CHAPTER 5 HEREDITY AND GENETICS
What is heredity?
OBJECTIVE: Explain why offspring have some of the traits of their parents but not
all.
Key Terms
trait: characteristic
inherited (ihn-HEHR-ih-tuhd) trait: trait that are passed from parents to their offspring
heredity (huh-REHD-ih-tee): passing of traits from parents to offspring
genetics (juh-NEHT-ihks): study of heredity
Lesson Summary
• Traits are the characteristics of an organism.
• The passing of traits from parents to offspring is called heredity.
• The field of biology that studies heredity is called genetics.
Key Terms
sperm cell: male reproductive cell
egg cell: female reproductive cell
gamete (GAM-eet): reproductive cell
Lesson Summary
• Every body cell of an animal contains the same number of chromosomes.
• Sperm cells and egg cells are gametes, or reproductive cells. Gametes contain half the
number of chromosomes of a body cell.
• Gametes from through a process called meiosis.
What is DNA?
OBJECTIVES: Describe the molecular makeup of DNA. Explain
the role of DNA in living organisms.
Key Terms
DNA: large molecule contained in chromosomes
replication (rehp-lih-KAY-shuhn): process by which DNA is duplicated
protein synthesis: process by which proteins are made
Lesson Summary
• Chromosomes are made of large molecules called DNA.
• DNA contains sugars, phosphates, and four nitrogen bases: adenine, thymine,
cytosine, and guanine.
• The structure of DNA was discovered by James Watson and Francis Crick, with the
help of other scientists.
• Before mitosis can occur, DNA molecules make copies of themselves during
replication. This results in two identical DNA strands.
• DNA contains a genetic code that determines the kinds of inherited traits an organism
will have.
• Proteins are produced during a process called protein synthesis. RNA is a molecule
involved in protein synthesis.
What are chromosomes?
OBJECTIVE: Describe how genes and chromosomes are
involved in heredity.
Key Terms
genes (JEENZ): parts of a chromosome that control inherited traits
allele (uh-LEEL): one of two or more forms of a particular gene
Lesson Summary
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Chromosomes are made up of chromatin.
Chromatin is composed of long, thin strands of DNA.
Genes control inherited traits such as eye color and hair color. .
Organisms that carry out sexual reproduction have chromosomes that exist in pairs.
Humans have 23 pairs of chromosomes.
• Alleles are different versions of the same gene. For example, the gene for hair color
may have one allele for brown hair and one for black hair.
Why can offspring differ from their parents?
OBJECTIVE: Explain the difference between dominant and
recessive traits.
Key Terms
homozygous (hoh-moh-ZY-guhs): having two like genes for the same trait
heterozygous (heht-uhr-oh-ZY-guhs): having two unlike genes for the same trait
dominant gene: gene whose trait always shows itself
recessive gene: gene of a trait that is hidden when the dominant gene is present
Lesson Summary
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Each trait of an organism is determined by at least one gene from each parent.
Organisms that are homozygous have two like genes for a specific trait.
Organisms that are heterozygous have two unlike genes for a specific trait.
In heterozygous organisms, the trait of the dominant gene is always expressed.
The recessive gene’s trait does not show itself in the organism.
3-6 How do genes combine in offspring?
OBJECTIVE: Identify what causes differences in the traits of parents and their
offspring.
Key Term
Punnett square: chart that shows possible gene combinations
Lesson Summary
• In organisms with at least two genes for each trait, each parent contributes one of the
genes.
• Letters are used to represent genes when written. Uppercase letters represent
dominant genes. Lowercase letters represent recessive genes.
• Punnett squares are charts used to show the possible combinations a set of genes may
exhibit.
• If one parent is homozygous dominant and the other parent is homozygous recessive,
all offspring will be heterozygous dominant.
• If both parents are heterozygous for a trait, the offspring have a 25% chance of being
homozygous dominant, a 50% chance of being heterozygous dominant, and a 25%
chance of being homozygous recessive.