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

Używając grochu, Gregor Mendel pierwotnie wykonywał krzyżówki monohybrydowe. Eksperymenty hodowlane między organizmami, które różnią się jedną cechą, taką jak kolor strąków.

Na przykład w pierwotnym pokoleniu rodzicielskim, P zero, jeden rodzic ma wszystkie zielone strąki. Jego genotyp jest homozygotyczny pod względem allelu koloru zielonego strąka reprezentowanego przez wielką literę G. Drugi rodzic ma tylko żółte strąki i jest homozygotyczny pod względem allelu żółtego strąka oznaczonego małą literą g.

Po skrzyżowaniu pierwszego potomstwa, pokolenie potomne lub F one wydaje się identyczne, wyrażając ten sam fenotyp zielonych strąków. Wynik ten ilustruje zasadę jednolitości i wskazuje, że zielony jest dominującą cechą tych dwóch kolorów.

Teraz, gdy rośliny z pierwszego pokolenia F mogą się samozapładniać, ich potomstwo, rośliny F dwa, zawiera niektóre z zielonymi strąkami, a inne z żółtymi w stosunku trzy do jednego. Co potwierdza, że żółty jest rzeczywiście cechą recesywną, ukrytą formą odziedziczoną po pierwotnych rodzicach.

Griffithsa, Anthony'ego J.F., Jeffreya H. Millera, Davida T. Suzukiego, Richarda C. Lewontina i Williama M. Gelbarta. 2000. Wprowadzenie do analizy genetycznej. Nowy Jork: W.H. Freeman. [<a href="https://www.ncbi.nlm.nih.gov/books/NBK22098/" target="__blank">Źródło</a>]

Miko, Ilona. 2008. "Krzyże próbne." Edukacja przyrodnicza 1 (1): 136. [<a href="https://www.nature.com/scitable/topicpage/test-crosses-585" target="__blank">Źródło</a>]

krzyżówki monohybrydowe

<h4>Przegląd</h4>

W latach pięćdziesiątych i sześćdziesiątych XIX wieku Gregor Mendel badał dziedziczenie, przeprowadzając krzyżówki monohybrydowe u grochu. Skrzyżował dwie rośliny, które rzeczywiście rozmnażały się pod względem różnych cech. Na podstawie swoich obserwacji Mendel zaproponował, że organizmy dziedziczą dwie kopie każdej cechy, po jednej od każdego z rodziców, oraz że cechy dominujące mogą ukrywać cechy recesywne. Wyniki te stworzyły podstawę dwóch podstawowych zasad genetyki: zasady jednolitości i prawa segregacji.

<h4>Krzyżówki monohybrydowe ujawniają cechy dominujące i recesywne</h4>

Przez osiem lat, obejmujących lata pięćdziesiąte i sześćdziesiąte XIX wieku, austriacki mnich Gregor Mendel przeprowadzał eksperymenty z hodowlą nasion grochu. Eksperymenty te wykazały podstawowe zasady dziedziczenia, dzięki czemu zyskał przydomek “ojca współczesnej genetyki“. Eksperymenty Mendla skupiały się na siedmiu cechach grochu, z których każda objawiała się jedną z dwóch cech określonych przez pojedynczy locus genowy.

Mendel zauważył, że gdy niektóre jego rośliny grochu rozmnażały się poprzez samozapłodnienie, ich potomstwo zawsze wykazywało tę samą cechę. Innymi słowy, byli prawdziwymi wychowankami. Na przykład niektóre rośliny z żółtymi strąkami dawały potomstwo tylko z żółtymi strąkami. Po skrzyżowaniu z innymi roślinami, które rozmnażały się z żółtymi strąkami, rośliny te również dawały wyłącznie potomstwo z żółtymi strąkami. Podobnie Mendel zaobserwował prawdziwe rośliny grochu, które rodziły wyłącznie potomstwo z zielonymi strąkami.

W tamtym czasie uważano, że cechy dziedziczne są mieszaniną cech rodzicielskich. Zamiast tego Mendel zaobserwował odrębne fenotypy, takie jak zielone i żółte strąki. Zaproponował, że zamiast mieszania się cech u potomstwa, oddzielne czynniki (obecnie znane jako geny) są dziedziczone od rodziców i pozostają oddzielne u potomstwa. W przypadkach, gdy cecha pomija pokolenie, Mendel zaproponował, że widoczna cecha jedynie maskuje obecność innej odziedziczonej cechy. Innymi słowy, dziedziczenie jest cząstkowe, a cechy dominujące ukrywają cechy recesywne. Aby określić, która cecha jest dominująca, Mendel przeprowadził krzyżówki monohybrydowe. Krzyżówki monohybrydowe łączą dwa organizmy prawdziwie hodowlane, które różnią się jedną cechą. Całe potomstwo takich krzyżówek jest monohybrydami lub heterozygotami i wykazuje cechę dominującą.

Na przykład Mendel skrzyżował rośliny grochu, które rozmnażały się w kierunku żółtych strąków, z tymi, które wyhodowały prawdziwe w przypadku zielonych strąków, aby określić dominujący kolor strąków. To pokolenie rodzicielskie (P0) wydało na świat potomstwo, pierwsze pokolenie potomne (F1), które było monohybrydami z zielonymi strąkami. Wielokrotne obserwacje tych odkryć ustaliły, że zielone strąki są cechą dominującą i wykazały zasadę jednorodności Mendla: heterozygoty pod względem jednej cechy genu wykazują ten sam fenotyp.

<h4>Allele rodzicielskie są losowo przydzielane do gamet</h4>

Następnie Mendel wywołał samozapłodnienie w roślinach F1, tworząc pokolenie F2. Rośliny grochu F2 z zielonymi strąkami przewyższały liczebnie rośliny z żółtymi strąkami w stosunku 3:1. Mendel wielokrotnie obserwował ten wzór dziedziczenia 3:1 dla każdej z siedmiu cech grochu.

Prawo Mendla segregacji wyjaśnia ten powtarzający się stosunek. Prawo segregacji mówi, że organizm rozdziela jedną ze swoich dwóch kopii genów do każdej gamety (komórki jajowej lub plemnika). Co ważne, rozkład ten jest losowy, tak że heterozygota (Gg) z równym prawdopodobieństwem wytworzy gamety z allelami dominującymi (G) i recesywnymi (g).

Jeśli heterozygota dokona samozapłodnienia (Gg x Gg), allele rodzicielskie mogą łączyć się na cztery możliwe sposoby: G ojcowski z G matczynym (GG), ojcowski G z matczynym g (Gg), g ojcowski z G matczynym (Gg) i ojcowskie g z matczynym g (gg). Trzy wyniki dają zielone strąki (genotypy GG i Gg), a jeden wytwarza żółte strąki (genotyp gg), w stosunku 3:1. Tak więc, jeśli wszystkie wyniki są równie prawdopodobne, samozapłodniające się heterozygoty spłodzą troje potomstwa z zielonymi strąkami na każde jedno z żółtymi strąkami. Jest to niezwykle zbliżone do stosunku fenotypowego zaobserwowanego przez Mendla, co potwierdza zaproponowane przez niego prawo segregacji.

<h4>Cechy dominujące nie zawsze są powszechne</h4>

W przeciwieństwie do zielonych strąków, groszek zielony jest recesywny, podczas gdy groszek żółty jest dominujący. Dlaczego zatem groszek, który regularnie spotykamy, jest zielony? Krótko mówiąc, ludzie wolą zielony groszek od żółtego. Jak pokazują eksperymenty Mendla, homozygoty dają potomstwo o tej samej cesze lub fenotypie, gdy dochodzi do samozapłodnienia lub skrzyżowania z innymi homozygotami. Jeśli rolnicy w dalszym ciągu będą wykluczać groszek żółty ze swoich krzyżówek, będą nadal produkować wyłącznie groszek zielony. Ten przykład ilustruje kolejny ważny punkt: cechy dominujące niekoniecznie są cechami najczęściej spotykanymi. Można na przykład selekcjonować szkodliwe cechy dominujące.

Krzyżówki monohybrydowe

Przegląd

W latach pięćdziesiątych i sześćdziesiątych XIX wieku Gregor Mendel badał dziedziczenie, przeprowadzając krzyżówki monohybrydowe w roślinach grochu. Skrzyżował się z dwoma roślinami, które były ...

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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...

monohybrid crosses

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,...

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...

probability laws

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sex-linked disorders

X- and Y-Linked Disorder Patterns

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

Barr Bodies and X-Chromosome Balance

Barr bodies and X-chromosome balance help explain how female mammalian cells control gene activity from the X chromosome. The human X chromosome carries far more genes than the Y chromosome. Because males usually have one X chromosome and females have two, females would make twice as many of some proteins without a control system. To prevent that imbalance, female mammalian cells inactivate nearly all the genes on one X chromosome during early embryonic development. The inactive X condenses...

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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...

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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Mendel’s Pea Plant Inheritance Ratios

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