![]() ![]() An example of this can be seen in Figure 4. The easiest way to do this is to determine the probability of the individual inheriting each trait separately, then multiply those probabilities together. Sometimes, we want to determine the probability of an individual inheriting two different traits. Organisms don’t just inherit one trait at a time, though. Again, it’s just like flipping a coin: if you flip heads the first time, that doesn’t change the probability of getting heads on the next flip. The probability that the first offspring will have the genotype “aa” is 25% and the probability of the second offspring having the genotype “aa” is still 25%. It’s just like flipping a coin: you expect 50% heads, but you wouldn’t be too surprised to see 7 heads out of 10 coin flips. Additionally, the probability does not change for successive offspring. It does not mean that these parents must have 4 offspring and that they will have the ratio 1 AA : 2 Aa : 1 aa. ![]() For example, the Punnett square in Figure 3 shows that there is a 25% chance that a homozygous recessive offspring will result from the cross Aa x Aa. This concept is called Mendel’s Law of Segregation.įigure 3: A Punnett square showing a cross between two individuals who are both heterozygous for A.Ī Punnett square shows the probability of an offspring with a given genotype resulting from a cross. It does not show actual offspring. That means that an offspring inherits one allele of each gene from each of its two parents. This is because meiosis produces haploid gametes that contain one copy of each chromosome, and those chromosomes are assorted into gametes randomly. Since genes are present on chromosomes, this means they will pass one copy of each gene to their offspring. ![]() Regardless of their genotype, they will randomly pass only one copy of each chromosome to their offspring. They can have two of the same alleles (homozygous) or two different alleles (heterozygous). Punnett SquaresĪs discussed above, diploid individuals have two copies of each chromosome: one from their male parent, one from their female parent. This means they have two copies of each gene. The first female has the condition being shown in the pedigree. In this pedigree, the parents (at the top) have produced three children: a male and two females. ![]()
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