Gregor Mendel's experiments with pea plants in the mid-1800s laid the foundations for genetics. Through his work, Mendel discovered the basic principles of heredity, including the concepts of genes, alleles, dominant and recessive traits, and how traits are passed from parents to offspring. Mendel developed theories like the Law of Segregation and the Law of Independent Assortment to explain inheritance patterns. His work allowed scientists to better understand and predict outcomes of genetic crosses using tools like Punnett squares and pedigree charts.

1. “Mendel and Heredity”

2. I. The Origins of Genetics
 A. Heredity – the passing of traits from parents to
offspring.
 B. Genetics – the study of heredity.
 C. Gregor Mendel – a monk whose experiments with
pea plants gave us our basic understanding of heredity.

3. Gregor Mendel

4. II. Mendel’s Theory
 A. Gene – is a segment of DNA that controls a
hereditary trait. These are carried on the
chromosomes.
 1. Alleles – the different versions of a gene.
 a. dominant – allele that is the expressed form of the trait.
Always shown by a capital letter. Ex: T
 b. recessive – allele that is not expressed when the dominant
allele is present. Always shown by a lower case letter. Ex: t

5. Chromosome & Genes

6.  B. homozygous – when the paired alleles for a trait are the same, either both
dominant or both recessive.
Ex: RR (homozygous dominant) or rr (homozygous recessive)
 C. heterozygous – when the paired alleles for a trait are different, one dominant and
one recessive. Ex: Rr
 D. genotype – the gene combination present in an organism.
Ex: RR, Rr, rr
 E. phenotype - the physical appearance resulting from the genes. Ex: round
seeds, wrinkled seeds
 F. law of segregation – when the paired alleles for a trait are separated during
meiosis and the formation of the gametes.
 G. Law of Independent Assortment - alleles for a trait separate when gametes are
formed.
 EX: There are 4 ways the genes from DdRr can sort: DR, Dr, dR, dr

7. III. Studying Heredity
 A. Punnett square – a diagram used to predict the
outcome of a genetic cross.
 B. Probability – the likelihood that a specific event will
occur. Can be expressed in
words, decimals, percentages, or as fractions.

8.  C. Pedigree – a diagram that shows a family history
and how a trait is inherited over several generations.

9. IV. Complex Patterns of Heredity
 A. Sex-linked – any trait whose allele is carried on the X
chromosome.
 1. Because males have just one X chromosome, any allele on
the X chromosome is expressed.
Ex: hemophilia, color blindness
 B. Polygenic trait – when several genes influence a trait.
Ex: haircolor, eye color

10.  C. Incomplete dominance – when an individual
displays a trait that is intermediate (a blending)
between the two parents.
Ex: straight haired X curly haired = wavy haired
red flower X white flower = pink flower
 D. Multiple alleles – genes with 3 or more alleles.
Ex: blood types in humans (A, B, O)
 E. Codominance – two dominant alleles are expressed
at the same time.
Ex: blood type AB

11.  F. Mutations – changes in the chromosome structure.
The harmful effects produced by inherited mutations
are called genetic disorders.
 1. sickle-cell anemia – produces a defective form of the
protein hemoglobin. Most common among African-
Americans.
 2. cystic fibrosis – most common fatal genetic disease
among Caucasians.
 3. hemophilia – sex-linked recessive disorder. Condition
that prevents the blood from clotting properly.
 4. Huntington’s disease – does not appear until age 30-
40. Loss of muscle control, physical spasms, severe
mental illness, and finally death.