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In humans, a cleft chin is dominant and no-cleft is recessive. What will the generations look like? Assume that Mendel's method of crossing two true-breeding parents with opposite traits is followed. Check all that apply.


  • The $P$ generation, $F_1$ generation, and $F_2$ generation will have all cleft chins.
  • The $P$ generation, $F_1$ generation, and $F_2$ generation will have all no-cleft chins.
  • The P generation and $F _1$ generation will have all cleft chins.
  • The $F _1$ generation will have all cleft chins.
  • The P generation and $F _2$ generation will have all no-cleft chins.
  • The $P$ generation and $F_2$ generation will have cleft chins and no-cleft chins.
  • The $P$ generation, $F_1$ generation, and $F_2$ generation will have cleft chins and no-cleft chins.

Verified Answer :

  • The \(P\) generation consists of both cleft and no-cleft phenotypes because they are true-breeding.
  • The \(F_1\) generation is entirely heterozygous (\(Cc\)), resulting in all individuals having a cleft chin.
  • The \(F_2\) generation exhibits both cleft and no-cleft phenotypes in a \(3:1\) ratio.
  • The correct descriptions are:
  • The \(F_1\) generation will have all cleft chins.
  • The \(P\) generation and \(F_2\) generation will have cleft chins and no-cleft chins.

Explanation

  1. Analyze the P Generation and Traits

In Mendelian genetics, traits are determined by alleles. Let \(C\) represent the dominant allele for a cleft chin and \(c\) represent the recessive allele for a no-cleft chin. A "true-breeding" parent means the individual is homozygous for that trait. Therefore, the \(P\) (parental) generation consists of one homozygous dominant parent (\(CC\)) with a cleft chin and one homozygous recessive parent (\(cc\)) with no-cleft chin. This means the \(P\) generation includes individuals with both cleft and no-cleft phenotypes.

  1. Analyze the F1 Generation

When the \(P\) generation (\(CC\) and \(cc\)) is crossed, every offspring receives one \(C\) allele from the first parent and one \(c\) allele from the second parent. Consequently, all offspring in the \(F_1\) generation have the genotype \(Cc\). Since the cleft chin trait is dominant, all individuals in the \(F_1\) generation will display the cleft chin phenotype. Therefore, the \(F_1\) generation consists entirely of individuals with cleft chins.

  1. Analyze the F2 Generation

The \(F_2\) generation is produced by crossing two members of the \(F_1\) generation (\(Cc \times Cc\)). Using a Punnett square, we find the possible genotypes: one-quarter will be \(CC\), one-half will be \(Cc\), and one-quarter will be \(cc\). Phenotypically, this results in three-quarters having a cleft chin (\(CC\) and \(Cc\)) and one-quarter having a no-cleft chin (\(cc\)). Thus, the \(F_2\) generation contains both cleft chin and no-cleft chin phenotypes.

  1. Evaluate the Options and Conclude

Now we evaluate the provided statements:

  • "The F1 generation will have all cleft chins" is correct, as all are \(Cc\).
  • "The P generation and F2 generation will have cleft chins and no-cleft chins" is correct, as the \(P\) generation contains both types (from true-breeding parents) and the \(F_2\) generation results in a 3:1 ratio of cleft to no-cleft.

All other statements are incorrect as they misidentify the phenotype composition of the generations.

Examples

Understanding Mendelian genetics allows us to predict the probability of specific traits appearing in subsequent generations. For example, if a couple knows their genetic status regarding a dominant trait, such as a cleft chin, they can calculate the likelihood of their children having that trait, which is a foundational concept in genetics and healthcare counseling. \(P(cleft) = 0.75\) for the \(F_2\) offspring of a heterozygous cross.

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