How do Punnett squares help us make valid predictions about offspring?
In the last lesson we discussed properties and how we represent them when we talk about organisms. You should have a good understanding of what phenotype and genotype mean and what combinations of letters make a genotype homozygous or heterozygous.
In this lesson, we'll learn more about how to do this.predictthe type of offspring that results from mating. Scientists would make predictions based on observations of these traits and then test their ideas about how a genetic trait is inherited by breeding the organisms in question and examining the offspring.
understand probability
To understand genetics and make predictions, you need to understand probabilities.
The traits that are passed from parent to offspring are determined by the combination of genes (genotype) in the gametes. Which allele is passed from a parent is aarbitrarilyevent in nature. The probability that a given event will occur is the fraction of outcomes where that event will occur.probabilityit is the probable outcome of a certain event happening by chance.
The toss of a coin is a random (but fair) decision.
In soccer or anywhere else, every time a coin is tossed, there is a 50% chance of heads and a 50% chance of tails. Likewise, the probability of inheriting one of the two alleles from one parent is also 50%, or 1/2. Take a look at the image below, which shows how sex chromosomes are inherited from parents.
Remember that females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). Females only donate X to their offspring in their egg, but males can donate their X or Y chromosome in their sperm. This means that there is a 50/50 chance that a child will receive an X from a parent. Or a 50/50 chance that they get a Y.
Stop and think:Which parents determine the sex of a baby? Is it the father or the mother?(Responder:Father. The father contributes an X or Y chromosome. The mother contributes only one X.)
Look again at the diagram above. If the probability of having one child is 1/2, then the probability of having two children at the same time or two children in a row is 1/2 x 1/2 = 1/4 or 25%. Each event (birth) has its own probability, so the probability that the next child will be a boy is still 1/2.
Stop and think:See family above. Mom and Dad already have three girls in their family. What is the probability that the fourth child is a boy?(responder:each child has the same probability of 1/2, 50%)
What is the probability of rolling two fives with a pair of dice? Each die has a 1/6 chance of rolling a five. To roll two at the same time, multiply the probability of rolling a five by the probability of rolling another five: 1/6 x 1/6 = 1/36 probability of rolling two fives.
What is a Punnett square?
APunnett Square(named after its creator, Reginald C. Punnett) is a chart created to determine the likely outcomes of a genetic cross. show you everythingpossibleCombination of offspring resulting from a cross. Therefore, a Punnett square is aforecastwho appreciates what we should see in nature.
How does a Punnett square work?
To draw a Punnett square, you need to know the genotypes of both parents.
The parents' genes, represented by associated letters, are written to the left and top of the Punnett square. The alleles of each gene are separated in each column (top) or in rows (bottom).
For example, if the letter Y represents the pea pod color gene, where yellow dominates green. One parent is heterozygous Yy and the other is homozygous yy, the complete Punnett square would look like the one on the right.
A filled Punnett square indicates the likely outcome of a specific cross. In this case, the probability of each phenotype (green vs. yellow) is 50%. The probability of offspring with the Yy genotype is 50%, as is the probability of the yy genotype. We can write these probabilities of offspring as ratios to simplify our results:
phenotype relationship: 2 yellow: 2 green
genotype ratio: 2JJ: 2JJ
Using Punnett Squares to Solve Genetic Problems: Step by Step
Follow the tutorial in this section to learn how to set up your own Punnett squares to solve genetic problems. You'll be using this skill for the remainder of the module, so don't rush it.
Example of problem #1:In guinea pigs, straight hair (H) dominates over curly hair (h). What would be the results of crossing a curly-coated guinea pig with a straight-coated heterozygous guinea pig?
go throughtutorial. (pdf version)
You can find a printable version of this tutorial in the sidebar.
Example of problem #2:In peas, round seeds (R) predominate over wrinkled ones (r). Show the possible offspring of a cross between two pea plants that are both heterozygous for this trait.
Step 1: Identify the cross:Rr x Rr
Step 2: Which gametes can each parent produce?Each Rr parent produces R and R gametes.
Steps 3 and 4: Assemble and solve your cross as follows:
What genotypes does this square predict? (to respond:One RR (homozygous dominant), Two Rr (heterozygous) and One rr (homozygous rezessiv))
What phenotypes does this square predict? (to respond:Three red, one white)
Record the genotype and phenotype ratios for the cross: (Answer:genotype ratio: 1RR: 2Rr: 1rr; Phenotype ratio: 3 red: 1 white)
Now that we understand how to use Punnett squares to predict mating outcomes, let's review Mendel's original experiment. click throughinteractiveto refine your understanding of what Mendel saw.
The Testcross: A Punnett square solving a "dominant" puzzle
Sometimes scientists need to figure out whether an organism's genotype is pure dominant (homozygous) or hybrid (heterozygous) for a trait. A test cross is performed to solve this puzzle.
Atest crossconsists of crossing the unknown organism with a known organismhomozygous recessive. If one of the offspring is homozygous recessive and shows the recessive phenotype, then the unknown had to be heterozygous.
Take a look at the two possible outcomes of a testcross below. In both, one of the individuals is known to be recessive (green pods: aa).
- If the mystery plant is heterozygous (Yy), it will see recessive phenotypes in the offspring.
- If the mystery plant is homozygous (YY), you will see all dominant phenotypes in the offspring.
monohybrid v. dihybrid crosses
So far, all of the Punnett square problems you've solved involve just one property, also known asmonohybrid cross.Adihybrid cross, on the other hand, is a cross in which two pairs of opposing genes (such as flower color and seed color) are studied simultaneously.
A dihybrid cross would result in a Punnett square, which is usually larger as more types of gametes are possible. Each gamete (row or column) would have two letters and each descending square would have four letters.
A short-tailed cat (SS) first king (Bed & Breakfast) can be crossed with another cat of the same genotype (SsBb). To make such a cross, a Punnett square of 16 squares must be used.
SsBb x SsBb
To determine which alleles to place outside the squares, remember how meiosis would combineSSYBed & Breakfast...onlyS o Sit is ab o bin each gamete. There are four possible gamete combinations for both parents (since they have the same SsBb genotype):
SB, SB, SBYjdn
Take a look at the Punnett square below, which shows the results of the same cross. Characteristics are long tail (s), short tail (S), brown coat (B) and white coat (b).
Whenever two individuals heterozygous for both traits are involved in a Punnett chart, we see four distinct phenotypic groups with a typicalPhenotype Ratio 9:3:3:1.
Stop and think:Take another look at the offspring of this cross and see if you can separate each of the four different phenotypic groups.
Phenotype relationship:9_____;3_____;3_____;1_____ (Responder:9 short tail, brown fur (purple) - 3 short tail, white fur (pink) - 3 long tail, brown fur (blue) - 1 long tail, white fur (green))