L5 Predicting Offspring Genotypes, Phenotypes & Probabilities

[T5] What is a Punnett Square used for?

If you know the genotypes of two parents and the relationship between the alleles, a Punnett Square can be used to predict offspring genotypes and phenotypes from a cross.

[T5] What is a Punnett Square structurally?

A matrix or square grid.

[T5] What is listed on one axis of a Punnett Square?

The gamete genotypes that can be produced by one parent.

[T5] What is listed on the other axis of a Punnett Square?

The gamete genotypes that can be produced by the other parent.

[T5] What is shown inside the boxes of a Punnett Square?

Each possible combination of parental gametes.

[T5] What does each square inside a Punnett Square represent?

The genotype of an equally probable fertilized egg.

[IF5] On the Punnett Square on page 8, what do the labels along the outside axes represent?

The possible gamete genotypes produced by each parent.

[IF5] On the Punnett Square on page 8, what do the internal boxes represent?

Possible offspring/fertilized-egg genotypes.

[A5] A male parent is Ff. What gamete genotypes can he produce for the F gene?

F and f.

[A5] A female parent is Ff. What gamete genotypes can she produce for the F gene?

F and f.

[A5] What is the first thing you need to determine from each parental genotype before filling a Punnett Square?

The possible gamete genotypes each parent can produce.

[A5] Once a Punnett Square is filled, how do you determine offspring phenotype probabilities?

Determine the phenotype associated with each offspring genotype, then calculate the fraction/proportion of boxes with the desired phenotype.

[A5] Once a Punnett Square is filled, how do you determine offspring genotype probabilities?

Calculate the fraction/proportion of equally probable boxes containing the desired genotype.

[A5] If black fur allele F is dominant to white fur allele f, what phenotype does FF produce?

Black fur.

[A5] If black fur allele F is dominant to white fur allele f, what phenotype does Ff produce?

Black fur.

[A5] If black fur allele F is dominant to white fur allele f, what phenotype does ff produce?

White fur.

[A5] In the cross Ff × ff, what gametes can the Ff parent produce?

F and f.

[A5] In the cross Ff × ff, what gametes can the ff parent produce?

f only.

[A5] In the cross Ff × ff, what offspring genotypes are possible?

Ff and ff.

[A5] In the cross Ff × ff, what offspring phenotypes are possible if F is dominant?

Black fur and white fur.

[A5] In the cross Ff × ff, what is the probability of black-furred offspring?

1/2 or 50%.

[A5] In the cross Ff × ff, what is the probability of white-furred offspring?

1/2 or 50%.

[A5] In the cross Ff × Ff, what offspring genotypes are possible?

FF, Ff, and ff.

[A5] In the cross Ff × Ff, what is the expected genotype ratio?

1 FF : 2 Ff : 1 ff.

[A5] In the cross Ff × Ff, what is the probability of FF offspring?

1/4 or 25%.

[A5] In the cross Ff × Ff, what is the probability of Ff offspring?

1/2 or 50%.

[A5] In the cross Ff × Ff, what is the probability of ff offspring?

1/4 or 25%.

[A5] In the cross Ff × Ff, what is the probability of the dominant phenotype?

3/4 or 75%.

[A5] In the cross Ff × Ff, what is the probability of the recessive phenotype?

1/4 or 25%.

[T5] Are the outcomes of successive offspring from the same parental cross dependent on previous offspring?

No. The outcome of one cross/offspring is independent of the outcomes of previous crosses.

[A5] Two Ff bears produce five black cubs in a row. What is the probability that the next cub will be white?

1/4 or 25%.

[A5] Why does having five black cubs in a row not change the probability that the next Ff × Ff cub is white?

Each offspring outcome is independent of previous offspring outcomes.

[A5] If a cross has a 25% probability of producing the recessive phenotype, does producing several dominant-phenotype offspring in a row change the probability for the next offspring?

No. The probability for the next offspring remains 25%.

[T5] What does true-breeding or pure-breeding mean?

The individual is homozygous for the gene.

[T5] Can a true-breeding individual be homozygous dominant?

Yes.

[T5] Can a true-breeding individual be homozygous recessive?

Yes.

[T5] Give two example genotypes that are true-breeding for an F gene.

FF and ff.

[A5] A true-breeding individual with a dominant phenotype has what genotype under complete dominance?

Homozygous dominant.

[A5] A true-breeding individual with a recessive phenotype has what genotype under complete dominance?

Homozygous recessive.

[A5] If round ears are dominant and a true-breeding round-eared male is crossed with a true-breeding pointy-eared female, what parental genotypes are implied?

Homozygous dominant × homozygous recessive, e.g. RR × rr.

[A5] What offspring genotype results from RR × rr?

All Rr.

[A5] If R is dominant to r, what phenotype do all offspring from RR × rr have?

The dominant phenotype.

[A5] What is the probability of the dominant phenotype from RR × rr?

100%.

[A5] Two heterozygotes for ear shape, Ee × Ee, are crossed. If pointy ears are recessive, what is the probability of pointy-eared offspring?

1/4 or 25%.

[T5] Does every Punnett Square problem assume a dominant/recessive relationship?

No. The allele relationship must be given or known; alleles can also have a non-dominant relationship.

[T5] In a non-dominant relationship, can the heterozygote have a phenotype different from both homozygotes?

Yes.

[A5] Straight-hair allele H^S and curly-hair allele H^C have a non-dominant relationship, and H^SH^C produces wavy hair. What phenotype does H^SH^S produce in the lecture example?

Straight hair.

[A5] What phenotype does H^SH^C produce in the lecture example?

Wavy hair.

[A5] If a straight-haired dog H^SH^S is crossed with a wavy-haired dog H^SH^C, what offspring genotypes are possible?

H^SH^S and H^SH^C.

[A5] In the cross H^SH^S × H^SH^C, what is the probability of curly-haired offspring H^CH^C?

0%.

[A5] If two heterozygotes R1R2 × R1R2 are crossed under a non-dominant relationship, what offspring genotypes are possible?

R1R1, R1R2, and R2R2.

[A5] In R1R2 × R1R2, what is the probability of R2R2 offspring?

1/4 or 25%.

[A5] Why must you know the relationship between alleles before converting genotype probabilities into phenotype probabilities?

Because different allele relationships can cause the same genotype combinations to produce different phenotype patterns.

[T5] In Lecture 5 probability problems, what mathematical operation corresponds to “AND”?

Multiply probabilities.

[T5] In Lecture 5 probability problems, what mathematical operation corresponds to “OR”?

Add probabilities.

[A5] What should you do when a question asks for phenotype A AND phenotype B?

Multiply the probabilities of the two events.

[A5] What should you do when a question asks for outcome A OR outcome B?

Add the probabilities of the alternative events.

[A5] White fur AND female requires what probability operation?

Multiply.

[A5] Black fur OR white fur requires what probability operation?

Add.

[T5] What method does the instructor highly recommend for questions involving two or more traits?

Use separate Punnett Squares for each trait to reduce the risk of error.

[A5] Why can separate Punnett Squares be useful for multi-trait questions?

They let you calculate each trait's probability separately and then combine the probabilities using AND/multiply or OR/add.

[F5] Is a large combined Punnett Square the only valid method for multi-trait problems?

No. The lecture shows it as an alternative but recommends separate Punnett Squares.

[T5] What are the typical sex chromosome genotypes used in this lecture for biological females and males?

Female = XX; male = XY.

[A5] For an autosomal trait, what is the probability of a child being male under the lecture's XX/XY model?

1/2 or 50%.

[A5] For an autosomal trait, what is the probability of a child being female under the lecture's XX/XY model?

1/2 or 50%.

[A5] A Bb male is crossed with a bb female, where B is dominant to b. What is the probability of bb offspring?

1/2.

[A5] In Bb × bb, what is the probability of a son with the bb phenotype?

1/2 × 1/2 = 1/4 or 25%.

[A5] Why do you multiply 1/2 for being male by 1/2 for having bb in the Bb × bb example?

The question asks for male AND bb.

[A5] In Bb × bb, what is the probability of the dominant B_ phenotype?

1/2.

[A5] In Bb × bb, what is the probability of a daughter with the dominant B_ phenotype?

1/2 × 1/2 = 1/4.

[A5] In Bb × bb, what is the probability of a son with bb OR a daughter with B_?

1/4 + 1/4 = 1/2.

[A5] When a problem asks for one combination OR another combination, what should you do after calculating each combination separately?

Add the probabilities.

[A5] A female bear is Ff and a male bear is Ff. What is the probability that their cub will have white fur ff?

1/4.

[A5] For the same Ff × Ff cross, what is the probability that the cub will be female?

1/2.

[A5] What is the probability of a white female cub from Ff × Ff?

1/4 × 1/2 = 1/8 = 12.5%.

[A5] Why is the probability of a white female cub 1/8 rather than 1/4?

The cub must satisfy two conditions: white fur AND female, so the probabilities are multiplied.

[IF5] On page 35, what two separate Punnett Squares are used to solve the white-female-cub problem?

One for the autosomal fur-colour gene and one for offspring sex.

[IF5] What calculation is illustrated on page 35 for a white female cub?

1/4 for white fur × 1/2 for female = 1/8 = 12.5%.

[A5] Two individuals with genotype Q1/Q1;R3/R5;T2/T6 are crossed. What is the probability of Q1/Q1 at the Q locus?

1.

[A5] In R3/R5 × R3/R5, what is the probability of R3/R5 offspring?

1/2.

[A5] In T2/T6 × T2/T6, what is the probability of T2/T6 offspring?

1/2.

[A5] For Q1/Q1;R3/R5;T2/T6 × the same genotype, what is the probability of offspring with the parental genotype Q1/Q1;R3/R5;T2/T6?

1 × 1/2 × 1/2 = 1/4.

[A5] In R3/R5 × R3/R5, what is the probability of R3/R3 offspring?

1/4.

[A5] In T2/T6 × T2/T6, what is the probability of T2/T2 offspring?

1/4.

[A5] What is the probability of Q1/Q1;R3/R3;T2/T2 offspring from two Q1/Q1;R3/R5;T2/T6 parents?

1 × 1/4 × 1/4 = 1/16.

[A5] What is the probability of Q1/Q1;R3/R5;T2/T6 OR Q1/Q1;R3/R3;T2/T2 from that cross?

1/4 + 1/16 = 5/16 = 0.3125.

[A5] Why are the probabilities for the parental genotype and Q1/Q1;R3/R3;T2/T2 added in this problem?

The question asks for one genotype OR the other.

[A5] In Ee × Ee, what is the probability of ee offspring?

1/4.

[A5] In Ee × Ee, what is the probability of the dominant phenotype E_?

3/4.

[A5] What does E_ mean in the lecture's probability notation?

Any genotype containing at least one dominant E allele, such as EE or Ee.

[A5] In Aa × Aa, what is the probability of aa offspring?

1/4.

[A5] In Aa × Aa, what is the probability of the dominant phenotype A_?

3/4.

[A5] What does A_ mean?

Any genotype with at least one dominant A allele, such as AA or Aa.

[A5] EeAa × EeAa: what is the probability of eeaa if the genes are not physically linked?

1/4 × 1/4 = 1/16.

[A5] EeAa × EeAa: what is the probability of E_A_?

3/4 × 3/4 = 9/16.

[A5] EeAa × EeAa: what is the probability of a female with eeaa?

1/2 × 1/4 × 1/4 = 1/32 = 0.03125.

[A5] EeAa × EeAa: what is the probability of a male with E_A_?

1/2 × 3/4 × 3/4 = 9/32 = 0.28125.

[A5] EeAa × EeAa: what is the probability of a female with eeaa OR a male with E_A_?

1/32 + 9/32 = 10/32 = 5/16 = 0.3125.

[A5] When calculating the probability of female AND ee AND aa, what operations are used?

Multiply all three probabilities.

[A5] When calculating female-ee-aa OR male-E_-A_, what final operation combines the two complete outcomes?

Add.