Chromosomes are made of proteins and deoxyribonucleic acid, or DNA—an organism’s genetic material.DNA The Structure of DNA A gene is a segment of DNA on a chromosome.

DNA is shaped like a twisted ladder which is referred to as a double helix. The Structure of DNA (cont.)

A nucleotide is a molecule made of a nitrogen base, a sugar, and a phosphate group.nucleotide Sugar-phosphate groups form the sides of the DNA ladder. The nitrogen bases, adenine (A), cytosine (C), thymine (T), and guanine (G), bond and form the rungs of the ladder. The Structure of DNA (cont.)

A and T always bond together, and C and G always bonds together. The Structure of DNA (cont.)

ReplicationReplication is the process of copying a DNA molecule to make another DNA molecule.

The DNA of each cell carries the complete set of genes that provide instructions for making all the proteins a cell requires. Proteins are made with the help of ribonucleic acid (RNA)—a type of nucleic acid that carries the code for making proteins from the nucleus to the cytoplasm.RNA Making Proteins

RNA: is made of nucleotides. is single-stranded. has the nitrogen base uracil (U) instead of thymine (T), which is found in DNA. Making Proteins (cont.)

TranscriptionTranscription—the process of making RNA from DNA—is the first step in making a protein.

Lesson 3 The process of making a protein from RNA is called translation. translation

A change in the nucleotide sequence of a gene is called a mutation.mutation The 46 human chromosomes contain between 20,000 and 25,000 genes that are copied during replication. Mutations can be triggered by exposure to X-rays, ultraviolet light, radioactive materials, and some kinds of chemicals. Mutations

There are several types of mutations. Three types of mutations are substitution, insertion, and deletion.

Mutations (cont.) In a deletion mutation, one or more nitrogen base is left out of the DNA sequence. In an insertion mutation, one or more nitrogen bases is added to the DNA. In a substitution mutation, one nitrogen base is replaced by a different nitrogen base.

The effects of a mutation depend on where in the DNA sequence the mutation happens and the type of mutation. Some mutations in human DNA cause genetic disorders. Mutations (cont.)

Heredity is the passing of traits from parents to offspring. Heredity Gregor Mendel is known as the father of genetics—the study of how traits are passed from parents to offspring. genetics Early Ideas About Heredity

Mendel studied genetics by doing controlled breeding experiments with pea plants. There are two types of pollination: self-pollination cross-pollination Mendel’s Experimental Methods

Self-Pollination

When a true-breeding plant self-pollinates, it always produces offspring with traits that match the parent. Mendel cross-pollinated pea plants himself and recorded the traits that appeared. Mendel’s Experimental Methods (cont.)

Cross-Pollination

Once Mendel had enough true-breeding plants for a trait he wanted to test, he cross-pollinated selected plants. Plants are called hybrids if they come from true- breeding parent plants with different forms of the same trait. Mendel’s Results

First-Generation Crosses

Mendel also cross-pollinated hybrid plants. He observed that offspring of hybrid crosses always showed traits in a 3:1 ratio. Mendel’s Results (cont.)

Second-Generation (Hybrid) Crosses

Mendel’s Results (cont.) Mendel recorded traits of offspring from many hybrid crosses.

Mendel concluded that two factors, one from each sperm and one from each egg, control each inherited trait. Mendel’s Conclusions

Mendel’s Conclusions (cont.) A dominant trait is a genetic factor that blocks another genetic factor.dominant trait A recessive trait is a genetic factor that is blocked by the presence of a dominant factor.recessive trait

Mendel concluded that two factors—one from each parent—control each trait. Mendel’s “factors” are part of chromosomes which exist as pairs—one chromosome from each parent. Each cell in an offspring organism contains chromosomes from both parents. What Controls Traits

A gene is a section on a chromosome that has genetic information for one trait.gene The different forms of a gene are called alleles. Each chromosome has one allele for every gene on it.alleles The two chromosomes in an offspring cell may have the same or different alleles. What Controls Traits (cont.)

Geneticists call how a trait appears, or is expressed, the trait’s phenotype. phenotype The two alleles that control the phenotype of a trait are called the trait’s genotype. genotype What Controls Traits (cont.)

When the two alleles of a gene are the same, the genotype is homozygous. homozygous If the two alleles of a gene are different, the genotype is heterozygous. heterozygous What Controls Traits (cont.)

Scientists use uppercase and lowercase letters as symbols to represent the alleles in a genotype.

A Punnett square is a model used to predict possible genotypes and phenotypes of offspring.Punnett square Modeling Inheritance

A pedigree shows phenotypes of genetically related family members. Modeling Inheritance (cont.)

Sometimes traits appear to be blends of alleles. Alleles show incomplete dominance when the offspring’s phenotype is a blend of the parents’ phenotypes.incomplete dominance Codominance occurs when both alleles can be observed in a phenotype. Codominance Complex Patterns of Inheritance

Some traits, such as human ABO blood type, are determined by more than one allele. Complex Patterns of Inheritance (cont.)

Polygenic inheritancePolygenic inheritance occurs when multiple genes determine the phenotype of a trait. Complex Patterns of Inheritance (cont.)

An organism’s environment can affect its phenotype. Some examples of environmental factors that affect phenotype are soil type that a flower is growing in or time of year that a butterfly develops. Genes and the Environment