Because each possibility is equally likely, genotypic ratios can be determined from a Punnett square. If the pattern of inheritance dominant or recessive is known, the phenotypic ratios can be inferred as well. For a monohybrid cross of two true-breeding parents, each parent contributes one type of allele. In this case, only one genotype is possible. All offspring are Yy and have yellow seeds. Punnett square analysis of a monohytbrid cross : In the P generation, pea plants that are true-breeding for the dominant yellow phenotype are crossed with plants with the recessive green phenotype.
This cross produces F1 heterozygotes with a yellow phenotype. Punnett square analysis can be used to predict the genotypes of the F2 generation. Therefore, the offspring can potentially have one of four allele combinations: YY, Yy, yY, or yy.
Notice that there are two ways to obtain the Yy genotype: a Y from the egg and a y from the sperm, or a y from the egg and a Y from the sperm.
Both of these possibilities must be counted. Therefore, the two possible heterozygous combinations produce offspring that are genotypically and phenotypically identical despite their dominant and recessive alleles deriving from different parents. They are grouped together. Because fertilization is a random event, we expect each combination to be equally likely and for the offspring to exhibit a ratio of YY:Yy:yy genotypes of Furthermore, because the YY and Yy offspring have yellow seeds and are phenotypically identical, applying the sum rule of probability, we expect the offspring to exhibit a phenotypic ratio of 3 yellow:1 green.
Indeed, working with large sample sizes, Mendel observed approximately this ratio in every F 2 generation resulting from crosses for individual traits. Beyond predicting the offspring of a cross between known homozygous or heterozygous parents, Mendel also developed a way to determine whether an organism that expressed a dominant trait was a heterozygote or a homozygote. Called the test cross, this technique is still used by plant and animal breeders.
In a test cross, the dominant-expressing organism is crossed with an organism that is homozygous recessive for the same characteristic. If the dominant-expressing organism is a homozygote, then all F 1 offspring will be heterozygotes expressing the dominant trait. Alternatively, if the dominant expressing organism is a heterozygote, the F 1 offspring will exhibit a ratio of heterozygotes and recessive homozygotes. Example of a test cross : A test cross can be performed to determine whether an organism expressing a dominant trait is a homozygote or a heterozygote.
The observable traits expressed by an organism are referred to as its phenotype and its underlying genetic makeup is called its genotype. The observable traits expressed by an organism are referred to as its phenotype. Mendel crossed or mated two true-breeding self-pollinating garden peas, Pisum saivum , by manually transferring pollen from the anther of a mature pea plant of one variety to the stigma of a separate mature pea plant of the second variety.
Plants used in first-generation crosses were called P 0 , or parental generation one, plants. Mendel collected the seeds belonging to the P 0 plants that resulted from each cross and grew them the following season.
Once Mendel examined the characteristics in the F 1 generation of plants, he allowed them to self-fertilize naturally. He then collected and grew the seeds from the F 1 plants to produce the F 2 , or second filial, generation. Mendelian crosses : In one of his experiments on inheritance patterns, Mendel crossed plants that were true-breeding for violet flower color with plants true-breeding for white flower color the P generation.
The resulting hybrids in the F 1 generation all had violet flowers. In the F 2 generation, approximately three-quarters of the plants had violet flowers, and one-quarter had white flowers.
When true-breeding plants in which one parent had white flowers and one had violet flowers were cross-fertilized, all of the F1 hybrid offspring had violet flowers. That is, the hybrid offspring were phenotypically identical to the true-breeding parent with violet flowers. However, we know that the allele donated by the parent with white flowers was not simply lost because it reappeared in some of the F2 offspring. Therefore, the F1 plants must have been genotypically different from the parent with violet flowers.
In his publication, Mendel reported the results of his crosses involving seven different phenotypes, each with two contrasting traits. A trait is defined as a variation in the physical appearance of a heritable characteristic.
The characteristics included plant height, seed texture, seed color, flower color, pea pod size, pea pod color, and flower position. To fully examine each characteristic, Mendel generated large numbers of F 1 and F 2 plants, reporting results from 19, F 2 plants alone. His findings were consistent. First, Mendel confirmed that he had plants that bred true for white or violet flower color. Regardless of how many generations Mendel examined, all self-crossed offspring of parents with white flowers had white flowers, and all self-crossed offspring of parents with violet flowers had violet flowers.
In addition, Mendel confirmed that, other than flower color, the pea plants were physically identical. The garden pea has several advantageous characteristics that allowed Mendel to develop the laws of modern genetics. Pea plant reproduction is easily manipulated; large quantities of garden peas could be cultivated simultaneously, allowing Mendel to conclude that his results did not occur simply by chance.
Some genetic conditions are caused by variants also known as mutations in a single gene. These conditions are usually inherited in one of several patterns, depending on the gene involved:. One altered copy of the gene in each cell is sufficient for a person to be affected by an autosomal dominant disorder. In some cases, an affected person inherits the condition from an affected parent. In others, the condition may result from a new variant in the gene and occur in people with no history of the disorder in their family.
Huntington disease , Marfan syndrome. In autosomal recessive inheritance , variants occur in both copies of the gene in each cell. The parents of an individual with an autosomal recessive condition each carry one copy of the altered gene, but they typically do not show signs and symptoms of the condition. Autosomal recessive disorders are typically not seen in every generation of an affected family. X-linked dominant disorders are caused by variants in genes on the X chromosome.
In males who have only one X chromosome , a variant in the only copy of the gene in each cell causes the disorder. In females who have two X chromosomes , a variant in one of the two copies of the gene in each cell is sufficient to cause the disorder. Females may experience less severe symptoms of the disorder than males.
A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons no male-to-male transmission. X-linked recessive disorders are also caused by variants in genes on the X chromosome. In males who have only one X chromosome , one altered copy of the gene in each cell is sufficient to cause the condition.
In females who have two X chromosomes , a variant would have to occur in both copies of the gene to cause the disorder. Because it is unlikely that females will have two altered copies of this gene, males are affected by X-linked recessive disorders much more frequently than females. Because the inheritance pattern of many X-linked disorders is not clearly dominant or recessive, some experts suggest that conditions be considered X-linked rather than X-linked dominant or X-linked recessive.
X-linked disorders are caused by variants in genes on the X chromosome , one of the two sex chromosomes in each cell. In males who have only one X chromosome , an alteration in the only copy of the gene in each cell is sufficient to cause the condition. In females who have two X chromosomes , one altered copy of the gene usually leads to less severe health problems than those in affected males, or it may cause no signs or symptoms at all.
A condition is considered Y-linked if the altered gene that causes the disorder is located on the Y chromosome , one of the two sex chromosomes in each of a male's cells. Because only males have a Y chromosome, in Y-linked inheritance, a variant can only be passed from father to son.
Y chromosome infertility , some cases of Swyer syndrome. In codominant inheritance , two different versions alleles of a gene are expressed, and each version makes a slightly different protein. Both alleles influence the genetic trait or determine the characteristics of the genetic condition.
ABO blood group, alpha-1 antitrypsin deficiency. Mitochondrial inheritance , also known as maternal inheritance, applies to genes in mitochondrial DNA. Mitochondria, which are structures in each cell that convert molecules into energy, each contain a small amount of DNA.
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