Skip to content

Menu

ExamPassoLog in

GED Science: Heredity and DNA Concepts

Importance ★★★★★ · 20% (estimate)

Overview

This chapter explains how traits are passed from parents to children. GED questions often ask you to complete a Punnett square, read a pedigree chart, or explain how genes and the environment work together.

DNA in chromosomes carries genes, the instructions for traits. Genetic information flows from DNA to RNA to protein, which is called the central dogma. Each person has two alleles (versions) of most genes, one from each parent. A dominant allele shows its trait even with one copy; a recessive trait shows only when both alleles are recessive. The genotype is the pair of alleles, such as Pp, and the phenotype is the trait you can see, such as purple flowers.

A Punnett square shows the possible offspring of two parents. A cross of two carriers (Pp × Pp) gives 1 PP : 2 Pp : 1 pp, so there is a 25% chance of the recessive trait. Mutations are changes in DNA that can create new alleles, and meiosis mixes alleles so that brothers and sisters differ. The environment can also affect how a gene is expressed; for example, the dark fur on a Siamese cat's cooler body parts depends on temperature.

Key concepts

  • Allele

    One version of a gene. A person usually has two alleles for each gene, one from each parent.

    Memory tipOne from mom, one from dad.

  • Dominant and recessive

    A dominant allele (capital letter) shows with one copy. A recessive allele (small letter) shows only when there are two copies.

    Memory tipPp shows the dominant trait; only pp shows the recessive one.

  • Genotype and phenotype

    Genotype is the allele pair (such as Pp). Phenotype is the observable trait (such as purple flowers).

    Memory tipGenotype = letters; phenotype = what you see.

  • Punnett square

    A grid that shows possible offspring. Pp × Pp → 25% PP, 50% Pp, 25% pp. Pp × pp → 50% Pp, 50% pp.

    Memory tipEach box is 25%.

  • Central dogma

    DNA → RNA → protein. DNA is copied into RNA, and RNA is read to build a protein.

    Memory tipDNA → RNA → protein, in that order.

  • Mutation

    A change in the DNA base sequence. Mutations are the source of new alleles.

    Memory tipMutations can be harmful, helpful, or have no effect.

Often tested

  • Two parents without a trait who have a child with it: the trait is recessive and both parents are carriers.
  • Check whether the question asks for the dominant or recessive trait before choosing a percent.
  • The environment can change how a gene is expressed without changing the DNA.

3 sample questions

Verified practice questions from this unit.

Practice questionReviewedLife Science › Heredity and DNA★★★★★

Question 1. Which molecule carries the genetic instructions in a cell?

  1. ① Glucose
  2. ② Starch
  3. ③ Protein
  4. ④ DNA
▼ Show answer and explanation▲ Hide answer and explanation

Answer: ④ DNA

Key point: DNA stores the genetic instructions

DNA (deoxyribonucleic acid) is found in chromosomes. Its sequence of bases holds the instructions (genes) for making proteins, which shape an organism's traits.

Wrong choices

  • Glucose: Glucose is a sugar used for energy.
  • Starch: Starch is a stored form of sugar in plants.
  • Protein: Proteins are made by following DNA's instructions. They do not store the instructions.
Practice questionReviewedLife Science › Heredity and DNA★★★★★

Question 2. In pea plants, the allele for purple flowers (P) is dominant over the allele for white flowers (p). What color are the flowers of a plant with the genotype Pp?

  1. ① White
  2. ② Purple
  3. ③ Half purple and half white
  4. ④ Pink
▼ Show answer and explanation▲ Hide answer and explanation

Answer: ② Purple

Key point: One dominant allele is enough to show the dominant trait

A dominant allele shows its trait even when only one copy is present. Pp has one dominant P, so the flowers are purple. The recessive p is hidden.

Wrong choices

  • White: White flowers appear only when a plant has two recessive alleles (pp).
  • Half purple and half white: The dominant allele controls the whole trait. The flowers are not split.
  • Pink: With complete dominance, the traits do not blend into pink.
Practice questionReviewedLife Science › Heredity and DNA★★★★★

Question 3. In pea plants, purple flowers (P) are dominant over white flowers (p). Two Pp plants are crossed. What is the chance that an offspring plant has white flowers?

  1. ① 75%
  2. ② 0%
  3. ③ 25%
  4. ④ 50%
▼ Show answer and explanation▲ Hide answer and explanation

Answer: ③ 25%

Key point: Pp × Pp → 1 PP : 2 Pp : 1 pp

  1. Each parent can pass on P or p.
  2. The Punnett square gives PP, Pp, Pp, and pp.
  3. Only pp is white: 1 out of 4 = 25%.

Check: 1 PP + 2 Pp + 1 pp = 4 boxes.

Wrong choices

  • 75%: 75% is the chance of purple flowers (PP or Pp), not white.
  • 0%: Both parents carry p, so a pp offspring is possible.
  • 50%: 50% is the chance of a Pp offspring, which is purple.
Practice 5 questions from this unit →

AI-written · Verified · Written by AI. Only units that passed a separate AI check (re-reading facts and numbers) are published. Tell us if you find a mistake.