Worked examples: Punnett squares | Inheritance and variation | Middle school biology | Khan Academy

Summary

This educational script explains the genetic inheritance of seed shape in pea plants, focusing on the dominance of the round seed allele (R) over the wrinkled seed allele (r). It guides viewers through creating a Punnett square to predict offspring ratios for two scenarios: a cross between heterozygous plants (Rr x Rr) resulting in a 3:1 ratio of round to wrinkled seeds, and a cross between a homozygous wrinkled seed plant (rr) and a heterozygous one (Rr), leading to a 1:1 ratio. The explanation demystifies Mendelian genetics and engages learners by encouraging them to solve the problem before the solution is revealed.

Takeaways

• 🌱 The script discusses pea plant genetics, focusing on seed shape determined by two alleles: a dominant allele for round seeds (R) and a recessive allele for wrinkled seeds (r).
• 🔍 It explains the concept of heterozygous pea plants, which have one allele for round seeds and one for wrinkled seeds (Rr).
• 📊 The script uses a Punnett square to illustrate the genetic cross between two heterozygous pea plants (Rr x Rr).
• 👶 It demonstrates that the offspring from such a cross can have four equally likely genotypes: RR, Rr, rR, and rr.
• 🍀 The dominant allele (R) results in round seeds, regardless of the other allele, while the recessive allele (r) only results in wrinkled seeds when homozygous (rr).
• 🔢 The expected ratio of round to wrinkled seeds in the offspring is three to one, as three genotypes result in round seeds and only one in wrinkled seeds.
• 🤔 The script challenges the viewer to pause and attempt to solve the genetic cross problem before revealing the solution.
• 📚 It introduces the concept of phenotypes, which are the expressed traits of an organism, in this case, the seed shape.
• 🌟 The script also explores a cross between a homozygous wrinkled seed pea plant (rr) and a heterozygous one (Rr).
• 🎯 In the second cross, the Punnett square shows that all offspring will have at least one R allele, resulting in a round seed phenotype, except for the scenario where both parents contribute the r allele.
• 🧬 The final ratio for the second cross is one to one, with an equal number of round and wrinkled seed offspring.
• 📝 The script provides a clear, step-by-step explanation of Mendelian genetics principles as they apply to pea plant seed shape inheritance.

Q & A

What are alleles, and how many alleles does the gene for seed shape in pea plants have?

Alleles are different versions of a gene that determine a particular trait. In the case of the gene for seed shape in pea plants, there are two possible alleles: one for round seeds (denoted as capital R) and one for wrinkled seeds (denoted as lowercase r).

What is the difference between a dominant and a recessive allele in genetics?

A dominant allele is one that expresses its trait even when present alongside a different allele (heterozygous condition). A recessive allele, on the other hand, only expresses its trait when both alleles for a trait are the same (homozygous recessive condition). In the pea plant example, the capital R allele for round seeds is dominant.

What does heterozygous mean in the context of genetics?

Heterozygous refers to an organism having two different alleles for a particular gene. In the script, the pea plants are heterozygous for the seed shape gene, meaning they have one allele for round seeds (R) and one for wrinkled seeds (r).

How is a Punnett square used in genetics to predict offspring traits?

A Punnett square is a diagram used to predict the genotypes of offspring in a genetic cross. It is created by listing the alleles of one parent along the top and the alleles of the other parent along the side, then filling in the squares with the possible combinations of alleles from both parents.

What is the expected ratio of offspring with round seeds to those with wrinkled seeds when two heterozygous pea plants are crossed?

The expected ratio is 3:1, with three offspring having round seeds for every one offspring with wrinkled seeds. This is because the round seed allele is dominant, and most of the possible genotypes result in round seeds.

What genotypes result in round seeds in the pea plant cross?

Round seeds can result from the genotypes RR (homozygous dominant) or Rr (heterozygous), as the round seed allele R is dominant over the wrinkled seed allele r.

What is the only genotype that results in wrinkled seeds in the pea plant cross?

The only genotype that results in wrinkled seeds is rr (homozygous recessive), where both alleles are for wrinkled seeds.

What is a phenotype in genetics, and how is it related to the pea plant seed shape example?

A phenotype is the observable physical or biochemical characteristics of an organism, as determined by the interaction of its genotype with the environment. In the pea plant example, the phenotype for seed shape is either round or wrinkled, depending on the genotype.

What is the expected ratio of round to wrinkled seeds when a homozygous wrinkled seed pea plant is crossed with a heterozygous one?

The expected ratio is 1:1, as each offspring has an equal chance of inheriting the dominant round seed allele (R) or the recessive wrinkled seed allele (r) from the heterozygous parent.

How can the dominance of an allele affect the observed traits in a population?

The dominance of an allele can lead to certain traits being more common in a population. If a dominant allele masks the expression of a recessive allele, as seen in the pea plant example, most individuals will display the trait associated with the dominant allele.

What happens when a homozygous recessive individual is crossed with a heterozygous one, and what is the resulting phenotype ratio?

When a homozygous recessive individual (rr) is crossed with a heterozygous one (Rr), all offspring will have at least one recessive allele (r). The resulting phenotype ratio will be 1 round seed (Rr) to 1 wrinkled seed (rr), as the dominant allele from the heterozygous parent can only produce round seeds when paired with another dominant allele.