Mendel’s Law of Segregation is a basic principle in genetics that was first discovered by Gregor Mendel. When working with pea plants, he found that each plant would produce two types of peas: one wrinkled and the other smooth. From this experiment, Mendel determined there were two versions of what we now know to be the gene for wrinkling (R), which can be dominant or recessive depending on what gene it is paired with (S).
What can we observe in order to visualize Mendel’s Law of Segregation? One way to do this is through Punnett squares. A Punnett square is a diagram used to predict the probability of getting different types of offspring if two parents with certain genetic information have children.
In this example, Parent Alpha has alleles (A) for both genes S and R. This means that he could produce any combination: RS or AS. And because alpha’s gene only determines the type of pea plant seedlings will be made, we’ll use “R” as shorthand for it hereinafter so you can see how these work in terms of Mendelian inheritance principles.
Mendel’s Law states that when there are two versions (alleles) of an inheritable trait like wrinkling, one version must always show up on each side. So in this case, Parent Alpha can’t have RR or SR offspring.
Parent Beta will be represented by (A) for one gene and a “B” for the other. So it could produce any of these combinations: AS or AB, but not AR. The same is true with allele “S.” That means that Mendel’s Law again states that beta won’t have SS, RS or BW children because they are impossible according to his rules about how genetic traits work in inheritance patterns.
If Parent Alpha and Parent Beta both had two copies of Alleles A on their single chromosome then they would each make RRS offspring when mating together as predicted by Mendelian Laws governing heredity based on dominant genes .
If Parent Alpha and Parent Beta both had one copy of Alleles A on their single chromosome then they would each make SSA offspring when mating together. This is because the alleles are dominant, so even though only one parent has them in this case, the child will have two copies of that allele.
If Parent Alpha and Parent Beta both had a recessive version (AR) for an allele on their single chromosome then they would each produce BW children when mating with other members of its own type. Mendel’s Laws predict it won’t happen any other way.
The same is true if either parent has the recessive versions R or s: RR or RS must be combined to get XYZ offspring as predicted by Mendel’s Laws.
If Parent Alpha and Parent Beta both have one recessive allele, then they would each produce WN children when mating with other members of the same type because a single parent can only pass on a recessive gene to its offspring if it has two copies for that gene-one dominant and one recessive.
Mendel predicted this by saying “the laws governing inheritance are as follows: Of two parents possessing different characters in ditto proportions, or what is called their reciprocal crosses, the child will show first the character of whichever progenitor prevails in number.” In order for there to be an even 50% chance of inheriting either version (A or R) from any given parent, we need to have a ratio of AA:AR=RR:WR.
Mendel’s law of segregation is that “the two alleles segregate independently from each other so that the gametes produced by an individual during meiosis will contain one allele or the other, but not both.” This means that when any organism (a cell in this case) undergoes sexual reproduction and produces gametes, it randomly separates its chromosomes into sperm/eggs with only one copy of either A or R for each characteristic.
This can be observed through what Mendel called Fortunate Mating which was mating between parents who were homozygous dominant and heterozygous recessive and their offspring became all phenotypically dominant because they inherited the dominant allele from both parents. This is what Mendel called “Fortunate Mating” because they were able to show that a recessive trait could be hidden in an organism’s genotype and passed on to their offspring under the right circumstances of mating with someone who has two copies of that gene.
Mendel showed how alleles segregate independently during meiosis by fertilizing pea plants (parent generation). He mated male, red-flowered peas crossed with female white-flowered peas and observed 69% purebred red flowers for the first filial generation or F I . The ratio he found was:
AA x AR = RR x WR
where AA represents the dominant homozygous background, AR represents the recessive homozygous background, RR represents heterozgotes or a dominant and an allele of another type (heteroalleles), WR represent recessive alleles.
The ratios that Mendel found for F I were:
AA x AR = 69% to 31%; AR x AA= 39% to 61%. When he looked at F II , what emerged was 60% red flowers with 40% white flowers. He also had a number of plants in which there was only one flower color – all purebred flowers or two colors where both parents carried a different gene. For example, if one parent is RrRr (RR) then they would have 50/50 chance of producing offspring who are either RR or RrR, an RR would produce 50% of its offspring as RR and 50% as rr.
Mendel found that the ratio for F I was 69:31 in favor of red flowers when crosses were done with purebred plants. When he looked at F II what emerged was 60% red flowers to 40% white flowers. He also had a number of plants where there is only one flower color – all pure bred or two colors where both parents carried different genes. For example if one parent is Rrr (RR) then they have a fifty percent chance of producing either rr or RR depending on how their child’s sex hormones work out. An RR will always make half its children be RR but it can’t make rr.
And a parent who is Rr (R) has the same chance of producing one child as RR, two children as rr or three children as either and in these cases F I equals 50% red to 50% white flowers. Mendel found that when he crossed purebred plants together what emerged was 69:31 ratio for favor of red flowers to 31%:71 for white flowers and 60-40 in favor of only one color over both colors.”