Biology Notes Chapter 8

“Mendel and Heredity”

I. The Origins of GeneticsA. 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.

Gregor Mendel

II. Mendel’s TheoryA. 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

Chromosome & Genes

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

III. Studying HeredityA. 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.

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

IV. Complex Patterns of HeredityA. 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

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

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.