Mendel's second law: The law of independent assortment By combining them we obtain heterozygous genotypes. In the gamete line we can see the different alleles presented by the parents. All offspring are heterozygous and have the dominant phenotype (yellow). By crossing a homozygous dominant parent (yellow pea plant) with a recessive homozygous (green pea plant) we obtain a homogeneous first generation (F1). Crossing of pure lines allows to postulate the principle of uniformity. In Figure 4 we can see the explanation represented visually.įigure 4. Let us see an example for the color character (A > a the 'A' allele (yellow) dominates over the 'a' allele (green)). The dominant allele is represented in upper case and the recessive allele in lower case. The phenotype will be determined by the dominant allele. If two inbred lines are crossed, the offspring of the first generation will be equal to each other both at the phenotypic level (appearance) and at the genotypic level (alleles).Īlso, the offspring will be the same in appearance (phenotype) as one of the parents. Mendel's First Law: The law of equal segregation It was in 1900 when his work was rediscovered and his laws were finally valued. Unfortunately, nobody understood the value his laws and Mendel, the father of genetics, died without knowing the great contribution he had made to science in general and to genetics in particular. This is despite the fact that he made handwritten copies for all the recognized scientists in the area.

However, this recognition was obtained long after he published his work in 1858. Thanks to all the information he obtained from his experiments, Mendel postulated three important laws that help us understand genetics.

Neither offspring will be color blind, but both will be carriers of color blindness and may have color blind offspring (25% chance). Color blindness is transmitted on the X chromosome (X d ). Crossing with an X-linked recessive inheritance pattern. Let's look at the example of colour blindness (X-linked recessive inheritance, Figure 3).įigure 3. However, for men a single copy is enough to manifest the disease since they only have one X chromosome and there is no other area that 'compensates' for that defective allele). In the case of women, we will need two copies of the recessive allele for them to manifest the disease. The inheritance linked to the X chromosome will be dominant (a single copy is enough) or recessive (we need two copies of the defective allele).įor this last type of inheritance linked to the X chromosome, we will find differences according to gender. Women have two copies of the X chromosome (XX), while men have only one (XY). Alleles are inherited solely through the X chromosome. This type of inheritance is a bit more complex. In this example, none of the descendants are sick, but all individuals are carriers of the disease and will be able to transmit it to their descendants. The A allele (normal erythrocytes) predominates over the a allele (sickle cell anemia) which means that individual with two copies of the a allele would suffer the disease. In fact, this is the most common form of AD. A dominant allele is one that predominates over the rest of the alleles, whether we have one copy (heterozygous) or two (homozygous dominant).Īn example of AD inheritance is achondroplasia (dwarfism). These are all those that are neither the X chromosome nor the Y chromosome. This type of inheritance affects autosomal chromosomes. Mendelian inheritance patterns explain how character is inherited and what determines the phenotype they acquire.Īs we will see in the section on “exceptions to Mendel's laws”, these patterns do not apply to all loci in the genome. Once he knew how the transmission of each character worked separately (explained in 'Mendelian inheritance patterns'), he began to make crosses between parents that differed in two features. Mendel observed that the offspring (F1) resulting from two parents that differed only in one of the features mentioned earlier. The real reason is that peas are cheap, they don't require much space, they have abundant offspring that can be obtained in a short period of time, they have genetic variability (multiple varieties with different features) and it is a selfing plant.Īlso, doing crosses between varieties of peas can be easily controlled. This would be an interesting reason, but it isn't the case. Characteristics studied by MendelĪnd why peas? Were peas his favourite food?

Principle of dominance states that