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Recessiveness, in genetics, the failure of one of a pair of genes (alleles) present in an individual to express itself in an observable manner because of the greater influence, or dominance, of its opposite-acting partner.
- The Editors of Encyclopaedia Britannica
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Dec 20, 2023 · What does it mean for a gene to be dominant or recessive? In genetics, a dominant gene is one that is always expressed, even if the individual only has one copy of it. A recessive gene, on the other hand, is one that is only expressed if the individual has two copies of it.
- Overview
- Key points:
- Introduction
- Mendel's model: It started with a 3:1 ratio
- Mendel's model of inheritance
- Mendel's model: The law of segregation
- The test cross
- Is that Mendel's complete model of inheritance?
- Check your understanding
Mendel's law of segregation. Genotype, phenotype, and alleles. Heterozygous/homozygous. 2 x 2 Punnett squares.
•Gregor Mendel studied inheritance of traits in pea plants. He proposed a model where pairs of "heritable elements," or genes, specified traits.
•Genes come in different versions, or alleles. A dominant allele hides a recessive allele and determines the organism's appearance.
•When an organism makes gametes, each gamete receives just one gene copy, which is selected randomly. This is known as the law of segregation.
•A Punnett square can be used to predict genotypes (allele combinations) and phenotypes (observable traits) of offspring from genetic crosses.
•A test cross can be used to determine whether an organism with a dominant phenotype is homozygous or heterozygous.
•Gregor Mendel studied inheritance of traits in pea plants. He proposed a model where pairs of "heritable elements," or genes, specified traits.
•Genes come in different versions, or alleles. A dominant allele hides a recessive allele and determines the organism's appearance.
•When an organism makes gametes, each gamete receives just one gene copy, which is selected randomly. This is known as the law of segregation.
•A Punnett square can be used to predict genotypes (allele combinations) and phenotypes (observable traits) of offspring from genetic crosses.
Today, we know that many of people's characteristics, from hair color to height to risk of diabetes, are influenced by genes. We also know that genes are the way parents pass characteristics on to their children (including things like dimples, or—in the case of me and my father—a terrible singing voice). In the last hundred years, we've come to understand that genes are actually pieces of DNA that are found on chromosomes and specify proteins.
But did we always know those things? Not by a long shot! About 150 years ago, a monk named Gregor Mendel published a paper that first proposed the existence of genes and presented a model for how they were inherited. Mendel's work was the first step on a long road, involving many hard-working scientists, that's led to our present understanding of genes and what they do.
Mendel studied the genetics of pea plants, and he traced the inheritance of a variety of characteristics, including flower color, flower position, seed color, and seed shape. To do so, he started by crossing pure-breeding parent plants with different forms of a characteristic, such as violet and white flowers. Pure-breeding just means that the plant will always make more offspring like itself, when self-fertilized over many generations.
[What is self-fertilization?]
What results did Mendel find in his crosses for flower color? In the parental, or P generation, Mendel crossed a pure-breeding violet-flowered plant to a pure-breeding white-flowered plant. When he gathered and planted the seeds produced in this cross, Mendel found that 100 percent of the plants in the next generation, or F1 generation, had violet flowers.
Conventional wisdom at that time would have predicted that the hybrid flowers should be pale violet—that is, that the parents' traits should blend in the offspring. Instead, Mendel’s results showed that the white flower trait had completely disappeared. He called the trait that was visible in the F1 generation (violet flowers) the dominant trait, and the trait that was hidden or lost (white flowers) the recessive trait.
Importantly, Mendel did not stop his experimentation there. Instead, he let the F1 plants self-fertilize. Among their offspring, called the F2 generation, he found that 705 plants had violet flowers and 224 had white flowers. This was a ratio of 3.15 violet flowers to one white flower, or approximately 3:1 .
This 3:1 ratio was no fluke. For the other six characteristics that Mendel examined, both the F1 and F2 generations behaved in the same way they did for flower color. One of the two traits would disappear completely from the F1 generation, only to reappear in the F2 generation in a ratio of roughly 3:1
Based on his results (including that magic 3:1 ratio), Mendel came up with a model for the inheritance of individual characteristics, such as flower color.
In Mendel's model, parents pass along “heritable factors," which we now call genes, that determine the traits of the offspring. Each individual has two copies of a given gene, such as the gene for seed color (Y gene) shown below. If these copies represent different versions, or alleles, of the gene, one allele—the dominant one—may hide the other allele—the recessive one. For seed color, the dominant yellow allele Y hides the recessive green allele y.
So far, so good. But this model alone doesn't explain why Mendel saw the exact patterns of inheritance he did. In particular, it doesn't account for the 3:1 ratio. For that, we need Mendel's law of segregation.
According to the law of segregation, only one of the two gene copies present in an organism is distributed to each gamete (egg or sperm cell) that it makes, and the allocation of the gene copies is random. When an egg and a sperm join in fertilization, they form a new organism, whose genotype consists of the alleles contained in the gametes. The diagram below illustrates this idea:
The four-squared box shown for the F2 generation is known as a Punnett square. To prepare a Punnett square, all possible gametes made by the parents are written along the top (for the father) and side (for the mother) of a grid. Here, since it is self-fertilization, the same plant is both mother and father.
The combinations of egg and sperm are then made in the boxes in the table, representing fertilization to make new individuals. Because each square represents an equally likely event, we can determine genotype and phenotype ratios by counting the squares.
Mendel also came up with a way to figure out whether an organism with a dominant phenotype (such as a yellow-seeded pea plant) was a heterozygote (Yy) or a homozygote (YY). This technique is called a test cross and is still used by plant and animal breeders today.
In a test cross, the organism with the dominant phenotype is crossed with an organism that is homozygous recessive (e.g., green-seeded):
If the organism with the dominant phenotype is homozygous, then all of the F1 offspring will get a dominant allele from that parent, be heterozygous, and show the dominant phenotype. If the organism with the dominant phenotype organism is instead a heterozygote, the F1 offspring will be half heterozygotes (dominant phenotype) and half recessive homozygotes (recessive phenotype).
The fact that we get a 1:1 ratio in this second case is another confirmation of Mendel’s law of segregation.
Not quite! We've seen all of Mendel's model for the inheritance of single genes. However, Mendel's complete model also addressed whether genes for different characteristics (such as flower color and seed shape) influence each other's inheritance. You can learn more about Mendel's model for the inheritance of multiple genes in the law of independent assortment article.
One thing I find pretty amazing is that Mendel was able to figure out his entire model of inheritance simply from his observations of pea plants. This wasn't because he was some kind of crazy super genius, but rather, because he was very careful, persistent, and curious, and also because he thought about his results mathematically (for instance, the 3:1 ratio). These are some of the qualities of a great scientist—ones that anyone, anywhere, can develop!
1.Imagine that you are a rabbit breeder with two purebred rabbits, a male with black fur and a female with tan fur. When you cross your rabbits, all of the F1 kits (baby rabbits) have tan fur.
Which trait is dominant, and which is recessive?
Choose 1 answer:
Choose 1 answer:
•(Choice A)
Black fur is dominant, while tan fur is recessive
The terms dominant and recessive describe the inheritance patterns of certain traits. That is, they describe how likely it is for a certain phenotype to pass from parent offspring. Sexually reproducing species, including people and other animals, have two copies of each gene.
If an allele is recessive, then the gene needs to have two copies (or be homozygous) to express the recessive phenotype. If an organism is a heterozygote, or has one copy of each allele type, then it will show the dominant phenotype.
The recessive trait will only be expressed by offspring that have two copies of this allele (Figure 12.3.1 12.3. 1 ), and these offspring will breed true when self-crossed. Since Mendel’s experiments with pea plants, other researchers have found that the law of dominance does not always hold true.
A recessive allele only determines the phenotype if there is no dominant allele present. A recessive allele is usually shown as a lowercase letter. A Punnett square is a model that represents a cross, or breeding event, between two organisms.
