genetic diagrams

What is a Punnett square used for?

To determine the possible genotypes and phenotypes of offspring from a genetic cross.

What can Punnett squares help determine?

The genotypic and phenotypic ratios of possible offspring.

Genotype

The genetic makeup of an organism (e.g., TT, Tt, tt).

Phenotype

The physical expression of a trait (e.g., tall or short plant).

Homozygous dominant

An organism with two dominant alleles (e.g., TT).

Homozygous recessive

An organism with two recessive alleles (e.g., tt).

Heterozygous

An organism with one dominant and one recessive allele (e.g., Tt).

What happens in a TT x tt cross?

All offspring will be heterozygous (Tt) and show the dominant phenotype.

Genotypic ratio for TT x tt

4 Tt : 0 TT : 0 tt → 4:0

Phenotypic ratio for TT x tt

4 dominant : 0 recessive → 4:0

What happens in a TT x Tt cross?

50% offspring will be TT, 50% will be Tt; all show dominant phenotype.

Genotypic ratio for TT x Tt

2 TT : 2 Tt → 1:1

Phenotypic ratio for TT x Tt

4 dominant : 0 recessive → 4:0

What happens in a tt x Tt cross?

50% Tt (dominant phenotype), 50% tt (recessive phenotype).

Genotypic ratio for tt x Tt

2 Tt : 2 tt → 1:1

Phenotypic ratio for tt x Tt

2 dominant : 2 recessive → 1:1

What happens in a Tt x Tt cross?

25% TT, 50% Tt, 25% tt.

Genotypic ratio for Tt x Tt

1 TT : 2 Tt : 1 tt → 1:2:1

Phenotypic ratio for Tt x Tt

3 dominant : 1 recessive → 3:1

What percentage of offspring would be heterozygous in a TT x tt cross?

100%

In a Tt x Tt cross, what percentage of offspring are expected to show the recessive phenotype?

25%

If two organisms with genotype Tt are crossed, what is the probability of getting a homozygous dominant offspring?

25%

What are the possible genotypes of offspring from a TT x Tt cross?

TT and Tt

Which cross results in a 1:1 genotypic ratio and 1:1 phenotypic ratio?

tt x Tt

If 100 plants are produced from a Tt x Tt cross, how many would you expect to have the recessive phenotype?

25 plants

In a TT x tt cross, what is the expected genotypic ratio?

4:0 (all Tt)

Why do all offspring from a TT x Tt cross show the dominant phenotype?

Because both TT and Tt express the dominant allele.

What is the phenotypic ratio in a heterozygous x heterozygous cross?

3:1

Which cross can produce offspring with the recessive phenotype?

Tt x Tt or tt x Tt

What does a 1:2:1 genotypic ratio indicate?

A heterozygous x heterozygous (Tt x Tt) cross

What is the expected outcome of a tt x Tt cross in terms of phenotype?

50% dominant, 50% recessive

Figure 1: Punnett square for TT x tt.

Figure 2: Punnett square for TT x Tt.

Figure 3: Punnett square for tt x Tt.

Figure 4: Punnett square for Tt x Tt.