Even without influencing each other, they stay together in their pure form. They mix or do not mix. Therefore, the law of segregation is also called the law of purity of gametes for this reason. During gamete formation, segregation of two alleles of a gene usually occurs due to segregation of homologous chromosomes during meiosis. The tetrads (where each tetrad consists of four chromatids of a homologous pair formed by synapse) separate during anaphase I, and then the sister chromatids of the homologous chromosomes separate during anaphase II. A Punnett square shows the probability that offspring with a certain genotype results from crossbreeding. It does not show real offspring. For example, the Punnett square in Figure 3 shows that there is a 25% chance that homozygous recessive offspring will result from crossing Aa x Aa. This does not mean that these parents must have 4 descendants and that they will have the ratio 1 AA:2 Aa:1:1 aa. It`s like tossing a coin: you`re expecting 50% heads, but you wouldn`t be too surprised to see 7 heads out of 10 throws. In addition, the probability of successive offspring does not change. The probability that the first offspring will have the “aa” genotype is 25% and the probability that the second offspring will have the “aa” genotype is always 25%. Again, it`s like tossing a coin: turning heads the first time doesn`t change the likelihood of having heads the next time you throw.
For generation F2, the law of segregation requires that each gamete receive either an R allele or an R allele with a Y allele or a Y allele. The law of independent sorting states that a gamete into which an r allele is sorted is also likely to contain a Y allele or a Y allele. Therefore, there are four equally probable gametes that can be formed when the heterozygous YyRr itself is crossed, as follows: YR, Yr, yR and yr. The arrangement of these gametes along the top and back of a 4 × 4 square of punnets (Figure 12.18) yields 16 equally likely genotypic combinations. From these genotypes, we conclude a phenotypic ratio of 9 round/yellow:3 round/green:3 wrinkled/yellow:1 wrinkled/green (Figure 12.18). These are the progeny ratios we would expect, assuming we crossed with a sufficiently large sample size. The F2 generation was created by the self-marketing of F1 systems. This can be represented graphically in a Punnett square.
From these results, Mendel coined several other terms and formulated his first law. First, Punnett Square is shown. A gamete is a cell involved in fertilization. The egg and sperm are the female and male gametes in humans, respectively. Human eggs contain only one type of sex chromosome, and that is the X chromosome. Human sperm contain X or Y chromosomes. This determines the gender of the successors. According to the law of segregation, a gamete receives one of two alleles for each trait, including the dominant or recessive trait. Due to the independent assortment and dominance, the 9:3:3:1 dihybrid phenotypic ratio can be summarized into two 3:1 ratios characteristic of any monohybrid cross that follows a dominant and recessive pattern. If we ignore the color of the seeds and only look at the texture of the seeds in the dihybrid cross above, we would expect three-quarters of the offspring of the F2 generation to be around and a quarter wrinkled.
If we isolated only the color of the seeds, we would assume that three-quarters of F2 offspring would be yellow and one-quarter green. Sorting alleles by texture and color are independent events, so we can apply the product rule. Therefore, the proportion of round and yellow F2 offspring (3/4) should be × (3/4) = 9/16 and the proportion of wrinkled and green offspring (1/4) × (1/4) = 1/16. These proportions are identical to those obtained with a Punnett square. Round, green and wrinkled, yellow offspring can also be calculated using the product rule, since each of these genotypes contains a dominant phenotype and a recessive phenotype. Therefore, the proportion of each is calculated as follows: (3/4) × (1/4) = 3/16. With these observations, Mendel was able to hypothesize segregation. To test this hypothesis, Mendel himself used F2 plants.
If his law were correct, he could predict what the results would be. And indeed, the results he expected have come true. Figure Possible genotypes are PpYY, PpYy, ppYY and ppYy. The first two genotypes would give purple-flowering plants and yellow peas, while the last two genotypes would give white-flowering plants with yellow peas, for a ratio of 1:1 of each phenotype. You only need a 2 × 2 punnett square (four squares in total) to perform this analysis because two of the alleles are homozygous. The principle of segregation defined that the individual has two alleles for each individual trait, and during gamete development, these alleles are separated. In other words, there is an allele in each gamete. The principle of segregation is crucial because it describes how genotypic ratios are created in haploid gametes. Although all the characteristics of Mendel`s pea behaved according to the law of independent assortment, we now know that some combinations of alleles are not inherited independently. Genes located on distinct non-homologous chromosomes are always sorted independently. However, each chromosome contains hundreds or thousands of genes that are linearly organized on chromosomes like beads on a string. Allele segregation in gametes can be influenced by binding, where genes that are physically close to each other on the same chromosome are more likely to be inherited as a pair.
However, due to the process of recombination or crossbreeding, it is possible for two genes on the same chromosome to behave independently or as if they were not connected. To understand this, we look at the biological basis of gene coupling and recombination. For a trihybrid crossover, writing the fork line method is tedious, but not as laborious as using Punnett`s square method. However, to fully demonstrate the power of the probability method, we can consider specific genetic calculations. For example, in the case of a tetrahybrid cross between heterozygous individuals for all four genes and in which the four genes are sorted independently and according to a dominant and recessive pattern, what proportion of offspring should be homozygous recessive for all four alleles? Instead of writing down all possible genotypes, we can use the probability method. We know that for each gene, the proportion of homozygous recessive offspring is 1/4. Thus, if we multiply this proportion for each of the four genes (1/4) × (1/4) × (1/4) × (1/4), we determine that 1/256 of the offspring is quadruple homozygous recessive. As previously described, Mendel proposed that genes are inherited as pairs of alleles that behave in a dominant and recessive pattern.
During meiosis, the alleles separate or separate, so each gamete is also likely to receive one of the two alleles present in the diploid individual. Mendel called this phenomenon the law of segregation, which can be detected in a single-hybrid crossing. In addition, genes carried on different chromosomes independently sort into gametes. This is Mendel`s law of independent assortment.