Mendelian inheritance facts for kids
Mendelian inheritance is a set of rules about genetic inheritance.
The basic rules of genetics were first discovered by a monk named Gregor Mendel in the 1850s, and published in 1866. For thousands of years, people had noticed how traits are inherited from parents to their children. However, Mendel's work was different because he did experiments on plants, and designed those experiments very carefully.
In his experiments, Mendel studied how traits were passed on in pea plants. He started his crosses with plants that bred true, and counted characters that were either/or in nature (either tall or short). He bred large numbers of plants, and expressed his results numerically. He used test crosses to reveal the presence and proportion of recessive characters.
Contents
Mendelian genetics
Limitations
Mendel's laws apply widely, but not to all living things. They apply to any organism which is diploid (has two paired sets of chromosomes) and which engages in sexual reproduction. They would not apply to bacteria, for example, or to asexual reproduction. They do apply to the great majority of plants and animals.
Mendel's laws
Mendel explained the results of his experiment using two scientific laws:
- 1. Factors, later called genes, normally occur in pairs in ordinary body cells, yet separate during the formation of sex cells. This happens in meiosis, the production of gametes. Of each pair of chromosomes, a gamete only gets one.
The factors (genes) determine the organism's traits, and are inherited from its parents. As the pair of chromosomes separate, each gamete only receives one of each factor. This Mendel called the Law of segregation. - 2. Alleles of different genes separate independently of one another when gametes are formed. This he called the Law of independent assortment. So Mendel thought that different traits are inherited independently of one another.
- The second law is only true if the genes are not on the same chromosome. If they are, then they are linked to each other. This was the next great discovery after Mendel: that genes were carried on chromosomes. The closer they were on the chromosomes, the less likely was crossing over between them.
Mendel's laws explained the results he got with his pea plants. Later, geneticists discovered that his laws were also true for other living things, even humans. Mendel's findings from his work on the garden pea plants helped to establish the field of genetics. His contributions were not limited to the basic rules that he discovered. Mendel's care towards controlling experiment conditions along with his attention to his numerical results set a standard for future experiments.
Consequences
- When the chromosome pairs are separated in a gamete, they are randomly segregated. A gamete might have any proportion from 100% maternally derived to 100% paternally derived chromosomes.
- In crossing-over, sections are exchanged between pairs of chromosomes during meiosis. This increases the number of genetically different individuals in a population, which is important in evolution.
- As a consequence of 1 & 2, except for identical twins, no two siblings have identical genetics.
Diagammatic examples
- The ratios in the diagrams below are statistical predictions. In a large number of crosses, the numbers of offspring with these features will approximate to the ratios given.
Figure 1: Dominant and recessive phenotypes.
(1) Parental generation. (2) F1 generation. (3) F2 generation. Dominant (red) and recessive (white) phenotype look alike in the F1 generation and show a 3:1 ratio in the F2 generation |
Figure 3: The color alleles of Mirabilis jalapa are not dominant or recessive.
(1) Parental generation. (2) F1 generation. (3) F2 generation. The "red" and "white" allele together make a "pink" phenotype, resulting in a 1:2:1 ratio of red:pink:white in the F2 generation. |
Images for kids
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Characteristics Mendel used in his experiments
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P-Generation and F1-Generation: The dominant allele for purple-red flower hides the phenotypic effect of the recessive allele for white flowers. F2-Generation: The recessive trait from the P-Generation phenotypically reappears in the individuals that are homozygous with the recessive genetic trait.
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Myosotis: Colour and distribution of colours are inherited independently.
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F1 generation: All individuals have the same genotype and same phenotype expressing the dominant trait (red). F2 generation: The phenotypes in the second generation show a 3 : 1 ratio. In the genotype 25 % are homozygous with the dominant trait, 50 % are heterozygous genetic carriers of the recessive trait, 25 % are homozygous with the recessive genetic trait and expressing the recessive character.
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In Mirabilis jalapa and Antirrhinum majus are examples for intermediate inheritance. As seen in the F1-generation, heterozygous plants have "light pink" flowers—a mix of "red" and "white". The F2-generation shows a 1:2:1 ratio of red : light pink : white
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When the parents are homozygous for two different genetic traits (llSS and LL sP sP), their children in the F1 generation are heterozygous at both loci and only show the dominant phenotypes (Ll S sP). P-Generation: Each parent possesses one dominant and one recessive trait purebred (homozygous). In this example, solid coat color is indicated by S (dominant), Piebald spotting by sP (recessive), while fur length is indicated by L (short, dominant) or l (long, recessive). All individuals are equal in genotype and phenotype. In the F2 generation all combinations of coat color and fur length occur: 9 are short haired with solid colour, 3 are short haired with spotting, 3 are long haired with solid colour and 1 is long haired with spotting. The traits are inherited independently, so that new combinations can occur. Average number ratio of phenotypes 9:3:3:1
See also
In Spanish: Leyes de Mendel para niños
