele to the allele A would be a, which, let's say, produces green pigment. In fact, both of these alleles get expressed. However, a heterozygous alligator with the genotype Aa would be observed to have "red" scales, because the allele A was expressed. The proteins from the recessive allele a did not function as effectively, and thus did not produce green pigment in the alligator's scales.
However, if the alligator had both recessive alleles for green pigment, the scales would, well, be green, thus "recessive inheritance." The alleles produced proteins that are functional, which therefore contribute to the scale's color in alligators(The alligator example I used is made up; this is just to show how it works.)
In brief, the recessive allele is expressed, but it does not have effective gene products. This is seen in molecular analysis :)(17 votes)
How is Dominant cells not a most common trait but it is Dominant.(8 votes)
How does the dominant allele mask the recessive one at a molecular level? Does the recessive allele not code for proteins? (If, so how does the cell know which genes to code for and which not to code for?)(3 votes)
"dominant" only refers to the fact that the allele is expressed over another allele.(3 votes)
Can genes be altered after birth ?(2 votes)- Can two parents with dominant characters produce an offspring with a recessive character?(0 votes)
If their alleles are something like Aa and Bb, then they have a 25% chance of producing offspring with recessive genes (ab). Aa and BB or AA and Bb have two homozygous dominant genes and two heterozygous dominant genes. If it's something like AA and BB, then no.(5 votes)
- so, my dad and my mom, have brown eye and my older sister get green eyes.
my two brothers and I get brown eyes.
and my little brother gets like blue eye color.
how come my older sister get green?(2 votes)
The most probable case here would be that the trait has been passed down from older generations as a recessive trait, and that it is now appearing in your sister because a dominant allele wasn't present to mask it.(1 vote)
- Can you destroy the genetic genes in our bodies?(0 votes)
You can't make them completely disappear. But there's lots of stuff you can do to harm them (if you really want to). You can eat the destroying angel (amanita bisporigera), a type of mushroom. You can also undergo radiation poisoning. But I wouldn't recommend either of them. The effects aren't pleasant.(4 votes)
- Why does the capital letter always have to be the dominate trait?(1 vote)
It's the other way around, dominant trait is represted with caps(2 votes)
- how do you know when to use a lower case letter ?(1 vote)
An upper case letter is usually used in reference to an allele of a gene that is dominant and the lower case letter is used for an allele of a gene that is recessive. Therefore if a person is homozygous dominant he may have something like BB, a person who is homozygous recessive would have something like bb and a heterozygote would have both dominant and recessive which is Bb.(2 votes)
