Name: mendel’s pea plant inheritance ratios
Uploaded: 2019-03-11T09:21:16.000Z
Duration: 1 min 20 s
Channel: JoVE

K12 Standards

Virginia  Standards of Learning

HIgh School

Life Science

LS.10

LS.10.c

Monohybrid Crosses

Using the pea plant Gregor Mendel originally performed monohybrid crosses. Breeding experiments between organisms that vary by a single trait like pod color.

For example, in the original parental generation, P zero, one parent has all green pods. It&#39;s genotype is homozygous for the green pod color allele represented as upper case G&#39;s . The other parent has only yellow pods and is homozygous for the yellow pod allele indicated with lower case g&#39;s.

Upon crossing the first offspring, the filial or F one generation appear identical expressing the same phenotype of green pods. This result illustrates the Principle of Uniformity and indicates that green is the dominant trait of the two colors.

Now when plants of the F one generation are allowed to self-fertilize, their progeny, F two plants, contain some with green pods and others with yellow ones in a ratio of three to one. Which confirms that yellow is indeed the recessive trait, a hidden form carried down from the initial parents.

monohybrid crosses

<h4>Overview</h4>

In the 1850s and 1860s, Gregor Mendel investigated inheritance by performing monohybrid crosses in pea plants. He crossed two plants that were true-breeding for different traits. Based on his observations, Mendel proposed that organisms inherit two copies of each trait, one from each parent, and that dominant traits can hide recessive traits. These results formed the basis of two fundamental principles in genetics: the Principle of Uniformity and the Law of Segregation.

<h4>Monohybrid Crosses Reveal Dominant and Recessive Traits</h4>

Over eight years spanning the 1850s and 1860s, an Austrian monk named Gregor Mendel carried out seminal breeding experiments with pea plants. These experiments demonstrated the fundamental principles of inheritance, earning him the moniker &ldquo;the father of modern genetics.&rdquo; Mendel&rsquo;s experiments focused on seven pea plant characteristics, each manifesting as one of two traits that are determined by a single gene locus.

Mendel noticed that, when some of his pea plants reproduced by self-fertilization, their progeny always displayed the same trait. In other words, they were true-breeding. For example, some plants with yellow pods only produced offspring with yellow pods. When crossed with other plants that bred true for yellow pods, these plants also produced only progeny with yellow pods. Similarly, Mendel observed true-breeding pea plants that produced only offspring with green pods.

At the time, inherited traits were thought to be some mixture of parental features. Mendel instead observed discrete phenotypes, like green and yellow pods. He proposed that, rather than traits mixing in offspring, discrete factors (now known as genes) are inherited from parents and remain separate in the offspring. In cases where a trait skips a generation, Mendel proposed that the visible trait merely masks the presence of the other inherited trait. In other words, inheritance is particulate, and dominant traits hide recessive traits. To determine which trait was dominant, Mendel conducted monohybrid crosses. Monohybrid crosses combine two true-breeding organisms that differ by a single trait. All offspring of such crosses are monohybrids, or heterozygotes, and display the dominant trait.

For example, Mendel crossed pea plants that bred true for yellow pods with those that bred true for green pods to determine the dominant pod color. This parental generation (P0) produced offspring, the first filial generation (F1), that were all monohybrids with green pods. Repeatedly observing these findings established green pods as the dominant trait and demonstrated Mendel&rsquo;s principle of uniformity: heterozygotes for a single gene trait display the same phenotype.

<h4>Parental Alleles Are Randomly Distributed to Gametes</h4>

Mendel then induced self-fertilization in the F1 plants, producing the F2 generation. F2 pea plants with green pods outnumbered those with yellow pods by a ratio of 3:1. Mendel repeatedly observed this 3:1 inheritance pattern for each of the seven pea plant characteristics.

Mendel&rsquo;s law of segregation explains this recurring ratio. The law of segregation states that an organism distributes one of its two gene copies to each gamete (egg or sperm cell). Importantly, this distribution is random, such that a heterozygote (Gg) is equally likely to produce gametes with dominant (G) and recessive (g) alleles.

If a heterozygote self-fertilizes (Gg x Gg), the parental alleles can combine in four possible ways: paternal G with maternal G (GG), paternal G with maternal g (Gg), paternal g with maternal G (Gg), and paternal g with maternal g (gg). Three outcomes produce green pods (the GG and Gg genotypes) and one produces yellow pods (the gg genotype), a 3:1 ratio. Thus, if all outcomes are equally likely, self-fertilizing heterozygotes will produce three offspring with green pods for every one with yellow pods. This is remarkably close to the phenotypic ratio that Mendel observed, confirming his proposed law of segregation.

<h4>Dominant Traits Are Not Always Common</h4>

Unlike green pods, green peas are recessive, whereas yellow peas are dominant. Why, then, are the peas we regularly encounter green? In short, people prefer green peas over yellow ones. As Mendel&rsquo;s experiments demonstrate, homozygotes produce offspring with the same trait, or phenotype, when self-fertilized or crossed with other homozygotes. If farmers continue to exclude yellow peas from their crop crosses, they will continue producing only green peas. This example illustrates another important point: dominant traits are not necessarily the most common traits. Harmful dominant traits, for example, may be selected against.

Overview

In the 1850s and 1860s, Gregor Mendel investigated inheritance by performing monohybrid crosses in pea plants. He crossed two plants that were ...

Education

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Biology

Genetics

Classical and Modern Genetics

KEY TERMS AND CONCEPTS

mendel’s pea plant inheritance ratios

Gregor Mendel used pea plants to study how traits are passed from parents to offspring. His breeding experiments in the 1850s and 1860s showed that inheritance follows clear patterns. He crossed true-breeding plants that differed in one trait and found that dominant traits can hide recessive traits.

Mendel focused on seven pea plant traits, and each trait had two forms controlled by a single gene locus. He noticed that true-breeding plants always produced offspring with the same phenotype, or visible trait. For example, plants with yellow pods produced yellow-pod offspring, and plants with green pods produced green-pod offspring. At a time when many people thought traits blended together, Mendel proposed that traits are inherited as separate units, now called genes.

To test which trait was dominant, Mendel performed monohybrid crosses. A monohybrid cross is a cross between two true-breeding organisms that differ in one trait. The offspring from these crosses are monohybrids, or heterozygotes, and they show the dominant trait. When Mendel crossed pea plants true-breeding for yellow pods with plants true-breeding for green pods, the first filial generation, or F1, all had green pods. These results supported the principle of uniformity, which says that heterozygotes for a single-gene trait have the same phenotype.

Mendel then let the F1 plants self-fertilize to produce the F2 generation. In this generation, green-pod plants outnumbered yellow-pod plants by about 3:1. Mendel saw this same pattern in each of the seven pea traits. He explained the ratio with the law of segregation, which states that an organism gives one of its two gene copies to each gamete, or egg or sperm cell. This distribution is random, so a heterozygote such as Gg makes gametes with either the G or g allele.

If two heterozygotes self-fertilize, as in Gg x Gg, their alleles can combine in four ways: GG, Gg, Gg, and gg. Three of these combinations produce green pods, and one produces yellow pods. That gives the expected 3:1 phenotype ratio seen in the F2 generation. Mendel’s results therefore supported his law of segregation.

Mendel’s pea work also showed that dominant traits are not always the most common traits. In peas, green pods are recessive and yellow pods are dominant, yet people often grow and eat green peas because they prefer them. If farmers keep choosing green peas for breeding, they will keep producing green pods. This example shows that a dominant trait can still be uncommon, and even harmful dominant traits may be selected against.

Mendel’s Pea Plant Inheritance Ratios

genetic lingo

Genotype, Phenotype, and Inheritance

Genotype, phenotype, and inheritance help explain how traits and disease risk are passed from parents to offspring. In diploid organisms, including humans, most other animals, and many plants, somatic cells carry a duplicate set of chromosomes. These chromosome pairs are homologous, with similar length, centromere position, and gene locations. Diploid organisms inherit two versions of each gene, one from each parent. These versions are called alleles, and they sit at the same relative locus,...

punnett squares

Punnett Squares for Predicting Traits

Punnett squares help predict which traits offspring may inherit from two parents. They show possible genotypes, which are pairs of alleles, and can also show the chance of a phenotype, or visible trait, when the inheritance pattern is known. These grids work best for traits controlled by a single gene locus and passed on independently during meiosis. Punnett squares were developed in the early 1900s by Reginald Punnett. His work came after Gregor Mendel’s pea plant experiments revealed the...

Gregor Mendel used pea plants to study how traits are passed from parents to offspring. His breeding experiments in the 1850s and 1860s showed that inheritance follows clear patterns. He crossed true-breeding plants that differed in one trait and found that dominant traits can hide recessive traits. Mendel focused on seven pea plant traits, and each trait had two forms controlled by a single gene locus. He noticed that true-breeding plants always produced offspring with the same phenotype, or...

dihybrid crosses

Mendel's Law of Independent Assortment

Mendel’s law of independent assortment explains how two traits can be inherited separately. Gregor Mendel tested this idea with pea plants that differed in two characteristics, such as pea color and pea shape. He used true-breeding parents that were homozygous for both traits but showed different phenotypes. In one example, Mendel crossed round, yellow peas with wrinkled, green peas. The parental generation had RRYY and rryy genotypes. Their first-generation offspring were all dihybrids,...

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Mendel’s Pea Cross Ratios

Mendel’s pea plant crosses show how traits can disappear in one generation and return in the next. When he crossed plants, one parental trait sometimes seemed to vanish in the F1 generation and reappear in the F2 generation. Mendel used this pattern to explain why some traits are dominant and others are recessive. He proposed that traits are controlled by alleles, which are different versions of a gene. An organism inherits two copies of each gene, one from each parent. In his model, a...

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Inheritance Risk with Sum and Product Rules

Inheritance risk can be calculated with the sum rule and the product rule. The sum rule applies to mutually exclusive events, or events that cannot happen at the same time. The product rule applies to independent events, or events that do not affect one another. These rules describe theoretical probability, which is the chance of an event before it happens. Empirical probability is different because it is based on events that have already occurred. These probability rules are useful in...

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Mendel’s Dihybrid Cross and Trait Ratios

Mendel’s dihybrid cross shows how two traits are inherited at the same time. A dihybrid cross is a cross in which both parents are homozygous for two distinct traits, and the F1 generation is heterozygous for both traits. Mendel used pea plants with round yellow peas and wrinkled green peas as an example. One parent had genotype YYRR, and the other had genotype yyrr. In the F1 generation, every plant showed the dominant traits, so the plants were YyRr and had yellow, round peas. The F2...

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Using Test Crosses to Identify Genotypes

Test crosses are used to identify the genotype of an organism with a dominant trait. A genotype is the pair of alleles inherited for a gene, and alleles are different forms of the same gene. In diploid organisms, including humans, each gene usually has two alleles, one from each parent. An allele is recessive when its effect is hidden by another allele at the same gene location. Pea plants provide a clear example with flower color. White flowers are recessive, because one copy of the purple...

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Gene Masking in Rabbit Fur Color

Epistasis describes how one gene can mask or change the effect of another gene. In high school genetics, this helps explain why many traits do not follow a simple single-gene pattern. Instead, alleles at different locations can work together to shape the final phenotype. Mendel studied seven unrelated pea traits, but most traits involve multiple gene interactions. Epistasis often happens when one gene interferes with the expression of another gene. It can also occur when genes are part of the...

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Pleiotropy is when one gene affects more than one trait. The SOX10 gene is a clear example because it can influence pigmentation, hearing, and intestinal movement. These traits may seem unrelated, but they can all be linked to the same gene change. In humans, a deleted region in SOX10 can cause Waardenburg Syndrome Type 4, or WS4. WS4 is an autosomal recessive disorder, which means a person usually inherits two altered copies of the gene. Common signs include a white forelock,...

sex-linked disorders

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X- and Y-linked disorders come from mutations in genes on the sex chromosomes. These genes, like those on autosomes, are needed for normal body function. When a mutation causes a biological problem, the condition is called sex-linked. Y-linked disorders come from changes in the Y chromosome. They affect only males, because only males carry a Y chromosome. Mutations in this small chromosome can affect male sexual function and secondary sex characteristics. One example is Y-chromosome...

x-linked traits

X-Linked Inheritance in Families

X-linked inheritance explains how traits on the X chromosome are passed through families. In most mammalian species, females have two X sex chromosomes, while males have one X and one Y. A mutation on one X chromosome may be masked in females by a normal allele on the other X. In males, that same mutation is more likely to show because there is no second normal X to balance it. Traits caused by changes on the X chromosome are called X-linked traits. Color blindness is a well-studied example.

x-inactivation

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chromosomal theory of inheritance

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Genes map to chromosomes, and this idea explains how Mendel’s inheritance patterns work. Gregor Mendel published his pea plant breeding results in 1866. He showed that physical traits follow predictable patterns. At the time, scientists did not yet know that genes existed. Mendel used the word "factors" for the hereditary units he studied. The details behind Mendel’s laws of segregation and independent assortment were still unknown for many years. In the late 1800s, better microscopes and...

key terms and concepts

Learn how Mendel’s pea plant crosses revealed dominant and recessive traits, the 3:1 F2 ratio, and the laws of uniformity and segregation.

Mendel’s Pea Plant Inheritance Ratios | JoVE K12

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Next Generation Science Standards

High School

heredity-inheritance-and-variation-of-traits

Heredity: Inheritance and Variation of Traits

HS-LS3-1

new-york-state-science-learning-standards

New York State Science Learning Standards

Inheritance and Variation of Traits

HS-LS3-1.

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9-12

SB3.b

Florida Department of Education

SC.912.L.16

SC.912.L.16.1

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Mendel’s Pea Plant Inheritance Ratios

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