Name: mendel's law of independent assortment
Uploaded: 2026-08-26T08:19:38.096Z
Duration: 1 min 22 s
Channel: JoVE

K12 Standards

Virginia  Standards of Learning

HIgh School

Life Science

LS.10

LS.10.c

Dihybrid Crosses

In pea plants, Gregor Mendel also performed dihybrid crosses, breeding experiments between organisms that differ in two traits, such as seed shape and seed color.For example, one plant is homozygous dominant for round and yellow seeds, while the other is homozygous recessive for wrinkled and green seeds.When crossed, all plants in the F1 generation are dihybrids, meaning they are heterozygous for both genes.The F1 plants display the dominant phenotype for both traits and produce round, yellow seeds.When these plants self-fertilize, their progeny form the F2 generation.This generation includes plants with round yellow, round green, wrinkled yellow, and wrinkled green seeds in a 9:3:3:1 ratio.The ratio of round to wrinkled seeds and yellow to green seeds is 3:1 for each trait.Because each trait retains its own 3:1 inheritance pattern, seed shape alleles are inherited independently of seed color alleles. This is known as the principle of independent assortment.

dihybrid crosses

To determine whether traits are inherited together or separately, Gregor Mendel crossed pea plants that differed in two traits. These parental plants were homozygous for both traits but displayed different phenotypes. The first generation of offspring were all dihybrids, heterozygotes exhibiting the two dominant phenotypes. When self-fertilized, the dihybrids consistently produced progeny with a 9:3:3:1 ratio of four possible phenotype combinations. This ratio suggested that inheriting one trait did not affect the likelihood of inheriting the other, establishing Mendel&#x2019;s law of independent assortment.<h4>Mendel&#x2019;s Dihybrid Crosses Demonstrate the Principle of Independent Assortment</h4>Gregor Mendel&#x2019;s monohybrid crosses, between pea plants that differed in a single trait, demonstrated that (1) organisms randomly inherit one of two copies of each gene from each parent (Mendel&#x2019;s first law, segregation), and (2) the dominant allele can mask the recessive allele&#x2019;s effects on phenotype (the principle of uniformity).To determine whether two traits were inherited separately or together, Mendel also performed crosses with pea plants that differed in two traits, such as pea color and pea shape. For these dihybrid crosses, Mendel first mated plants that were true breeding (i.e., homozygous) for different traits of the same two characteristics. For example, he crossed plants that bred true for round, yellow peas (RRYY genotype) with those that bred true for wrinkled, green peas (rryy genotype). This parental (P0) generation produced offspring (F1 generation) that were all heterozygous with dominant phenotypes. These dihybrids had RrYy genotypes and round, yellow peas.Mendel then induced self-pollination in the F1 dihybrids. Of the sixteen possible parental allele combinations, nine produce offspring with both dominant traits, yellow and round peas. Six fertilization events confer one dominant trait, with three producing yellow (dominant), wrinkled peas, and three creating green, round (dominant) peas. The one remaining possibility results in green, wrinkled peas, the two recessive phenotypes.The proportion of phenotypes that Mendel observed in F2 plants was consistently similar to this 9:3:3:1 ratio, which is expected only if each fertilization event is equally probable. Thus, observing this phenotypic ratio suggests that inheriting one of these traits (e.g., yellow or green pea color) does not influence the likelihood of inheriting one of the others (e.g., round or wrinkled peas). This finding is the crux of Mendel&#x2019;s second law, the principle (or law) of independent assortment.<h4>Linkage and Recombination Influence Trait Co-inheritance</h4>Genes on separate, non-homologous chromosomes are independently assorted into gametes during meiosis. However, genes close to one another on the same chromosome are more likely to be distributed into the same gametes; a phenomenon called linkage. Thus, inheriting one trait can be linked to the likelihood of inheriting another. Mendel never reported linkage, although not all of the traits he studied are determined by loci on different chromosomes.The alleles determining pod color and pea shape are on chromosomes 5 and 7, respectively, and are thus unlinked. For most of the other traits, the lack of linkage can be accounted for by recombination, which can cause the inheritance patterns of genes on the same chromosome to mimic independent assortment. During prophase I of meiosis, chromosome pairs line up, cross over, and swap homologous genetic segments, a process known as recombination. The closer two loci are to each other on a chromosome, the more likely they are to be on the same recombined segment and thus inherited together. Likewise, loci that are far apart are more likely to be inherited separately due to more recombination events dividing them.Returning to Mendel&#x2019;s traits, pea and flower color are determined by two chromosome 1 loci that are far apart. Similarly, the locus for flower position is far from the other chromosome 4 loci, pod shape and plant height. Due to recombination, it is unsurprising that linkage never manifested in these crosses. Loci for pod shape and plant height, however, are close enough to each other on chromosome 4 that some linkage is likely. Mendel never published the results of this particular crossing, so it is possible that he simply never carried out these experiments, making him one cross short of discovering linkage.

Overview

To determine whether traits are inherited together or separately, Gregor Mendel crossed pea plants that differed in two traits. These parental ...

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Classical and Modern Genetics

KEY TERMS AND CONCEPTS

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, meaning heterozygotes for both genes, and they showed the dominant round, yellow phenotype.

Mendel then let the F1 dihybrids self-pollinate. The F2 generation produced four phenotype combinations in a consistent 9:3:3:1 ratio. Nine offspring showed both dominant traits, three showed yellow but wrinkled peas, three showed green but round peas, and one showed both recessive traits.

This pattern suggested that one trait did not affect the chance of inheriting the other. That result became the basis of Mendel’s second law, the principle of independent assortment. It also built on his earlier monohybrid crosses, which showed segregation and the way a dominant allele can hide a recessive one.

Independent assortment fits best when genes are on separate, non-homologous chromosomes. Genes that are close together on the same chromosome can be linked, which means they are more likely to travel into the same gamete. So, inheriting one trait can sometimes raise the chance of inheriting another.

Mendel did not report linkage in his pea experiments. Some of the traits he studied were unlinked because their genes were on different chromosomes, such as pod color and pea shape on chromosomes 5 and 7. For other traits, recombination during prophase I of meiosis can make genes on the same chromosome behave as if they assort independently.

During recombination, homologous chromosomes line up, cross over, and swap segments. Genes that are far apart on a chromosome are more likely to be separated by these exchange events. Genes that are close together are more likely to stay together on the same recombined segment.

This helps explain why Mendel’s pea and flower color traits, as well as flower position, did not show obvious linkage. Their loci are far apart on chromosome 1 or chromosome 4. The pod shape and plant height loci are closer together on chromosome 4, so some linkage would be expected. Mendel never published that cross, so he may simply have missed the experiment that could have revealed linkage.

Mendel's Law of Independent Assortment

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

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monohybrid crosses

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...

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,...

law of segregation

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...

law of independent assortment

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...

test cross

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multiple allele traits

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epistasis

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...

polygenic traits

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nature and nurture

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Genes and environment both shape human height and other traits. Genes are the inherited instructions we get from our parents. Environment includes factors such as diet, stress, and social conditions. Many phenotypes, or observable traits, reflect both influences. Some also show up later in life, including cancer and other diseases. Height is a clear example of this interaction. Scientists estimate that 70-90% of height variation comes from genetic differences among people. The remaining 10-30%...

chromosomal theory of inheritance

How Genes Map to Chromosomes

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...

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x-linked traits

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sex-linked disorders

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key terms and concepts

Explore Mendel’s law of independent assortment, the 9:3:3:1 F2 ratio, and how linkage and recombination can affect trait inheritance in pea plants.

Mendel's Law of Independent Assortment | 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 Law of Independent Assortment

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