Apply Mendel's laws and use Punnett squares to predict the genotype and phenotype ratios of monohybrid crosses, and identify patterns of inheritance including dominant, recessive, codominant, and incomplete dominance (TEKS Biology, Reporting Category 2; patterns; using mathematics).

What this topic is asking

The Biology TEKS ask you to apply Mendel's laws and use Punnett squares to predict the outcomes of genetic crosses, and to identify inheritance patterns including dominant, recessive, codominant, and incomplete dominance. For STAAR Reporting Category 2 this is the most quantitative topic, so the practice of using mathematics (ratios and probabilities) is central, alongside patterns.

Alleles, genotype, and phenotype

Alleles are written as letters: a capital for the dominant allele and the same letter in lowercase for the recessive allele. For height in pea plants, $T$ is tall (dominant) and $t$ is short (recessive). An organism with two of the same allele ($TT$ or $tt$) is homozygous; with two different alleles ($Tt$) it is heterozygous.

Dominant and recessive

Punnett squares: predicting a cross

A Punnett square sets out the alleles each parent can pass and combines them to show the possible offspring. To use one: write each parent's alleles, place one parent's along the top and the other's down the side, then fill each box by combining the row and column allele. Counting the boxes gives the expected ratio and the probability of each genotype and phenotype.

For a cross between two heterozygous tall plants ($Tt \times Tt$), the four boxes are $TT$, $Tt$, $Tt$, $tt$: a genotype ratio of $1:2:1$ and a phenotype ratio of 3 tall to 1 short. Each offspring has a $\frac{3}{4}$ probability of being tall and a $\frac{1}{4}$ probability of being short.

Codominance and incomplete dominance

Not every trait is a simple dominant-or-recessive pattern. STAAR also tests two variations:

• Incomplete dominance. The heterozygote shows a blend of the two homozygous phenotypes. A red flower ($RR$) crossed with a white flower ($WW$) gives pink ($RW$) offspring, because neither allele fully masks the other.
• Codominance. Both alleles show fully and at the same time in the heterozygote. In some cattle, a red allele and a white allele together give a coat with both red and white hairs (roan), and the human ABO blood group AB shows both A and B.

The difference: incomplete dominance is a mix (pink), while codominance shows both phenotypes together (spotted).

Try this

Q1. Define genotype and phenotype. [2]

• Cue. Genotype is the alleles an organism has (for example $Tt$); phenotype is the observable trait that results (for example tall).

Q2. A red flower ($RR$) is crossed with a white flower ($WW$) and all offspring are pink. State the inheritance pattern and explain. [2]

• Cue. Incomplete dominance: the heterozygote ($RW$) is a blend of the two homozygous colors because neither allele fully masks the other.